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design

engineering

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EDITORIALJ. D. Beadle

ART EDITOREdna A . Moore

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the devel-the points-

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g chaptersF Fastening

idcompar-int perfor-ms well asner' brings

pins , riv-

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o chapters. g.washersdevoted to

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For details of other books in this series please contactthe Publisher

1

Design Data

Chapter 1 FASTENERS - ORIGIN, EVOLUTION & SELECTIOND. N. Pearce.

Page 5

Chapter 2 RETAINING RINGS & FIXESF. H. Bowler.

Moulded Fasteners Ltd.

,

Plastics Div. , Geo. Salter & Co. Ltd.

Page 13

Chapter 3 EYELETSW.T.J. Bownes,Geo. Tucker Eyelet Co. Ltd.

Page 18

Chapter 4 INSERTED FASTENERSH. D. Chambers, C. Eng, M.I. MechE.Armstrong Patents Co. Ltd.

Page 26

Chapter 5 NUTS - CAGEDE. Larner,Firth Cleveland Fastenings Ltd.

Page 35

Chapter 6 NUTS - CLINCH & ANCHORA. Jordan,G.K.N. Bolts & Nuts Ltd.

Page 36

Chapter 7 NUTS - LOCKINGT.E. Harris.

Page 44

Chapter 8 SINGLE THREADED FASTENERSB. M. Wright,

Carr Fastener Co. Ltd.

Page 54

Chapter 9 NUTS - PLAIN & WELDR.W. Lowe,G.K.N. Screws & Fasteners Ltd.

Page 63

Chapter 10 PLASTICS FASTENERSA. Griffiths.

Page 70

Chapter 11 PINS - SOLID & TUBULARR.G. Thatcher,

Spirol Pins Ltd.

Page 76

Chapter 12 PROJECTION WELDED FASTENERSC.H. Meader,K. S. M. Stud Welding Ltd.

Page 79

Chapter 1 3 QUICK RELEASE FASTENERSH.J. Smith and M. R. P. Knight, A.M. B.I. M.Dzus Fastener Europe Lid.

Page 93

Chapter 14 RIVETS - BLIND (METAL & PLASTICS)J. S. Sanders, B. Eng.

,

Avdel Ltd.

Page 98

Chapter 15 RIVETS - SOLID & TUBULAR' J.M.A. Paterson, M. A..J.P.

,

The Bifurcated & Tubular Rivet Co. Ltd.

Page 108

Chapter 16 SCREWS - MACHINED. S. Thompson,G. K. N. Screws & Fasteners Ltd.

Page 114

Chapter 17 SCREWS - SELF TAPPING ETCT.E. Harris.

Page 124

Chapter 18 SCREWS - SETDennis Troop and Barbara Shorter,

Unbrako Ltd.

Page 132

Chapter 19 SCREWS - WOODJ. M. Humphrey, C. Eng. , M. I. Mech. E.G.K.N. Screws & Fasteners Ltd.

Page 138

Chapter 20 SPRING STEEL FASTENERSH. D. Browne,Firth Cleveland Ltd.

Page 144

Chapter 21 WASHERSR. M. Billington, M. Inst. M. S. M.

,

Morlock Industries Ltd.

Page 150

Chapter 22 STRUCTURAL ADHESIVESE. B. McMullon and D. T. S. Ilett

CIBA (ARL) Ltd.

Page 155

Chapter 23 SELECTED SPECIAL FASTENERSA. Griffiths.

Page 161

Directory

EQUIPMENT DIRECTORY

MANUFACTURERS ADDRESSES

INDEX TO ADVERTISERS

Page 169

Page 173

Page 176

ACKNOWLEDGMENT

The Editor and Publisher gratefully acknowledge the help and assistance that have been given in the comp-ilation of this handbook by many companies in the Fastener Industry.

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Fasteners - origin, evolution

and selectionby D.N. Pearce.

A suitable definition of a fastener is as follows:

'A device that will position and hold two or moremembers in a desired relationship to each other'.

To understand when man first used fasteners it is

necessary to study the origins of toolmaking. Per-haps with the Pliocene, certainly by the dawn of

the Pleistocene, that is about a million years ago,

the typically human level of cerebral developmenthad been reached. Stone artifacts of standardised

types have been found in lower Pleistocene deposits

in various parts of Africa, and in deposits only

slightly more recent in Asia and Western Europe.They show that toolmaking was no longer merelyoccasional, but served permanent needs of theseearliest men. Examples have been found of chop-pers, crudely fashioned from quartz stone andbroken animal bones, flint axes were also usedduring this period.

The making of cord and rope by plaiting or twisting

fibres, hair and strips of hide presumably beganin Palaeothic times, since Stone Age man neededcordage for fishing equipment and for the construc-tion of traps. The idea of lashing parts together,

for example, could have originated in association

with a free-mutation, but it became established as

the basis of a general method by which a desiredconnection between any two suitable componentscould be effected or reinforced. As just suggested,the idea of attaching a stone blade to a wooden haft

may have arisen out of some incidental method of

holding the blade temporarily in position in a bent

haft, and perhaps only at a later stage with a lash-

ing for security.

It is tempting to suggest that man discovered the

'principle' of the sleeve, or the socket, and, sayrotary motion, but what he actually discovered wasthat, in the case of the sleeve and the socket, theseparticular features of form facilitated the union of

two components, and, in the case of rotary motion,that circularity in certain mechanisms had its ad-

vantages.

The tripartite disc is demonstrably the oldest aswell as the most wide- spread form of simple wheel.About 2,750 years B. C. , wooden pegs were fixed

through the axle to prevent the wheel coming off.

In a tomb at Susa (2000 B. C. ) the peg is replacedby a copper bolt with a decorative head, preciselylike the linch pins familiar in later periods. Thewheels were generally mortised together and some200 or 300 copper nails were driven into the cir-

cumference to protect the rims from wear. By the

year 2000 B. C. copper tyres were being used on

chariots and these were attached to the rim with

copper nails. In 1475 B. C. Egyptian wheelwrightsbegan making spoked wheels, these consisted of a

hub with axle hole and sockets for the spokes anda felloe or felloes. The Egyptians normally madetheir felloes as several segments of wood, carvedseparately to fit on the same circle and then con-nected by mortise and tenon joints. By 500 B. C.Celtic wheelwrights in Bohemia were already shap-ing a felloe from a single length of timber, bentinto a circular form with heat, the ends were bevel-led and overlapped, and the junction held together

by a metal swathe, which was nailed or riveted to

the rim.

An insistent problem for the metalworkers was thejoining of several pieces of gold or copper, theycould be fastened with pins or rivets, which wereindeed commonly used for fixing handles to a dag-ger or knife, or for sheet metal work, as in a type

of drinking vessel, the body of which was built upof separate pieces. In Ireland, the goldsmith fast-

ened plates by folding the edges together or by sew-ing them with wire. In the Near East, an observantcraftsman, melting together two nuggets of goldfrom various sources, noticed that some fused

earlier than others and, spreading over the rest,

bonded them together.

Moreover, he found that it was always the nuggetsfrom a particular source that melted first. Ofcourse he could not have known, as we do, that

native gold is always alloyed with some other met-als, and that gold with a proportion of copper orsilver melts at a lower temperature than purergold. Nevertheless, such easily fusible gold was,in fact, the earliest solder, and long preceded anyconscious attempt to make solder by adding copperor silver to gold.

After this discovery, search was naturally madefor a similar material by which copper or bronzemight be joined. Modern brazing materials (i. e.

materials for joining copper and its alloys) areusually composed of alloys of copper and zinc. TheRomans appear to have made brass by smeltingcopper ores with callamine. Probably the earliest

example of essentially pure zinc is a coin of YungLo (A. D. 1402) Ming dynasty, China. Brazingmaterials, like all other hard solders, requirea high working temperature and form much strong-er joints than soft solders.

A solder is a metal or alloy which, having a lowermelting point than the pieces to be joined, may becaused to flow between those pieces and, on cool-

ing, bond them. There is a clear distinction bet-

ween hard soldering which needs a temperature of

550-900 C or higher, employed for jewellery, sil-

ver work and better class copper and bronze work,and soft soldering for joining tin plate, lead, etc. ,

which may need only 183°C or less. Copper and

gold melt at nearly the same temperature (1083°Cas against 1063°C) but if 10 parts by weight of- cop-per are added to 90 of gold the melting point of thealloy falls to 940 C, which suffices to make it asafe solder for pure gold. If 18 parts of copper areadded to 82 parts of gold, the alloy will melt at

878°C, the lowest melting point of any gold/copperalloy. If a lower melting pint is required a propor-tion of another metal, such as zinc (melting point at

419°C) must be added. There has been confusion

in technical literature as to the methods 'actually

employed by the early craftsmen in joining pieces

of gold, electrum, silver, bronze or copper. State-

ments that they were fused together by autogenouswelding, without solder, or welded together byhammer as a blacksmith welds iron, are erroneous.

By 2500 B. C. the soldering of gold and silver wasas well known as it is today. The ancient gold-

smiths, to whom so easy a process as soldering

was available, would not have attempted the dif-

ficult, if not impossible task, of welding. Burningtogether was practised from the Bronze Age on-

wards. By this method a joint can be made on a

bronze tool or weapon without the aid of solder. Abronze sword, broken at the hilt, might thus berepaired. A smith fitted the pieces together and

formed a mould in clay around them. He left a

passage all round the joint and provided the mouldwith a funnel shaped pour for the introduction of

the metal, and an overflow hole. Then he pouredinto and through the mould several pounds of moltenbronze. The metal flowed between and heated upthe broken parts of the sword partly melting them.

Most of the molten metal escaped through the over-flow hole, but enough remained to make the joint

strong. Superfluous metal could be cut away later.

Welding is the art of joining separate pieces of

metal by heat or mechanical treatment without sold-

er. For wrought iron it requires a temperature of

about 1350°C. At this temperature scales of iron

oxide flake off continually from incandescent sur-

faces, leaving them clean. The metal is in a pasty

state and the surfaces to be united can come into

intimate contact. The crystals at the surface breakup under the hammer blows, and the fragmentsgrow into new crystals interlocking across the joint.

The welding of metal has been practised from early

times in Asia Minor as, for example, on the iron

head rest of Tutankhamen (1350 B. C. ) when as yet

iron was practically unknown in Eqypt. This headrest was probably a gift from some ruler in Syria,

where iron working was more advanced. Not until

welding and the making and hardening of steel be-

came well understood, which in Syria was betweenthe 11th and 9th Centuries B. C. , can a true Iron

Age be said to have begun.

The principle of the wedge was known to man from

early times. Examples have been found holding

together the pieces of a Greek mining tool (300

B. C. ). The Romans used wooden wedges for rock

splitting, driving them into the rock then saturating

them with water so that they swelled and split the

rock. The wedge was used for fixing together Ro-

man lever and screw presses, used for extracting

the juice from olives and grapes.

The so called stick furniture was of a very simple

construction and was used for Roman times. Seats

of chairs and stools and the tops of tables wereslabs of wood upheld on three or four legs. The

tops of the legs penetrated the seat and were held

tight by wedges. Other good examples of the wedge

as a fixing device can be seen in early Roman cata-

pults and cross bows.

The peg was used for fastening from very early

times, particularly in ships. During the late Bronze

age this method of building was used for the Home-ric ships and its application was almost the sameas that employed for some wooden ships today. Thevessels had keels, stem and stern posts, and ribs

covered with outside planking. The construction

was fastened together with wooden pegs (tree nails),

a method which has only recently been generally

superseded by metal fastenings.

Rivets are known to have been used since 2,000

B. C. Good examples of riveted copper metal workhave been discovered in the shrine of a temple at

Drecros, Crete (750 B. C. ). Probably no finer

instance of riveting has survived from the ancient

world than a bronze trumpet of the late Celtic per-

iod in Ireland, it is 8 ft. long and made from sheet

bronze bent round to form a tube. The abutting

edges are riveted to a strip of bronze about § in.

wide, and there are no fewer than 638 rivets along

the seam.

Nails were widely used by the Romans, thus follow-

ing the methods used by the Egyptians a thousand

years earlier. The Greeks used iron nails of vari-

ous forms to fix terra- cotta facings to timber or

stone structures. Viking vessels found at Nydamlate 4th Century A. D. showed planks attached to

ribs with iron nails.

Other good examples of the use of nails can be seen

in Tudor furniture and also in Gothic doors, wheremany wrought iron nails with square heads wereused. The method of producing these nails wasextremely primitive and it involved hammeringthe metal through a graded series of holes until

the correct diameter was obtained.

Perhaps no other device has played such an import-

ant role in the development of fastening techniques

as the screw, and it is worth examining its evolu-

tion in some detail. The auger, translating a cir-

cular motion to a linear motion along its axis of

rotation, is related to the screw which was certain-

ly known before Archimedes (287-212 B. C. ), to

whom it has been falsely ascribed. It may have

been invented by Archytas of Tarentum, a Pytha-

gorean philosopher and mathematician (400 B. C. ),

though the evidence is unreliable. Screws of metalwere, however, known in classical antiquity. Manymachines for working metals are illustrated by

Leonardo though it is uncertain whether they re-

present his own ideas or apparatus already known.

One of the most original of Leonardo's machinesis the screw cutter, a model of which can be seenin the Science Museum, South Kensington. Multi-plication of forces by pulleys had been known sincethe invention of pulley blocks in antiquity. Reduc-tion of velocity by the screw was, however, probab-ly a Mediaeval invention. The earliest known re-cord is in the chronicle of Gervais the Monk (1200

A. D. ) who mentions the use of screws for lifting

loads. By the 15th Century it was commonly usedin bending the cross bow.

The use of the screw stopper on pottery bottles wasknown in the Mediaeval period. A wooden chasingdevice was used to form the thread in the neck of

the bottle, the top was made from a cast of the

vessel.

The development of light engineering and toolmak-ing is closely associated with the extension of the

use of the screw. Although taps and dies wereunderstood and are sketched by Hero of Alexandria,

screws were made with the simples hand tools.

Screw cutting lathes were first in existence in the16th Century but appeared to have been intendedfor use in ornamental work. Despite the adequacyof these machines in principle, they could not beused in practice and long screws in wood or metalwere cut with chisel or file, much as in antiquity.

Short screws both coarse and fine and in metal orwood, were commonly used for scientific purposesafter 1650, for focusing microscopes and on manymeasuring instruments. Long screws were, how-ever, expensive and likely to be inaccurate. Theuse of the long lead screw was obstructed by thesedifficulties in production. It is significant that

lathe work was developed on an alternative prin-ciple that presented less technical difficulty. Theso called mandrel lathe was controlled by one ormore short screws which gave the work a traverseof a few inches. Small pieces could be turned withthe guidance of these screws supplemented by someform of fixed support for the cutting tool. It is

difficult to trace the development of the lathe in the17th Century before Plumiers account of 1701. It

was still used principally for ornamental turning,

but it embodied principles that were later to be of

industrial significance, especially in clock andwatch making. In Plumiers time it was possibleto cut the screws for the arbors of the mandrel ona lathe. Plumier was anxious to do so because it

was difficult to produce a perfectly cylindrical man-drel with a file, but he found only two workmen in

Europe capable of turning satisfactory mandrels in

iron and steel. They used lathes of special con-struction firmly fixed between floor and ceiling

and backing against the wall. A model of the mand-rel was made in wood, somewhat larger in dia-meter than the finished article. The iron was first

forged to this copy and turned to the shape requiredin the lathe. A thread was then cut upon the endof the turned mandrel. The mandrel was rotatedby means of a cord looped around it which was at-

tached to a foot treadle and a pole.

The production of screws by mechanical meanswas thus severely limited. Techniques of casting

should have been applicable to the production of

screws in bronze. Cast iron would hardly havebeen satisfactory.

Long before the forging process was introduced

bolts and screws were hammered out by hand (it

was not until the 19th Century that bolts were pro-

duced by forging). These early bolts were manu-factured from a square steel bar which was heated.

The cylindrical shank of the bolt was hammeredout of the square. This crude product was the fore-

runner of the present square head machine bolt.

For precision work or for cutting lead screws, the

following precedure was adopted. On a rectangular

sheet of paper transverse lines were drawn, the

spacing and angle of inclination of which corres-ponded to the thread to be traced. The paper waswound around the rod to be made into a screw andthe threads were traced by following the line with

a sharp file, the cutting was done first with a trian-

gular file and finally with a steel chaser havingteeth spaced to correspond with the pitch of the

screw to be made.

Such methods took a long time and the quality of

the result depended entirely upon the skill of the

operator. To overcome these difficulties the great

English inventor and engineer, Jesse Ramsden(1735-1800), invented in 1770 the screw cutting

lathe which was certainly the first machine of this

type ever constructed and which gave satisfactory

results.

Henry Maudsley (1771-1831), introduced in 1797a lathe fitted with a slide rest, this was another

major step forward and it was widely adopted in

the screw making trade.

Maudsley gave much attention to the initial forma-tion of accurate screw threads. In the method final-

ly adopted a hard wood cylinder was rotated in a

suitable holder against a crescent shaped knife held

obliquely to its axis. The knife, in cutting into the

cylinder, caused it to traverse, thus generating a

screw which could be copied in steel. Using anaccurately made screw Maudsley was also able to

make a bench micrometer accurate to 0. 0001 in. ,

which served him as a workshop standard.

Sir Joseph Whitworth (1803-1887) was the son of

a school master. At the age of 22 he went to Lon-don and joined Maudsley. In 1833 he started his

own machine tool manufacturing business in Man-chester. He produced a very wide range of mach-ines which were quite revolutionary in their con-cept and were certainly years ahead of their time,after a few years he was manufacturing lathes,

planing machines, shapers, slotters, plane and rad-ial drills, punching and shearing machines, nut

shapers, screwing machines, wheel cutting anddividing machines. Whitworth was responsiblefor bringing about the standardisation of screwthreads. He collected and compared screws fromas many work shops as possible throughout Englandand in 1841 proposed, in a paper to the Institution

of Civil Engineers, the use of a constant angle (55^between the sides of the threads, and a specifica-

tion for the number of threads to the inch for the

various screw diameters. The Whitworth threadremained standard in engineering until 1948.

In America in the year 1855, Robbins and Lawrenceproduced what they called a screw milling machine,because it was used primarily for making screws.Today we would call this an eight tool turret lathe.

However, at the time of its invention it certainly

represented the most advanced evolution of the

lathe and substantially reduced the manufacturing

costs of screws.

The Civil War (1861-1865) stimulated in the UnitedStates a need for higher output with less expendi-ture of labour, and this played an important part

in the evolution of automatic machine tools. Auto-matic lathes for the mass production of screwswere built during the war, but the machine havingthe most far reaching influence on the developmentof automatic manufacture was designed by C. M.Spencer, shortly after the war. Spencer built alathe which incorporated cylindrical cams, later

known as 'brain wheels'. Movement of. the cuttingtools and turret was controlled by adjustable camsfitted on the cam cylinders, which were geared to

the spindle drive. So long as the machine was fedwith bar stock it automatically manufactured until

wear or breakage of the tools required them tobe changed. Spencer's lathe was widely used in

America for the production of screws and similarcomponents, and subsequently small automaticlathes have always been known in America as auto-matic screw-machines.

A British automatic machine for the production of

screws was patented by C. W. Parker in 1879 andbuilt by Greenwood and Batley. The bar stock wasfed through the head stock and turned to the correctsize by stationary tools, which were then withdrawnto allow the screw die to advance and cut the thread.The screw was cut from the bar by a parting tool.

The machine was originally designed to finish thehead of the screw, an operation later carried out

on a separate machine. The machine could producescrews t in. diameter at the rate of 80-150 an hour,according to their length. The movement of the

cutting tools were derived from a shaft carryingcams that ran along the bed of the machine. A rol-

ler feed for the bar stock was incorporated in the

machine.

In 1895 an important new principle was introduced

into the construction of the automatic lathe whena multi- spindle automatic was built in the UnitedStates for the manufacture of sewing machine com-ponents. A prototype of the first five-spindle auto-matic was built in the USA in 1897, and by the endof the Century four- spindle machines were com-mercially available both in the USA and in Sweden.

In the early 19th Century small nails were shearedfrom sheet. The sheet was cut to the width requir-

ed for the length of the nail and was fed forwardinto a shear blade set at a small angle, being turn-

ed over between each stroke of the blade. The tap-

ered nail, of rectangular section, was headed in

another machine. Before the end of the Century,

however, nail and rivet machines using mild steel

wire were capable of turning out 300 componentsa minute, cutting and, in the case of the nail, point-

ing simultaneously, heading was done in the move-ment which ejected the component from the machine.

This Chapter would not be complete without somemention of the Guest Keen and Nettlefold organisa-tion which probably had more effect on the growthof the fastener business than any other company in

Great Britain.

The oldest part of the Guest Keen and Nettlefolds

group is the steel works which was founded by JohnGuest in 1759 when he started making steel at Dow-lais in South Wales. John Guest and his successorswere very accomplished steel masters making rail-

way rails and other steel products which came with

the industrial revolution.

The second name commemorates Arthur Keen whostarted work as a railway clerk in Smethwick andeventually went into his landlord's business andthen married his bosses daughter, eventually buy-ing a patent nut and forming the Patent Bolt andNut Company Limited. This company bought Lon-don Works from Fox Henderson Limited and madebolts and nuts, on the site of which is now the reg-istered office of Guest Keen and Nettlefolds Limit-ed. In 1900 Guest and Company Limited amalga-mated with the Patent Bolt and Nut Company, to

become Guest Keen and Company Limited.

In the early 1830's John Sutton Nettlefold left the

family's business of ironmongers in London andcommenced making woodscrews in a water mill at

Sunbury on Thames, and shortly afterwards movedto gain the advantages of being near the Black Coun-try and founded a factory in the centre of Birming-ham. He progressed for approximately 20 yearswhen he bought an American patent for putting agimlet point on woodscrews. To exploit this hecommenced to build a factory at Heath Street; onthe borders of Smethwick and Birmingham whichis the site of the present works. In order to pur-chase the patent he borrowed money from his father-in-law, Joseph Chamberlain the 1st, and took into

partnership Joseph Chamberlain the 2nd, the part-

nership being known as Nettlefolds and Chamberlain.This partnership flourished until 1874 when Cham-berlain decided to go into politics full time, andJohn Henry Nettlefold, son of the founder, continuedthe company under the name of Nettlefolds until

1880 when a limited company was formed as Nettle-folds Limited. This company amalgamated withGuest Keen and Company to become Guest Keenand Nettlefolds Limited, in 1901.

An so we come to the 20th Century where the devel-opment of new types of fasteners and fastening sys-tems has been extremely rapid, particularly duringthe last 35 years.

Whereas the designer in the 19th Century had avery limited number of different fasteners avail-

able to him, the situation today is completely re-versed. There are many thousands of different

types of fasteners to choose from, and the problem

facing today's designer is to decide which of thesefasteners is the best for his particular application.He should, therefore, keep in mind these funda-mental considerations in fastening selection:

1. Is the fastening necessary?2. Is the minimum number of fastenings specified?3. Does the fastener specified perform the jobbest?

4. Is the fastener simple to apply?5. Will the fastener have to be removed duringservice and if so will it be easily removable ?

6. Does the fastener have the proper specifica-tions for material?7. Does the fastener have the proper specifica-tions for finish ?

Now let us elaborate on each of the foregoing con-siderations:

Is the fastening necessary?

It is probably true to say that with careful productdesign and with the application of value analysistechniques, a considerable number of the fastenersin use today could be eliminated, thus reducingmaterial and assembly costs and at the same timeupgrading the product performance.

In many instances, especially on stampings andinjection moulded plastics components, the functionof several components can be combined, therebyeliminating separate fasteners entirely. Springmembers can often be attached by latching methods.

Is the minimum number of fastenings specified?

A good example of this is a cover plate where often

four screws are used. One screw would hold thecover down provided the design caters for properlocation, which can be achieved by recessing thecover into the unit or by providing locating tabs onthe cover or the unit. Alternatively the cover couldbe an injection moulding provided with a 'tuck under'locating tongue or tongues and an integrally mould-ed fastener detail which would engage in a hole in

the unit. Also always check strength requirementsto avoid wasteful 'over engineering'.

Does the fastener specified perform the job best?

A fastener that fails in service is both unreliableand uneconomical, therefore, the operating environ-ment should always be checked and a fastener selec-ted that will withstand the physical effects involved.

Always consider what forces will act on the fasten-er and whether extremely high temperatures will beinvolved during manufacturing or in service. Do notexpect fasteners to overcome faulty design of com-ponents or assembly. The proper fastener canonly be selected after, or even better at the time,the joint or assembly has been properly designed.

Every part of a fastener, i. e. in the case of the

screw, the head, the thread, the point and the wash-er, should be selected to perform a specific func-tion. Consider each feature as a means of improv-ing performance. It is most important when con-

sidering function to ascertain whether the assemblywill be subjected to vibration. This particular

aspect is discussed in more detail towards the endof this Chapter. If the fastener is one of a groupwith inter- related hole centres, which have to ac-cept a 'mating' component, it is desirable to specify

fasteners which will float in their mounting holesand thus enable wide manufacturing tolerances tobe used, i. e. plastics captive nuts, caged nuts orsimilar.

Is the fastener simple to apply?

Use one-piece multi- function fasteners whereverpossible. They are best suited for both automaticassembly and manual application. If the article

to which the fastener is attached will be subjectedto a finishing process, i. e. plating or painting,

prior to final assembly, then it is essential wher-ever possible to specify a fastener which can befitted after the finished process, thus obviating a

costly and time consuming re-tapping operation to

clear contamination from the threads. Always try

to specify a fastener which can be easily assembledby hand, or automatically, to the upper side of the

work piece. Generally speaking welding and stak-

ing operations are expensive because they are timeconsuming and it is difficult to replace a fastenerif it is damaged in the subsequent assembly opera-tions.

It is not uncommon in manufacturing today to find

instances where the cost of wages or overheads

equal or exceed all other costs of the finished pro-duct. To reduce these basic costs and increaseprofits it is necessary to produce more in the sameamount of time. Production efficiency and economycan be markedly improved by the selection of afastener which can save assembly- line man hours.

It has been established that 19 per cent of the realcost of fastening is in the piece part price. 81 percent of the cost of fastening is in the application

on the assembly line.

The above point is .extremely important, alwaysremember that a 10 per cent saving in assemblycosts can be more significant than a 40 per centsaving in piece part price.

Will the fastener have to be removed duringservice and if so will it be easily removable?

This is an important consideration which is fre-

quently overlooked, some designers only concernthemselves with the initial assembly and give little

or no consideration to removal and replacementduring service. A designer's responsibility doesnot finish when the finished product leaves the fact-

ory because, in the event of the product having to

be dismantled to rectify a fault or for routine serv-icing, it is important that these operations can becarried out by the service engineer or mechanicin the minimum of time. In many cases particular-ly in the event of a warranty claim, the cost of this

work has to be borne by the manufacturer, there-fore affecting his overall profitability.

•

9

If it is known that the fastener will be in a posi-

tion that is subject to rapid corrosion, the designershould give consideration to using fasteners whichwill not freeze up. There are a wide range of excel-

lent injection moulded plastics captive nuts avail-

able today which are easily snapped into place dur-ing final assembly and are self retaining. Theyprovide insulation at the fastening point and arecorrosion free. They also have a prevailing torquetype locking action and automatically accommodatefor any panel misalignment. It is virtually imposs-ible to overtighten them and with the latest designsof 'reverse' nuts with split heads the metal screwwill often fail before the nylon nut.

Does the fastener have the proper specificationsfor material?

Quite often stainless steel is specified for fastenerswhen brass or aluminium could do the job just aswell, and with considerable cost savings.

Does the fastener have the proper specifications

for finish?

Always define the function of the component andselect a plating or paint specification which hasthe minimum cost but which will meet the designspecification. Particular attention must be givento the avoidance of hydrogen embrittlement whenconsidering finishes for spring steel components,this point is covered in more detail at the end of

this Chapter.

VIBRATION

Some fasteners must, of course, carry heavy loads.

They resist various combinations of tension andshear loading, usually without permitting any sig-

nificant relative movement of the fastened parts.

Most threaded fasteners are screwed up tight so

that they clamp the fastened parts together. It is

desirable to maintain this initial clamping force-

or as large a portion of it as possible.

Although the primary function is to permit conveni-ent assembly and disassembly, threaded fastenersare expected to stay in place between those events,without fail!

Characteristics the designer wants then, are, re-liability, strength, tightness and convenience in

service. He looks for ways to combine all these

ideals economically.

If a fastener loosens and falls off during service it

has failed as completely as if it had broken. A bolt

that is strong enough to carry its load when tight

may fail from fatigue if the joint loosens enough to

permit 'fretting' - or even if some of the initial

clamping force is lost. In fact, in the case of apre-stretched joint, failure has occured as soon as

the pre- stress is lost, which may be a long timebefore the bolt 'rattles'.

It is well known that nuts and bolts tend to loosenif the components they fasten are subjected to vibra-

tion or repeated impacts. A generally accepted

theory explains how motion of the fastened parts

can cause turning of a nut on a bolt. To visualise

this situation, consider a weight resting on an in-

clined plane. If static friction exceeds the com-ponent of weight that tends to cause sliding, the

body remains at rest. If the plane surface is vi-

brated or if mechanical shocks are applied to it,

the effective coefficient of friction is reduced. Asvibratory motion of the plane surface becomesmore intense, a point can be reached when a weight

begins to slide down the plane. A loose nut on an

axially vibrating bolt will tend to 'walk' up and

down the bolt. The mechanism is much the sameas the sliding weight.

Vibration reduces the effective coefficient of fric-

tion and provides energy. Masses and shapes are

never perfectly symmetrical, and consequently

that energy produces motion. A few hours after

assembly a 'settling down' process takes place.

The mechanical fits and finishes involved deter-

mine to a great extent how much initial clamping

load will be lost. With precise, well finished,

parts, this relaxation may be limited to 2 or 3 per

cent of pre- stress. With rough surfaces, loose

thread tolerances and lack of squareness, as muchas 10 per cent of the original loading may be lost.

If fastener loosening is caused by repeated mech-anical shocks which set up extremely high frequ-

ency vibrations, in fastener systems, there is not

much hope of solving problems by eliminating these

shocks, they are characteristic of the fastener

environment and cannot be avoided. There is nopractical way to 'tune out' all the exciting forces.

The factors that tend to prevent loosening are, high

pre-stress or bolt tension, the length of bolt understress and vibration energy dampening. Of these

three, bolt tension and length are relatively inflex-

ible, being determined by the individual fastening

application. Dampening, however, is of special

importance. Measured against all the practical

requirements a fastener dampening material mustmeet, nylon plastics emerges as a clear first

choice. It is a good damper of high frequency elas-

tic waves in fasteners. It stays in place. It lasts

indefinitely in service. Nylon has a memory of

its initial shape and it tends to recover after de-

forming forces are released. It serves as a lubri-

cant during assembly and disassembly. The plas-

tics does not harden, flake, powder or crumble,however, nylon is not a usable material at temp-eratures above 350°F, and it is this one limitation

that prevents almost universal use of nylon in self

locking fastener systems.

In conclusion then, a designer should give consider-

ation to specifying a nylon captive nut or a metalself locking nut incorporating a full nylon locking

ring. For high temperature applications an all

metal self locking nut of the distorted thread orbeam type should be considered.

CORROSION AND PROTECTIVEFINISHES

Corrosion protection for a fastened joint encompas-

10

ses much more than a consideration of the corros-ion resistance of the fastener itself. Actually re-quired is an analysis of the entire assembled jointas a system. This system includes structural de-sign, materials, protective coatings, stresses, pro-duct life expectancy and environmental conditions.

Consequently, designing for maximum fastenerjoint corrosion resistance is a complex problem thatcannot be readily resolved by applying a few gen-eral rules of thumb. As a matter of fact, corrosionis one of the least understood design considerationsin fastened assemblies.

The need for adequate protection against corrosionin fastened joints is increasing, owing to the longeroperating life and current warranty periods of mech-anical equipment.

Furthermore, environments are becoming morecorrosive, normal operating temperatures for sometypes of equipment are going up, load stresses areincreasing, and optimised designs in some casesare leaving less margin for strength losses. Allof these factors point to the need for greater con-trol of corrosion in fastened assemblies.

The first step in designing for optimum corrosionresistance in fastened joints is an analysis of thefactors producing corrosion, among which are time,environment, stresses, and the effects of joiningdissimilar materials. Designers must ask them-selves how long the assembled product should last.

Corrosion may be no problem in a product which is

intended to be used up or destroyed shortly aftermanufacture. Storage life, also, is a factor thatmust be considered in the corrosion analysis.

What are the environments to which the fastenerand joint will be exposed during the useful life of theproduct? How will salt on the roads, or sulphide,

smoke, ash or smog in the air affect the assembly?What will humidity and atmospheric conditions at

coastal airports do to an international continentaljet while it is on the ground between flights? Whatcorrosive liquids, cutting oils or sealants will splashon the machine tool? Will the assembly be usedin a vacuum or a relatively air-tight enclosure?A condition that affects cadmium coatings.

These questions are typical of those that the designengineer must evaluate in his study of fastenerjoint design with optimum resistance to corrosion.

Moisture and humidity are environmental conditionsthat must be considered in such a study, since cor-rosion, generally, is an electro-chemical processand the presence of an electrolyte encourages chem-ical reactions. Temperature is also a factor, be-cause high temperatures accelerate chemical re-actions. Static charges and electric currents thatare normal in electronic equipment and electricalequipment may create or accelerate corrosive con-ditions by providing circuits for galvanic reactionsbetween dissimilar metals.

End use of the product is still another factor to beconsidered. Will protective coating be abraded by

wrenching during assembly or destroyed by care-less handling? If not, it may be alright to let thefastener corrode in place along with the rest of theassembly. On the other hand, if it will be necessaryto remove and re-use the fastener, then in all pro-bability no appreciable fastener joint corrosion maybe tolerated.

The economic factors of the design also must beconsidered. Cost can be one of the most importantfactors in the design analysis. An assembly maybe completely protected from corrosion, if cost in

terms of money or performance is no object. Forexample, corrosion resistant high strength fasten-ers can be produced from some materials that costfrom upwards of 40s. per lb. On the other hand,design requirements may be relaxed to permit fast-

eners to be specified that are larger than actuallyrequired, thereby, making the loss of strengthfrom corrosion unimportant. Or the fastener usedmay be made of low strength material with highcorrosion resistance to a particular chemical to

be encountered.

Generally, the analysis of corrosion protection in-volves a detailed consideration of the followingbasic elements in the fastener joint system.

If the design problem is one of direct corrosiveattack, the first line of approach probably will beto choose a material that offers high resistance tothe corrosive element in the particular environ-ment involved. Another consideration in choosingmaterials, however, is the possible incompati-bility of mating metals. Where similar metals can-not be used, the choice should be metals which areclose together in the galvanic series. Metal couplesthat are far removed in galvanic potential shouldbe avoided. For example, a bare stainless steelinsert in a bare magnesium plate probably wouldloosen from galvanic corrosion in only one or twodays after assembly. Where metals close in thegalvanic series are not possible, the designer mayapply a fastener material that is cathodic to thejoint material and rely on the area rule principleto control corrosion. The area rule principle isbased on the idea that the rate of galvanic corro-sion is a function of the relative areas of anodic(less noble) and cathodic (more noble) metals. Thegreater the area of the anodic metal, which is themetal that corrodes, the less severe the corrosion.

In practise, it is sometimes possible to use incom-patible metals such as steel fasteners in an alumi-nium structure without serious corrosion providedthe area of aluminium is relatively large. If thematerials are reversed, and the aluminium rivetsare used in a steel structure, corrosion will berapid because of the relatively small area of thealuminium anode.

Protective coatings are normally used as economi-cal substitutes for expensive, corrosion resistantbase materials or to prevent galvanic corrosionbetween incompatible metals.

Low cost coatings include paint, hot dip zinc andphosphate oils.

11

Zinc galvanising is widely used as a protective

coating for industrial fasteners with broad toler-

ances. Thick coatings of galvanised zinc, however,are unsuited for precision threaded fasteners.

Where cost is a governing factor, and corrosion is

not likely to be severe, conversion-type coatings

provide economical protection for close toleranceindustrial fasteners. Included in this category arevarious phosphate base coatings for carbon andalloy steel fasteners.

Passivation, another form of conversion treatment,makes many stainless steel alloys more resistantto corrosion.

Electroplating, generally, is a superior processfor providing corrosion protection for fastenedjoints. Chromium plating, for example, which is

known as a barrier plating, provides a layer of

metal that is more noble and therefore less sus-ceptible to corrosion than the base metal. Anotherform of electroplating is known as a sacrificial type.

This type of plating uses cadmium, for example,because it is less noble than the base metal, so it

corrodes, thereby protecting the base metal of thefastener.

Economical corrosion protection is provided in

many non- fastener applications by use of noblemetal barrier coatings such as chromium plating.

However, to be effective, a noble metal coatingmust be at least 0.001 in. thick, to bridge over theimpurities common to deposited platings. If the

coating is thinner than 0. 001 in. the plating may beworse than no protection at all because breaks in anoble metal coating expose the less noble metalbelow to rapid deterioration by galvanic action.

The two most widely used sacrificial platings forthreaded fasteners are cadmium and zinc. Sincecadmium and zinc are considered toxic to humans,tin is often used in food industry applications. Fre-quently, cadmium and zinc coatings are renderedeven more corrosion resistant by post plating chro-mate conversion treatment.

If cadmium plating is exposed to temperaturesabove 450°F it begins to melt and attacks the basematerial. Cadmium should not be used in airtight

applications since, in the absence of oxygen, it

forms whiskers of cadmium salts.

HYDROGEN EMBRITTLEMENT

Care must be taken to prevent hydrogen embrittle-

ment when some metal fasteners are electroplated.

Delayed embrittlement failure caused by the absorp-tion of free hydrogen during cleaning and electro-plating occurs primarily in plated carbon and alloysteels.

The cause of the embrittlement is hydrogen whichis trapped beneath the surface of the metal, a

source of which is the acid cleaning prior to elec-

troplating and the plating process itself. In both

cases, atomic hydrogen is liberated at the surface

of the metal being treated. It is a well established

fact that atomic hydrogen can and will diffuse

through steel, whereas steel is opaque to molecu-lar hydrogen. Under loading, which causes the

components to flex, bend or flatten, the atomic hy-

drogen will migrate ahead of the stress and collect

at dislocations (usually grain boundaries) and formmolecular hydrogen which cannot further diffuse.

Pressure will build up at these points until it ex-ceeds the tensile strength of the steel at which timerupture occurs. Each of these ruptures acts as a

sharp notch which effectively lowers the ductility,

and as this occurs at countless points throughoutthe component, it exhibits a very brittle nature.

Unless the part is charged very heavily with hydro-gen, it will exhibit good properties when first load-

ed, failure will occur later from a few minutes to

ninety hours. Components which show no failure

after being loaded for 96 hours are considered to

be free from embrittlement.

Prevention of hydrogen embrittlement begins with

good heat treatment. All oil and grease from prior

manufacturing operations should be removed. Aproper atmosphere must be maintained in the hard-ening furnace to prevent the formation of scale orsoot. After quenching, the work should be cleanedof the quench oil before the tempering operation.

As zinc is less noble than cadmium in the electro-

motive series, more hydrogen will be liberated andabsorbed during electroplating with zinc than cad-mium. For this reason zinc plated parts are moresusceptible to hydrogen embrittlement than cad-mium plated parts.

After plating, the work should be baked to removeas much of the hydrogen as possible, as the speedand completeness of the hydrogen removal variesdirectly with the temperature, the highest tempera-ture possible should be used. The limiting factorusually is the ability of the plating material to with-

stand oxidisation. For the customary finishes,

such as zinc and cadmium this upper limit is about425 F. Whilst four hours at this temperature will

remove most of the hydrogen and is generally ad-

equate for parts loaded in pure tension, a minimumof eight hours is required for parts which are load-

ed in bending.

As a measurement of the effectiveness of the dif-

ferent processes of production, it is good practiceto daily test load random samples from each typeof plating in accordance with acceptable, internal

A. Q. L. sampling practices.

It can be seen that simple or idealised solutions to

corrosion problems rarely are practicable andsince the designer usually must work within a bud-get and an established framework of functional re-quirements, it may be concluded that designing foroptimum corrosion resistance requires carefulstudy, intelligent analysis and wise compromise.The designer must select from the many ways ofreducing corrosion, the materials, coatings, sea-lants and environmental controls that will provideadequate corrosion protection to meet the majordesign parameters at an acceptable cost.

12

Retaining rings and fixes

by F.H. Bowler (Moulded fasteners Ltd. , Plastics Div. , Geo. Salter & Co. Ltd.)

The retaining ring or circlip is designed basically

to provide a shoulder, on a shaft or in a bore, andin so doing offers an economic and mechanically

sound method of positioning and retaining compo-nent parts.

This Chapter deals mainly with retaining rings pro-

duced, by high speed press methods, from metalstrip. The range manufactured to-day is now very

wide, and from the original basic types, alterna-

tive shapes and designs have been developed to suit

and satisfy specific applications.

Fig.1 .

INTERNAL BASIC EXTERNAL BASIC

OAXIAL ASSEMBLY

The basic types illustrated. Fig. 1, serve the large

majority of retaining ring applications, where axial

assembly is possible.

The tapered section, decreasing symmetricallyfrom mid section to the free ends, ensures that the

ring maintains circularity when expanded or con-tracted within the working limits of its normal use,

this being approximately 10 per cent of its dia-

meter. Their design also provides for a constant

pressure against the bottom of the groove, makingthem secure against heavy thrust loads. The ex-

ternal ring may be used in assemblies subjected

to strong centrifugal forces and is secure against

high rev. /min.

The inverted lug type rings shown in Fig. 2 incor-porate certain modifications to the basic types, in

order to satisfy certain specific fastening problems.

Fig. 2.

INTERNAL INVERTED EXTERNAL INVERTED

o o

They differ from the standard internal and external

rings in two ways - the section height is increased

and the lugs inverted, so that they abut the bottomof the groove. Due to the lug design these rings

provide less contact with the groove wall and there-

fore have a lower thrust load capacity than the

basic rings. For certain applications, however,

the following characteristics may well prove either

advantageous or desirable:

a. Due to the lug design on the internal ring a

larger clearance diameter is possible through the

ring. On the external ring, a smaller overall dia-

meter is possible - a useful feature when an as-

sembly is required to pass through, or locate in,

a minimum diameter housing.

b. This design provides a higher shoulder than the

standard rings and one that is uniformly concentric

to the shaft or housing. For this reason the inver-

ted rings are suitable for locating and retaining

lenses, seals and other components having curved

surfaces. The higher shoulder also makes it pos-

sible for these rings to accommodate ball, needle

and roller bearings and other items with large cor-

ner radii or chamfers.

c. When used externally, this ring looks better

than the basic ring and for this reason is especial-

ly suited to external applications on cameras, of-

fice machinery, domestic appliances and other

products where appearance is important.

The rings illustrated in Fig. 3 have been developed

to obtain an exceptionally high shoulder, and asfar as possible a uniformly distributed abutting

area around the circumference. The external con-tour of the lugs of the external type and the inter-

nal contour of the lugs of the internal type havebeen designed so that they lie on a circle concen-tric with shaft or bore respectively. These ringsare mainly used for retention of bearings or com-ponents with large corner radii. This design is

widely used on the Continent but is not so commonin this country or America.

In many assemblies, dimensional tolerances in ring

thickness, groove location or the overall length of

13

Fig. 4.

INTERNAL BOWED EXTERNAL BOWED

the machine components being retained add up to

a degree of clearance, or end-play, between theabutting surfaces of the ring and the retained part.

A useful development from the basic internal andexternal rings has been the bowed rings and bevel-led rings. The former are widely used in this coun-

try, but the latter type which originated in Americahave not been so readily accepted. The bowed ringprovides a resilient end-play take up, whilst the

bevelled ring is intended for rigid end-play take up.

The bowed rings illustrated in Fig. 4 are designedto take up end-play resiliently and to dampen vib-

rations and oscillations. They are intended forrelatively small assemblies where diameters of

shaft, bore or housing do not exceed l|- in. As canbe seen from the illustration, they differ from thebasic rings in that they are bowed cylindrically

around an axis normal to the diameter bisecting

the ring gap. It can be seen that this bowing makesit possible for the rings to take up end-play causedby tolerances in groove location or the parts to beretained.

The bowed rings provide resilient end-play take-

up in an axial direction while maintaining a tight

grip against the bottom of the groove. Properorientation of the rings is important for optimumperformance. Internal rings should be assembledwith the convex surface abutting the retained part;

the external ring should be installed with the con-cave surface against the part. In addition to pro-viding resilient end-play take-up in an assembly,these rings may be used to pre-load bearings, pre-vent rattle in machine linkages and provide springtension on adjusting screws. In the event of groovewear, or if the groove for a flat basic ring hasbeen cut oversize, then the bowed ring can use-fully be used to salvage the assembly. Averageamount of take-up possible with both internal andexternal rings is 0. 010 in.

The bevelled rings, see Fig. 5, are designed to

provide rigid end-play in assemblies and other ap-plications where manufacturing tolerances - orperhaps wear in the parts being retained - causeend-play between the ring and the retained part.

These rings differ from the flat basic rings in that

the edge in contact with the groove is bevelled to

an angle of 15°. The bevel is therefore located

Fig. 5.

INTERNAL BEVELLED EXTERNALBEVELLED

around the outer circumference of the internal ring

and around the inner circumference of the externalring. The groove required for these rings has a

corresponding 15° bevel on the load bearing wall

of the groove. The ring should be seated at least

half way in the groove to provide sufficient contact

area with the load-bearing groove wall.

When a bevelled ring is installed in its groove, it

acts as a wedge between the outer groove wall andthe part being retained. When there is end-playbetween the ring and the abutting face of the retain-

ed part, the ring's spring action causes it to con-tract or expand more deeply into the groove, thus

compensating for the end-play. It also exerts an

axial force against the retained part. If necessary,the axial force can be calculated from an analysisof the forces caused by ihe spring action of the ring

on the bevelled groove.

RADIAL ASSEMBLY

In many assemblies it may be impossible or im-practical to install external retaining rings - of the

types already described - axially along the shaft.

The rings described in this section have been de-veloped to accommodate this type of assembly.Whilst it must be generally accepted that the partsto be described will not withstand the loads sup-ported by axial assembled rings, the radial assem-bled rings offer two very important features - lowunit cost and rapid assembly. With these types,

methods of dispensing and application are available

which make them ideal for high speed mass pro-duction.

The E-ring illustrated in Fig. 6 is probably the

most widely used and most popular ring of the

radial type. It provides a relatively large shoulderon small diameter spindles. Although contact with

the groove is provided only through three prongs,spaced approximately 120° apart, a comparativelydeep groove serves to increase this fastener's

thrust load capacity.

Fig. 6.

cFig. 7.

CRESCENT(REGISTEREDTRADE MARK)

nThe 'Crescent' ring illustrated in Fig. 7 is anotherpopular radial type ring and because of its shallowsection height and uniform shoulder, is ideal forassemblies in which clearance dimensions arecritical, secure against moderate thrust loads andvibration, neat in appearance and easily applied.

The E-ring and 'Crescent' ring retainers are read-ily available through a wide range of spindle sizes.Both types are more easily assembled with the helpof an applicator as illustrated in Fig. 8, and to

load the rings, a fixture as illustrated in Fig. 9

may be used. The ring is pushed forward against

14

Fig. 8.

MILLED RECESS

Fig. 9.

ASSEMBLY FIXTURE

the vertical section of the fixture, and the recessedjaws of the applicator spring round the ring holdingit firmly.

Being held under spring tension, the ring cannot bedislodged until it is applied to the groove. As the

ring's gripping power on the shaft is greater thanthe tension of the applicator jaws, the ring remainsin the groove when the applicator is withdrawn.Applicators can be angled or cranked to suit cer-tain locations.

The rings illustrated in Fig. 10 show variations in

design from the E-ring and 'Crescent' ring types.None of these are so widely known or accepted asthe two previous types, and the size range for eachis limited - however, each appears to be preferredfor certain applications in industry.

To conclude the radial assembled rings, are twotypes for end-play take-up. The bowed E-ringillustrated in Fig. 11 is similar in construction

to the flat E-ring but differs in that it is bowedcylindrically around an axis normal to the diameterbisecting the ring gap.

Fig. 11

SECTION I-

Fig.12.

PRONG-LOCK (REGISTERED TRADE MARK)

Fig. 13.

*^a.TJ

(a)(b)

INSTALLATION

(a) RING IS PLACED NEXTTO SHAFT AND COM-PRESSED WITH SCREWDRIVER (OR APPLICATOR)UNTIL LOCKING PRONGSENTER GROOVE

(b) RING IS THEN PUSHEDFORWARD UNTILPRONGS PASS OUTERCIRCUMFERENCE OFSHAFT AT WHICHTIME RING SPRINGSBACK TO NORMALBOWED POSITION ANDPRONGS LOCK AROUNDSHAFT

The bowed ring is designed to provide resilient

end-play take-up similar to that of the basic types- for best results the ring is installed with the con-cave surface abutting the retained part. Theserings cannot be used with a dispenser, due to thebow - but may be assembled with an applicator.

The ring illustrated in Fig. 12 is an excellent bow-ed type ring. It provides end-play take-up, but in

addition the two small 'ears' provide a positivelock behind the groove and ensure that the partcannot dislodge. Two flats, one on each side, givea good bearing surface. Assembled as shown in

Fig. 13.

PUSH-ON AND SELF LOCKING TYPES

The final group of retainers to be described, is

one which provides a range of parts invaluable to

industry in general, where rapid assembly of largequantities of components is required. In many as-semblies, it is impractical, or may be undesir-able, due to cost, to cut a groove in a shaft orhousing. This is particularly true in the case of

toys, small appliances, plastics products and otherapplications where the shoulder provided by theretainer is not required to withstand any sizeableload, but merely to position or act as a lockingdevice.

For applications such as these the push-on-fixesare essential, and although there are many typesfor the engineer to choose from, they are all basedon the same simple but effective design. Table 1

shows a selection of parts currently ayailable andin use to-day, each one has prongs which are de-flected backwards as the fix is pushed down theshaft. Whilst it is possible to continue movementof the part in the direction of the assembly, thegrip of the inclined prongs will prevent movementin the opposite direction. Ideally suited for die-cast and plastics studs, and certain types will caterfor rivets, tubing and wire.

15

oTable 1 .

Push—on fix with an arched rim For increased strength and thrust load capaci-

ty. Extra long prongs accommodate wide shaft tolerances.

oPush-on fix with a flat rim, has shorter prongs and smaller outside dia-

meter. Ideal where flat contact surface with retained part is required or

clearance dimensions are critical

.

© Push-on fix with three prongs only, which provides stronger fixing than the

above parts. Also provides a large shoulder relative to spindle diameter.

©Push-on fix where the inside form is star shaped - this design is normally

used on very small spindles, i.e. is to 4 . Particularly suitable for miniature

assemblies where smallest possible outside diameter is necessary.

O Push—on fix with only two prongs, diametrically opposed, and the design al-

lows for considerable flexing, allowing quite wide tolerance on spindle.

IE3IPush-on fix similar to the above, but rectangular in shape. Rectangular

part normally used for tight load applications.

Upturned end ensures that fastener will not dig into abutment surface. Allows

quite wide tolerance on spindle.

The inexpensive tool illustrated in Fig. 14 simpli-fies the assembly operation of pushing on the cir-

cular push-on fasteners. It provides clearance forthe locking prongs to flex as the fastener movesalong the shaft and exerts an even thrust aroundthe periphery. A similar tool exists for the rect-

angular parts, but with the latter parts applicationpressure is only exerted on the two long sides of

the fastener.

It will be appreciated that the push-on fix detail

can be incorporated into clips of a special naturewhere the quantity to be used warrants specialtooling.

A comparatively recent addition to the varioustypes of self locking fasteners has been the so

called 'Gripring' illustrated in Fig. 15. This is

an extremely useful part, similar in shape to the

basic external rings but differing in several re-

spects. Firstly, it is pressed from a thicker gauge

and has a larger section height - the ratio betweenthe section height and free diameter is quite dif-

ferent from the standard ring. The overall size

of the 'Gripring' is much larger than the basic ringfor a given spindle size, providing a higher retain-ing shoulder.

The ring is applied and removed with pliers, usually

a heavier or stronger type, to cope with the heaviergauge - for suitable production assemblies a fix-

ture can be designed to incorporate a wedge movingbetween the lugs to spread the ring, the spindle canthen be fed into'the ring. The 'Gripring' can be usedon tubes where the groove for a conventional ringwould be impossible, on plastics spindles, castingsand other parts not normally machined to close

tolerances.

On a mild steel shaft the 'Gripring' offers good re-

sistance to thrust loads, e. g. for a \ in. dia. shaft

± 0. 002 in. , the appropriate ring will withstandloads of up to approximately 35 lb.

Fig. 15.

GRIPRING (REGISTERED TRADE MARK)

16

MATERIALS - FINISHES - PACKING

The standard material for most types of retainingrings and fasteners is carbon spring steel En42or CS70. On certain small type sizes, berylliumcopper is standard. Providing a sufficient quantityis required to justify purchase of the material,most parts can be manufactured in beryllium cop-per or phosphor bronze, should the application call

for a non-ferrous part.

Generally speaking, it is found that stamped re-taining rings and wire formed retaining rings arecomplimentary to each other, both serving indus-try generally in a very wide field of application.

Production of stainless steel retaining rings in this

country is now practically nil, due mainly to the

difficulties of obtaining suitable strip material andthe limited demand which makes economic produc-tion impossible. As a result most stainless ringsare imported from the USA.

The normal standard finishes for most parts in

spring steel are 'chemical black' or 'blued' finish.

Where desirable cadmium and zinc plating can beapplied and zinc chromate paint is also used for

certain parts and applications.

Basic type rings, E-rings and 'Crescent' rings, canall be tape stacked, and this type of packing offers

several advantages. The rings are easily handled.

counted and identified - the tape has the ring typeand size printed on it - there can be no tanglingof rings.

Development of special parts is often undertaken,and these mostly occur in the 'push-on' field wherea specific fastening for a component can be satis-fied by a clip incorporating the 'fix' detail.

Quantities have to be sufficient to justify designand tooling and a requirement in excess of 100, 000parts would normally be necessary if a low piecepart price is to be achieved.

WIRE FORMED RETAINING RINGS

These are normally coiled automatically from colddrawn spring wire of a uniform section and shape.The gap ends are cut according to the design re-quirement and may be square or angled. The wirering is available in various cross sectional shapes,the most popular, however, being round, squareand rectangular.

Probably the biggest single advantage of the wirering is its ability to expand or contract over amuch wider size range than the pressed ring, thisbeing due to the material grain structure. As aresult of this spring action, they are able to com-pensate large shaft or bore tolerances if seatedwithout radial play. They are particularly usefulfor shafts or housings of non-standard dimensions,i. e. not covered by the pressed rings, and wherethe quantity does not justify press tooling.

c

salten I

OMPONENTSGROUP

Saltersprings

Salter Precision

Presswork Ltd.

Salterfix Ltd.

Salter Machining

Salter HeatTreatment

Saltercast

components group

All types of springs from wire,

for all trades.

Specialists in precision

presswork.

Standard circlipsand fasteners

of many types.

Auto turning, capstan turning,

milling and gear hobbing.

Austempering capacity for bulk

quantity work.

Grey iron casting, aluminiumsand casting and pressure die

casting.

London Spring

Co. Ltd.

MouldedFasteners Ltd.

Concise Tools

Ltd.

Multi-slide presswork.

Injection moulding capacity upto 17oz. Experience in all

thermoplastic materials.

All types of Press tooling,

Multi-Slide tooling. Experiencein high class multi-stage tools.

Salter Components Group. Spring Road Smethwick. Warley. Worcs.

17

3

Eyelets

by W.T.J.Bownes (Geo. Tucker Eyelet Co. Ltd.)

The dictionary definition of an 'eyelet' is simply

'a small hole 1 but the term is generally acceptedas denoting a metal re-inforcement or neatener

for a manufactured hole in some less rigid mate-rial. The usage of metal eyelets in this context

goes back a century or so, notably on sails and tar-

paulins, and the smaller varieties later began to

be used on boots and corsets. During the interven-

ing years hole reinforcement eyeletting, latterly

by automatic and semi-automatic means, of labels

and swing-tickets, tents and camping equipment,industrial aprons, waterproof clothing, bedding,

travel goods, etc. , has assumed increasing cur-rency and perhaps the most recent extension of this

is the eyeletting of reinforced polyethylene shroud-ing to enable building work to continue during the

winter season.

Between the wars eyelets began to be used as fas-

teners for file fittings as a logical extension of

their usage on other stationery items and the idea

was quickly taken up by the radio industry wherenumerous applications for a lightly stressed fas-

tener were beginning to appear.

The demands of these industries for high-rate in-

sertion machinery inspired improved manufacturingtechniques with closer tolerances and from thencedeveloped the wide range of eyelets and associatedinserting tools available today.

DESCRIPTION

Applications for metal eyelets are legion and in-

volve the whole spectrum of light industry but for

the purposes of this Chapter we can roughly sub-divide them into three main categories:

1. Assembly types

2. Contact types and3. Grommet types

Fig.1 . Drawn eyelet.

Assembly eyelets

Drawn. Assembly or fastener eyelets are madefrom brass, copper, steel, nickel, monel and alum-

inium in diameters from 0. 047 to 0. 750 in. and in

lengths up to 2. 5 in. , see Fig. 1. These are pro-duced by three basic means dependent upon length

to diameter ratio. The larger proportion of these

eyelets have a length: diameter ratio of less than

4:1 and are produced from the surface of metalstrip by progression or follow-on drawing opera-

tion. This method produces an eyelet of good mec-hanical and visual quality, having a degree of taper

Fig. 2. Seamed eyelet

.

in the barrel (shank) and with some thinning downin the wall toward the shank end. The stock mat-erial thickness will vary with the size of eyelet butis generally within the region of 0. 010 - 0. 020 in.

(heavier gauges can be adopted for special pur-poses) and the average wall thickness will be some-where below these figures. The flange or head onthis type of eyelet can be of more or less infinite

diameter if so required and of one of three basicconfigurations: round-rim, flat-rim or counter-sunk (funnel). The average flange diameter approx-imates to a 50 per cent increase on the barrel dia-meter but special flange forms can be readily pro-duced to order. The majority of such eyelets canbe automatically fed.

Seamed. Where the length: diameter ratio needs toexceed 4:1, or for reasons which will suggest them-selves later, the second basic manufacturing meth-od is to blank from strip and roll the eyelet with

18

a longitudinal butted seam, see Fig. 2. By this

means brass or steel eyelets of 0. 050 in. diameterwith a length of say 0. 500 in. can be produced withparallel barrels whilst lengths of up to 2. in. canbe offered in larger diameters. Limitations exist

on the flange diameters that can be offered withthis type of eyelet and all will exhibit a segmentalslot in the flange relative to the butted seam. Themajority of these eyelets are not suitable for auto-matic insertion.

Tube. Where, for reaons of mechanical strengthor for aesthetic considerations, the butt-seamedtype of eyelet cannot be adopted, eyelets of diametersfrom 0. 047 in. upwards and of lengths of up to 3.

in. are fabricated from brass, copper or alumin-ium tube, see Fig. 3. Head diameters of up to 50

Fig. 3. Tube eyelet.

per cent above the shank diameter are offered andof the three basic types available with the drawneyelets, i. e. rolled-rim, flat-rim and countersunk(funnel). Wall thicknesses tend to be of the samebasic order as the stock material for the drawneyelet, i. e. 0. 010-0. 020 in. dependent upon dia-

meter and in general those eyelets having a length:

diameter ratio of 4. 5:1 or less can be automaticallyfed, although this ratio can be exceeded in somecircumstances, as discussed later in this Chapter.

Contact type eyelets , including eyelet tags

Eyelets are used in various ways to promote elec-trical continuity and some of these are mentionedhere.

Single and double-winged tags with integral roundor square eyelet barrels are used on transformerbobbins, coil formers, etc. The single-wingedtype, see Fig. 4, are supplied with the wing bentat various angles and are generally hand assem-bled, although at the time of going to press an auto-matically fed machine is being developed. Double-winged tags, see Fig. 5, some with blades suitablefor receptacle (quick- connect) connection, are gen-erally supplied unbent for automatic insertion bymeans of a modified eyelet machine. This machineinserts and clenches the eyelet barrelled tag at thesame time forming up one or both wings at 90° to

the plane of the Danel or bobbin. These tags aregenerally of brass, suitably finished for soldering.

n

n

Fig. 4. Single wing tag

Wire-end tags are similar to the single-winged tagsdescribed above but having long wings of up to 2.0in. or so, generally 0.031 in. wide, see Fig. 6.

These are usually of brass or phosphor-bronze,suitably finished, and are used to terminate capa-citors of various types. They are not suitable forautomatic assembly.

A range of terminal eyelets, having internal dia-meters when set, suitable to accommodate BA to

8 BA screws, are available. Manufactured frombrass and suitably finished, they are used to ter-minate the motor leads in refrigerators and vacuum

/^\

KJ

Fig. 5. Double wing tag.

19

^jFig.6. Wire-end tag

.

Fig. 7. Terminal eyelet

cleaners, and the mains cables of electric irons,

etc. They are automatically fed and set by a ver-

sion of the eyelet machine which forms a loop in

the stripped lead end and clenches the appropriate

diameter eyelet on to the preformed wire (Fig. 7).

Eyelets are used on ceramic feed-through devices

and on glass /metal seals, of steel, Nilo-K or Ko-var, see Fig. 8. Brass eyelets are similarly used

on feed-through capacitors, usually being slit to

accommodate the diametral variation encountered

in the ceramic bodies of these devices, see Fig. -9.

Grommet type eyelets

This term is used in the broad sense to cover the

use of an eyelet to bush a hole in rigid or flexible

material for any purpose. This type of application

extends from the thin- walled brass eyelet used onlabels through zinc, brass and aluminium eyelets

used on garments and footwear to sail eyelets andspur-toothed grommet eyelets up to 2.0 in. dia-

meter. Oval eyelets are also included in this cate-

gory and are available in a range of sizes and fin-

ishes, generally made from brass (Figs. 10-14).

Fig. 9. Body eyelet

capacitor.

High-speed inserting machines, having sequential

punching and eyeletting operations, are supplied

for the smaller sizes used by the garment and foot-

wear industries, see Fig. 15. The larger sizes

are generally hand-fed as it is often necessary to

use mobile tools owing to the nature of materialsinvolved.

MATERIALS AND FINISHES

The normal materials involved in eyelet manufac-ture have been mentioned when describing the vari-

ous types, however, some comment on the proper-

ties of each will assist designers. By far the lar-

ger proportion of assembly eyelets are of brass,

with steel and aluminium following in that order.

The drawing qualities of brass lend themselvesadmirably to the fashioning of an acceptable eye-

let in terms of appearance and general perform-ance whilst being non-ferrous and of good electri-

cal conductivity it can be used widely on electrical

Fig. 10. Stationery

eyelet.

Fig.11. Shoe eyelet

(nicked)

.

apparatus. Shear and tensile strengths are gene-rally adequate for the type of application found in

this class of assembly, and increased mechanicalperformance can be obtained where necessary bythe adoption of a tube eyelet having a greater wallthickness.

Steel is the most widely used alternative to brassin the general assembly field, having two advan-

20

New Unbrako Loc-Wel socket screws have a radically

different kind of locking element. They represent a

significant advance over all other conventional self-locking

screws. The locking element is a thin skin of nylon fused

onto the threads: no drilling or slotting is involved.

It has an exceptional plastic memory that has enabled

Loc-Wel to be used many, many times.

Loc-Wel is the first full-strength self-locking socket screw.

No grain flow lines cut. No metal removed. No hardness

"let down". And since the Loc-Wel element is spread

over 4, 5 or more threads a greater surface tension is

obtained, allowing adjustment over a wide range.

Loc-Wel is available now from Unbrako on mostUnbrako socket, cap, or set screws. With all normal

finishes (another exclusive Loc-Wel advantage).

Get together with Unbrako Limited_ _ pp_ pHppH ppH Burnaby Road, Coventry

A member of the SPS group of companies

No drilling

No slotting

No burrs

No chips

No moisture traps

No screw softening

No lock nuts

No lock washersNo spring washers

No kidding Loc-Wel^

Fig. 12. Sail eyelet and ring.

tages, i. e. reduced cost and increased mechanicalefficiency. However, the problems of corrosion,

etc. , outweigh these in most instances and brasscontinues to be favoured. Aluminium is used wherethe joint is only lightly stressed as the harder al-

loys do not respond well either to the drawing meth-od of manufacture or subsequent processing. How-ever, it has the advantage of cheapness and is some-times used preferentially for this reason in verylight assemblies. Tube eyelets manufactured fromaluminium have the improved mechanical qualities

consistent with their greater wall thickness and canbe used in place of drawn brass eyelets or wherelength:diameter ratios of greater than 4:1 are re-

quired.

Monel is one of the stainless group of alloys andhas most of the properties desirable in an eyelet.

The setting loads dictated by the relative hardnessof this material are greater than for brass or steel

drawn eyelets and its use is therefore restricted

to applications where its stainless properties are

considered essential.

The finish required in a fastener type eyelet will

necessarily vary with the desired performance andenvironmental conditions. Brass and copper eye-

lets are supplied bright -dipped for decorative pur-poses or moderate environmental conditions, other-

wise normally nickel -plated. They can be electro -

tinned or stannate tin- dipped, the former being a

solderable finish, with or without a prior nickel

flash to prevent zinc migration. Brass, copper orphosphor-bronze solder tags will be normally elec-tro-tinned or solder coated (hot-tin-dipped) withor without a prior nickel flash.

Steel eyelets will normally be nickel-plated to pre-vent corrosion, or alternatively cadmium plated

to special order. A brass finish can be applied for

decorative purposes or very moderate environmen-tal conditions.

Nickel and monel eyelets are normally used as

made, no additional treatment being necessary.

Aluminium eyelets are used as made in moderateenvironmental conditions but where electrolytic

problems are likely to be encountered, anodisingis standard practice.

Fig. 14. Oval eyelet.

ADVANTAGES OF EYELETS OVERALTERNATIVE FASTENERS

a. Low product price.

b. Low installed cost using unskilled labour.

c. Flexibility.

The prime advantage of the eyelet system of as-sembly is undoubtedly its low installed cost. Thedrawn type assembly eyelet compares very favour-ably in product price with rivets, screws, etc. ,

and having regard to the moderately priced high-

speed feed machines available, often at low rentals,

the system is easy to install and operate, requir-ing normally only un- skilled or semi-skilled fe-male operators. Assembly times per fastener will

obviously vary with the complexity of the piece-

Fig. 15. Highspeedinserting machine for

grommet eyelets (Re-produced by courtesyof George TuckerEyelet Co. Ltd.).

22

^1 Fig. 16. A poweredeyelet machine (Re-produced by courtesy

of George TuckerEyelet Co. Ltd.).

Fig. 17. A poweredbench-mountedeyeletter (Repro-

duced by courtesy

of George TuckerEyelet Co. Ltd.). [

parts involved but will show great economies over

threaded fasteners, etc.

Setting pressures can easily be regulated to allow

movement of one piece-part relative to others and

the system can be adopted therefore to provide ar-

ticulated joints of various types in lightly stressed

assemblies, in toys and models for example and

on metering devices, watch bracelets, etc. Eye-lets having double diameters can be used to replace

relatively expensive shouldered turned parts with

the extra advantage of semi-automatic assemblyadded. Given that the eyelet system of assemblyis used in its correct context as a light fastening

system with full cognisance of the mechanical pro-

perties of the eyelet involved, this system has no

disadvantage compared to alternative methods.

DESIGN CONSIDERATIONS

Having regard to the availability of the thousands of

assorted sizes of fastener eyelets and to the easily

installed nature of the eyelet system, usually at

most requiring a mains electrical supply, design-

ers are sometimes inclined to assume the fact of

eyeletting and leave this detail until too late a stage

of development. Great advantages can follow fromconsidering the fastening aspect of the design at

the earliest possible stage so as to (a) enable the

use of a standard eyelet with its obvious cost ad-

vantage and (b) to utilise this eyelet in the mosteconomical way in terms of operator and assem-bly machine efficiency.

Counterbores in one or both of the external compo-nents can often be arranged, subject to the strength

requirement involved, to enable a standard drawneyelet to be used in place of the more expensive

tube eyelet, or to permit semi-automatic assem-bly. Such design considerations, to be effective

from the cost view-point, must be incorporated at

Fig.18. A manuallyoperated bench-mounted eyeletter

(Reproduced by court-esy of George TuckerEyelet Co. Ltd.).

a sufficiently early stage, as secondary operations

to effect them later are invariably stop-gap and ex-

pensive.

To take full advantage of the low installed cost fac-

tor, designs should take into account the require-

ments of the hopper-fed semi-automatic eyelet

machine in terms of accessibility and clearance

diameters for the necessary pierced or drilled

holes in the piece-parts. Such holes should have

approximately 0. 008 in. clearance over the nom-inal external diameter of the associated eyelet and

should be at sufficient distance from any obstruc-

tion as to allow the access by the tool-post. Tool-

post diameters will of course vary with the eyelet

diameter but a high proportion of fastener eyelets

are clenched on tools of 0. 250 in. diameter held

in approximately 0.500 in. diameter tool posts. It

will be seen therefore that in these circumstances

a hole -cent re distance from the obstructing mem-ber of at least 0. 250 in. is normal but this dis-

tance can be reduced slightly where the height of

the obstructing member is below say 1.0 in. and

the eyelet diameter will allow. Tool post heights

of 1.5 in. and 3.0 in. are standard but posts of up

23

to 10. in. in height can be specially supplied oncertain machines to enable eyelets to be clenchedinside deep box assemblies.

In general the utilisation of a powered eyeletter asillustrated in Fig. 16 requires clear access to theface of the piece-parts on which the eyelet flangewill show although obstructions of a maximum heightof say 0. 75 in. can be tolerated provided adequatehole-centre to obstruction distance to accommo-date the eyelet flange support tool is maintained.Powered and manually operated bench mountedmachines are shown in Figs. 17 and 18.

Small tube eyelets down to 0. 047 in. diameter andwith length:diameter ratios of up to 8:1 can be suc-cessfully fed and set by means of a pneumaticallyoperated bench-mounted eyelet machine illustratedin Fig. 19. This type of machine has a vibratoryfeed system and presents the eyelet flange down-

Fig. 19. A pneumatically operated bench-mountedeyelet machine (Reproduced by courtesy of GeorgeTucker Eyelet Co . Ltd . )

.

wards for the operator to assemble the piece-partsthereon. Hole diameters can therefore be held towithin plus 0. 003 in. of the nominal eyelet diameterand multiple piece-parts can be effectively assem-bled.

Where the eyelet length:diameter ratio or the con-figuration of the piece-parts precludes the usageof a powered or hopper-fed eyeletter, a hand-fedtreadle operated press, see Fig. 20, can be util-

ised and tooling can usually accommodate most sit-

uations. Such presses are made in a range of framesizes to provide adequate clearance for the piece-parts.

Pliers having a back clearance of approximately1.0 in. and accommodating eyelets of up to 0. 275in diameter are illustrated in Fig. 21. These arenot normally utilised for production batches butare useful model- shop or service tools.

The setting tools for all the machines so far des-cribed will necessarily vary dimensionally to suitthe machine for which they are intended but in one

Fig. 20. A hand-fedmanually operatedeyelet press (Re-produced by courtesyof George TuckerEyelet Co. Ltd.).

important aspect they must be alike. This aspectis performance, and for this they depend on gooddesign and workmanship. The major design con-sideration is involved with providing the maximumstrength in tension consistent with the base mater-ial of the eyelet and this strength is a function ofa clean rolled setting. Such settings are the resultof a correct setting tool profile, perfectly temper-ed and polished. Incorrectly made or badly worntools can cause collapse of the eyelet barrel whichmay give rise to the erroneous impression that theeyelet is too short (the average length allowancefor setting is 0. 060 in. ) or may cause the eyeletsetting to split badly with consequent poor appear-ance and some reduction in strength. Making ofcorrect tools is something of an art and the eyeletmanufacturers can generally be relied upon in theirown interests to provide a good service in these.

APPLICATIONS

Some typical applications for assembly eyelets inapproximate order of ascending diameter size areexampled below.

Assembly of switch contacts to S. R. B. P. or moul-ded stators, using drawn brass eyelets, silveredfinish, by means of hopper-fed pedestal eyeletters;or tube eyelets, silvered finish by means of hopper-fed bench-mounted pneumatic machines. The lowsetting pressure results in reduced reject rate com-pared to alternative fastening methods.

Fig. 21 . Eyeletpliers (Repro-duced by courtesyof George TuckerEyelet Co. Ltd.).

24

Assembly of terminals on low voltage batteries,

using drawn brass eyelets by means of poweredeyeletters. The usage of an eyelet enables a satis-

factory electrical connection to be established with

or without re-inforcement by soldering.

Assembly of automotive switches using drawn brasseyelet with rectangular flange, the flange acting aselectrical contact, thus eliminating separate com-ponent.

Assembly of ceramic bodied lamp holders for spe-cial lighting using brass tube eyelets, the low rateof radial expansion and tolerance of changes in as-sembly thickness proving more efficient than alter-native methods.

Assembly of socket panels, valve holders, etc. , to

chassis and cabinets in hi-fi, TV domestic radioand tape equipments, using drawn brass nickel-

plated eyelets, in hopper-fed powered eyeletters.

The low installed cost of the eyelet assembly sys-tem has proved to be unassailable in this very widefield. The above mentioned socket panels are typical

of the wide range of components used in the radio

industry which are themselves assembled by meansof eyelets.

In fact the eyelet assembly system is used through-

out industry wherever a lightly stressed perman-ent fastener is required and its versatility will en-

sure for it a logical place in future light industrial

designs.

PRICES AND ORDERING QUANTITIES,STANDARD AND SPECIALS

It will be seen from the previous discussions that

low installed cost is the mainstay of the eyelet as-

sembly system and the two major components of

this are (a) low product price and (b) efficient in-

serting machinery. The high rate of production of

the drawn brass eyelet and the enormous quantities

made contribute toward keeping the product price

at a desirable level. Designers should bear the

quantity component of this price in mind, however,

and select, where possible, eyelets from the stan-

dard range in order to achieve as economic a cost

as possible. Standard eyelets are bulked packed

(that is to say not in multiples of a given quantity)

and are priced by the thousand. Price differentials

relative to quantities ordered apply and therefore

advantages accrue to both sides if larger quanti-

ties with scheduled deliveries are ordered. This

will enable production planning by both parties to

be advantageously implemented.

Seamed eyelets and tube eyelets in brass have a

cost factor of very approximately 2x and 3x re-spectively in relation to drawn eyelets of similar

dimensions, and the same considerations apply to

price/quantity ordered. Steel and aluminium, whereavailable, are approximately equal in price quan-

tatively due to the weight factor and are currently

cheaper than the brass equivalents.

The above comments apply to 'standard' eyelets

already tooled and in production, but inevitably

there will be requirements from time to time for

special manufacture of one or the other of the threebasic types of assembly eyelet. In this event thevarious methods of manufacture dictate differing

economic ordering quantities and these are broadlyas follows:

Drawn assembly eyelets in brass, steel, alumini-um, etc. , minimum initial order for special sizes250, 000-300, 000 off, usually plus part cost of tool-

ing varying with eyelet size and with possibility of

using part of existing tooling.

Seamed eyelets in brass or steel, minimum initial

order for special sizes 50, 000-100, 000 plus partcost of tooling varying with the blank size involved.It is not possible to utilise part of existing toolingfor new sizes of this type of eyelet so that the parttool cost is an inevitable corollary.

In the case of tube eyelets in brass or aluminium,

the method of manufacture from tube involves little

or no- special tooling with no liability, therefore, to

the purchaser in this respect. By the same token

short runs can be undertaken economically by the

manufacturers although it is true to say that longer

runs can produce a higher degree of price reduc-

tion pro-rata than the other two basic types. Thereare virtually no standard sizes therefore with this

type of eyelet - the available tube diameters con-

stituting the basic limitations. Tube of any prac-

ticable diameter and wall thickness can be obtained

specially and the economic minimum is approxim-

ately 200 ft. run. It will be seen therefore that

minimum quantities with this type of eyelet can beas low as 10, 000-20, 000 off even for a special size

not previously made.

FUTURE TRENDS

There seems no doubt that the eyelet assembly sys-tem will continue to find a ready place as the pre-mier light fastener in a broad range of industryand that, with increasing cost of labour, its lowinstalled cost will find it new applications every-where. However, its utilisation to the fullest ad-vantage requires intelligent use of the standardranges by design and production engineers togetherwith optimum utilisation of automatic eyeletting

machines. It seems likely, therefore, that the ex-

traordinary diversity of sizes and types currentlymade will give way to a rationalised range of sizes,

with steady increments of diameter and length,

which will at once enable designers and methodengineers to plan with the same certainty of ap-proach as they can with, say, the BA range of

screws, and the manufacturers to offer an impro-ved performance from the viewpoint of deliveryand tool supply based on increased quantities of

far fewer types.

25

Inserted fasteners

by H.D. Chambers, C.Eng. , M.I.Mech.E. (Armstrong Patents Co. Ltd.)

Fig.1 . External

view of a wirethread insert.

(By courtesy of

Armstrong Pat-ents Co. Ltd.)

Fig. 2. Threaded bushes for insertion into

tapped hole

.

Fig.3a. and 3b. Inserts for

'moulding into' component.

Inserted fasteners for engineering products fall

within one of the following categories:

a. For insertion into a previously threaded hole.

b. Moulded or cast 'in situ'.

c. For insertion into a plain drilled, cored ormoulded hole.

Type (c), logically, would include rivets, but thisfield is adequately covered in Chapters 14 and 15.

Rivet bushes, being threaded fasteners for use in

sheet or panel material, are covered below.

Types (a) and (b) will provide for higher strengthfastening then type (c) in many engineering mater-ials, although this is not so with components mould-ed from many of the thermosetting plastics, norwith certain die cast alloys.

Type (a) fasteners include wire thread inserts(Fig. 1) and threaded bushes (Fig. 2).

Type (b) are threaded bushes so formed on the outersurface as to be secure against axial and torsional

Fig. 4. Rivet bush. Axialforce applied by hand orpower press using a

special tool first piercesthe hole and 'splines' the

panel and then swagesthe sheet metal to pro-vide retention against

tensile loading and a

'tight' spl ine .

Fig. 5. Rivet bush. Afterinserting fastener into

previously pierced ordrilled hole, axial forceis applied by hand punchor press , rivets the flangeand causes the serratedface to bite into the panelsurface

.

Fig. 4. by courtesy of

Prestincert Ltd. andFig. 5. by courtesy of

Benton Engineering Co.Ltd.

Fig. 6. 'Push type'

insert.

26

SHEET METAL?

WITH

ROSAN PRESS NUTS

THE SIMPLE ANSWER

SO EASYSO QUICKSO PROFITABLE

SO WHAT?-so send for some free ones!

PROVE FOR YOURSELF THAT THISEASILY INSTALLED PRESS-NUT:

• provides a deep tapped hole in sheet metal

* can be fixed from one side

cannot rotate

• is smaller, lighter

» requires no riveting or clinching

There is a full range of British, American

and Metric threads — so just write

asking for your sample requirements &

details to INSTRUMENT SCREW CO..

LTD.. NORTHOLT ROAD, SOUTH

HARROW, MIDDX. Tel: 01-422 1141

ROSAN PRESS NUTSactual size of

a 2 BA nut

27

Fig. 7. For thermosetting plastics or aluminiumalloys. Held in position by the action of theinsert itself. (By courtesy of Armstrong Pat-ents Co. Ltd.)

Fig. 8. (Left) Insert locked in

position by the action of the

screw which expands the fast-

ener. (By coutesy of the Pre-cision Screw & Mfg. Co. Ltd.)

Fig. 9. (Centre) See text reference.

(By courtesy of Heli-Coil Corp.)f^ig.10. (Right) A self tapping wire thread insert.

(By courtesy of Armstrong Patents Co . Ltd .

)

forces when moulded or cast into the object to befastened (Fig. 3).

Type (c) fasteners, other than the self tapping type,

achieve security against exial and torsional forcesby inducing a radial force, producing 'hoop' stressin the component in which the fastener is located.

In the case of rivet bushes, this radial force is

sometimes replaced by the fastener splining theplate or panel, or by gripping axially with serra-tions on the fastener flange biting into the surfaceof the panel (Figs. 4 and 5).

Type (c) fasteners, locating and holding by radialforce, take a number of forms, viz. , the expansioninsert which is either expanded by the action of theinsert itself (Figs. 6 and 7), or by the action of the

screw which is inserted after assembly of the matingcomponents (Fig. 8).

A variant of type (c), which works other than byinduced radial force, is available for thermoplasticcomponents. This insert (Fig. 9), is pushed into

a moulded hole and the plastics material immedia-tely adjacent is than heated by inducing vibrationor high speed rotation of the fastener. This causeslocal melting and, on resetting of the plastics, thefastener has chaged its type from (c) to (b) as it is

now, effectively, moulded-in.

Fig. 11a. For rolling

threads into preparedhole without cutting.

Fig. 11b. Insert cuts

its own thread . Boreis broached for in-

sertion with an hex-agonal key

.

The self tapping types are useful in a fairly re-stricted range of main component materials andtypical types are shown in Figs. 10 and 11.

ADVANTAGES AND DISADVANTAGESOF THE VARIOUS TYPES

Type (a) inserts (for Insertion into a previouslytapped hole) are used when the main componentmaterial is such that either the thread tapped in it

is substantially weaker than the screw or stud to beused to make the fastening, or the resistance towear is inadequate and /or corrosion (electrolyticor chemical) is likely to be a problem.

The above may be reasons for 'designing in' the in-

sert or for using it as 'salvage' where service ex-

perience shows this to be necessary.

Wire thread inserts (Fig. 1) have the advantageover threaded bushes (Fig. 2) in requiring lessspace. Effectively, only half the thread depth ofthe fastener to be used is added to the standardtapped hole diameter as can be seen from Fig. 12.The driving tang can be supplied 'notched' to facili-

tate removal in the case where it is necessary for

the screw to be engaged through the entire length orinserted from either end. Generally, wire threadinserts are specified as 'notched' as these are

Fig.12. An installed wire thread insert. (By court-esy of Armstrong Patents Co. Ltd.)

Fig.13. Wire thread

insert with screwlocking facility. (Bycourtesy of Arm-strong Patents Co . Ltd)

universally applicable. A cost saving can be madeby using unnotched inserts in specific blind holes.

Further advantages accruing from the wire threadinsert are that the tensile strength and the threadsurface quality can be very much higher than is

possible using a tapped hole. Furthermore, a

degree of compliance is provided, allowing pitchand thread angle errors in the tapped hole in the

main component to be accommodated.

A disadvantage lies in the necessity of using specialtaps. These are fully covered by BS specifications

and equivalent foreign specifications, as are theinserts themselves, and are readily available in all

the manufacturing countries of the world.

A range of wire thread inserts are available provid-ing a screw locking function (Fig- 13). This provid-es a prevailing torque lock built into the femalethread thereby avoiding the need for loose compon-ents which can be lost. They also avoid the need for

special screws and the danger of being lost and re-

placed incorrectly with standard screws in service.

The threaded bushes shown in Fig. 2 may show a

saving in prime cost compared with wire threadinserts, but the provision of means for locking the

insert in a tapped hole may easily cancel this ad-vantage. Various forms are available to overcomethis problem using separate locking rings, or with

a keyway along the external length into which a

locking key strip is driven. There is the ever pre-sent risk that the locking facility may be omitted

accidentally or lost.

Type (b) (moulded or cast 'in situ') are only usablewhen either the main component is produced as amoulding or casting (generally only die cast metalcomponents are suitable due to the difficulty of

location in the casting) or when the ecomonics of

the manufacturing process permit the increased'cycle time' occasioned by the need to position the

bushes in the die or mould.

Advantages lie in the low prime cost of the bush.Disadvantages are the danger of omission and in-

creased 'floor to floor' time for moulding or casting.

Type (c) (for insertion into a plain drilled, coredor moulded hole) are normally used when either

the cost of tapping the main component is not accep-table, or material is unsuitable for tapping. In

certain very soft materials a stronger fastening

can be achieved by using this type of insert than bytapping and insertion.

The simplest fixing is probably the 'push type'

shown in Fig. 6, and the strongest shown in Fig. 7.

Both have the advantage of permitting insertion to

be carried out at any convenient stage after mould-ing or casting.

The type shown in Fig. 8 is not 'captive' in the maincomponent until the screw is fitted. Its advantageis that it is sometimes stronger than the type shownin Fig. 6 (depending on how the latter is used), andlower prime cost than those shown in Fig. 7. Its dis-

advantage lies in its lack of captivity when first in-

serted and the possibility of the insert turning in

the hole when the screw is engaged.

The self tapping type shown in Fig. 10 is a wirethread insert manufactured in a diamond profile

carbon steel wire. It is suitable for use in fibrous

material such as wood, chipboard and building

board.

Fig. 11 type self tapping inserts are suitable for

fibrous materials and certain moulded plastics.

APPLICATIONS

Type (a) fasteners are used most extensively in

light alloys or die cast parts. The aircraft, auto-

Fig. 14. Fig . 1 5 . Fig. 16.

29

threaded bush inserts in stainless steel or brass.For high temperature applications in excess of

450°C, special alloys, such as the Nimonics, areoften used.

FINISHES

On certain applications it is necessary to call for

a plated finish, zinc or cadmium being the usualfinishes for threaded bushes.

With stainless steel wire thread inserts, plating

is not normally required against electrolytic corro-sion, although zinc chromate paste is sometimesapplied to the threads if the insert is to be used in

a magnesium alloy.

Cadmium, or even silver plating, may be called for

on wire thread inserts, if the screw or stud spec-ification is likely to seize up or gall on tightening.

On some applications where it is necessary to stan-dardise the specification of the metal fastener, it

is more economic to specify that all thread inserts

are plated rather than risk the mating of incompat-ible fixings.

Cost factors involved in the specification of other

than natural finish

Unlike nuts where zinc and cadmium plating is verycommon, plating of inserted fasteners is avoidablein most applications.

As stated previously, the use of stainless steel is

normal for wire thread inserts for use in metal,and phosphor bronze for use in plastics.

The question of cost penalty for special finishes is

restricted to cadmium plating for wire thread orbush inserts. Except for extremely high temper-ature applications when silver plating is used oninserts manufactured from the Nimonic range of

alloys.

A surcharge of 35 per cent for cadmium on stain-

less steel, and 40 per cent for silver plating onNimonic alloy is a rough guide to the extra costsinvolved.

OVERALL PRICE COMPARISONS

Due particularly to the small cash value of any typeof insert, prices are very 'quantity sensitive' thisis also because of the disproportionately high costsof order processing, packing and invoicing.

Companies using inserted fasteners on many appli-cations are advised to schedule their supplies andthus effect economies.

The following prices (Table 1) are typical for theinserts that have been described; the figures refer

to ordering quantities of 20, 000.

Table 1

Type Diameter Price/100

Fig.

6

BA 1 8s . 1 d .

4 BA 5s.6d.

Fig. 7 BA 18s.4d.4 BA 10s.4d.

Fig. 8 BA 15s.5d.4 BA 7s.11d.

Fig. 10 tin. 13s.6d.

Fig. 11 i in. 42s .3d.

For supply ex works, 500 off in the smallest sizeand 100 off in the largest size represents a typicalminimum order. Smaller quantities are obtainablefrom manufacturers for prototype work.

Normally a 50, 000 run will be necessary for anypart requiring special tooling unless, with the new-er ranges, the resulting fastener consitutes a logi-

cal addition to the catalogue sizes.

Unless the prospective user has considerable ex-perience of the use of all the listed types, it is

important to obtain manufacturers advice in deter-mining the best type for any new application. Quiteapart from strength, life and cost factors of theinserts themselves, a very wide variety of toolingfor insertion exists and the economics of the pro-ject may well be affected more by 'floor to floor'time than in the cost of the actual insert.

ASSEMBLY METHODS

For Fig. 1 type wire thread inserts, tooling forinsertion is available in manual, power and semi-automatic forms. The choice of method will de-pend upon the quantity to be fitted and on the sizeof the inserted fastener.

Fig.22. Installation of tape fed air motor type

power insertion tool for wire thread inserts.

(By courtesy of O. T.A. L. U . , Chambery.)

32

x*1 Fig. 23. Installation of drill press

operated 'power' insertion tool

for wire thread inserts above 3£ in.

diameter . (By courtesy of Arm-strong Patents Co . Ltd .

)

Fig.26. Foot pedal operated

power' insertion equipment for

Fig. 7 type inserts.

The three basic forms comprise the .hand insertion

tool (Fig. 21) and the reversible air motor tool with

tape feed located in a roving arm which ensures

true axial alignment as shown in Fig. 22. For in-

serts above iin. diameter the tool shown in Fig. 23

is very successful and can be driven by a standard

pillar drill or a hand held drill as reversibility is

not required.

In all cases, the inserts are pitch controlled in the

nozzle through which the fastener passes. This

ensures, in effect, a continuous thread from tool

to work piece as the former is spring loaded into

contact with the latter in operation.

Fig.24. Simple punch for inserting Fig. 7. type

inserts . (By courtesy of Armstrong Patents

Co . Ltd .

)

Fig.25. This tool locates the expansion plate

of the type of inserts shown in Fig.7 . (Bycourtesy of Armstrong Patents Co. Ltd.)

With the smaller diameters the mandrel which eng-ages the tang on the insert is often threaded giving

complete pitch control. As this type of mandrelmust be screwed both into the fastener before in-

sertion and then screwed out again, a reversible

drive is necessary.

The speed of the whole operation, using power, is

such that a typical 'floor to floor' time is 7 seconds

and applications have been tooled down to 5 secondsper insert.

Fig. 7 inserts may be inserted by hand and the ex-

pansion plate then pushed to the bottom of the hole

by a simple punch (Fig. 24). Higher speed is ach-ieved without the use of power tooling by the semi-automatic tool shown in Fig. 25 which locates the

expansion plate with a spring loaded co- axial pin.

For insertion in large hatch or production line pro-

ducts, the hopper fed power insertion equipment

(Fig. 26) enables much faster assemblies to be com-pleted.

FUTURE TRENDS

With the exception of the self tapping inserts, the

use of all the types covered so far lies mainly in

metals and plastics, although highly satisfactory

applications of Fig. 1 type inserts in wood do exist.

Building and constructional fasteners are not within

the scope of the Chapter, but the author believes

that fasteners for containers should receive at

least a mention as many products depend very muchon the use of lighweight and/or re-usable containers,

particularly where air transportation is a require-ment.

The captive screw device shown in Fig. 27 is usedin conjunction with a Fig. 10 insert for lid or collap-

Fig .27 . (By courtesy of Armstrong Patents

Co. Ltd.)

33

Fig.28. For fastening through a panel or as an'expansion fastener' in fibrous materials.Spring legs 'bite' into component on tightening.

sible container walls. The fastener shown in Fig. 28threads into a drilled hole in fibrous materials suchas wood or wood products where size limitationprohibits the use of the Fig. 10 type which are notavailable below number 10 screw size.

Fig. 28 type also provides a fastening at the backof the wall or panel, being inserted on the screwfrom the outside. Whilst small, light and inexpen-sive, pull test loads in excess of 50 lb. and shearloads above 200 lb. can be demonstrated. The in-sert is also very suitable for fastening metal cladd-ing to timber frames.

Test work on the Fig. 9 type insert for use in thermo-plastic materials is well advanced and certain appli-cations already exist.

Fig. 27 type is well established and Fig. 28 andFig. 9 inserts will shortly be available on the UKmarket.

In conclusion, techniques of manufacture and auto-mation of tooling for insertion are continually ad-vancing in pace with the increasing use of threadedinserts in engineering and consumer products.

Never this-

SAY NYLONSELF LOCKINGSELF SEALINGNON CORROSIVELIGHTWEIGHTCOLOURSSAY NYLOY

Nyloy Screws Ltd.274 King StreetHammersmith,

01-748 9973 London, W.6.- WITH NYLON

34

s

Nuts-caged

by E. Lamer (Firth Cleveland Fastenings Ltd.).

A caged nut is a full threaded nut enclosed within aspring steel retainer. It is a fastening device thathas the high strength characteristics associatedwith full threaded fasteners, and the versatilityand self-retaining features of spring-steel fasteners.

The retaining portions, or cages, are normallymade of high-carbon spring steel. The threadedmembers are mild steel nuts.

Fig. 1 shows a cage-type nut retainer which is usedto secure standard square nuts to sheet-metalpanels and other assembly components. As can beseen, the cage is in fact a loose box-like retainerwhich fits over the threaded nut. Two sides areopen with tabs bent over to retain the threaded nut;the other two sides extend underneath to form thepanel engaging elements. The nut floats within thecage to compensate for assembly mis-alignment.

The nuts used in these fasteners can be low coststandard square nuts, nylon nuts, or any specialdesign to suit specific requirements.

Full-thread nut retainers are particularly usefulin blind fastening locations. Their self-retainingfeature eliminates the need for welding, clinching,or staking nuts in place. They can be snapped intoplace at any convenient spot along the productionline. They can be installed after painting or enam-elling, thus masking or re-tapping is unnecessary.

In most nut retainer designs the nut floats withinthe spring steel cage, allowing enough tolerance tooffset normal assembly mis-alignment. But byelongating the mounting hole, even greater mis-alignment can be accommodated.

Fig. 2 shows the 'J' type of caged nut which operatesin much the same way. The 'J' type has a shortleg designed to embrace the panel and is startedover the edge of a panel and pressed into position,

Fig. 2. £± Fig.3.£>

until the mounting hole is engaged by the loca-tion means. A typical application for the 'J'' cagednut is the replacement of reinforcing rings andblind threaded bushes on headlight assemblies inthe automobile industry where, clipped into screwreceiving positions on the wing apperture, theshort leg on the front side of the nut ensures agood seal between gasket and wing, precluding mudand water leakage.

'J' type nuts are extensively specified in the bodyassemblies of the Land Rover and the Rover 2000.Each body has slots or holes pierced to receivethe correct type of nut at the appropriate stage ofconstruction. They are used extensively in themounting of a fascia, making full use of their blindassembly advantages and vibration-proof qualities.

On the Rover 2000, for example, door and roofpanels are prepared as sub-assemblies. The doorlatch remote control assembly uses three'

J

1 nutretainers. Their 'floating' characteristic speedsup assembly while still incorporating the full threadengagement of a conventional nut. The boot lidcatch, a heavily loaded application on most moderncars which has to withstand harsh treatment, isalso secured by 'J' nuts.

Another popular type of caged nut is the circularvariety (Fig. 3). Again it is designed to. providea full-threaded nut for assembly where access isfrom one side only. If required, it can be fitted tothe panel before final assembly with a special appli-cation tool or a flush fixing can be made by counter-sinking the panel to contain the flange of the cage.

It is also possible to make a satisfactory assemblywithout the application tool by exerting pressure onthe outer panel to prevent the cage from rotating.

Summing up, the caged nut is invaluable for heavyduty blind applications. They are thus used ex-tensively on automobiles, farm equipment, officefurniture, domestic appliances and in any productwhere the design requirements necessitate blindfastening with high strength combined with a degreeof 'float'.

Nuts - clinch and anchor

by A. Jordan (G.K.N. Screws & Fasteners Ltd.)

These fasteners provide a means of obtaining deeptapped holes, to take conventional machine screws,in parent metal that is too thin to be tapped, or ex-truded and tapped. They are also beneficial in

those applications where access to tightening onfinal assembly is severely restricted and does not

allow adequate wrench engagement. Although gen-erally applied to sheet metal sections the nuts canbe used on other materials that do not lend them-selves easily to welding, i. e. light alloy, glass

fibre and plastics.

The use of such nuts also eliminates the need for

locally strengthening the parent material by the oldestablished methods of fabrication, such as the

welding on of bosses, or bolting on of flanges. Onceriveted in position the nuts permit the 'blind' as-

sembly of the bolt on final installation. The sameadvantages are obtained as with the fully tappedthicker materials, in that the nut and anchor sheet

are one unit, without interface movement betweennut and attached sheet.

CLINCH NUTS

Basic design features of most types of clinch nuts

is of a common nature, in that a nut of normal com-mercial proportions is mounted on a spigot. Tofix the nuts in position a hole to match the nut spi-

got is drilled, or pierced, in the attachment plate,

the spigot end is then riveted to secure the nut to

the plate.

In order to prevent damage to the threaded sec-tion of the nut the spigot is countersunk to a depthslightly exceeding the spigot depth. The top face

of the nut also is slightly dished, so that in the

Fig.1 . Typical nut

profile showingsetting action with

sheet metal inter-

lock.

Clinch nut inserted in

hole to become integral

part of the product

.

Fig. 2. Assembly de-tails of square spigottype clinch nuts.

Special punch quickly

swages a clinch nut to

work

.

riveting process the thread run out is protected.

A flat face is recommended on the closing tool

for clinching the spigot end; on the large sizes of

nuts a convex punch may be necessary to spreadthe spigot initially but a flat tool should be usedfor final setting. Conical or pointed tools shouldbe avoided in case damage to the thread start is

incurred. Fig. 1 outlines the nut shape and setting

technique.

Precise design configuration will depend on instal-

lation requirements, viz. spigot shape and length,

body shape, resistance to turning. The followingillustrations serve as a guide to the range of nutsavailable, but do not cover the combination of fea-tures that are available.

Non circular spigot

This type of nut is somewhat of a 'special' and is

used in heavy installations that require an extreme-ly high resistance to torsional rotation of the as-sembled nut. The spigot shape, hexagonal, or Dsection is located in a pierced hole of the sameshape in the attachment plate. Abutment surfaceof the nut is usually flat, and the spigot is riveted

to clamp the plate. This means that the clampingpressure, or resistance to pull out is derived fromthe riveting operation, and the resistance to turnachieved from the spigot-hole keying action. Fig. 2

shows this type of nut.

Circular spigot

Hexagon nut body - normal duty. For use with plate

thickness in the 20 swg. to 11 swg range. One such

36

Fig. 3. Standard hexagon body, showingundercut abutment face locking indentations.

Fig. 5. Round body 'blind' sealing clinch nut.

type is illustrated in Fig. 3 and represents a typicalcommercial variety, the spigot is of a length to ac-commodate a limited range of plate thicknessesdependent on nut size. The abutment undersurfaceof the nut body is back tapered to facilitate the flush

fitting of the spigot with the inner face of the attach-ment plate. In the riveting operation the plate is

deformed into the relieved abutment surface, whichis usually indented, thus generating a nut to plate

keying interference which gives the anti- rotational

properties.

The degree of 'flushness' that can be achieved will

depend on the proportion of spigot length and plate

thickness. For absolute flushness a slight counter-

sink in the drilled hole may be necessary, wherespigot length and plate thickness is not wholly com-patible, i.e. plate too thin to accommodate dis-

placed spigot material.

Hexagon nut body — heavy duty. For heavy duty

installations, embracing plate thicknesses of up to

6 swg. , some forms of nut body have an annularserrated ring on the underside of the nut abutmentface (Fig. 4). The serrated teeth embed in the at-

Fig.4. Hexagon body with serrated abutment

face for heavy duty installations.

tachment plate during the spigot setting operation,

thus giving strong anti-rotational properties. Thistype is generally used when the plate thickness in-

creases to such an extent that it will not deform, to

give sufficient torsional lock, under the riveting

pressure.

Nuts of this type are usually available with variousspigot lengths, to suit a wide range of plate thick-

nesses, in all thread sizes.

Round body nuts

There is a range of round bodied nuts, instead ofhexagon, having the same application performance

and following the same basic designs as those pre-viously mentioned.

This type of body acts as a safety feature on thoseapplications where field servicing may be required;

the round body prevents inadvertent loosening of

the nut by the application of a spanner to the clinch

nut body. Viewed from the wrong side a hexagonclinch nut may be mistaken for a normal nut bolt

assembly, by the uninitiated.

Splined or serrated spigot

The basic nut configuration, and design, is similar

to other clinch nuts except that the anti -rotational

properties are achieved by the use of a serratedspigot. Installation techniques are the same as

for other nuts; the riveting operation forces the

spigot serrations into the drilled hole, and into

the clamped face of the plate, giving high torsional

resistance.

Tank sealing nuts

This type of nut is used for making leak proof at-

tachments to water tanks, and other liquid con-tainers (Fig. 5).

'The nut body is blind, i.e. there is no throughthread. Spigot design and installation techniquesare similar to other types. Flush fitting of the

riveted end is obtained by back countersinking the

abutment surface, and anti- rotational propertiesare achieved by indents in this surface. To pre-vent accidental unscrewing the nut body is cylind-rical in form.

'Specials'

Clinch nuts having a 'self-locking' or 'stiff' fea-

ture in the threaded section are also available;

Fig. 6a. Riveting tool

arrangement for pre-vailing torque clinch

nuts

.

37

Fig. 6b. (Top) Clinch nut with 'all metal'

thread friction prevailing torque feature.

Fig. 7. (Above) Contamination free 'cap'

clinch nut with splined spigot torsional lock.

mainly used in the aircraft industry, the threadsare of the UNF range. The prevailing torquethread locking feature can be either the 'all metal'type (Fig. 6b), or the annular nylon insert variety.

Installation of this type requires the bottom rivet-

ing tool to be counterbored so as to accept the nutbody, and prevent damage to the friction element,the pressure bearing surface being the annularsurface at the top face of the nut body. To meetaircraft requirements. Air Ministry specification

A 122 must be met, which states minimum rotation-

al torque values for various plate thicknesses.

Another example of a special nut is the 'plastics

cap' type shown in Fig. 7, which is used in thoseapplications - electronics mainly - which require a

contamination free atmosphere. The cap formsa seal over the open thread end so that any plating

dust, or metal throw out that occurs during the

bolt insertion is contained within the nut body.

DESIGN CONSIDERATIONS

It is important to obtain the correct relationship

between metal thickness, size of thread used, andtype of nut; spigot length and attached materialthickness must be directly compatible. Excessspigot length, from using too large a nut, or too

thin sheet, leads to excessive riveting pressureto achieve satisfactory clamping, and adequateflushness. Over setting in this manner will causelocal deformation, or dimpling, of the sheet. Suchexcess riveting can also deform the lead in coun-

tersink of the spigot; the extra metal which has to

be displaced is spread radially inwards, and out-wards, thus creating interference to the bolt entry.

Sheet metal too thick for the nut spigot raises the

reverse problems, in that insufficient material is

available for riveting, resulting in poor clampedconditions. Attendant problems in this type of in-

stallation occur when mechanical riveting is used,

and the setting tool operates to a fixed height.

Where 'spigot' length/material thickness are mar-ginally close, due account must be taken in vari-

ations in spigot length, due to normal commercialmanufacturing tolerances. In such critical con-

ditions the disparity between maximum and mini-mum spigot length - usually of the order of 0. 010

in. - can make the difference between satisfactory

and unsatisfactory installations. The use of a coun-tersink, or counterbore, can be employed in this

situation to achieve the correct relationship of spi-

got to sheet, as a last resort; the rivet setting tool

diameter must, however, be adjusted accordingly

to clear the parent sheet.

Correct relationship between hole size and spigotdiameter must be maintained. Insufficient clear-ance can lead to interference fits between spigotand sheet, due to manufacturing tolerances on bothhole and spigot; this will increase insertion time,and cause incorrect seating of nut body. Too largehole diameters permit swelling of the nut body,generate poor bolt-nut thread engagement, and re-duce effective clamping. As a general guide, holesizes should be 0. 002 to 0. 005 in. bigger in dia-

meter than the maximum specified spigot diameter.

The buckling strength of the sheet metal used is

quite often the weakest part of an assembly; thus,without adequate support, thin sheet sections canbe deformed under the induced loads of a properlytightened assembly, e.g. a mild steel i BSF screwtightened to its correct pre-load, induces a tensileload of 1500 lb. approx. On the other hand, if thesheet metal strength is the dominating design fac-tor, a lower strength bolt may be employed.

Axial loads induced in the assembly should, of

course, act against the abutment face of the nut,

the pull should never be against the riveted spigot.

A guide to the general clinching performance of astandard range of steel hexagon bodied nuts, ap-

Table 1.

Torque to Pull out Rivet set-

Sizeturn load ting load

lb. in. lb. lb.

6 BA 15 130 4500

4 BA 20 150 5600

2 BA 30 200 5600

i in. 35 250 6700

& in. 150 400 1 1 ,200

1 in. 200 500 15,700

38

make fastfaster

-with Long-Lok self-locking screws and bolts

Long-Lok self-locking screws and bolts are designed to help designers by reducing

the number of locking components, cutting assembly time and providing a

vibration-resistant lock at any degree of torque. The locking action is effected by a

strip of special resilient material held in a longitudinal slot which imposes a

metal-to-metal drag between the threads opposite. Lock washers, split pins, safety

wires, popping - all are unnecessary. Long-Lok self-locking nuts and bolts lock as

they are inserted, reducing component and assembly costs. They also assist

after-sale maintenance and inspection : they can be re-used up to 1 5 times without

loss of lock.

^ Strip-Lokisa commercial version of the proven

M Bl||ftf"***#*lH Long-Lok product.

m UlMwUITllV It is available at lower cost, where high volume™^^^^requirements apply. Recommended re-usability: 5 times.

Special feature of Strip- Lok is sealing against fluid pressures, when screw thread

has been fully torqued down (seals as it locks).

T-Sert This thin- wall insert, which has received fine acceptance, locks

externally and internally, and is used in soft materials such as

aluminium and plastics. It possesses high strength characteristics while

offering a re-usable locking method.For full information about range and applications of Long-Lok products,

j

please send for catalogue.

Long-Lok LimitedBuckingham Avenue Trading Estate, Slough, Bucks.

Telephone Slough 26741 . Telex 841 65.

39

plied to recommended material thicknesses is giv-en in Table 1

.

Torque to turn is the measurement of the force re-quired to rotate the hexagon body after setting.

Pull out - push out - load, registers the resistanceto pull out the nut against the riveted spigot.

Setting pressures are those necessary to adequate-ly set the spigot to obtain suitable flushness andclamp performance, and serve as a guide to presscapacity requirements.

Operating temperatures for steel nuts should belimited to 200°C, and 125°C for brass and alum-inium nuts.

SIZE RANGE AND MATERIALS

The most common materials used for clinch nutsare steel and brass, and these should be availableas stocked items. Stainless steel (En58M) and lightalloy are available for specialised applications.

The lack of a British Standard for this productmakes the permutation of material, thread type,nut type and availability, a daunting proposition.Although all thread forms BA-BSF/W-UNF/C arecatered for, usually, in the i to i in. range, notall are readily available in all types of nut. Keydimensions controlling installation features, spigotdiameter and length, nut body height may vary be-tween suppliers, and even types of nut. Thus, it

is essential to establish precise control dimen-sions, and supply conditions, at the earliest designstage.

ANCHOR NUTS

Anchor nuts provide a means of obtaining a captivenut, in the pre-assembly stage, in those areas offinal assembly that prohibit the use, through re-stricted space, of the normal wrenching means.This type of nut is widely used in the aircraft in-dustry where assembly of wing sections, etc.

,

present many problems of restricted accessibilityin the final stages of construction.

ADVANTAGES

The anchor nuts are affixed to the requisite mem-ber in the early stages of jigging, where installa-tion is easily achieved; thus final nut-bolt assem-bly can be obtained by bolt driving only, permittingassembly from one side. There are by-product ad-vantages in that they can reduce assembly man-power, and hence costs, and eliminate the dangerof incorrect fastening because of poor nut spannerconditions, which are inherent in such situationswhere the use of open-ended spanners only is pos-sible. Assembly conditions are more stable, byhaving one common driving member, this in turngives a more uniformly loaded assembly, torque-tension relationship being more stable.

TYPES

The type of nut to be used will be decided by theinstallation conditions prevailing, the relationshipof nut-bolt axis, and attachment planes availablewill determine the shape of the nut, single lug,double lug, countersunk, etc. Environmental con-ditions will determine other requirements, align-ment problems in the long run assemblies, seal-ing necessity in tank construction, operating tem-perature, and tensile requirements of the assem-bly. Weight considerations will determine the needfor standard or miniature assemblies.

To meet these design requirements there is a largevariety of anchor nuts available, the most commontypes in use being as follows :

Fixed anchor

This type is used when bolt misalignment in thefinal assembly is reduced to a minimum.

Fig. 8. (Top) Fixed anchor , single long lug stiffnut-ig.9. (Centre) Fixed anchor , double lug stiffnut.Fig. 10. (Bottom) Fixed anchor, corner attach-ment stiffnut.

Single lug (Fig. 8). Generally used where attach-

ment to the plate is only possible on one side of the

nut axis, it allows the nut body to abut to a vertical

adjoining plane.

Double lug (Fig. 9). Attached to the plate in two

places equally disposed from the axis of the nut,

generally used where greater freedom of attach-

ment is available.

Comer lug (Fig. 10). Used in the restricted areas

where three adjoining plates, forming a corner,

prohibit the use of either a single or double lug.

In the three types outlined above a restricted am-ount of misalignment of the bolt and nut in the final

assembly is permitted by the slightly oversize

clearance hole in the anchor lug plate. This clear-

ance is generally of the order of 0. 004/0. 005 in.

in excess of the nominal bolt diameter.

"Floating' assemblies

In assemblies that require a greater degree of flex-

ibility in final construction alignment, a range of

anchor nuts can be obtained in the 'floating', oradjustable condition (Fig. 11). The nut is contained

in the lug assembly, but is permitted to move lat-

erally and vertically to a limited degree to take upany out of line conditions that exist on final assem-bly. This lateral movement is permitted again byemploying an oversize bore in the lug base plate,

the amount of movement available is dependent onthread size; as a guide the oversize hole is of the

order of 0. 040/0. 050 in. in excess of the bolt dia-

meter. There are also, however, 'special' float-

ing assemblies that will give excessive movementfor extreme cases of adjustment.

In straight line multiple unit applications the useof gang channel strip is advised (Fig. 12). The'floating' anchor nuts are contained in a continuousstrip, in various specified nut spacings in lengths

up to six feet.

— ig .11 . (Below)Floating anchor .double lug stiffnut.

Fig .12. (Bottom)Floating anchor ,counterborednut gang channel

.

Fig. 13. Floating anchor nut, self sealing.

By this means installation costs are reduced, andassembly time shortened. The same precaution

for accommodating mis-alignment is available, as

for the single floating anchor assemblies.

Self sealing

In applications requiring liquid or pneumatic seal-

ing, i.e. fuel tanks and pressurised cabins, a rangeof self sealing anchor nuts are available (Fig. 13).

These are steel capped nuts, that contain the bolt

engagement within the cap, having an annular rub-ber sealing ring in the bearing face that expands

on tightening, giving a pressure tight seal.

Pressure range of such nuts is -14 to +50 lb. /sq.

in. within an operating temperature range of -80°Fto +250°F.

Deep counterbored nuts

A range of nut body heights is available which ac-commodates height variation in assembly clamped

Fig. 14. Floating anchor , two lug, deep countei

—

bored stiffnut.

Fig. 15. Weight saving achieved by the use of

deep counterbored anchor nuts.

OLD METHOD NEW ME I HOD

41

Table 2.

Details of Material , Finish & Performance .

Pe r formance

.

Material Finish Min . Tensile Max . Operating• Temp.

Carbon steel Cadmium plated, molybdenumdisulphide dry film lubricant

coated after plating.

160,000Ib./sq.in.

250OC,

Carbon steel Cadmium plated molybdenumdisulphide dry film lubricant

coated after plating.

125,000Ib./sq.in.

250oc

Corrosion Molybdenum dry film 125,000 250°Cresistant lubricant. Ib./sq. in.

steel (A286)

Corrosion Silver plated. 125,000 450°Cresistant Ib./sq.in.(A286)

members (Fig. 14). By this means a standard bolt

length can be employed, and the elimination of pack-ing shims, with consequent reduction in weight, is

achieved (Fig. 15).

ATTACHMENT OF ANCHOR NUTS

The usual method of attachment is by riveting,during a pre-assembly jigging operation. In ex-treme cases, however, the nuts can be riveted 'in-

situ' by locating the nut on the bolt and 'spotting'through the rivet holes; that is, using the nut as atemplate.

Friction welding is also used, this method howeveris generally confined to the heat and corrosion re-sistant steel nuts. In these applications, weldingnibs are provided on the lug of the anchor nut, inplace of the rivet holes. The use of welded attach-ments are necessary in those applications wherethe drilling of rivet holes is unacceptable, for rea-sons of stress limitation. Such installations areof a permanent nature.

LIGHTWEIGHT FASTENERS

The present trend is towards the lightweight, 'stiff

anchor nut assembly, drawn from relatively thin,

heat-treatable steels, a high quality lightweight

Fig. 16. Beam offset stiff anchor nut for high

temperature installations

.

all metal fastener is obtained. Basic metal thick-

ness ranges from 0. 01 7 to 0. 048 in. for most vari-eties of nuts. Carbon steels are used for nuts in

applications where the operating temperatures donot exceed 250°C; above this temperature corro-sion resisting steels are used. Table 2 outlinesthe nut steels used, limiting operational tempera-tures, tensile performance and the appropriatefinishes applied.

In order to improve vibration and shock resistancethe nuts are provided with a 'stiff feature, or fric-

tion element, which induces a prevailing torquewhen the bolt is assembled. This is achieved onthe 'all metal' type of nut, by elliptically deform-ing the upper portion of the threaded section duringmanufacture. To prevent thread seizure, or gal-ling, and induce uniform torque, the nuts are final-

ly lubricated, the type of lubrication being depend-ent on nut material and finish, see Table 2.

The flexibility of the nut body, together with thecontrolled lubrication, premits the nuts to be re-

Fig.17a and 17b. Two types of fixed anchor nuts

showing cage, cap, nut and the assembly.

42

Fig. 18. Fixed anchor nuts solid body weldedto attachment plate.

Fig. 20. Fixed lug assemblies with annular

nylon inserts for inducing prevailing torque.

used with consistent performance, and to retain

prevailing torque.

Prevailing torque characteristics can be achievedby the use of the nylon insert type nut where a cap-tivated annular nylon ring, at the thread sectionremote from bolt entry is compressed by the pas-sage of the bolt. The 'memory or recovery of the

nylon provides the frictional prevailing torque onthe bolt; withdrawal of the boit, allows the nylonto reform to its original shape, thus allowing re-

application without loss of torsional characteristics.

Nylon inserts retain their effectiveness in opera-ting temperatures up to 125°C; heat resistant nyl-ons, or polyamides, are required for temperaturesabove this range.

In applications subjected to prolonged high temper-atures (450°C), it is advisable to compensate thefrictional element for temperature changes, so that

at operating temperature the nut is not overstress-ed. The 'Beam offset' type of nut (Fig. 16) is de-signed for this purpose; the multiple axially slot-

ted body is deformed, and retains flexibility suchthat the locking torque remains consistent at highoperating temperatures.

The bolts used for such installations, such as ex-haust manifold systems, are also compensated fortemperature change by having a 0. 003 in. relievedpitch diameter.

In areas that are extremely confined, or whereweight reduction is of prime importance a range of

'miniature' assemblies is available; the same mech-anical properties are obtainable with these nuts aswith the standard range. Weight reduction is at-

tained mainly by the reduction in the size of theattachment lugs; an indication of the weight of theseassemblies can be appreciated from the following:

iUNF Single Lug Standard 0. 47 lb. per 100

i UNF Single Lug Miniature 0.33 1b. " "

i UNF Corner Lug Standard 0. 52 lb. " "

i UNF Corner Lug Miniature 0.30 1b. " "

Fig. 19. Caged assembly for welded installation.

Size range

Lightweight 'all metal 1 nuts cater generally for the

smaller sizes of the Unified thread series. Sizes

4-40 to 8-32 in the coarse thread range, and 10-32

to 1-24 in the fine thread range, class 3B threads.

DESIGN CONSIDERATIONS

The lightweight range of anchor nuts is replacing

the original captive nut assemblies, which covered

the larger diameters and British thread systems.

Standard type and sizes of nuts were contained in

a cup which was then encompassed in an enveloping

anchor plate (Fig. 17). The same system of lug

configuration, floating, static and strip assemblies

were, and are still, available. Variation in nut

type, material, and thread sizes were many, viz.

Fixed nuts

Floating nuts

6BA to

6BA to

iBSF&BSF

Unified thread 6 UNC to I UNF are confined to the

solid, or one piece, nut body, and are attached byspigot welding rather than encaged (Fig. 18), Nutmaterials include, carbon steel, corrosion resis-

tant steel, light alloy, brass, phosphor bronze.

It is recommended that nut, cup and anchor plates

in these assemblies are of the same material class.

This system does permit the use of a wide permu-tation of proprietary nuts, materials, and threadtypes, in applications where weight is not of primeimportance. Whilst the accent has been placed onthe aircraft industry for the use of anchor nuts, the

advantages and benefits to be derived can apply to

any installation where accessibility and loss of con-trolled installation is encountered. Fig. 19 illus-

trates one such type of the welded spigot varietythat is currently in use in the automative industry.

AVAILABILITY

The various types, and thread sizes, outlined aboveare but a guide to the whole range available in theanchor nut field. It is advisable, at the earliest

possible design stage, to contact suppliers for com-plete range and specifications. Small modificationsat an early stage may well permit the use of a stan-dard, or stock item, at a cost much reduced to

that which one must pay for specials, which becomea necessity if the design is too far advanced.

43

Nuts - locking

by T.E. Harris

There are many different terms for nuts with lock-ing media, but for the purpose of this Chapter twotypes of locking nuts will be considered and referr-ed to throughout as 'stiffnuts 1 and 'free spinninglockouts 1

..

Stiffnuts. This is the term used for a prevailingtorque locking nut, which is provided with someelement which grips the bolt threads, thus realis-ing a continuous, or prevailing, torque whereverthe nut is positioned on the bolt. This torque hasto be overcome before the nut can be moved in

either direction along the bolt.

Free spinning locknuts. This descriptive nameimplies that the locknut can be freely spun up to the

abutment or joint face until the locking medium orelement acts with the abutment face to give a lock-

ing action.

Before detailed study of locking nuts can be appre-ciated it is important to understand the basic theoryof threaded joints, which is taken for granted bymost design engineers.

F2

Fi

u.

u

11

o/

D/'

Fo

... F~"p*p

jr PiTP-AF

\ ,

o

Fi

TENSION "^COMPRESSION

DEFORMATION e

g.1 . Force ~ Deformation diagram.

—f//////<K\\\\\\M^

M

•ezzzzz^ sssssa-

Fig. 2.

Clamped joint

.

THEORY OF THREADED JOINTS

Reference to Fig. 1 will show the behaviour of anut and bolt connection of two metal plates beingclamped together as illustrated in Fig. 2.

Within the elastic range. Hook's Law applies anddeformation (e) is directly proportional to the app-lied force (F). The bolt deformation occurring ontightening can be represented by line OA in Fig. 1.

The connected parts deform in compression asthey are also assumed elastic and their deforma-tion can be represented by line BA. Because it is

usual that the components being clamped are morerigid than the bolt, then CB is shorter than OC. In

other words the clamped components take less de-formation than the bolt for the same load.

The joint is tightened to a point at A where theforce on the bolt and the clamped parts is Fi andthen tightening is stopped. This is now a norm-ally tightened connection and we need to study whateffect externally applied loads (service loads)would have on the joint.

Assume an external load is applied to the joint.

This results in a further elongation Ae of the bolt

and the compression of the connected parts de-creases by Ae.

The load on the bolt increases by an amount AFand if the connected parts are more rigid than thebolt the load on these parts decreases by a greateramount than AF shown by P-AF.

It is obvious that since the external load on the

bolt follows a straight line law and the deforma-tion can reach point Q when the compression ofthe connected parts becomes zero (at B) then the

joint will begin to open, since the parts can nolonger expand to maintain contact.

It can be proved that the external load FQ in Fig. 1

required to open the joint can be given by:

'.-"to*]

where k^ = stiffness of bolt = ~

Pik„ = stiffness of connected parts = sr-

44

P EXTERNAL LOADF, = INITIAL TENSION

DEFORMATION e

Fig .3 . Force~DeformationDuctile bolt - rigid joint.

or conversely the pre-tension necessary to preventopening when a know external load FQ is applied to

the joint can be found from:

Pretension Fi = F°|V^cJ

(Formula 1

Practically F^ should be between 1. 5 and 2 timesthe value obtained from this formula.

If the joint is less stiff than the bolt (k <kij. the

term inside the bracket in Formula 1 becomessmall and the pretension necessary to prevent thejoint opening is low relative to the applied load.

Conversely, and the more usual case, if the value

of kc >kD then the bracket in Formula 1 approachesunity and the pretension necessary approaches thevalue of the applied load, if the joint is not to open.

This can be illustrated by Fig. 3, which comparesthe force/ deformation diagram obtained by tighten-

ing a very long ductile bolt to clamp a very rigid

joint material with that in Fig. 4 which shows the

diagram obtained when a short stiff bolt is used to

clamp a relatively unstiff, perhaps gasketed, joint.

P « EXTERNAL LOADF, = INITIAL TENSION

DEFORMATION e

Fig. 4. Force~DeformationStiff bolt - unstiff joint.

In both cases the same external load P is applied,

but the effect is vastly different. It can be seenthat in all cases the actual load is greater than theinitial tension.

Actual Load > FiAlso Actual Load < Fi + P

In Fig. 3 a smaller increase in load is felt by the

bolt so that any dynamic load P would have less

effect on fatigue life than with the arrangement of

Fig.. 4 where a very large proportion of the exter-nal load is added to the bolt. It can be said, there-

fore, that the more ductile the bolt relative to the

joint material the smaller the effect of dynamicload on the joint and the greater the fatigue life.

The converse is true in the joint represented byFig. 4 which could be a gasketed or spring loadedjoint where high initial tension can be dangerousdue to subsequent loading overstressing the bolt.

Fig. 5. Torque/Tension equipment.

Pretension can also be examined from the formula:

Young's Modulus E =

so that strain =

strain =

stress

strain

stress

E

Elongation

original length

Elongation = stress x —E (Formula 2)

Where L = Length in inches, E = 30 million lb./sq.in. for steel. Therefore for each inch of bolt lengthand each 30, 000 lb. /sq. in. of bolt tensile strengththe bolt elongation will be 0. 001 in.

From this theory it can be seen that the longer thebolt the greater stretch possible for the same load-ing, which in the case of a dynamically loaded joint,

we have seen, is desirable.

Consider a hypothetical case of two similar jointsof the type shown in Fig. 2, one with a 1^ in. long

45

clamped length and the other with a 3 in. long clam-ped length, both bolts being 'S' quality (50 ton/sq.in.

minimum tensile strength) steel. Consider bothbolts tightened to 60, 000 lb. /sq. in.

From formula 2:

1. Short Bolt: Elongation = ^tg—fp^ = 0. 003 in.

2. Long Bolt: Elongation =60,000 x 3

30 x 10g— = 0. 006 in.

Now assume, as a result of burrs under the headof the bolt or on the abutment face of the nut, flat-

tening due to cold flow during the joints early ser-vice life, that both joints relax in grip by 0. 001 in.

This would result in the joint with \\ in. clampedlength losing 33 per cent of its original tension,

thus retaining 67 per cent, whilst the longer bolt

joint with 3 in. clamped length would retain 83 percent of its original tension.

This is a simple way of illustrating that the longerbolt is more likely to retain its tension and there-fore perform more reliably in service in resisting

dynamic loads, with subsequent increase in fatigue

life.

It is because in practice there are many thousandsof cases where long bolts are not specified, that

the locking media are necessary on the nuts.

NUT FUNCTION

The function of a nut is to engineer or stress the

bolt to its full potential of strength and to maintainthe loads resulting, throughout the life of the joint.

How is the necessary tension in the bolt predicted

and attained? Once the joint design is finalised

and the required pretension calculated it can beattained in a number of ways. Three of these are:

1

.

Turn of the nut method.2. Bolt length increase method.3. Torque/tension method.

—VIELD TENSION

_ _ . nacr-.-n ibc"V ^_^—""" POINT

2PFhJ

01

^^YIELD TORQUE

APPLIED TORQUE

Fig. 6. Bolt tension ~ Applied torque.

Turn of the nut method

This is probably the most accurate practical meth-od, but not widely used except in heavy constructionindustries. Here the pitch of the thread gives thedegree of turn of the nut necessary to stretch thebolt a given amount and thus engineer a given loadinto it.

Bolt length increase method

This is not very practical but is very accurate.The overall length of the bolt is measured beforeand after tightening and by formula 2 the bolt loadis predictable.

Torque/tension method

This method is by far the most widely used in arriv-ing at the correct pre-tension in a bolt. The cor-rect torque to engineer a given tension is predictedin two ways:

a. By use of torque/ loading test equipment of thetype shown in Fig. 5 which simultaneously measuresbolt tension and torque to turn the nut in order to

simulate actual conditions, a curve of load againsttorque can be plotted for each case. A typicalcurve is illustrated in Fig. 6.

Since there is a more or less linear relationshipbetween tension and torque, a percentage of the

yield torque can be taken to give the same percent-age of the yield tension, so that if a yield torqueof 20 lb. ft. were obtained, 15 lb. ft applied tighten-

ing torque would give 75 per cent of the yield loadas the bolt pretension. This figure of 75 per centis quite often used as the utilisation of full bolt po-tential, but higher figures can be used if the joint

arrangement is suitable, as in the case of long duc-tile bolt fixing a stiff joint.

The relationship between torque and tension is ex-tremely inconsistent with factors such as threadroughness, plating finish, squareness of the faceof the nut to the axis of the thread, material, lubri-cation present and thread fit, being only a few of

the multiplicity of factors involved.

b. By calculation from the formula:

T = Fj [^B (a+9) + Tm nl (Formula 3)

whereT - Application torque.

Fi= Bolt pretension.

de = Effective diameter of thread.<* = Thread helix angle.

8 = Friction angle of thread.

m= Mean radius oi abutment face of nut.

I1 = Coefficient of friction.

This formula is quite commonly used, but usuallya simplified version is employed:

T = KFid (Formula 4)

where d = bolt major diameter. Normally Ej is in

46

pounds force, giving torque in pounds inches whend is in inches, and in pounds feet when d is in feet.

K is a friction factor which varies according to thecondition of finish, lubrication, etc. , already men-tioned. The value of K is found to lie between 0. 1

for MoS2finished nut on a highly ground abutment

surface, to over 0. 2 for dry self finish nuts onrough finished bolts and with rough abutment sur-faces.

EXAMPLE . Assume a joint consisting of Sin.

UNF x \\ in. long bolt tightened with a dry self

finish nut with a bolt tension of 4, 000 lb. requiredCalculated the applied tightening torque required:

Solution Consider a K factor of 0. 18 from the con-dition described:

Thread shear

The nut shown in Fig. 7 is subject to high shearforce on the first threads. If the nut materiallacks the ductility that enables it to deform undertightening, thus allowing enough threads to engageto distribute the load more evenly, then progres-sive thread shear can occur. Relative materialstrength of nut and bolt to ensure satisfactory re-sults in this respect are described later.

Crushing

Nuts must have sufficient abutment face area to re-

sist the crushing force, avoiding the high crushing

stresses which would result from small surface

areas. This crushing will result in relaxation in

bolt tension with possible adverse results.

T = 0. 18 x 4000 x 0.312

= 225 lb. in. or 18. 7 lb. ft.

NU DESIGN

A high tensile bolt is only as useful as the tension

that can be loaded into it, making the nut as criti-

cal an element as the bolt.

The thread of a nut is subject to a force duringtightening which can be expressed as two compo-nents: (1) horizontal or radial force acting outwardsand tending to dilate the nut at the base; (2) vertical

or shear force acting in a line parallel to the axis

of the bolt.

Wall dilation

A nut must have sufficient wall thickness and mat-erial strength to resist the radial force which is

trying to spread the base of the nut. However, oneadvantage from this is the spreading of the load to

threads futher away from the abutment face, com-pared with an over rigid nut in which the load dis-

tribution is one of excessive load on the first threadsfollowed by a rapid dropping away as shown in

Fig. 7.

i

\

/y

—A ^3

Fig. 7.

Load distribution

over threads.

COARSE OR FINE THREADS

Yet another important consideration affecting the

threaded joint is the selection between coarse orfine threads and the relative advantages and dis-

advantages of both are discussed here:

1. Coarse threads are easier to start than fine

threads.

2. Coarse threads are less likely to seize during

tightening.

3. The stress distribution with coarse threads is

more even than with fine - even when each is of

the same material.

4. Fine threads have approximately 10 to 15 per

cent load carrying advantage over coarse threads

in the Unified thread series.

5. Fine threads have greater torsional strength

than coarse threads, because of the higher value

of cross sectional area of the bolt core.

6. Fine threads tend to strip earlier than coarse

threads due to nut dilation causing early disengage-

ment from male threads.

7. Fine threads have a greater resistance to un-

screwing as a result of their lower helix angle.

MATERIAL. SELECTION

The material is selected for the bolts required in a

joint on the basis of service loads on the joint, size

of bolts, number of bolts, type of thread selected,

and whether the fastener has to be corrosion re-sistant. Once this bolt material has been selectedit is essential to select the correct nut material.While the bolt must be capable of sufficient strength

to resist failure by external joint load, the nut mustbe capable of engineering this bolt and we haveseen that, in order to do this, it must be ductile

enough to distribute the load as evenly as possibleover the maximum number of threads to minimisethread shear. The nut material is usually severalgrades of material strength lower than the bolt

material (usually about 75 to 85 per cent in termsof material used) and is expressed in the BritishStandard Specification for Unified precision hexa-gon bolts, screws and nuts (BS1768) in terms of

nut proof load. This is also the case in the Ameri-can Specification for prevailing type hexagon lock-nuts (Stiffnuts).

47

Table. 1 . Proof load for nuts - Unified hexagon series.

Bolt stress Grade Grade 1 Grade 3 Grade 5

Nut

size

Area Nuts Nuts Nuts Nuts

UNC UNF UNC UNF UNC UNF UNC UNF UNC UNF

in. sq.in. sq.in. ton. ton. ton. ton. ton. ton. ton. ton.

1

A16

i

.0324

0.05320.0786

0.03680.05870.0886

1 .134

1 .862

2.751

1 .288

2.0543.101

1 .6202.6603 . 930

1 .840

2.9354.430

1 .782

2.9264.323

2.0213.2284.873

2.4303.9905.895

2.7604.4026.645

iie

i

1

.10780.14380.184

. 1 1 980.16120.209

3.7735.0346.440

4.1935.6437.176

5.3907.1909.200

5.9908.06010.25

5.9297.91 1

10.12

6.5898.869

11 .28

8.08510.7813.80

8.98512.0915.37

13I

i

0.2290.3380.467

0.2580.3750.513

8.01611 .83

15.97

9.03113.1217.96

11 .45

16.9023.35

12.9012.6025.65

12.6018.5925.68

14.1920.6328.21

17.17

25.3535.02

19.3528.1238.47

1 0.612 0.667 21 .42 23.34 30.60 33.35 33.67 36.68 45.90 50.02

The proof load figures for BS1768 are shown in

Table 1 and are tested by assembling a samplenut on a hardened thread mandrel and the nut load-

ed to the appropriate load shown for the particular

size. The nut should resist the load without stripp-

ing. Nuts in this British Standard are classed as

either Grade 0, 1, 3 or 5.

In the American Specification, nut Grade A, B andC compare respectively with Grades 1, 3 and 5 in

British Standard. These values are shown in Table3. The underlined figures are the proof stressesused for calculation of all proof loads shown belowthese figures. Grade A locknuts are for use withbolts up to 35 ton/sq. in. Grade B locknuts are foruse with bolts from 45 ton/sq. in. to 55 ton/sq. in.

Grade C locknuts are for use with bolts from 60

ton/sq. in. to 70 ton/sq. in.

WHY USE LOCKNUTS?

We have examined, in the first part of this Chapter,the theory of joint design, and have seen that if cor-rectly selected fasteners are specified and the cor-rect pretension has been applied by properly app-lied tightening torques the joint will remain intact.

It is because of breakdown of abutment surface andother factors causing relaxation of tension and thedifficulty of arriving at the correct pretension in

the first place, that makes locknuts necessary.

Also, design limitations sometimes necessitate the

use of short bolts on joints which are not rigid in

that they may be gasketed, pivoting or sprung andhigh pretension and high bolt stretch cannot beachieved.

STIFFNUTS

The object of stiffnuts is, if they cannot prevent the

joint 'slackening', to prevent the joint from 'turning

loose'. To explain these terms we can say that if

a joint is secure against 'slackening' it is also sec-ure against 'turning loose', whereas a joint securedagainst 'turning loose' is not necessarily securedagainst 'slackening 1

.

Prevailing torque type locknuts or stiffnuts areprovided with locking elements of various typesusually at the end opposite the abutment face of thenut or at a point between the abutment face and thefree end of the hut. This element exerts a friction-al force on the bolt threads thus helping to preventthe nut turning loose during service.

There are two main standards which have been usedas a basis for testing of stiffnut performance, thefirst being a British Standard for stiffnuts (Unifiedthreads) for aircraft, the second, the previouslydiscussed American Specification for prevailingtorque type hexagon locknuts.

British Standard (aircraft)

This has been used for many years as the standardfor stiffnuts by many manufacturers. It specifiesa minimum unscrewing torque figure for the nutusing an unused bolt.

Six nuts are taken from a test sample and mountedon unused dry bolts and the average torque to resistunscrewing is measured over at least three turns

Table.2. Nut and bolt selection

.

Nut BoltGrade Tensile Strength (min.)

Grade P 35 ton/sq. in.

Grade 1 S 50 ton/sq . in

.

Grade 3 T 55 ton/sq. in.

Grade 5 V 65 ton/sq. in.

X 75 ton/sq. in.

48

Table. 3. Proof load stiffnut specification.

Proof load (lb.)

Nut Grade A Grade B Grade CSize UNC UNF UNC UNF UNC UNF

109,000 120,000 1 50 ,000

iin. 3,450 3,950 3,800 4,350 4,750 5,450

iin. 5,700 6,300 6,300 6,950 7,850 8,700iin. 8,450 9,550 9,300 10,500 11 ,600 13,150

&in. 11 ,600 12,900 12,800 14,200 15,900 17,800

iin. 15,500 17,400 17,000 19,200 21 ,300 24,000

Sin. 19,800 22,100 21 ,800 24,400 27,300 30,500iin. 24,600 27,900 27,100 30,700 33,900 38,400

Jin. 36,400 40,700 40,100 44,800 50,100 56,000104,000 115,000

iin. 48,000 52,000 53,100 58,500 69,300 76,4001 in. 63,000 69,000 69,700 76,200 90,900 99,500

at a uniform speed between 2 and 30 rev. /min.

,

the static torque being ignored. After this the samenut is mounted on the same bolt and immersed in alight oil and the test repeated.

The average of the six dry and the average of thesix oiled unscrewing torque readings are to be not

less than the values in Table 4, and the minimumindividual unscrewing torque reading is to be notless than 75 per cent of the values.

An endurance test is specified in which a nut is

screwed on a standard bolt for at least three threadsthrough the friction or locking element and then re-moved, the cycle being repeated 30 times.

The final unscrewing torque must be at least 50

per cent of the figure shown in Table 4 in each case.

Other tests performed are bolt tolerance test, tor-

sional strength test and high and low temperaturetests.

The first four nuts described below, i. e. 'Nyloc',

'Parlox', 'Aerotight' and 'Philidas', are all design-

ed to satisfy this specification, whilst all othersdescribed under stiffnuts are tested to the Indust-

rial Specification for prevailing torque type hexagonlocknuts.

Industrial specification

Grade A and B nuts which cater for bolts up to 'T'

quality (55 ton/sq. in. ) are required to satisfy maxi-

Table.4. Unscrewing torque - steel nuts.

Size Torque lb ./in.

iin. UNF 1 .3

fiin. UNF 2.4iin. UNF 4.0&in. UNF 6.1Jin. UNF 8.8Ain. UNF 12.4iin. UNF 16.4iin. UNF 27.0*in. UNF 41 .5

1 in. UNF 60.0

mum figures for prevailing torque (on) during first

application and first and fifth minimum breakawaytorques on removal as shown in Table 5.

An unused standard bolt of thread fit Class 2A is

taken and an unused stiffnut screwed on to it, the

prevailing torque being the 'on' torque measured onfirst installation with no load on the bolt and with

the locknut in motion, and with the bolt protruding

through the locknut between two and three threads.

The breakaway torque is measured on the first andfifth removal and is the torque required to start the

locknut in unscrewing motion at a point where the

bolt protrudes through the locknut by between twoand three threads, the bolt unloaded at this point.

Stiffnuts described below (except the first four) aretested to this specification.

•Nyloc'

This nut (Fig. 8) is probably the best known in this

category and is one of the most reliable of all stiff-

nuts. It consists of a plain nut portion surmountedby a shroud which has been rolled over and keyedafter insertion of a ring of nylon material. Thekeying is to prevent rotation of the insert duringapplication.

49

METAL—s.SHROUD ^—

,

,-—NYLON— ^r INSERT

The inside diameter and thickness of the nylonring is scientifically designed to satisfy the torque

and performance requirements previously des-cribed. The inside diameter gives a controlledpercentage engagement in the depth of the malethread before application to the bolt.

As the nut is tightened on to the bolt thread the nyl-

on is formed (not cut) with the profile of the thread

and a high pressure is therefore exerted on the

threads by the nylon. The nylon moves radially

inwards to make intimate contact with the full depthof the male thread giving a reasonable degree of

sealing effect against seepage of fluid, though noclaim is made by the manufacturer of sealing undera head of fluid.

Creep of the nylon under load does not occur be-

cause of the almost complete encapulation of the

nylon ring by the turned-over shroud (Fig. 9).

On removal of the nut from the bolt for service, the

nylon insert recovers towards its original shape sothat subsequent application shows a torque recovery.

The nut is therefore almost infinitely reusable andFig. 10 shows the performance curve of unscrewingtorque against number of removals of a 'Nyloc' nut.

The main advantages of this nut are reusability,

reliability in performance which is second to none,

no bolt damage and high resistance to turning loose

due to extreme vibrations.

The disadvantages are temperature limitations and

limitations on its use in chemicals which attack

nylon. Price is also slightly higher than some of

the metal types described later.

A wide range of inch and metric sizes in variousmaterials and finishes are available.

' Pariox'

The 'Parlox' nut is as the 'Nyloc' nut in its opera-tion with the same advantages and disadvantagesand has only very slight design differences whencompared with 'Nyloc'.

On the larger sizes the nut is of two piece construc-tion with the shroud formed from a tubular steel

section cut to length and staked to the nut body.The manufacturers claim that this method gives ahigher torque performance compared with a onepiece construction. This can only be so if the physi-cal dimensions of the nylon after closure or thegrade of nylon used are different.

The 'Parlox' nut is now available in a glass fibre

reinforced nylon insert which gives higher torqueand higher temperature performance.

The standard and glass fibre reinforced versions'

are available in a wide range of sizes, materialsand finishes.

'Aerotight'

With the 'Aerotight' nut (Fig. 11) the locking action

is obtained from the cantilever arms which areformed by cutting and slotting operations followed

by a downward deflection of the arms after tapping

of the nut.

When the nut is applied to a bolt, the bolt threadforces the arms upwards towards their original

position with the resultant pressure due to the re-silience of the nut material causing a locking action

on the bolt threads. The orientation of the beamsmakes for easier application than removal.

It is claimed that the 'Aerotight' nuts can be usedto 300°C with no problems. They are available in

various thread sizes, materials and finishes.

Advantages are high temperature performance,resilience, reliability, reusability and resistance

to many chemicals and oils.

There seems to be a disadvantage in the danger of

fatigue fracture of the cantilever arms, under veryextreme conditions of vibration, due to notch effect

in region of the root of the cantilever.

•Phil idas'

The 'Philidas' stiffnut (Fig. 12) is another which is

provided with a beam like locking element by cir-

cumferentially slotting the crown portion and de-

flecting the beam, so formed, after tapping.

The nut, it is claimed, can be used up to 500°cand retain its locking action after many removalsand applications.

50

Fig. 11. Aerotight nut. Fig. 12. Phil idas nut. Fiq.13. Cleveloc nut.

Fig. 14. Stover nut. Fig .15. Two-way nut

.

Fig. 16. Uni-torque nut.

'Philidas' nuts are available in all standard threadsand in many materials and finishes.

Advantages are high temperature performance,resilience, reliability, reusability and resistance

to many chemicals and oils. Notch effect of slott-

ing can be considered similar to 'Aerotight'.

'Cleveloc'

'Cleveloc' (Fig. 13) is an all metal prevailing torque

type stiffnut manufactured with an integral locking

crown portion which is given a controlled elliptical

deflection to provide an excellent locking medium.This elliptical design form eliminates thread in-

teruptions or excessive pitch error so that bolt

entry is met with a gradual and smooth increasein prevailing torque up to its maximum value.

The nut is prelubricated with a wax finish to assist

assembly with resulting greater consistency in the

prediction of correct tightening torques.

It is claimed that the nut performs satisfactorily

at temperatures between -70°C and +250°C.

Advantages are that the full height of the nut is

load bearing and high temperature resistance.

One of the big disadvantages as with many of the

all metal. stiffnuts is occasional bolt thread damageas a result of over deflection of the crown. If this

does not occur the locking performance is reliable.

'Stover'

As can be said for the 'Clevelock' nut the 'Stover 1

nut (Fig. 14) is one of the newer generation of all

metal prevailing torque stiffnuts and is formed bycontrolled deflection of the top portion of the nut at

two opposite flats of the hexagon. The control of

this deflection ensures that the locking action is not

sudden but is gradually applied over the last two

or three threads of the nut so that thread galling

on application is avoided.

The nuts are waxed to assist assembly with the

same advantage as with 'Cleveloc'.

The full height of the nut is load bearing and the

nut can be used up to 300°C.

Bolt thread damage on occasions can occur with

weakening of thread strength and possible removalof plating protection on bolt threads.

Hexagon distortion can occur over the whole wrenchlength of the nut as a result of the top deflection,

with possible spanner fit problems.

'Two—way'

The centre portion of this all metal stiffnut (Fig. 15)

is compressed at two opposite flats to form an el-

liptical thread centrally between top and bottom of

the nut.

The nut was originally a symmetrical plain nut so

that the formed stiffnut can be used from either end.

The spring like action of the elliptical section de-velops a progressive and strong locking action onthe bolt threads.

Advantages and disadvantages are similar to those

of 'Stover' nuts but with the added advantage of hav-ing no orientation porblems.

51

Fig .17. (Top) Action of the Uni-torque

.

Fig.18. (Centre) Eslok nut.

Fig. 19. (Bottom) Binks nut.

Table. 5. Locking performance grade A & B locknuts.

Size Prevailing

torque

lb. in. max

Breakaway torque lb . in . max

.

lin. 30

1st removal 5 th removal

5 3.5&in. 60 8 5.5fin. 80 12 8.5

A in. 100 17 12

Jin. 150 22 15

Sin- 200 30 21

Jin. 300 39 27

tin. 400 58 41

*in. 600 88 621 in. 800 120 84

'Uni-torque'

This one piece stiffnut (Fig. 16) is provided with

its locking element by a controlled deflection of

the top threads as illustrated in Fig. 17. The nut

starts freely on the bolt up to the deflected threadsthe prevailing torque then building up to its maxi-mum value very quickly.

Advantages and disadvantages as with the 'Cleveloc'

and 'Stover' types of nut.

'Eslok*

The 'Eslock' stiffnut (Fig. 18) is a comparativelynew design and consists of a plain nut with a per-manently applied patch of nylon on a controlled

area of thread to give predetermined levels of

locking torque.

'Eslock' nut offers excellent resistance to vibration

and has the advantage of not causing thread gallingcompared with some all metal types. The nylonpatch gives effective sealing against liquid seepagealong the bolt threads.

A disadvantage with this nut is the tendency for

the bolt threads to shear out fragments of the nylonpatch which, if it occurs, leaves low breakawaytorque figures. This can happen with bolt threads

which are not smooth and clean.

Maximum temperature for this nut is only 120 C.

Binks'

The 'Binks' nut (Fig. 19) is another of the slotted

deflection beam type of nuts, but with the two slots

moving at an angle from the top centre of the nut

downwards and outwards to stop at a designed di-

stance from the flats of the nuts.

The nut is tapped after slotting followed by a con-

trolled downwards deflection of the beam portions.

Advantages and disadvantages are similar to other

all metal types with the notch effect of slotting,

lowering fatigue strength, the only obvious dis-

advantage .

'Philidas MarkVThe 'Mark V one piece all metal nut has a turretsection which is accurately deformed at two diam-metrically opposite points in a plane between radialand axial, so that a flexible locking element is

created, in a similar manner to the Uni-torque nut.

All threads are fully load bearing so that shorterbolts can be used for the same strength of joint

,

compared with other types in which the lockingelement is not fully load bearing.

FREE SPINNING LOCKNUTS'Whiz Tite"

The 'Whiz Tite' nut (Fig. 20) is a free spinning lock-

nut with a series of spiralling serrations or teeth

Fig.20. Whiz Tite nuts.Fig. 21 . Whiz

Tite design details.

Fig. 22 . Keps nut -

external lock washer

.

(Fig. 21). The number, shape, height and curve of

the teeth are critical in the performance, which is

aimed at creating a higher breakloose (off) torque

than the application or tightening torque.

It is claimed by the manufacturer, that the 'Whiz

Tite' is designed so that when vibration or shockload are applied the teeth grip the abutment surface

with unequalled locking power.

•Keps-

'Keps' is the term applied to a nut and washer com-bination and work on the principle or spring action

in both the External Lock Washer Keps (Fig. 22)

and the Plain Dished Washer Keps.

Dished and external lock washer Keps come into

their own on short bolts where high bolt elonga-

tion cannot be expected. With high strength joints,

the collapsing load of the washer may be low com-

pared with the normal design pretension of the bolt,

and the washer effect is reduced to that of a plain

washer until the tension drops to below the flatten-

ing load of the washer.

ACKNOWLEDGMENTS

Firth Cleveland Fastenings Limited.

Parlox Limited.G.K.N. Limited.Whitehouse Industries Limited.

Glynwes Screws & Fastenings Limited.

E.S.N. A.Brown Brothers (Aircraft) Limited.

ITW Limited.

North Bar Tool Co.

C J. FOX & SONS LTD.SELLING AGENTS FOR:

KAYNAR HARTWELL

TWO-LUG FLOATING ANCHOR NUT

All metal lightweight stiffnuts,

Nut plates, Thin-wall inserts,

Self sealing nuts and also

Greer nylon insert nuts

SPECIAL TRIGGER CONFIGERATION

HOOK, pin, Rotary and trigger latches;

Quick-release pins: Nylon panel fastners

and Cable clips.

PLEASE CONTACT US FOR FULL PARTICULARS

C J. FOX & SONS LTD.

117, VICTORIA ST., LONDON S.W.I.

TEL: 834 0204/5 TELEX: 27661

53

8

Single threaded fasteners

by B.M. Wright (Carr Fastener Co.Ltd.)

The first pressed metal nut was designed for use ontop of a tightened load-carrying nut to prevent the

nut from backing off or loosening under severe vib-

ration. They can, however, be used singly for

light assemblies and, in many instances, replace

the three more conventional parts used, namely, a

plain nut, flat washer and lockwasher.

These types of fastener are single thread locknutsmade of hardened and tempered carbon steel andcan be assembled like ordinary nuts. The threadengaging area is a formed helix in true relationto the pitch of the screw thread, in which the innercontour is designed to provide maximum strengthfrom a single thread nut. At the same time, it is

sufficiently resilient to yield in a spring like manner,when tightened, to provide a vibration proof lock.Assembly can be by hand or power operated tools.

They have certain advantages over other methods offastening:

By the nature of their design they can save up to

65 per cent of the weight of plain nuts, 80 per centof a nut and washer and 85 per cent of nut, lock-washer and plain washer. In most cases, accord-ing to the type used, they require less space thanmany other fasteners. This is especially truewhere lock and flat washers are eliminated.

When assembling, it is only normally neccessaryto run the nut down the thread until the assembledparts are brought into contact and the first resi-stance to turning begins. A further *toi of a turnis all that is necessary to complete the assembly.

When the assembly is completed a double lockingaction is applied by powerful spring forces beingextended upwards by the helix on to the undersideof the threads and downwards by the underside ofthe nut on to the assembly. At the same time further

JUtrFig.1. Illustration

showing the doublelocking action. Thearrows indicate the

directions of locking

forces . (By courtesyof Carr FastenerCo. Ltd.)

fe^fcy- livp»)[W|

forces are exerted inward with the nut gripping thebolt like a chuck (Fig. 1).

TYPES OF SINGLEFASTENERS

Regular type

THREADED

They can be removed and reused repeatedly, withfull security, as long as the coned centre portion

has not been unduly flattened by excessive tighten-

ing torque. They are interchangeable with otherlocking devices and generally require no change in

design when used.

Because the fasteners are made from hardened andtempered spring steel and require no other mater-ial to effect a positive lock they will withstand, andare not affected by, temperatures up to 400 F. Forconvenience in assembly either the nut or the screwcan be driven.

These nuts are frequently used for assembly on to

threads which are affected by paint, burrs or dirt

,

but because of their single thread form they incor-porate a very effective self-cleaning action whenapplied.

The fastener can be used safely for assembly of

fragile or brittle parts and materials. The resi-

lience of thread form effects a firm but spring

cushioned pressure on the assembled parts.

The Regular type locknut (Fig. 2) is the nearest in

appearance to. an ordinary hexagon nut. For light

duty assemblies (Fig. 3) it can be used alone butwhere higher stress is involved, it can be used ontop of a solid nut (Fig. 4). The solid nut carries theload and the pressed metal nut is applied to main-tain the original tightness. Regular types can beapplied in confined spaces. Where it is not possible

Fig. 2. Regular type single thread pressed metalnut. (By courtesy of Carr Fastener Co. Ltd.)

54

fiveminutes

We'll put you wise to Dotlocs:

what they are, how they work, and why you should use 'em.

Dotlocs - what they are

Our Dotlocs are the most effective

way of retaining threaded parts.

They're unique. Single thread lock

nuts made of hardened and

tempered steel giving quick, secure

fastening ... at low cost ! There are

eight different designs in a host

of sizes. Take a look at two of 'em.

Acorn: Tension:Covers unsightly Holds adjusting

rough bolt ends and screws to desired

protects assembly.

Self-threading

version available.

setting.

Why they work so well

Take the regular type Dotloc. Its

engaging part is spirally formed in

true relation to the pitch of the

screw head. Its inner contour gives

maximum strength from a single

thread nut. When tightened,

powerful spring forces (AA) are

exerted upwards on the screw

threads and downwards (BB) on

the part. Spring forces (CC) are

exerted inwards making it grip the

bolt like a chuck. Hence the double

locking action - the most efficient

there is.

Regular type DotlocThe double locking is common to most.

B ^^ BWhy they're the bestDotlocs cost less. They're precision

made under careful control, yet,

because they're produced in such

large numbers, they're exceptionally

low priced.

They save weight. And lots of it

!

More than 65% of the weight of

plain nuts; 80% of nut and lock

washer; 85% of nut lock washer

and plain washer. They save space

and assembly time. A single Dotloc

replaces two, three, even four

fastening devices depending on the

application and type used. Forget

about lock, flat, seal washers etc. etc.

Regular type DotlocWhen used as load carrying nuts for

light duty assembly, they replace plain

nuts and/or lock washers.

With just the one part to handle

you'll fit them much much faster . .

.

and in a smaller space!

They're interchangeable. Dotlocs

get on well with most other locking

devices! Only in exceptional cases

will they call for a change in design.

They're tough. Real tough.

Temperatures of up to 400°F won't

affect them.

And you can use 'em over again.

Screw Dotlocs on and off as manytimes as you like and they'll be none

the worse for it!

thefirm with the best connections

CARR FASTENER

UNITEO-CARR GROUP

'I know something about Dotlocs.

II like what I know. But I'd like to

know more. Please send me the rest

Iof the gen.

To : Carr Fastener Co Ltd,

I Stapleford, Nottingham

I We make

. Name

' Position

| Company

IAddress

F4/DEH

55

Fig .3 . Regular type used for retaining a volumecontrol switch to radio chassis . (By courtesy ofCarr Fastener Co. Ltd.)

to use conventional wrenches for tightening, the in-side hexagon of the nut can be utilised using aninternal plug wrench. Table 1 shows available sizes.

Acorn type

This type of pressed metal nut (Fig. 5) is intendedfor covering unsightly bolt ends when a neat appear-ance is necessary. The single thread helix incor-porates the same self-locking feature as the othertypes. The Acorn type affords up to 25 per centmore inside screw length clearance than ordinarydome nuts, thus minimising failure to seat and as-semble correctly, a fault so often encountered withsolid tapped dome nuts. These nuts can be used bythemselves for light assemblies or as a locknut ontop of an ordinary nut for high stress assemblies(Fig. 6). Table 2 shows commonly available sizes.

Adjusting type

The Adjusting type single thread fastener (Fig. 7) issimilar to the Acorn type but in addition to the lock-ing action at the base, it has the top formed downand inward to provide an additional six point springgrip at the top (Fig. 8). This will provide a pre-vailing torque grip at any position on the screwthread (Fig. 9). Table 3 shows available sizes.

Fig.4. Regular type nut used on top of anordinary nut as a locknut. (By courtesy ofCarr Fastener Co. Ltd.)

The semi-Acorn shape approaches the smooth ap-pearance of a full Acorn nut, but provides for theextension of screws through the top should varia-tions of screw length be encountered. This typecan be used on its own as an adjusting nut. Thesix point grip at the top acts as a brake on thescrew threads, retaining itself in any pre-deter-mined position, making it suitable for adjustingpurposes (Fig. 10). It requires no seating to holdeffectively when used in this way.

Tension type

The Tension type nut (Fig. 11) is similar in appea-rance to the Regular type except for a small barbon the edge of each flat. This nut is applied in such

Fig. 5. Acorn type .

(By courtesy of CarrFastener Co. Ltd.)

Fig. 6. Acorn type used as a locknut on higherstress assemblies. (By courtesy of CarrFastener Co . Ltd

.

)

a way that the barbs will bite into the surface of theassembly (Fig. 12). They are used in instanceswhere adjustment of the screw is necessary. Oncethe nut is applied and tightened, the screw can beadjusted as many times as is necessary. Varyingdegrees of tension in the screw can be obtained byvarying the torque applied in seating the fastener.They grip satisfactorily on most materials such asmild steel, brass, aluminium or plastics, but arenot recommended for use on hardened steels, castiron or chromium plated surfaces. Table 4 showscommonly available thread sizes.

Wing type

A lightweight Wing nut (Fig. 13) incorporates allthe qualities and principles of the previously men-tioned types. These nuts are self locking when only

56

Table 1 . Commonly available thread sizes of

the Regular type locknut.

Thread Size Thread Size

4 BA i in . x 26

2 BA 1 in . x 26

ft in. WHIT & in . x 26

1 in. WHIT ftin. x UNF

i in. BSFftin. BSF| in . - 32

Fig. 7.. Adjusting

type . (By courtesyof Carr FastenerCo. Ltd.)

Fig. 8. Illustration

showing locking

actions applicable to

the Adjusting type

.

(By courtesy of CarrFastene r Co . Ltd

.

)

finger tightened and are proof against assembliessubject to vibration. Table 5 shows available sizes.

Washer type

The Washer type nut (Fig. 14) combines in a onepiece spring steel fastener the functions of a nut,

a lockwasher and a plain flat washer. The springlocking action and resilience of the large diameterwasher base result in firm assembly but will absorbthe shock of tightening and permit safe assembly of

fragile parts. The washer base enables the nut to

be used in conditions where the thread projectsthrough large diameter clearance holes or slots.

Table 6 shows available sizes.

Earthing type

The Earthing type (Fig. 15) is similar to the Washertype but has three tooth- like elements formed out of

the washer base. These are intended for effecting agood earth on electrical assemblies by penetratingany non- conductive coatings, unclean or corrodedsurfaces. They can be provided with a plastics

sealer to prevent water or dust seepage through to

the assembly. Table 7 shows available sizes.

Captive nuts

These fasteners (Fig. 16) were designed for appli-

cation to sheet metal, as a quick and simple fasten-ing for sheet metal and plastics where a threadedhole is required. A specified hole is prepunchedor predrilled in the sheet and the captive nut is

pushed on. The integral latch on the flat side dropsinto the hole positioning the nut in readiness for

the screw, but at the same time allowing somemovement for centring. They can also be used in

any position in a panel by inserting through a slot

(Figs. 17 and 18). Captive nuts are made fromhardened and tempered spring steel, and are avail-

able in a wide range of sizes to fit standard screwthreads. The engaging portion of the thread is aspecially formed spiral in true relation to the pitchof the thread in which the inner contour is designedto provide maximum strength from the single thread.At the same time, because the material is hardenedand tempered spring steel, it is sufficiently re-silient to yield in a spring like manner when tight-

ened, providing a vibration resistant lock. Thisfeature eliminates the use of other forms of lock-ing such as toothed washers, etc. It is only neces-sary for the screws to be turned to finger tightnessand then given a further half to three quarters of aturn to be fully locked, tightening the screws untilthey can be tightened no more is not necessary.

Other forms of single thread fastening are for appli-cation where access to one side of the panel only isavailable. Two types of single thread fastenersare manufactured for this purpose.

Blind Assembly panel nut

The Blind Assembly panel nuts (Figs. 19 and 20)are intended for snap-in application in a round holeand provide a single form thread. The part is so

Table 2. Commonly available thread sizes of

the Acorn type

.

Thread Size Thread Size

2 BA10-32ftin. WHITiin. WHIT

iSin. WHIT8-32i in. UNF| in. UNF

Fig. 9. Adjustingtype will retain

itself in any pre-determinedposition . (Bycourtesy of CarrFastener Co . Ltd .

)

arranged that when it is snapped into the preparedhole two spring legs open out behind the panel thuspreventing the fastener from being removed. Theselegs also help to prevent the fastener turning whenthe screw is tightened.

57

Fig.10. Adjusting

type used as springadjustment on indust-

rial equipment . (Bycourtesy of CarrFastener Co. Ltd.)

Fig. 1 1 . Tensiontype . (By courtesyof Carr FastenerCo. Ltd.

Fig. 12. Tension type

can be used singly.

They may also be usedon either side of sheet

metal as shown. (Bycourtesy of CarrFastener Co . Ltd .

)

45° turn nut

Another type of fastener for blind assembly is the45° turn nut (Fig. 21). It is intended for applicationinto a square hole from the front face of a panel.When inserted the fastener is turned through 45° to

lock in position (Fig. 22). Projecting tabs locate in

the corners of the hole preventing the fastener fromturning. At the same time the corners of the squarenut are positioned under the centre of the sides of

the hole/ securing the fastener ready to accept the

screw.

Helix washer

A fastener developed for use in the metal furnitureindustry incorporates a single thread helix (Fig. 23).

This fastener is manufactured from high qualityspring steel and enables two tubes to be joined at

right angles with considerable strength and no un-sightly welds. Table 9 shows available sizes.

A bolt is fixed through the diameter of one tube, andthe Helix washers are spun on to the protruding por-

F ig . 1 3 . Wing nut type . (By courtesy of CarrFastener Co . Ltd

.

)

Table 3 . Commonly available thread sizes of

the Adjusting type

.

Thread Size Thread Size

2 BA10-32& in. WHIT* in. WHIT

,-Sin. WHIT8-32J in. UNFi|in. UNF

tion with the concave side facing towards the headof the screw. The assembly is then pushed intothe end of the other tube, and screwed tight, mak-ing a strong right angle joint (Fig. 24). The outsideedge of the Helix washers bite into the sides of thetube in a powerful locking engagement which resistsremoval. Two washers are necessary for align-

ment and average strength. Where a stronger jointis required a third Helix washer can be introduced.

SELF THREADING FASTENERS

Although not for applications involving turned threads,the self threading fastener falls into the categoryof the single thread nut. When applied to plain rodsor studs it makes its own thread by means of theintegrally formed helix. The self threading nutsare made from hardened and tempered carbon steel.They are available as Washer types (Fig. 25) orAcorn types. Self threading fasteners retain approx-

Fig.14. Washer type.

Fastener Co. Ltd.)

(By courtesy of Carr

Fig. 15. Earthing type

Fastening Co . Ltd .

)

(By courtesy of Carr

58

studbolt, studiron, allworm, allthread, nippling, wormrod, screwed stick,

threaded rod, stemming, threaded stem .... Whatever you call it we cansupply it in brass, copper, mild, HT and stainless steel, aluminium, nylon and pvc. Threads?BA, Whit, BSF, UNF, UNC, BSP, CEI, ISO and left handed! Length? You name it!

There are even more applications than names for it. Suspending, clamping,

jigging, tensioning, jacking, and prototypes are just a few. But all you needto remember is Telcomatic Studding. There are stockists throughout the

country. Ask us for price list and further details. We also manufacture a vastrange of other fasteners and turned parts -ask us about things like tie

rods, allthreads, nuts and specials.

Telco Telco Limited, Alma Road, Enfield, Middx. Tel: 01-804 1282. Telex 21783Birmingham: Aston Brook Street, Birmingham 6.

Tel: 021-359 4828 Telex 33572

59

Table 4. Available sizes of the Tension type.

Thread Size Thread Size

6 ANC4 BA2 BA

A in. WHITi in . WHIT

Table 5. Available sizes of the Wing type.

Thread Size Thread Size

2 BAA in. WHITiin. WHITA in. WHIT

A in. 16 threads/in.1 - 32 in

.

1 in . 16 threads/ in .

A in. 26 threads/in.

Fig.16. Captive nut.

Fastener Co . Ltd .

)

(By courtesy of Carr

Fig.17. Illustration

showing method of

application of Captivenut and also the lock-ing arrangements

.

(By courtesy of CarrFastener Co. Ltd.)

Fig. 18. Captive nut used with sheet metal screwcan be inserted through a slot in any position onpanel . (By courtesy of Carr Fastener Co. Ltd.)

Table 6. Commonly available sizes of theWasher type.

Thread Size Base Dia.

in . mm

.

10.32 0.500 12.7010.32 0.625 15.8810.32 0.750 19.0510.24 0.500 12.708.32 0.470 11 .94

&.Fig. 19. BlindAssembly panel nut.(By courtesy of CarrFastener Co. Ltd.)

Fig.20. Illustration Showing the method of appli-cation of Blind Assembly panel nuts. (By courtesy)of Carr Fastener Co . Ltd . )

Table 7 . Available sizes of the Earthing type

,

Thread Size Base Dia.

in

.

mm.10.32 0.750 19.0510.32 0.500 12.7010.24 0.500 12.70

imately the same strength characteristics as con-ventional threaded members. Table 10 shows com-monly available sizes.

No special tools are required, standard socket,ring or open ended spanners are suitable alongwith power operated tools.

The self threading fastener can be removed andreused in the same way as a normal nut by un-threading. It can be applied to studs which areup to 20° off vertical and still seat on to the faceof the assembly making a perfectly safe and securefix (Fig. 26). In the same vein, a fastener of thistype does not have to be applied squarely to thestud. Since the device cannot cross thread, theassembler can be as much as 10 to 15° out of lineand complete a fully satisfactory assembly.

This type of fastener can be supplied with a bondedplastisol seal (Fig. 27).

Characteristics of self threading fasteners

The fasteners usually have a double lead with atotal gripping area on the stud of slightly less than

60

Fig.21. 45° turn nut.

Fastener Co. Ltd.)(By courtesy of Carr

Fig.22. Illustration showing method of appli-

cation of securing 45° turn nut. (By courtesyof Carr Fastener Co. Ltd.)

allowance will have to be made in the stud dimen-sions so that the finished stud diameter does not

exceed the tolerances. At the point where the fast-

ener will engage the stud, the following tolerances

should be specified: metal studs + 0. 002 in. (in-

cluding plating) - 0. 003 in. ; plastics studs + 0. 005

in. to -0.000 in. Table 11 shows typical assemblytorque and corresponding stud tensions developed

under average assembly conditions.

Zip Twist fastener

A variation on the self threading principle is the

'Zip Twist' fastener (Fig. 28). This fastener is in-

tended for use on 3 mm. dia. plain studs of the morefragile materials.

The 'Zip Twist' is positioned on the stud and then

pushed down until firmly seated. To effect a posi-

tive retention the fastener is then given aito 1 of

a turn to lock. The torque exerted by the fastener

on to the stud is very low, enabling it to be used onsuch materials as brittle and flexible plastics and

die cast metals.

one full thread. The pitch is coarse, usually about

five to seven threads per inch. The thread cutting

teeth are formed by generating a helical form in

the stamped nut.

Studs

The self threading fastener will yield optimum per-formance only when the stud material is softer

than that of the fastener itself.

The fastener can be removed by unthreading like astandard nut. No special tools are required as a

standard hexagon is incorporated.

It must be noted, however, that these fasteners do

not develop sufficient clamping force to pull upwarped or poor fitting sheet metal, or to compressany but the most flexible gaskets.

MATERIALS AND FINISHES

The stud must be fixed so that it cannot rotate as

the nut is applied. It must also (including its joint

if welded) be strong enough to withstand the highultimate torque and resulting tension exerted whenthe fastener is seated against the assembly.

To help in starting a self threading fastener the

stud is preferably chamfered at the tip.

The fasteners can be applied to die cast studs whichhave been nickel-chromium plated. In this instance

Table 8. Commonly available sizes of Captive nuts.

Choice of material for producing the above range

is limited to a high content carbon steel which when

Thread Size To Suit Panel Thicknessin. mm

.

iin. ACME 0.036 - 0.064 0.95-1 .65

10 PK 0.036 - 0.064 0.95-1 .65

&in. WHIT 0.036 - 0.064 0.95-1 .65

10-32 0.036 - 0.064 0.95 - 1 .65

2 BA 0.036 - 0.064 0.95-1 .65

8 PK 0.028 - 0.064 0.71 - 1 .65

8 PK 0.036 - 0.064 0.95-1 .65

2 BA 0.036 - 0.064 0.95-1 .65

10 -32 0.036 - 0.064 0.95-1 .65

i in. ACME 0.064 - 0.090 1 .65 -2.29i in. x 20 UNC . 036 - . 064 0.95 - 1 .65

8 PK 0.028 - 0.064 0.71 - 1 .65

8 PK 0.048 - 0.064 1 .22 - 1 .65

8 PK 0.060 - 0.100 1 . 52 - 2 . 54B 3.9 mm 0.028 -0.064 0.71 - 1 .65

Fig.23. Helix washer. (By courtesy of CarrFastener Co . Ltd .

)

Fig.24. Helix washers used to make right anglejoints in tubes . (By courtesy of Carr FastenerCo. Ltd.)

61

Table 9 . Availabl e sizes of the Helix washer.

For.Tube Size Thread SizeO/D gauge

f in . WHIT

in

.

mm.

16-180.750 19.05

i in. WHITJ in. BSF

& in . WHIT0.875 22.22 16-18

i in. WHITJ in. BSF£ in . WHIT

& in. WHIT1 .000 25.4 16-18 i in. WHIT

J in. BSF

Fig. 25. 'Dotloc' type

self threading fast-

ener. (By courtesy of

Carr Fastener Co. Ltd.)

heat treated would give the required hardness, andyet still have the 'built in' resilience necessary to

achieve the double locking actions. A 0. 45 to 0. 60per cent carbon steel (to BS1449 En43F) is used forall the parts covered in this Chapter. The heattreatment consists of passing through an electric-ally heated Austempering furnace and quenchingin a salt bath. This treatment produces a VPN of

500 to 580.

The standard finishes available for the aforemen-tioned products are as follows:

Black oil

A low cost finish for conditions where maximumcorrosion resistance is not important. The finish

is glossy black coated with a corrosion resistantoil.

Walterised finish

A blacK phosphate coating on the parts which arethen immersed in a corrosion resistant oil for fur-

ther protection. This finish is suitable for use in

Fig. 26. Self threading fasteners can be applied10-15 off square to the stud and still seatcorrectly, and also can be applied to studs whichare up to 20° off vertical . (By courtesy of CarrFastener Co. Ltd.)

ffi

Fig.27. Self threading fasteners with inte-grally bonded plastisol seal . (By courtesy ofCarr Fastener Co . Ltd . )

Table 10. Available sizes of self threading fasteners

Stud Size Type Washer Base Diesin. mm.

Washer

in

.

mm.Jin. 3.18 0.437 11.11ftin- 4.76 Washer 0.500 12.70*in- 4.76 Acorn - -

Jin. 6.37 Washer 0.539 15.08Jin. 3.18 Washer 0.531 13.49

conditions where the parts are not exposed directlyto the weather.

Phosphate black

Walterised for good bonding in initial corrosionresistance properties the parts are finished withphosphor etch stoving enamel. An even black mattsatin finish results.

Zinc chromate

This highly corrosion resistant finish consists of

a walterised coating followed by two coats of zincchromate stoving enamel. Appearance is olivegreen semi-matt.

Nickel plate

Bright nickel plate is a highly corrosion resistantfinish and does not tarnish with atmosphere sulphurcompounds.

Table 1 1 . Typical assembly torque and stud tensions

Stud size Torque (lb ./in.) Stud Tension (lb .

)

in

.

mm. Zinc(die cast)

Steel Zinc(die cast)

Steel

Jin.

Sin.

3.184.76

9

402265

40110

150280

Fig.28. Zip Twistfastener. (Bycourtesy of CarrFastener Co. Ltd.)

Nuts - plain and weld

by R.W. Lowe (GKN Bolts & Nuts Ltd.)

This Chapter is divided into two main sections,

plain nuts and weld nuts. There is also a short

note on torque-tension relationships.

Industry uses thousands of millions of nuts every

year. Apart from some precision and miniature

applications, they range in size from 8 BA (0.086

in. ) through BA (0. 236 in. ), i in. , 4 in. , I in.

,

A in. , i in. , etc. through to 6 in. Thereafter, non

'standard 1 nuts are almost exclusively specials anddiameters of two feet and more have been known.

By far the most commonly used sizes are t in. ,

3 in. and f in. and, particularly in the motor in-

dustry, these three sizes fulfil the majority of

applications.

THREAD FORMS

Nuts are supplied in a variety of thread forms, but

the most common are BA, BSW, BSF, UNC and

UNF. Again, the motor industry tends to prefer

the Unified range whilst general engineering mainly

uses BSW and BSF. With the advent of metrication,

however, BSW, BSF and BA have now been declared

obsolete. The 'recognised' thread forms are now ISOMetric and ISO Unified inch (both having a coarseand a fine pitch series). Although Unified is widely

used in the motor industry, it is now thought that

it will eventually give way to Metric. British motorcompanies have already began using ISO Metric

fasteners and even the Americans, who until re-

cently seemed to be against the use of Metric, havenow begun to seriously investigate the possibility

of changing over. Thus, although current usageof nuts covers most thread forms, by 1971/72 it

is anticipated that 25 per cent of them will be ISO

Metric.

Although ISO Metric has two series of pitches, only

the coarse pitch is currently available from stock.

It should be suitable for the majority of applications.

ISO Metric coarse is finer than Whitworth, but an

increased angle of thread and a larger root radius

compensate for this. With regard to the replace-

ment of BSF, similar considerations counterbalancethe difference in pitch between Metric coarse and

BSF making the substitution by Metric coarse a

practical and safe proposition. Nevertheless, de-

signers may require ISO Metric fine nuts and canthen take advantage of the fine pitch series. How-ever, before specifying 'fine' nuts, he should care-fully examine his reasons for doing so. Many haveadmitted that they have decided to use fine on tradi-

tional grounds alone, others because of the increas-

ed stress area which a fine pitch series gives. Whatthey frequently forget is that although the stress

area is higher with a fine thread this is only rele-

vant when considering the bolt strength. Further-

more, unless the tolerance class, i. e. the class

of fit, of a nut on a bolt, is carefully controlled, a

fine thread is far more likely to fail through strip-

ping than is a coarse thread.

It is desirable that the length of the internal threadand its dimensions be such that, taking into account

differences in the strength of material of the inter-

nal and external threads, the threaded portion of

the external thread will break before either the

external or internal threads strip. The reasonquite simply is that bolt fracture is readily noticed;

stripping of the nut threads is not.

With this in mind therefore, standard nuts are de-

signed to give sufficient length of engagement to

cause the bolt to fracture rather than the nut to

strip. Advantage can, however, be taken of the

increased area in the nut over which the load is

taken compared to the bolt, and, in most cases,nuts are of a lower tensile strength material than

the bolts on which they are used.

TOLERANCING

As far as the class of fit is concerned, it is worthnoting at this point that the ISO recommended toler-

ancing system is specified by numbers and letters.

For example: 5H/4h; 6H/6g; 7H/8g. These corre-

spond to 3A/3B, 2A/2B, 1A/1B for the Unified

series.

The tolerance class is a combination of the toler-

ance grade and the tolerance position, signified bya number and a letter respectively. Nuts (i. e. in-

ternal threads) are referred to by capital letters.

The small letters refer to the externally threadedmembers.

MANUFACTURE

It is often thought that most nuts are turned fromhexagon or square bar. This is not now the case

for sizes up to I in. diameter, which can be cold

forged. There are several nut forging and press-ing processes, but the most common is one in whicha nut forming (transfer) machine cuts off a slug

of 'round' wire and forms it into a nut blank. Tap-ping is all that is then required. A typical pro-gression is shown in Fig. 1.

63

Fig.1nut.

Stages in the manufacture of a standard

Fig. 2. Washer Faced nut.

Larger nuts can also be forged, but this is doneon automatic hot forging machinery. When the sizeexceeds about 1 i in. diameter, nuts are hot forgedon an indenting machine, which cuts hexagon blanksfrom rectangular section bar. Very large nuts,3 in. diameter and over, are forged 'by hand' onhammer forging machines.

STANDARDMARKINGS

DIMENSIONS AND

Below are given details of nuts to various BritishStandards, their shape and size, grades marking,etc. As ISO Metric is the thread form of the future,

greater detail has been gone into for nuts to MetricStandards.

BS1768 grades of nuts

BS1768.(1963) gives guidance on the correct gradeof nut to be used with each grade of bolt.

Grade Nuts suitable for use with bolts, grades'A', >B' and 'P'. These nuts should be capa-ble of withstanding a proof load based onminimum tensile strength of Grade 'P'

bolts.

Grade 1 Nuts suitable for use with bolts grade 'S'.

These nuts should be capable of withstand-ing a proof load based on minimum tensilestrength of the Grade 'S' bolt.

Grade 3 Nuts suitable for use with bolts grade 'T',

These nuts should be capable of withstand-ing a proof load, based on minimum ten-sile strength of Grade 'T' bolts.

Grade 5 Muts suitable for use with bolts grades 'Vand 'X'. These nuts should be capable ofwithstanding a proof load based on the

minimum tensile strength of Grade 'X 1

bolts.

The nuts must satisfactorily resist the proof loadwithout the threads stripping and must be remov-able by the fingers after the test.

Brinell hardness. BS1768(1963) gives guidance onthe Brinell Hardness requirements for grades ofnuts. For nuts over 1 in. diameter, the BrinellHardness should not be less than the minimum hard-ness specified. For nuts up to 1 in. diameter theBrinell Hardness numbers are recorded for guid-ance only.

Brinell Hardness NumbersUp to 1 in. Over 1 in.

Min Max Min

Grade 1 163 240 180Grade 3 183 300 230Grade 5 270 335 270

Marking of nuts. Grades 1, 3 and 5 will be markedwith the Grade number on the non-bearing face ofthe nut. In addition either:

i. A circular groove of semi-circular section in-dented in the non-bearing face (for cold formeddouble chamfered nuts and locknuts);

orii. A recess in the non-bearing face of the nut (forcold formed washer faced nuts only);

oriii. A line of contiguous circles indented on one ormore of the flats of the hexagon and parallel to theaxis of the nut (for nuts made from the bar).

BS1 083 grades of nuts

BS1083 gives guidance on the correct grade ofnutto be used with each grade of bolt.

Ultimate BrinellTensile Hard-Strength ness

Gradeton/sq. in.

A For use with

Grade 'R' BoltsP For use with

28 min 121/235

Grade 'T' Bolts

R For use with35 min 152/240

Grade 'V Bolts

T For use with45 min 201/271

Grade 'X' Bolts 55 min 248/335

For nuts which are manufactured from the bar,the Brinell Hardness numbers are given for guid-ance only and are not part of the requirements ofthe Standard. When nuts are manufactured by coldforming from round wire, with or without subse-quent heat-treatment, the Brinell Hardness num-bers apply as part of the requirements laid downby this Standard.

Marking. Grades of nuts P, R and T should have thegrade letter marked on one of the hexagon flats.

64

BS916

Nuts to this standard must possess a minimumtensile strength of 26 ton/sq. in.

ISO Metric nuts (BS3692, 4190, etc)

The designation system for steel nuts should be a

number which is-feth of the specified proof load

stress in kg. /sq. mm. The proof load stress is the

minimum ultimate tensile strength of the highest

grade of bolt with which the nut is to be used.

Designation of nut 4 5 6 8 12 14

Proof load stress (kg/sq. mm) 40 50 60 80 120 140

Thus the correct nut to use with a Grade 8. 8 bolt

is a Grade 8 nut.

Nut marking

Nut marking is in the form of a code symbol basedon a clock face with a single dot indicating twelve

o'clock. The second mark, a bar, indicates the

grade, i. e. in the case of Grade 8 nut the bar is at

the eight o'clock position on the top of the nut. Themarks on nuts are indented.

rniiFig. 3. Grade 8 nut.

STRENGTHGRAOe 4 5 B e* 12* 14*

SYMBOL i S e 8 12 14

'CLOCKFACETMARKINGSYSTEM

-j-kfJml fjis

(k-^!

l|pj ^ P^marking of strength grade is manoatory

Fig. 4. Strength grade designation marking of

nuts.

Fig. 5. Examples of marking of forged nuts.

Fig .6. Example of marking of bar turned nut.

Table 1 .

MECHANICAL PROPERTIES OF STEEL NUTS

Strength grade designation 4 5 6 8 12 14

Proof load

stress

kg./sq .mm 40 50 60 80 120 140 All nuts other than

those exempted byagreement betweenthe purchaser andthe manufacturer.

N/sq.mm 392 490 588 785 1177 1373

Brinell hardness HB max. 302 302 302 302 353 375 All nuts

Rockwell hardnessHRC max

.

30 30 30 30 36 39 All nuts

Vickers hardness HVmax. 310 310 310 310 370 395 All nuts

Table 2

.

RECOMMENDED BOLT AND NUT COMBINATIONS

Grade of bolt 4.6 4.8 5.6 5.8 6.6 6.8 8.8 10.9 12.9 14.9

Recommended grade of nut 4 4 5 5 6 6 8 12 12 14

NOTE. Nuts of a higher strength grade may be substituted for nuts of a lower strength grade.

Preferred diameters

These are as follows:

Ml. 6 M4 M10 M24 M48M2 M5 M12 M30 M56M2.5 M6 M16 M36 M64M3 M8 M20 M42

TORQUE-TENSION RELATIONSHIPS

Apart from certain types of load indicating deviceson high strength friction grip bolts, the most com-monly used method of controlling the tightening of

nuts is by torquing thorn up to a pre-determinedvalue and by using simple formulae, relating this

to axial load.

Table 3. BS3139 (High strength bolting).

Nominal T.P.I. Stress Area Proof Load*Size of Nut

UNC sq . in

.

tons lb. kg.

i in. 13 . 1 41

9

9.12 20,450 9,276| in. 11 0.226 14.53 32,550 14,764i in. 10 0.334 21 .47 48,100 21 ,818i in. 9 0.462 29.71 66,550 30 , 1 861 in. 8 0.606 38.95 87,250 39,576

1 A in. 7 0.763 49.62 109,900 49,8501 i in. 7 0.969 62.30 1 39 , 550 63,2981 | in. 6 1 .405 90.31 202 ,300 91 .762*Basedon 64.73 ton/sq.in. (144,000 Ib./sq.in.) (10.124 kg/sq.cm.)on tfr e equivalent stress area ofthe corresponding bolt.

Table 4. BS1750 (Bolting for the petroleum industry).

*Based on Minimum Tensile of Grade X bolts 75 ton/sq

.

in

.

Nominal T.P.I. Stress Area Proof Load*Size of Nut

UNC sq. in. tons lb. kg.

i n

.

13 . 1 41 9 10.64 23,839 10,813i n. 11 0.226 16.95 37,968 17,222i n

.

10 0.334 25.05 56,112 25,452i n. 9 0.462 34.65 77,616 35,2071 n. 8 0.606 45.45 101 ,808 46 , 1 80

1 h n. .7 0.763 57.23 128,184 58,1441 i in. 7 0.969 72.68 162,792 73,842

Table 5. Nut materials. Grades of carbon and alloy steel for nuts to BS1750 and ASTMA.A 193.

Grade of Nut and Marking Symbol

Service Conditions

Material Specifications

2 or 2H (see Fig. 8.)

High Temperature

BS1506 - 162

High Temperature

BS1506 - 240

L4

Low Temperature

BS1506 - 240BS1510 - LT.100

There are two methods of calculating torque. First-

ly, it must be recognised that this torque has to:

1. Overcome friction between the underside of the

nut, and the washer.2. Overcome friction in the threads.

3. Induce tension in the bolt.

TORQUE FIGURES

Recommended torque figures for ISO Unified, BSW,BSF and ISO Metric threads are shown in Table 6.

The torque figures quoted in this Table are aver-

age figures and apply to fasteners in the 'self col-

our' condition only. They do not take into account

special lubricants, plating or the effect of hard, and

smooth mating surfaces (e.g. hardened washers).

All of these factors may reduce frictional conditions

and have a significant effect on the torque figures.

WELD NUTS

There are many applications where a nut has to befixed to the parent metal. This can be done quite

simply either mechanically or by welding and it

has been found that by designing a special type of

nut, namely a weld nut, this can be effected quick-

ly, easily and cheaply. Resistance welding (by the

application of heat and pressure) has proved to bethe best method of fixing nuts in this way. It is

clean, no filler or flux is required, it is fast, con-

trolled, needs little skill and it is repetitive. Un-fortunately heavy, expensive machinery is neededwhich must deliver high instantaneous power andit must be frequently maintained. The electrodesmust be clean and flat and the component itself

must also be clean.

Nuts can be welded on by arc welding but this is

only employed for very large sizes. If an attemptwas made to weld an ordinary nut to sheet, the

heat would be dissipated throughout the nut andthis would cause it to deform and collapse. Someform of heat concentration is therefore needed andso by designing weld nuts to have a limited contact

area (i. e. small projections) the heat can be con-centrated in small specific areas. Having thus

established a basic design, a means of location

must then be provided and there are two basic typesof weld nut:

1. Locating collar weld nuts

2. Collarless weld nuts

Locating collar nuts

This is the most accurate location method, butbecause of the physical depth of the collar there is

a limitation to sheet thickness to which it can beattached. Furthermore, a collar has to be made(thus making the nut more expensive to produce)and because the nut needs to be located by hand,it is slower in use. The hole in the plate mustalso be punched to a closer tolerance. However,a collared nut does have welding advantages. Weld

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67

spatter is reduced because the collar protects thethreads of the nut from molten metal during thewelding process. Secondly, the collar preventsdirect shearing of the welding projections by itself

bearing on the parent metal.

Collarless nuts

The collarless nut requires a retractable spigot

in the electrode usually made of a non conductingheat resistant material. It is cheaper to makebut its location is not as accurate as the collarednut. However, it is much easier to use on auto-matic machinery and it can be attached to very thin

plate.

Its disadvantages are that there is little protectionfor the threads, which unless the welding para-meters are carefully controlled, are frequentlysplashed with molten metal. It also cannot with-

stand such high torque loadings because it tendsto shear on one projection rather than on three orfour.

The following are the basic types of weld nuts:

1. Standard locating collar - suitable for use on0. 036 to 0. 048 in. (20 swg - 18 swg) material.

2. Deep locating collar - suitable for use onmaterial 0. 064 to 0. 160 in. (16 swg - 8 swg)material.

3. Standard collarless nut - suitable for use onmaterial of less than 0. 036 in. (20 swg) mat-erial.

4. The McLaughlin Patent square weld nut -

(collarless) - suitable for most sheet thick-nesses and has increased welding projections.

5. Cone weld nuts - suitable for sheet thickerthan 0. 160 in. (8 swg) material. This nut

Fig.10. Deep locating collar weld nut.

gives an annular ring weld and is extremelystrong but 'large capacity' machines are nec-essary for its use.

The above are the most common types of weld nut,but other special varieties such as the round andthe grommet type are available for special applica-tions. The grommet type of weld nut has muchheavier projections and can be used on thicker mat-erial than the deep locating collar variety can copewith. Round weld nuts are usually required forsome automatic feeding systems.

It is difficult, if not impossible, to specify the ex-act settings of welding machinery for these nutsbecause there are so many inter-related variables.It is preferable, therefore, to carry out a few shorttests with the welding machinery.

Fig. 11 . Standard collarless weld nut.

The normal sequence of events of welding is:

1. Squeeze (air pressure on).

2. Weld (squeeze still on) current on (for a pre-set number of cycles).

3. Hold (current off).

Pressure, current (heat) and time (also a type ofheat control) can all be varied but , unfortunately,are not independent of each other. Furthermore,the nut condition and the plate condition can havea considerable effect on the quality of the weld ascan the size of the plate to which the nut is beingwelded. This latter problem arises because thelarger the amount of magnetic material in the sec-ondary circuit, the lower is the current developedat the electrodes.

Of the above variables, time is the least influen-tial and current the most. Current is affected bypressure (because of the alteration of electricalresistance). If the resistance is increased the heatincreases and therefore lowering of the pressurecan increase the heat.

Pressure

This must be large enough to break any oxide filmand to follow-up the collapse of the projections.Failure to achieve either will result in weld spat-ter. The pressure must be small enough not todamage the small projections (and cause prema-ture collapse).

68

Time

This must be long enough to allow the projectionsto collapse and short enough to limit overheating.This time normally lies between 5 and 10 cycles.

Current

This is the most important variable of all and mustbe high enough to ensure good fusion but not so

high as to give excessive heat or spatter.

What to look for in a good weld

(a) Top (or nut) side:

1. The heat pattern around the projections should

extend 50 - 75% of the way to the next cornerof the nut.

2. The top face and top threads of the nut should

be free from overheating and distortion.

3. The nut should be 'down' on the plate (i. e.the

projections should be fully collapsed).

(b) Bottom side:

1. There should be little or no weld splash.

2. A small nugget (or blister) should be visible.

3. The weld should not extend past the projection

area.

Unfortunately, the afore mentioned variables fre-

quently give rise to bad welds and nuts are seen to

fall off sheet under quite small torque loadings.

The following points should give guidance as to the

cause of a bad weld:

1. The main cause is obviously incorrect weld

settings as described above.

2. The electrodes may not be flat and are often

damaged .

3. The machine may be in poor condition.

Fig. 13. Cone weld nut.

4. The sheet on to which the nut is being welded

may be rusty or dirty (oil does not usually

matter).

5. There may be some detergent on the nuts (re-

sulting from incorrect washing procedures).

6. If a collar nut is used the collar could possi-

bly have projected below the sheet and thus

short circuited the current through the bodyof the nut and not through the projections.

7. The collar may be too tight in the hole thus

shunting the heat away from the projections.

8. The projections may not be formed properlyor are possibly damaged.

9. The wrong nut has been chosen for the appli-

cation.

To check the quality of the weld there are two re-

cognised methods:

1. Torque shear loading.

2. Push out loading.

The torque method is normally preferred and, again,

this varies from application to application. Sometypical torque shear loads are shown in Table 7.

Table 7

.

TORQUE SHEAR LOADS - LB ./FT.

DIAMETER 16 swg 1 8 swg 20 swgOF NUT Sheet Sheet Sheet

Thickness Thickness Thickness

10 N.F. — — 9

& In. 16 16 16

i in. 27 27 27

A in. 36 36 36

The above torque values are well in e>ccess of the

torque loads which will be transmitted to the nut

during tightenings

.

69

10

Plastics fasteners

by A. Griffiths (Consultant Editor)

For the purpose of this Chapter, a plastics fasten-er is considered to be a fixing device made from athermoplastic material as used in an engineeringassembly. Other fixings manufactured from plas-tics are also to be found in miscellaneous indus-tries, such as the garment and horticultural in-dustries. Whilst these applications are of con-siderable consequence, it is proposed to concen-trate on those devices available to the engineer.

Plastics fasteners, as they are recognised today,were not available until the early 1950's. At theend of the Second War the spring steel clip was theforerunner of modern fastening techniques and asmore suitable plastics became available some metaldevices were replaced by thermoplastic fixes.

Designers in the USA were first to use the modernplastics fastener and their acceptance of this newform of fixing can be related to the following facts:

1.

3.

Plastics technology was advancing rapidly andnew engineering plastics were being widelyaccepted by forward thinking designers.Labour costs were rising rapidly which in-creased the costs of 'secondary operationsassociated with regular fixing devices. Plas-tics do not normally require any after opera-tions.

Engineers were becoming more concernedwith installed costs rather than the price ofthe actual fastener unit. Thus plastics fixings,which are normally more expensive than met-al counterparts, were accepted for their full

value.

ADVANTAGES

The main advantages of plastics fastenings arelisted as follows:

Non-corrosive. This particular property of plas-tics is probably one of the most important. Gener-ally a polymer can be chosen that will withstandpractically any environmental attack - many poly-mers are therefore unaffected by common solventsand acids. Above all, plastics do not rust or cor-rode when weathered.

Non-conductive. In normal terms plastics do notconduct electricity and are, therefore, capable ofacting as electrical insulators as well as fasteners.This is shown in Fig. 1 where a radio chassis isisolated by way of plastics 'nuts'.

B.S.SPECIFICATION

415

IMPROVEDVERSION

Fig .1 . A nylon

nut that can besnapped into aradio chassis.The self tappingscrew is isolated

from the chassiswhen assembled.

Light weight. The majority of plastics are lightin weight, with specific gravities of about 1.2,compared with mild steel at 7. 87. Whilst the in-dividual saving in weight of one fastener may besmall, the overall reduction, when the total perautomobile, appliance or the suchlike is consider-ed, can be great. This is particularly so in theaircraft industry where a multitude of fastenersare used in connection with cable and pipe fittings,and for attaching insulative and decorative panels.Fig. 2 shows a lightweight, non- conductive, self-fixing cable clamping arrangement.

Fig. 2. A speciallightweight cablestrap that can befixed in a varietyof ways

.

70

Fig. 3. Refrigerator

shelf supports that

have integral rivets

incorporated in the

design . The rivet pins

are shown in the

undriven position.

Fig. 4. A refrigerator

door bearing bush that

is self retained by wayof a special leg detail..

Self-colouring. Unlike most traditional fasteners,

plastics fixings can be moulded in pigmented com-pounds, so as to be self-coloured. In consequence,

if scratched, the marks and witnesses are unlikely

to show. Also the component will not rust or cor-

rode if maltreated. The many plastics trim pad

fixings on modern automobiles illustrate good use

of the above properties.

Multi-functional. One of the most profitable advan-

tages of a plastics fastener is its ability to have a

multiplicity of uses or functions. Probably 60 percent of today's plastics fixes perform more than

one function when assembled. This is typical of the

refrigeration industry, where shelf- supports are

self- fixing into the liners, as in Fig. 3.

Self lubricating. Several 'engineering' thermoplas-

tics are self lubricating, for instance nylon andacetal. Therefore, a fastener may be used as a

glide button. Conversely, a bush or bearing maybe self-fixing and thus show a further saving. Asimple nylon hinge bush is shown in Fig. 4 whereit is used on the assembly of an appliance door.

Thermally insulating. Plastics fasteners are usedin low temperature applications where thermal in-

sulation is an important factor. For this reason,

plastics evaporator supports are also widely usedin refrigerators. This is illustrated in Fig. 5.

Non-rattling. As opposed to metallic fasteners,

plastics fixings do not tend to rattle in the assem-bled condition. In fact, they can be used to deaden

vibrations in mating components. These applica-

tions are particularly apparent in the automobile

industry.

Self locking. When used in conjunction with screws,

many polymers are self locking and prevent en-

gaged threads from rotating. This advantage is

particularly apparent where nylon inserts are used

Fig. 5. Two common designs for evaporator

supports . The arrangement shown is typical of

many assemblies incorporating plastics fixings.

as locking elements in nuts and bolts. Many pro-

prietary lock nuts and bolts feature small plastics

locking pieces.

DISADVANTAGES

As well as the above advantages, it is necessary

to consider the disadvantages of particular fasten-

ing methods. In the case of plastics these are gen-

erally as follows:

Plastics fasteners are expensive. The average

plastics fix costs about |d. each compared with

the equivalent cost of a metal fix of about |d. each.

However, the full advantages and disadvantages

must be considered before making the final choice.

It should also be remembered that an average fas-

tening usually costs less to purchase than to handle

and fix into position. Therefore, the installed cost

must be taken as the ultimate yardstick. Plastics

are also usually expensive to tool. However, if

the application is sound and warrants the develop-

ment of a special fixing, then it is wise to ignore

the tooling cost, to amortise it into the unit cost

and consider the economies of the combined price.

Fig. 6. An indication of the size of a simple single

impression mould. The part of the tool containing

the core is not shown.

71

Plastics fixings are difficult to prototype. Unlikemetal fixings, which can easily be hand made, plas-tics pieces are best produced from single cavityprototype tools. A mould of this nature, the sizeof which is shown in Fig. 6, will cost in the re-gion of £100. However, this enables the user toobtain many samples; very often in different plas-tics at a minimal cost. These sample tools canalso frequently be of added value when contempla-ting production tooling since some manufacturingdifficulties can be overcome in advance.

Plastics fasteners are not strong. This is true inas much that plastics are inherently weaker thanmetals and some other traditional materials. How-ever, many fixings made from metals are grosslyover engineered. For instance, one may ask whyan automobile number plate should be held on with5 in. mild steel bolts when plastics fixes mouldedin a suitable polymer would certainly be as stronga,s the application requires.

Heat affects plastics fixings. Most complaints ofplastics failing at elevated temperatures are causedby engineers and designers failing to adequatelytest their pre-production prototypes. There aremany instances of plastics fastenings being suc-cessfully and economically used on electric ovensand automobile under-bonnet applications. These ap-plications have been beneficial because careful con-sideration has been given to material choice, com-ponent design and thorough environmental testing

Plastics fixings are weakened by exposure to sun-light. It is true that ultra violet rays will often havea detrimental effect on certain plastics. However,stabilised grades of most plastics are available inmany instances. Generally these materials willneed to be black in colour, if the ultimate in per-formance is required.

APPLICATIONS

The merits of a fastener system are generally bestdescribed by illustrating actual proven examples

of successful applications. The following figurestherefore indicate some typical examples of plas-tics fixings in use. Fig. 7 shows two ferite rodclips which are used on radio assemblies. This isa double ended fastener also acting as an insulatorand spacer. It is moulded from nylon 66 and wouldcost about lid. each. Fig. 8 illustrates a self-fixing pivot. The pivot has a special rivet detailwhereby the fastener legs are pushed into a holeand the protruding pin is driven through the part soas to expand the legs behind the panel, thus givinga secure blind fix. A fastener of this nature wouldcost about Id. each moulded in nylon. Fig. 9 showsa simple plastics grommet that is used to securea front entry indicator lamp. It is moulded in nyl-on 66 and the head also acts as a decorative bezel.Some of the bezels are vacuum metallised for fur-ther effect.

POLYMERS USED FOR FASTENERS

The following thermoplastic materials are current-ly used in the fastener industry and they have beenlisted in an approximate order of importance.

Nylon 66. Probably 50 per cent of all plastics fas-teners are made from this material. Its main vir-tues are:

1. A 'springy' material.2. A relatively hard surface.3. Good chemical resistance.4. A 'tough' plastics.

5. Fair resistance to creep.6. High temperature performance relative to

other thermoplastics.

Nylon 66 is a fairly expensive raw material and it

costs the moulder about 3jd. per cubic inch.

Nylon 6. This grade of" nylon has many of the pro-perties of Nylon 66 but is less springy and has asofter surface.

Nylon 11 . A more specialised grade of nylon whichtheoretically has a lower temperature performancethan other nylons. It is relatively soft material and

72

easy to process. Most grades of nylon are hygro-scopic to quite a degree, nylon 11 is considerably

better in this respect. It is more expensive than

other nylons.

Acetals. This group of plastics are divided into

homopolymers and copolymers. Both types areused by leading manufacturers and there is little

to choose between the two materials. In many waysacetals have similar properties to nylon 66 but canusually be identified in their natural form by their

slightly whiter appearance. The choice of plastics

is best left to the experienced fastener producer,especially in the case of acetals and nylon. How-ever, it can be concluded that the acetals are gen-erally more springy than nylons and in consequenceprove to be a very useful fastener medium.

Polypropylene. The use of this material is gener-

ally confined to larger components which have someintegral fixing device. Examples of this material's

use would be found in cable clips, cable straps,

housings and covers. Polypropylene is strong and

relatively inexpensive and can therefore be usedfor larger fastening devices where the raw mater-ial is a greater proportion of the manufacturing

cost. Polypropylene may be 'waisted' so as to forma section that will hinge. Consequently it has beensuccessfully used to make hinges that incorporate

integral rivets and fixes.

Polystyrene. This material is mainly used in its

higher impact grades in the refrigeration industry.

The majority of self assembling shelf and evapor-ator fixings have been made in this material. It is

relatively cheap at id. per cubic inch, but it is not

as strong as the previously mentioned materials.Polystyrenes are particularly weak in light sec-tions and all high impact grades have a matt sur-face.

ABS (acrylonitrile/butadiene/.styrene). This mat-erial is similar to polystyrene but has a harder,glossier surface and is much stronger. It also hasa better high temperature performance. ABS costsabout twice as much as polystyrene.

Polythene (polyethylene). There are two maingrades of polythene - high density, which is hard,and low density, which is soft. Polythene is alsoknown by some users as polyethylene. Most fas-

tener applications call for the high density mater-ials. It is generally used for simple and non-critic-al applications. Its main characteristics are:

1. Inexpensive - about Jd. per cubic inch.

2. Light in weight (S. G. of 0. 95)3. Non-springy

The most common applications for polythene arehole plugs, stud anchors and cable ties.

PVC. Few fasteners are made from this material.If used in conjunction with other plastics or paintedsurfaces it must be ascertained that the materialsare compatible. This particularly applies to the

plasticised grades of PVC.

Polycarbonate- This is a specialised plastics with

good high temperature characteristics. It is strong,

can be transparent and very tough if correctly pro-cessed. Its main use for fasteners would be wherethe fastener was being used as a lens as well as a

fixing in areas where elevated temperatures could

be expected. Polycarbonate is an expensive rawmaterial.

PPO (polyphenylene oxide) and its derivatives arebeing considered for some fastener applications

where a performance similar, but superior, to

acetal is required. PPO is also expensive anddifficult to process, but it has yery good temper-ature characteristics.

Polysulphone. This material has many of the pro-perties of PPO but also has outstandingly goodelectrical and chemical properties.

Whilst many plastics have been described in this

Chapter it is interesting to note that about 75 percent of all plastics fasteners are made from eithernylon or acetal. Of the remaining 25 per cent about15 per cent are produced in polythene leaving therest of the plastics with a 10 per cent share. Thus,one can see that the more sophisticated materialsare only contempleted in very special instanceswhere peculiar environmental and operational con-ditions are envisaged.

FINISHES

.As mentioned previously, finishing is not normallyrequired on a plastics fastener. However, in someinstances the following secondary operations areencountered.

Annealing. This is a process which refers to the

conditioning of plastics components after mould-ing. It is carried out by heating the items in air ora liquid so that moulded-in stresses are relieved.

Moisture that has been dried out by processing canbe rapidly replaced by boiling in water. This is

particularly the case with nylon 66.

Vacuum metallising. In this process a thin metaldeposit, often aluminium, is deposited on to the

fastener. The metal is protected by applying a

transparent and tough lacquer. This finish is oc-casionally applied to hole plugs and the such like

where a chromium plated finish is required. Thefinish is not particularly durable and is only as

strong as its protective lacquer. Most thermo-plastics can be vacuum metallised.

Electroplating. Some plastics - notably ABS - canbe plated in the same way that metals are finished.

It is relatively expensive to apply and the applica-

tions are not numerous.

Painting and lacquering. Although one of the majoradvantages of a plastics fixing is that it can beself-coloured, there are instances where pigment-ed plastics are not available. Either because thereis not the time to prepare the material or wherethe quantity to be produced does not warrant a col-

73

our-matched raw material. When lacquering, caremust be exercised in selecting a paint that adhereswell to the base but at the same time has no unduesolvent effect on the material. This is particularlyso when finishing polystyrene.

Vapour blasting. On occasions it may be necessaryto impart a matt finish on to a fastener. This canbe done by either vapour blasting the cavities inthe mould, or blasting the mouldings themselves.The process is seldom used, but it does enable col-our/texture matches to be achieved more readily.

PRICES OF PLASTICS FASTENERS

The next section has been included to assist en-gineers in evaluating the cost of a plastics fixing.The figures given are typical of a particular type.However, it must be stressed that figures havenot been taken from any one manufacturer's pricelist. Therefore the illustrations must be regardedas typical of a type of fixing. As mentioned earlier,it is wise to consider the installed cost of the fix

when looking at prices. Also, as with all massproduced items, the unit cost is considerably in-fluenced by the extent of the tooling. Usually highertool charges result in lower unit costs.

Fig.10. Nylon/acetal blind rivet.

Unit cost: 20s to 60s per 1000.The price depends much on size and popularity.The smallest rivets are about i in. in diameterranging to i in. diameter

Fig.11. Polythenestud anchor

.

Unit cost: 10s to 15sper 1000. Used to

fix most automobilebadges

.

Fig.12. Nylon 66 or acetal push-in-fix.Unit cost: 17s to 50s per 1000.Costs will vary .rnmensely according to sizeand popularity of item.

Fig.13. Nylonpush-in-nut

.

Unit cost: 20s to 60sper 1000.Size ranges fromnumber 4 screwsupwards. The boresare not threaded.

Fig.14. Nylon/polythene hole plug.Unit cost: 20s per 1000.Materials will varyaccording to application

Tooling costs

These depend on the intricacies of the componentdesign and the number of impressions required toput into the tool. The average cost for a fastenermould is about £750. Tt may be considerably moreif the fastener is incorporated into a bracket or a

larger moulding. Also many fastener manufactur-ers contribute part oi' the tool cost so that they canassist the user and retain the right to produce forother customers on the same tooling.

Ordering quantities

Generally, all plastics fasteners are made to or-der and stocks are not kept on the shelf. Thereare few fixings that can be called 'standard parts'.The reason for this is that many pieces are madefrom a specific material or colour for each appli-cation. It is therefore important to specify fasten-ers early in the design stages.

Most manufacturers will consider designing andproducing a 'special', if the initial order is for250, 000 parts or more. Frequently, orders forless than 50, 000 pieces create problems, sincethe cost of 'setting up' to produce such small quan-tities would be prohibitive. Some manufacturerswill produce short runs of standard parts but theymay charge a premium to cover the extra costs.

CHOOSING A PLASTICS FASTENER

1. Call a specialised fastener manufacturer andtry to use a standard fixing.

74

2. Fully test all applications before releasing for

production. This particularly applies to appli-

cations where elevated temperatures are to beexpected.

3. Make sure that hole sizes are closely toler-

anced - most plastics fasteners require care-

ful attention to hole details.

4. If fixing holes are punched ensure that fasten-

ers are inserted from the 'punch' side.

5. Allow for paint build up in holes and test fas-

teners wherever possible in piercings that havebeen painted under production conditions.

6. Use correct installation tools wherever rec-commended.

FUTURE TRENDS

In 1960 there was practically no market for plas-

tics fixes in the UK. Today it is calculated that

each year over 700 million plastics fixing devices

are used in automobile industry alone - such has

been the growth of the business. No doubt the use

of plastics has been due to the many factors pre-

viously mentioned, however it is also sure that the

activities of practitioners in value engineering in

various companies has done much to highlight the

virtues of plastics fixings. The future trends al-

most certainly depend on their ever increasing use

as moulded-in fixing details in larger components.

The increased usage of existing devices can also be

foreseen as more sophisticated plastics becomewidely used. Furthermore, whilst metal fastenings

are generally becoming more expensive as rawmaterial prices rise, this should not be so in the

plastics industry, where price of raw material has

remained static or has even dropped.

75

11

Pins - solid and tubular

by R.G. Thatcher (Spirol Pins Utd.)

Because of their simplicity of design, pin fastenersoffer a neat and effective approach to assembly ina variety of applications. Pin fasteners representone of the basic methods of joining parts and canbe used as pivots, shafts, retainers, stops, locat-ors, etc., in most industries. Traditional formssuch as tapered, dowel and cotter pins which wereintended as location surfaces, are among the oldestfastening elements in use and are still finding validapplications in certain assembly functions. Althoughtoday some fasteners which have been in use foryears are still demanded by some designers, thegreatest potential in design for fastening service isoffered by a group of pin devices that are of morerecent origin. Although all of these fasteners arecharacterised by the inherent simplicity of the pin,details of design and construction vary widely.

Basically there are two types of pin available, csisting of machined pins and radial locking pins.

con-

MACHINED PINS

Hardened and ground dowel pins. These pins arehigh quality parts manufactured to exacting require-ments, the assembly of which necessitates a pressor tap fit into reamed holes.

Tapered pins . The wedging action of this type ofpin is obtained by a force fit assembly into a taper-ed hole which often necessitates a three drillingoperation and then finally a reaming operation witha tapered reamer. The standard pins have a taperof tin. per foot measured on the diameter. Asa simple low cost fastener element, the standardtaper pins have been widely used for light duty ser-vice in the attachment of wheels, levers and simi-lar components, to shafts. To provide a tight fit

the taper pin is usually driven into the hole untilit is fully seated. The taper on the pin aids holealignment in assembly.

Cotter pins. These are one of the oldest forms ofpin fasteners. The split cotter pin is characterisedby its simplicity and reliability. In assembly thepin is inserted into a hole and locked in place byspreading the split ends. Basically, however, thecotter pin is a locking device for other fasteners.

RADIAL LOCKING PINS

To allow for ease of assembly and low cost of pro-duction, radial locking pins of various types havebeen developed during the fairly recent past, and

in view of their demand and improved characteris-tics this section has been expanded to explain morefully their capabilities.

Even here there are two basic forms of pins - first-ly, the solid pin with grooved surfaces and secondlythe tubular pin either with a spirally wrapped coilor with a cross section of a tube with a longitudinalslot along the entire length.

The high resistance to vibration and impact loadsof these pins have proved to be the greatest attri-bute of this fastener. In assembly the radial forcesproduced by these parts put pressure on the sidewalls of the hole and develop a secure frictionallocking grip against them. In addition to the fric-tional locking action several other desirable cha-racteristics of these pin fasteners stem from theresilient surface construction. All of these pinsare re-usable, and can be removed and re-assem-bled many times without appreciable loss of fasten-ing effectiveness, although once a pin has damagedthe side walls of the hole it does begin to lose ef-fectiveness, and the designer should always ensurethat the pin used would not cause damage, other-wise this will result in the pin loosening in the holeand eventually becoming ineffective. With this typeof pin the need for accurate sizing of the holes is

eliminated as the design of these pins allow for thepin to be larger than the hole into which it is to beinserted - a reduction in the expanded diameter ofthe pin is made when tapped into the hole. Holesdrilled to standard production tolerances are usual-ly adequate and in Fig. 1 a comparison of hole tol-erances allowable with different types of pin canbe seen.

Grooved straight pins. Locking action of the groov-ed pin is provided by parallel longitudinal groovesuniformly spaced around the pin surface. The con-ventional practice is to use three grooves, rolled

Fig.1 . Graph showing permissible hole tolerancesof pin Fastener Forms .

DIAMETER IN INCHES

76

or pressed into solid pin stock. The grooves tend

to expand the effective diameter and when the pin

is driven into a drilled hole corresponding in size

to the nominal pin diameter, the deformation of

the raised grooved edges produces a force fit with

the side walls of the hole. The best results with

this type of pin are obtained under average assem-bly conditions when the holes are drilled closely

to th,e same size as the nominal pin diameter. Un-dersized hole specifications should be definitely

avoided. Also, when the part material is apprecia-bly harder than that of the pin, chamfered or round-ed hole edges should be specified to avoid shearingthe expanded pin section.

Spring pins. Resilience of walls under radial com-pression forces is the principle of two pin formsdeveloped for fastening applications. One designemploys a spirally wound metal strip to achievealmost a coiled spring effect, the other has theshape of a slotted tube to provide the desired effect.

Both of these pins are made to controlled diametersgreater than the holes into which they are inserted.

When compressed on being driven into the hole, the

pins exert spring pressure against the hole wall

during the entire engaged length to develop a stronglocking action.

Slotted tubular pins. Standard sizes of these pinfasteners provide a range of standard nominal dia-

meters from ii to i in. and in length from $ to 4 in.

Standard materials are heat treated carbon steel

with corrosion resistant steel available in somecases. These pins offer a tough yet resilient self

locking fastener that can withstand shock and vi-

bration loads. Under normal application conditions,

holes produced with standard fractional drills andheld within the practical tolerance shown in Fig. 1

for slotted tubular pins, will provide adequatelocking action where the material of the hole is

satisfactory. Because of their design, the slotted

tubular standard pin is not practical for use with

automatic assembly operations, although morerecently these pins have been manufactured with

the slotted section narrower than the thickness of

the material used in the manufacture of that dia-

meter. This, unfortunately, reduces the effective-

ness of the spring type action. The shear values of

these pins as shown in Table 1 are only obtainable

when the force of shear meets the pin with the slot-

ted section 90° from that direction. Higher shearstrength can be obtained with the use of these pins

if two pins are used in conjunction with each other,

but in double pin assemblies random orientation

of the slots is recommended.

Spirally wrapped pins. Standard sizes of these pins

cover a range of nominal diameters from ^to }

in. and in length from } to 6 in. Standard mater-ials are heat treated carbon steel, heat treatedchromium alloy stainless steel and work hardenednickel stainless steel. Three series of pins are

available to meet varying load and service require-

ments. Light duty pins are recommended for lowshear loading and are suggested for use in soft orbrittle materials and in delicate instruments. Themedium duty pin is the optimum in balance betweenhigh shear strength and great shock resistance.

Heavy duty pins are recommended for extremeservice conditions where shock and vibration loads

Table 1 . Minimum static double shear strength of equivalent pin fasteners .

Recommendedshaft sizes

Nominaldiameter

Minimum double shear in lb.

Slotted tubular pins Spirally wrapped Equiv. solid

cold rolledCarbon Carbon Carbon Steel

Steel (1

)

Steel &En 58A (2)

& En 58 A Steel pin (1

)

3/32 1/32 - — 75* -

3/32 0.039 - - 120* -

5/32 3/64 - - 170* -

5/32 0.052 - - 230* -

3/16 1/16 425 425 450 4007/32 5/64 650 650 700 6251/4 3/32 1 ,ooo 1 ,ooo 1 ,000 900

5/16 7/64 - - 1 ,400 -

3/8 1/8 2,100 1 ,840 2,100 1 ,600

7/16 - 1/2 5/32 3,000 3,000 3,000 2,5009/16 - 5/8 3/16 4,400 4,400 4,400 3,60011/16 7/32 5,700 5,700 5,700 4,9003/4 - 7/8 1/4 7,700 7,700 7,700 6,40015/16-1.1/16. 5/16 11 ,500 11 ,500+ 11 ,500 10,0001.1/8-1 .1/4 3/8 17,600 17,600+ 17,600 14,4001 .5/16 - 1 .7/16 7/16 20,000 20,000+ 22,500 19,6001 .1/2 - 1 .7/8 1/2 25,800 22,240+ 22,240 25,6001 .15/16 - 2.3/16 5/8 - - 46,000 —

2.1/4 and up 3/4 - - 66,000 -

1 . As given by Firth Cleveland Fastenings Limited

.

2. As given by G.E. Bissell and Company Limited.* En 58 A only

+ Carbon steel only

77

are severe. The coiled design, which is morerecent than any other type of pin fastener design,offers characteristics of the cross section whichwill withstand higher shock and vibration loads.The overlap on the outside surface is a slit edgewhich is slightly broken to prevent wear in the mov-ing parts. Orientation of the overlap in assemblywith directional applied load is not necessary. Pro-duction drilled holes are recommended. With toler-ances more liberal than those for other types ofpin, both plus and minus hole tolerances are per-missible on nominal diameters of & in. and greater.Standard, pins have a swaged chamfer on eitherend to facilitate assembly. The locking force de-veloped by the spirally wrapped pin is a functionof length engagement and pin diameter. Pin inser-tion and removal forces can be readily varied to

meet specific application requirements by controlof hole and pin sizes. These pins are suitable forapplication in blind or open locations.

The material most commonly used for pin fastenerconstruction is En49A carbon steel or mild steel,

although other materials are available, stainlesssteel in particular. The spirally wrapped pin is

stocked in the widest range of materials as stan-dard. There are various finishes available on thecarbon steel and the most common of all is the zincor cadmium plated finish, which both give a fair

resistance to corrosive conditions. There is alsothe phosphate coated finish which not only combatscorrosion but also, in certain cases, can increasethe frictional hold of the pin on the side walls.

Ordering quantities vary from manufacturer tomanufacturer and if only small quantities are re-quired, say less than 1000 in the smaller diameters,to 50 in the larger diameters, a surcharge is madeby companies who will supply in these smallerquantities.

Special types of pin fasteners are available, andbecause of their design the spirally wrapped pinscan offer the widest variety of special designs pos-sible. Some slotted tubular pins and grooved pinscan also be supplied in special forms; normally,however, most companies do insist on a minimumquantity of 50, 000 pins on a special production line.

The spirally wrapped pins can be supplied in quan-tities of as little as 1000 on small diameters, al-

though a set up charge is made.

One main point that requires a re-appraisal of en-gineering thinking which has been prevalent in thepast, is the understanding of dynamic situationswith which most fasteners are presented. Thereis a so-called 'rule of thumb 1 to double the staticshear strength requirements for applications sub-ject to shock or dynamic situations. This rule re-sults from the great difficulty in analysing dynamicsituations without performing the actual simulatedtests. The continued flexibility of a spring pin inthe hole creates a new relationship between staticshear strength and dynamic loading. It has beenproved that a flexible pin will repeatedly outlast asolid pin which has greater static shear strength,and it would be advisable to ensure that a pin beused that will remain flexible even after it is in-serted into the hole. These facts do not make theproblem of analysing a dynamic situation easier,but comparative tests will prove the results in eachapplication.

Obviously one of the prime considerations in design,is servicing and the cost of the finished article.During a survey on pin fasteners it was found thatonly 19 per cent of the assembled price was for theactual pin, the remaining 81 per cent of the costwas for fitting that pin. Therefore, when consider-ing the prices of pins, the very important factor ofease of assembly should be taken into considera-tion. In an effort to reduce this assembly cost, anew range of pin insertion machines are availablethat can feed, position and insert normal types ofpins at a rate of up to 20, 000 per day. These mach-ines are of definite interest to the production en-gineers, whilst also concerning the designer on anyfuture designs. Comparatively cheap to buy, thesemachines will facilitate ease of assembly especiallyin the smaller range of diameters and lengths thatare usually a menace on the assembly lines. Themachinery will take standard type of pins down to»in. in diameter.

Because pin fasteners can offer a real saving notonly on the piece part price, but also on assemblycosts, the designer not only has the task of findingthe situations where other fasteners can be replac-ed by a cheaper method, but, because of the num-ber of different types of pin fasteners available,

has the decision of which pin fastener to use, andit is hoped that this brief resume of pins available,

together with their characteristics, will make that

job a little easier.

78

12

Projection welded fasteners

by C.H. Meader (KSM Stud Welding Ltd.)

When looking into the possibility of utilising the

stud welding process in the manufacture of his com-pany's products, it is a distinct advantage for thedesigner to understand the welding process involvedin attaching the fastener, as well as knowing whattypes of fastener are available to him.

Two different forms of stud welding are in general

use, 'arc and 'capacitor discharge' (or 'CD'); the

fundamental principle by which they effect the weldsare similar, the two forms being complimentaryrather than competitive in their application.

There is now on the market a range of arc andcapacitor discharge equipments, which suit everyneed, whether it be a portable unit for general fast-

ening applications (Fig. 1 ) or solid state control

fully automatic machines for mass production re-quirements (Fig. 2).

Experience with the practical applications of both

forms of stud welding, coupled with due considera-tion of the economics involved, has proved the twoprocesses do not generally overlap in application,

although, on some occasions, some may give equal-

ly acceptable results.

For ease of fastener selection, it is better to treatthe arc and capacitor discharge processes separ-ately. It can be seen from Figs. 3 and 4 how thewelding operations for the two processes differ.

In both cases, the welding process in controlledautomatically by stud welding equipment and studpositioning can be controlled to as close as + 0. 003in. It is necessary, therefore, to give due regardthe capital outlay on equipment when consideringthe economics of stud welding.

Fig.1. Portablearc stud weldingcontrol unit withhand held gun.This unit has the

capacity to weldfasteners up to J in.

diameter at ratesof 10/12 per minute.

Fig. 2. Solid state

control automaticfeed pneumaticallyoperated capacitordischarge benchproduction machine.Multihead versionsof this type of mach-ine are available

each head being cap-able of welding up to

1800 fasteners perhour.

As an alternative to outright purchase of equipment,

it can be hired at low rates for periods of one weekor more, thereby enabling stud welding to be justi-

fied on a short run or contract basis.

SELECTION OF PROCESS

There are a few check points which will enable the

designer to ascertain what system and, therefore,

what type of fastener to select. These check points

are as follows:

a. Fastener size.

b. Parent metal thickness.

c. Material compositions.

d. Fastener shape.

Looking at the above check points in more detail

we have:

Fastener size. If the stud required is larger than

i|in. diameter the arc stud welding process mustbe used. CD stud welding limitation presently rests

at iJin. diameter maximum. Arc stud welding limi-tation reaches a maximum diameter of li in.

Parent metal thickness. If the parent metal is lessthan 16 swg. , the CD system must be used. If theparent metal is heavier, arc stud welding can beused.

Material composition. Mild steel and austeniticstainless steel is compatible with either process.Various aluminium alloys can also be welded witheither system. Copper, brass, and galvanisedsheet can only be welded with the CD process. Die-

79

3. 6.

Fig. 3. Stages in the arc stud welding process.

1 . The stud is located on the spot to which it is

to be welded

.

2. Pressure on the stud welder seats the arc shieldfirmly with the work

.

3. The trigger is pulled, the solenoid energisesand the lifting mechanism in the stud welder lifts

the stud , thereby creating a pilot arc between theend of the stud and the work surface

.

4. The welding contactor closes and as the studremains off the work the welding arc puddles themetal under the stud and melts a small portion ofthe end.

5. When the cycle is automatically completed, thesolenoid de-energises and the stud welder's mainspring plunges the molten end of the stud into themolten area of the work where a complete bondingof the metals forms at once

.

6. The molten metals solidify almost instantly,fusing the metals in a permanent bond . When thestud welder is removed from the stud, the arcshield is knocked off. The stud should appear asillustrated.

cast zinc and certain cast and sintered alloys canalso be satisfactorily welded with the CD process.

Fastener shapes . It is usual to consider fastenersof circular section; but unusual shapes, such assquare or rectangular pins, can be satisfactorilywelded with both processes.

FASTENER TYPES

'Capacitor discharge' (CD)

The CD process operates on the principle of capa-citor stored welding energy, which is instantaneous-ly discharged by the equipment system through a

1.

3.

Fig. 4. Stages in theCD welding process.

1 . The stud is 'located on the spot to which it is

to be welded and the stud welder footpiece is seatedon the plate.

2. The trigger is pulled, releasing the electricalenergy stored in the capacitors. The current pro-duced disintegrates the projection on the end of thestud and creates an arc between the stud and thework resulting in a molten state on the surface ofthe plate and the stud

.

3 . At the instant the tip is completely melted

,

spring pressure forces the stud into the moltenpool , completing the weld . The entire weld cycletakes place in approximately 6 milliseconds. Thecompleted fastening develops the full strength of

the stud and plate material and will not break in

the weld area.

special weld tip. This results in high temperature,which melts the weld end area of the stud and thearea of parent metal immediately below it. Thestud is forced into the molten metal and, upon cool-ing, a uniform cross sectional bond is achieved.

A vast selection of studs and fasteners are current-ly available as standards, varying in size from8 BA to ft in. These studs are available with all

forms of thread, including Metric.

It is normal for CD studs to be manufactured bya cold heading process; threads, where required,being rolled and not cut. Some of the more comp-lex types are manufactured by an automatic turningprocess, and where necessary further secondaryoperations, such as slotting or cross drilling, arecarried out.

CD studs are normally manufactured from the fol-lowing materials:

Mild steel

Stainless steel

AluminiumAluminium alloy

CopperBrass

(En 2A)(Eno8B)(Commercially pure)

(3^ per cent magnesium)(Electrolytic or lead free rolled)(lead free 63/35 or 70/30)

Table 1 will enable the designer to select the typeof fastener required for his application from thestandard range available. It should be remember-

80

I I ! «

I ! •Fig. 5. Shown above is a small selection of theinfinite variety of CD stud types; including,

threaded studs .nameplate, insulation and piercedhanging pins , tapped pads knock-off pins andsquare and rectangular shaped studs

.

directly end welded to the parent metal in a fraction

of a second, although these times are long whencompared with the CD process.

A source of direct current welding energy is re-

quired in addition to the welding controller and gun.

All arc studs are solid aluminium fluxed on the

welding end, and each stud is supplied with a cera-

mic ferrule (arc shield). Fig. 3 shows the weldingsequence.

Once again, a vast selection of studs and fasteners

are available as standards, varying in diameterfrom 1 to 11 in. Thread forms include RA, UNF,BSW, BSF and Metric. Arc studs are available in

the following materials:

ed that all material listed above can be offered in

this standard range.

Standard fasteners are flanged, but for some ap-

plications the designer finds this flange undesirable.

It is possible, therefore, to obtain the whole rangeof fasteners detailed in Table 1 with the flange re-

moved. On the question of economics, the non-

flanged studs are more expensive as this flange

removal involves a further operation during manu-facture.

In addition to the standard fasteners listed in Table

1, an extensive range of semi- standard and special

fasteners are available. An indication of the varied

selection available can be seen in Fig. 5.

'Arc'

The arc process is similar in many respects to

manual arc welding. The fastener (electrode) is

Mild steel

Stainless steel

Aluminium alloy

(En32A)(En58B)

(3| per cent magnesium)

Table 2 will enable the designer to select the type

of fastener required for his application from the

standard range available. In addition to these stan-

dard fasteners, an extensive range of semi- stand-

ard and special fasteners are available. An in-

dication of the varied selection available can be

seen in Fig. 6.

It would be impossible to include specifications of

all the various styles of studs that have been pro-duced and are readily available for the designersuse. Most normal machining operations such ascross drilling, slotting, bending, swaging, pierc-

ing, etc. , are available in combination with the

standard studs listed. In the case of design ap-

plications with special lengths or other secondary

machine operations, free advice is offered by the

stud welding fastener manufacturer.

Standard welding studs

Full threaded 'PD' studs

Collar studsStraight pins

Tapped pads'J' shaped support pins

Pierced rectangular pins

'T slotted rectangular pins

(i) Korr pins

(j) Multiple grooved korr pins

(k) Concrete anchors(I) Shear connectors(m) Insulation pins

(n) Stay bolts

(o) 'T pins

(p) Shoulder studs

(d)

u

u-S

(e)

(ft

CO

o

to)

n

CO (k) (1) Cm) Co)

fr3

Ch)

§CP)

Fig. 6. A small selection

of the infinite variety of arcstuds currently available.

81

Table 1 . Standard 'capacitor

Stud Size

Head Diameter 'D' (inches)

MildSteel

Stain-

less

Steel

PureAlum.

Alum.Alloy

Brass

Head Thickness 'T'(inches)

MildSteel

Stain-

less

Steel

PureAlum.

ftin. BSW &in. UNC

iin. Dia. pin

*in. BSF lin. UNF

i in . BSW J in . UNC BA M 6

]|in. Dia. pin M5

2 BA &in. BSW 10 UNF 10 UNC

8 UNF 8 UNC iin. Dia. pin

4 BA

M 4

6 UNF

6 UNC

4 UNF 4 UNC 6 BA

M 3

8 BA

0.40

0.350

0.325

0.315

0.280

0.250

0.250

0.250

0.250

0.250

0.220

0.220

0.220

0.160

0.40

0.335

0.325

0.315

0.280

0.250

0.250

0.220

0.250

0.220

0.220

0.220

0.220

0.160

0.350

0.350

0.312

0.280

0.250

0.250

0.250

0.250

0.220

0.220

0.205

0.220

0.187

0.350

0.350

0.312

0.280

0.250

0.250

0.250

0.250

0.220

0.220

0.205

0.220

0.187

0.330

0.312

0.312

0.280

0.250

0.250

0.250

0.250

0.220

0.220

0.220

0.220

0.187

0.050

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.050

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.032

0.050

0.050

0.050

0.050

0.050

0.050

0.050

0.050

0.050

0.050

0.050

0.050

0.050

Tolerances ±0.015 in

.

Tolerances ±0.010 in.

NOTE: ABOVE DIMENSIONS DETAIL TYPES A AND B. BUT ALL

STANDARD FLANGETHREADED STUDS

(a)

STANDARD FLANGEPLAIN PINS

TU

discharge' welding fasteners.

Head ThicknessT (inches) Maximum Length '

I_' (inches) Minimum Length 'L' (inches)

Allum.Alloy

BrassMild

Steel

Stain-less

Steel

PureAlum.

Alum

.

AlloyBrass

MildSteel

Stain-

less

Steel

PureAlum.

Alum.Alloy

Brass

- - 3.75 3.75 3.75 3.75 3.75 0.375 0.375 0.375 0.375 0.375

0.050' 0.050 3.75 3.75 3.75 3.75 3.75 0.312 0.312 0.312 0.312 0.312

0.050 0.050 3.75 3.75 3.75 3.75 3.75 0.312 0.312 0.312 0.312 0.312

0.050 0.050 3.75 3.75 3.75 3.75 3.75 0.312 0.312 0.312 0.312 0.312

0.050 0.050 2.0 2.0 2.0 2.0 2.0 0.250 0.250 0.250 0.250 0.250

0.050 0.050 2.0 2.0 2.0 2.0 2.0 0.250 0.250 0.250 0.250 0.250

0.050 0.050 2.0 2.0 2.0 2.0 2.0 0.250 0.250 0.250 0.250 0.250

0.050 0.050 1 .5 1 .5 1 .5 1 .5 1 .5 0.250 0.250 0.250 0.250 0.250

0.050 0.050 1 .5 1 .5 1 .5 1 .5 1 .5 0.250 0.250 0.250 0.250 0.250

0.050 0.050 1 .5 1 .5 1 .5 1 .5 1 .5 0.250 0.250 0.250 0.250 0.250

0.050 0.050 1 .5 1 .5 1 .5 1 .5 1 .5 0.250 0.250 0.250 0.250 0.250

0.050 0.050 1 .0 1 .0 1 .0 1 .0 1 .0 0.250 0.250 0.250 0.250 0.250

0.050 0.050 1 .0 1 .0 1 .0 1 .0 1 .0 0.250 0.250 0.250 0.250 0.250

0.050 0.050 1 .0 1 .0 0.75 0.75 1 .0 0.250 0.250 0.250 0.250 0.250

Tolerances Tolerances ± . 01 5 in

.

Tolerances ± . 015 in.

±0.010 in.

ARE AVAILABLE WITH FLANGE REMOVED AS TYPES C AND D.

NON-FLANGEDTHREADED STUDS

(c)

NON-FLANGEDPLAIN PINS

L

83

Table 2. Standard 'arc' weldingfasteners

.

MATERIALS: LOW CARBON MILD STEEL AND 18/8 OR 18/8-1 STAINLESS STEEL.AVAILABLE THREAD FORMS: BA.BSW.BSF .UNF .UNO ,ANF .

STANDARD THREADED STUDS

U^AFTER WELD

(AW)

REDUCED BASE STUDS

h^HAFTER WELD

(AW)

FULLY THREADED STUDS

AFTER WELD(AW)

STUD SPECIFICATION

Diameter

Ain. 2BAtin. OBA

| in.

iin.

fin.

Jin.

H

0.1600.2160.2750.3320.4450.5630.682

Min. (AW)Length

A L

A in- tin.iin. fin.iin. Jiniin. tinrain. Siniin. 1 in

Ain. 1 Ain

Weld BeadDimensions

iin.

Min.

Ain.gin.

iin.

iin.

ft1"-

a in.

sin.

iin.

ftin.

Ain.Ain.

Max

.

(AW)Length

4.0 in.

6.0 in,

8.0 in.

8.0 in.

10.0 in.

10.0 in.

10. in.

STUD SPECIFICATION

Diameter

DAin. 2BAiin. OBAAin.

Iin.

iin.

iin.

iin.

Dia.

H

0.1250.1750.2300.2830.3790.4940.606

Min. (AW)Length

1 iin.

1 Iin.

A

iin

iin

5sin

A in

as in

iin,

iin,

Weld BeadDimensions

iin.

u •

32 in

.

lain

.

Sin.

F

iin.

iin.

iin.

kin

.

A in

.

Ain

.

Ain

.

Max.(AW)Length

3.0 in

3.0 in

5.5 in

5.5 in,

5.5 in,

5.5 in

3.5 in,

STUD SPECIFICATION

Diameter

D

Ain. 2BAi in . OBAA in

.

iin.

iin.

iin.

iin.

Min (AW]Length

L

Sin.

Sin.

iin.

iin.

1 in.

1 Ain.

1 iin.

Max

ft m.Ain.Ain

.

A."in.

Ain

.

Ain

.

Ain

.

Weld BeadDimensions

in.

in

.

|4n.gin.ft in

.

iin.

1 in.

Bin.

i in.

32 in

.

Ain

.

Ain

.

Max (AW)Length

2.75 in.

2.75 in.

2.75 in.

2.75 in.

2.75 in.

2.75 in.

2.75 in.

PLAIN PINS

1-T

AFTER WELD(AW)

STUD SPECIFICATION

Diameter

D

Ain.

iin.

Ain-

Iin.

Ain.

iin.

iin.

Iin.

Min (AW)Length

L

iin.

S in '

iin.

iin.

1 in.

1in.

1 A in

.

1 iin.

Weld BeadDimensions

Bin.gin.

Ain.

i in.

'iin.

Sin.

iin.

1in.

iin.

«in.

ain.

iin.

Bin.

kin.*iin .

Max (AW)Length

4.0 in.

6.0 in.

8.0 in.

10.0 in.

10.0 in.

10.0 in.

10.0 in.

10.0 in.

NOTE: 1. A FULL RANGE OF ARC STUDS WITH METRIC THREAD FORMS ARE ALSO AVAILABLE.2. SHORTER STUDS OF 'BREAK-OFF' TYPE ARE ALSO AVAILABLE AS STANDARDS.3. STUDS OF GREATER LENGTH THAN THOSE LISTED ABOVE ARE AVAILABLE TO SPECIAL

ORDER

.

All shapes. All sizes. That's our versatile rangeof standard studs. If we don't have in stock

exactly what you need studs can be speedily

made up to your requirements. And remember,all of these studs can be welded in undera second. Saving time. Saving money. Try us.

P.S. We make stud welding equipments, too.

(rompton Parkinson Stud Welding

CromptonParkinson Ltd. . CromptonHouse, Aldwych, London WC2

f0f HAWKER SIDDE1.EY COMPANY

85

DESIGN CONSIDERATIONS

Arc stud welding

m. High application rates (upHo 1800 welds perhour with single head automatic bench machines).

In designing arc stud welding fastenings, thereis a ratio of parent metal thickness to stud dia-meter that should be followed for practical engin-eering and production quality results. The parentmetal thickness should be a minimum of a third of

the weld base diameter of the welding stud. Thereare, however, many applications where strengthis not the primary requirement. In cases such asthese the parent metal thickness may be reducedto a minimum of one -fifth the weld base diameter.Thickness above this will afford complete cross-sectional area weld fusion without burn-throughor excessive distortion of the parent material.

CD stud welding

In designing fasteners with the CD system, parentmetal material can be as thin as 0. 020 in. (0. 032in. aluminium) without burn-through occurring.Studs welded to this thickness will normally causesheet failure when loaded to ultimate.

FASTENER COSTS

To achieve the lowest fastener cost, it is recom-mended that first and fullest consideration shouldalways be given to:

1. Use of standard stud types, as detailed in manu-facturer's specification sheets.2. Use of standard lengths, diameters and threadforms (studs are available with length incrementsof i in. )

3. Use of supplier's standard materials.

ADVANTAGES OF STUD WELDING

Listed below are some of the main advantages ofstud welding when compared with other fasteningprocesses:

a. Low cost standard fasteners.b. Elimination of drilled or punched holes.c. Elimination of tapping operations.d. Reduction in gauge of parent materials.e. Aesthetically improved product.f. With CD welding, the ability to join dissimi-lar metals often having widely different meltingpoints.

g. Welding and fastening from one side which, -in

some cases, eliminates the need for two opera-tors.

h. With the well designed stud welding equipmentcurrently available on the market, unskilled op-erators can be taught to use it successfully in avery short time.i. Vibration proof permanent fastening,

j. Reverse side marking and burning consider-ably reduced or eliminated.k. Leak-proof fastening (lending itself to use oncontainers of all kinds).

1. Low cost. jigs and fixtures.

DISADVANTAGES OF STUD WELDING

The disadvantages of stud welding can be summar-ised as follows:

a. Difficulty in obtaining high strength welds oncertain base materials, i.e. high carbon steels,copper based aluminium alloys, cast iron, etc.

b. Difficulty in welding fasteners through pre-painted surfaces, unless the weld area is scrapedor ground clean beforehand.

c. Difficulty in welding through heavily platedzinc or cadmium surfaces. (It should be pointedout, however, that stud welding can be carried outsuccessfully through many plated surfaces includ-ing electro-galvanised zinc, chromium, nickel,etc.)

Ftg.7.

TYPICAL APPLICATIONS

Stud welding is currently being used in virtuallyevery section of industry, a few of the more inter-esting applications are described below:

1. A manufacturer of high class holloware pro-ducing utensils in stainless clad aluminium wishedto eliminate the conventional method of fasteninghandles, which involved punching holes and assemb-ling handles by a riveting process.

The solution was found by designing a square sec-tion aluminium alloy tapped CD welding fastenerwith a weld base diameter of 0. 375 in. (Fig. 7). Aspecial handle was designed to suit this fastener,the square section of which provided the locationto prevent the handle rotating. The handle is re-tained by a screw into the pad, and Fig. 8 showsthe arrangement for final assembly.

After final assembly, the utensil is considerablyimproved in appearance and has no marking what-

86

Fig. 8.

soever on the inside surface, which makes it easier

to clean and renders it leak- proof.

With this particular application, utensils can be

welded at the rate of 400 per hour and a cost saving

was effected over the original method of riveting.

2. Another application for capacitor discharge

stud welding which has proved to have considerable

advantages over the previous technique, is in the

production of high quality plastering trowels.

With this application, it is necessary to attach the

handle tang to a light gauge high carbon steel blade.

The original method was first to punch holes in the

blade, secondly to place the blade over the ready

drilled tang, passing through each of the holes a

mild steel countersunk head rivet. A skilled crafts-

man then peened over each rivet by hand, the final

operation being to grind flat the working surface of

the trowel.

The technique now adopted is to stud weld standard

flanged brass CD pins on to the blade, thus elimi-

nating reverse side marking and hence the need

for punching and grinding. The tang is more easily

assembled to the blade as the rivets .are weldedfirmly into position. The end result is that as-

sembly time has been considerably reduced and

the quality of the product improved.

Fig. 9 shows the trowel blade with the stud welded

in position and the finished article - note the ab-

sence of marking on the reverse side of the blade.

3. An application for arc stud welding, which is

highly successful and well-proven, is for inspec-

tion plate covers on industrial boilers and oil filled

transformers.

One application in question involved the attaching

of sixteen \ in. diameter threaded studs to a cir-

cular inspection plate cover flange. The original

drilling and tapping method involved seven opera-

tions and took 60 minutes to complete.

When arc stud welding was introduced, the numberof operations was reduced to three, and the com-plete operation was carried out in 8 minutes.

Arc stud fastener costs compared with those of

the threaded studding used in the original opera-

tion. The installed cost of the fastener was, how-

ever, considerably reduced.

MATERIAL SELECTIONAND SPECIFICATION

Arc and capacitor discharge stud welding can be

carried out on a variety of base materials. How-

ever, CD stud welding is more versatile in this

respect, the weldable range of base materials in-

cluding mild steel, medium carbon steel, stainless

steel (austenitic), lead free brass and copper, alu-

minium and aluminium alloys. With arc stud weld-

ing, applications are limited to low carbon mild

steel, stainless steel (austenitic) and magnesiumbased aluminium alloys.

Table 3 indicates the weldability of the above quoted

base materials related to CD studs produced in a

variety of materials.

With certain arc stud welding applications, it is

necessary to pre-heat the base material immediate-

ly prior to welding, for example when welding to

armour plate, or to special high yield structural

steels. It is essential, therefore, for technical

advice to be sought from the stud welding manu-

Fig.9.

facturer when designing for stud welding to special

base materials.

Tables 4 and 5 indicate typical standard load

strengths on CD and arc studs of different sizes

and materials. These values should be used as

a guide only, as it is impracticable to provide pre-

cise torque loadings for all conditions.

FINISHES

Finish must be considered firstly from the aspect

of protective coatings and secondly from the type

of protective finish through which studs can be

welded to the base materials.

In considering the finish on the studs, it is normal

to supply mild steel arc studs, self- finished and

slightly oiled, however, if a protective finish is

required on the stud, they can be supplied zinc or

cadmium electroplated. This plated finish is not

applied to the welding end of the stud, as the effect

of the zinc or cadmium is detrimental to the weld

quality.

In the case of CD welding studs, mild steel types

are normally supplied with a copper flash finish.

87

Table 3

.

BASE MATERIAL STUD MATERIALMILD STEELEN2

STAINLESS STEEL18/8

ALUMINIUMPURE AND3>i% Mg.

BASS65-35, 70-30

Mild Steel

0.3% C. Max.Excellent Excellent Excellent

Medium Carbon Steel0.3 - 0.55% C

Limited Limited Limited

Galvanised Sheet Excellent Excellent

Structural Steel Excellent Excellent ExcellentStainless Steel

(Austenitic)Excellent Excellent Excellent

Lead Free BrassElectrolytic CopperLead Free Rolled Copper

Limited Limited Excellent

Aluminium Alloys(Non-Heat Treatable) Excellent

Aluminium Alloys(Heat Treatable) Limited

Zinc Alloys(Die Cast)

— ...Limited Limited Excellent Limited

CODE: Excellent -All capacitor discharged flanged studs up to and including ,-Sin. diameterwelded with full strength results.

Limited - Generally full strength results; dependent upon stud size/parent materialcombination

.

the thickness of this coating being 0. 0001/0. 0003 in.

The main purpose of this copper flash coating is toprotect the stud during storage, which also has theadditional advantage of ensuring a good electricalcontact between the stud and the chuck during thewelding operation.

Mild steel CD fasteners can also be supplied witha protective nickel flash, if specifically requested.All studs in other materials are normally suppliedself- finish.

PLATING OF BASE MATERIALS

To ensure trouble-free welding conditions, the de-signer should always aim for the stud to be weldedto clean, unpainted, or unplated, surfaces. Failingthis, it is possible to CD weld satisfactorily throughelectroplated zinc and cadmium surfaces. It is notrecommended that CD welding be carried out onpainted surfaces of any kind, unless the area to bewelded is scraped or ground clean beforehand.

In the case of arc stud welding to pre-coated sur-faces, as a general rule, it is not recommended toweld through pre-plated surfaces of any kind. How-ever, using specially designed arc shields, it ispossible to achieve high quality welds when weldingthrough electroplated zinc surfaces. Welding throughhot dipped zinc coatings is defintely not advised.

If arc studs are required with a cadmium or zincplated finish the cost of the stud can increase byas much as 50 per cent. This high cost is due tothe fact that the weld end of the stud needs to beprotected during the plating process, each stud,therefore, has to be handled individually.

PRICES

When considering the justification for stud weldingfrom an economic point of view, it must firstly beremembered that this process offers the followingadvantages which, in themselves, save costs:

a. Elimination of punching or drilling operations.b. Elimination of tapping operations.c. Relatively simple positioning of jigs or tem-plates required.d. Equipment can be used by unskilled operators.e. Assembly can be made from one side.f. Thinner parent materials can be used.

After considering the above advantages to be gainedfrom using stud welding, it is wise then to considerthe most suitable, and of course, the cheapest typeof fastener for the application in question. Thecheapest fastener available is of the cold headedflanged type (Table 1), these varying in price from2s. per hundred upwards, depending upon the mat-erial used, the quantity purchased, and the sizerequired.

88

Do you spend hours lookingfor the right material ?

Do you then spend hourslocating a manufacturer?

If you answer 'yes' to the questions youare strongly advised to read on

Design Engineering has long since recognised that time is often wastedsearching for the right material and the best way to form it, and then tryingto find the most suitable manufacturer. It is to ensure that the best adviceis always on hand that Design Engineering Handbooks have been conceived.In the Design Engineering Handbook on Metals base metals, preciousmetals, refractory metals, irons and steels-, and coated metals are examined.Nine of the 34 chapters deal with the forming of metals, whilst the otherchapters discuss the advantages and limitations of each material, its

applications and design considerations, together with the latest developmentsFor more information and a list of Design Engineering Handbooks, write tothe Publications Manager, Product Journals Ltd., Summit House, Glebe Way,West Wickham, Kent.

89

Table 4. KSM standard CD stud load strengths.

MATERIAL SIZE FASTENING ULTIMATE MAXIMUMTORQUE TENSILE SHEAR LOAD(IN. LB.) (LB.) (LB.)

Steel 6 BA 4 400 280Low Carbon 4 BA 9 850 590Copper 2 BA 18 1 1 00 770Flashed i BSW 40 1850 1300

Stainless 6 BA 6 600 420Steel 4 BA 14 1250 89018/8 2 BA 28 1650 1150

* BSW 60 2770 1950

Aluminium 4 BA 3 260 160(Pure) 2 BA 6 360 220

i BSW 11 600 380

Aluminium 4 BA 6 520 320(3J£% Mg.) 2 BA 12 720 440

i BSW 22 1200 760

Brass 6 BA 5 400 2704 BA 10 850 5202 BA 17 1100 700J BSW 40 1850 1230

This same range of studs can be purchased withouta flange, but as this involves a further operationduring manufacture, it must, of course, be realis-

ed that the cost will be higher.

Internally threaded fasteners are manufactured bya slower auto turning process and the cost for a

fastener of this type would therefore be higher thanfor that of an externally threaded fastener.

Several thousand different types of fasteners arecurrently available and it must be appreciated,therefore, that in an article of this type it is dif-

ficult to provide an average cost. It must also be

remembered that it is the applied cost which mustbe considered and not just the fastener cost.

The selection of arc studs currently available en-ables a range of fasteners varying from 4 in. dia-meter to 1 i in. diameter to be satisfactorily endwelded to base materials, however, with this pro-cess it is necessary to utilise a ferrule and the verynature of the stud, therefore, makes its cost high-er than a comparable CD fastener of the same size.

If fasteners above A in. diameter are to be weldedwith portable equipment, it is essential to use thearc stud welding process. It is better on the groundsof economics, therefore, to design around a CDfastener in sizes up lo ,-| in. diameter.

As in the case of CD fasteners, several thousanddifferent types and shapes are currently available.

Table 5. KSM standard arc stud load strengths.

MATERIAL SIZE(THREADED BSW)

FASTENING TORQUE*(IN. LB.)

ULTIMATETENSILE (LB.)

MAXIMUM SHEARLOAD (LB

.

)

LOWCARBONSTEEL

Jin.

fin.

i in.

gin.

iin.

iin.

1in.

51 .5

112.0184.0405.0870.01090.01 660 .

2460.0

2,0003,2404,8208,750

14,20020,90029 ,000

38,000

1 ,500

2,4403,6206,65010,60015,65021 ,60028,400

STAINLESSSTEEL18/8 or1 8/8 - 1

iin.

,-fin.

§ in.

iin.

fin.

Jin.

iin.

1in.

75.2132.0236.0517.01110.01530.02328.

O

3440.0

2,8804,6806,92012,80020,20030,00041 ,600

54,500

2,1603,5005,1909,600

1 5 , 1 5022 , 500

31 ,20040,900

* These values should be used as a guide only, as it is impracticable to provide precise torqueloadings for all conditions.

and it is, therefore, difficult to give a cost for sucha fastener. Once again, the applied cost is the im-portant consideration.

Special tooling costs

If a designer requires a fastener to be manufactur-

ed for his application of a type not listed as stand-

ard in the manufacturers' catalogues, it is usually

necessary to pay a higher price for the fasteners,

in order to offset the special tooling costs involved.

Alternatively, tooling can be paid for separately.

In the case of special cold headed fasteners, a spec-

ial tooling cost of between £50 - £100 is involved,

and in the case of auto turned fasteners, wherespecial operations, such as cross drilling or cross

slotting, are involved, tooling charges could rise to

as much as £150 - £300.

It can be seen, therefore, that particularly with

short-run work, it is better to aim for the stand-

ard range of fasteners offered by the manufacturer.

Ordering quantities

When considering the use of CD welding, it should

be borne in mind that the minimum ordering quant-

ity for standard fasteners is 2000 off.

In the case of special fasteners, this minimumquantity can also be as low as 2000 off. However,

for special auto turned fasteners, this minimumquantity may be raised to 5000 off, depending upon

the setting up time and the tooling charges involved.

The stud welding manufacturer is, however, usual-

ly prepared to accept orders on a blanket cover or

scheduled call- off, provided the fasteners covered

on these orders are called-off within a maximumperiod of twelve months from the date that the or-

der was placed.

In this way, the customer can gain the advantage

of quantity discount. An indication of the scale of

discounts for varying quantities of CD and arc fast-

eners is shown in Table 6.

Table

Arc welding studs, because of their higher value

and the range of diameters available, can be pur-

chased in quantities as low as 100 off per size.

Once again, it is to the advantage of the designer,

when using this type of fastener, to direct his pur-

chasing department to place blanket order cover,

thereby ensuring maximum price advantage for

quantity.

FUTURE TRENDS

CD welding

Since its establishment in the UK some 8 years

ago, capacitor discharge stud welding equipment

has made rapid strides. When first introduced,

only portable machines were available, capable

of welding fasteners up to | in. diameter at rates

of 10/12 per minute.

Since this time, further developments have been

single and multi head bench production machineswith electro mechanical charging and control cir-

cuits. Following on from this, a similar range of

machines have been developed with all solid state

control. This change to solid state systems has

opened the field up even wider by enabling the stud

welding equipment manufacturers to offer single

and multi head machines with automatic stud andbase component feed, each head being capable of

welding at a rate of up to 1800 fasteners per hour.

The requirement for accurate positioning of fast-

eners to mass produced components has, at this

stage, led to the development of special purposeautomatic machines with logic control systems,which utilise a pre-programmed tape controlledindexing bed, thereby combining high applicationrates with accurate positioning of fasteners, a typi-

cal unit is illustrated in Fig. 10.

Arc stud welding

This process, although established in this countryfor a much longer period, has not developed alongthe same lines. The requirement for arc stud weld-

ARC WELDING FASTENERS

STUDQUANTITIES

DISCOUNTSCHEDULE

100TO249

PLUS60.0%

250TO499

PLUS30.0%

500TO1 ,999

LISTPRICE

2,000TO4,999

LESS4.5%

5,000TO9,999

LESS9.5%

10,000TO49,999

LESS11 .5%

50,000TO99,999

LESS14.0%

100,00TO199,999

LESS18.6%

200,000ANDUPWARDS

LESS21 .0%

C. D. WEL DING FASTENERS

STUDQUANTITIES

DISCOUNTSCHEDULE

2,000TO4,999

PLUS66.0%

5,000TO9,999

PLUS12.0%

10,000TO24 ,999

LISTPRICE

25,000TO49,999

LESS5.0%

50,000TO99,999

LESS8.0%

100,000TO249,999

LESS12.0%

250,000TO499,999

LESS16.0%

500,000ANDUPWARDS

LESS21 .0%

Fig. 10. Single head automatic Feed capacitordischarge stud welding unit with tape controlsystem

.

ing still lies mainly in the heavier fabrication field,where it is more convenient to use portable equip-ment with hand held guns. There is little need forautomatic feed equipments, although some specialpurpose machines have been developed which feed$ - i in. diameter fasteners at rates of 20/30 perminute. Applications here include boiler tube stud-ding and commercial vehicle assembly.

When looking at the future of stud welding, it isfairly obvious that the bias will be towards full

automation, thereby eliminating the need for cons-tant operator attendance. At the same time, theaccent will be on a greater degree of positionalaccuracy at high rates of application.

The stud welding manufacturers are constantly de-veloping along these lines, and the many advant-ages to be gained from using the drawn arc andcapacitor discharge stud welding process indicatesa very bright future.

want

to cut

fastening

costs ?

makeamM of

diffmce!

Contact us now to find out about our

free technical advice and service.

* How to eliminate costly drilling and

riveting.

* High application rates.

* Trouble free operation.

* Excellent delivery on all goods.

KSM Stud Welding Ltd.

^^>I I 1, Farnham Trading Estate, Farnham, Surrey.

Telephone: Farnham 21101-4.

92

13

Quick release fasteners

by H.J. Smith and M.R.P. Knight, A.M.B.I.M. (Dzus Fastener Europe Ltd.)

'Quick release fastener' is a generic term used to

cover any device which is designed to give a simple

and rapid means of closure and release. This mayrange from a clip device to secure kitchen cabinet

doors, to a highly sophisticated and complex mech-anism for use on aircraft. This range of devices

may be loosely grouped into five basic headings and

these are: rotary stud, toggle, latch, press button

and slide. Rotary stud devices are those most com-monly termed quick release fasteners (or in mili-

tary phraseology - turnlock fasteners), and it is in-

tended that this Chapter should be restricted to a

description of this type of fastener.

GENERAL DESCRIPTION

Rotary stud fasteners comprise a solid fastener

stud or pin which passes through a hole in the dis-

mountable panel or component and this is usually

held captive but free to rotate in the panel and an

anchor member, frequently called a receptacle, is

secured to the inner face of the fixed structure to

which the dismountable panel is to be attached. In

operation the dismountable panel is offered up to

the fixed structure, the fastener stud being brought

into preliminary engagement with the anchor mem-ber. A brief turn of the stud completes the engage-ment, thus providing a strong and vibration proof

fixing. With most rotary stud fasteners a 90° turn

is sufficient to lock the fastener, although a simi-

lar quick release function can be achieved with

fasteners based on multiple thread principles. Theseare usually two or four start threads and the femalethread is usually generated in the anchor memberor receptacle or, in some designs, as an internal

threading of the fastener stud. Various forms of

friction or depitching methods are used to lock the

threads against accidental release under vibration

or shock loads. The chief advantage of this type

of fast thread fastener is that they are capable of

pulling rigid materials together which may havedistorted or be subject to residual stresses after

Fig.1 . The multiple thread Dzus Universal fast-

ener.

^Bk

periods of cycling loads have been applied in ser-

vice. The fasteners are capable of resisting very

high shear and tension loads and the principle of

these fast threaded fasteners is generally well

known. Such types are often descriptively called

long reach, high shear fasteners. A typical fast

thread fastener is shown in Fig. 1.

However, the majority of rotary stud quick re-

lease fasteners are of the quarter-turn variety.

This type of fastener offers very rapid locking andunlocking, and when locked can withstand predeter-

mined loads. The use of quick release fasteners

can therefore save many valuable man hours in un-

locking panels where the equipment requires fre-

quent servicing. As the stud portion of the assem-bly is usually of a fixed length it is necessary to

supply different types of stud lengths for different

thicknesses of material. Designs differ according

to manufacture but generally the stud lengths in-

crease in increments of 0. 010 in. on miniaturetypes of fasteners to ranges between 0. 025 in. and

0. 050 in. on other larger types of fasteners. It

will be seen therefore that proper selection of stud

length is essential to meet total material conditions

and allowances should be carefully made for total

tolerance build up during fabrication, rubber strips,

paint or other surface finish layers.

Thus it may be difficult to standardise on one parti-

cular length of fastener where varying panel thick-

ness conditions may occur. The second point wheredifficulty sometimes occurs is that too little atten-

tion is given to the inclusion of quick release fast-

eners in the early design of equipment and conse-

quently the selection and call-up of any particular

fastener is generally left until the equipment is

either built or in a very advanced stage. It is ex-

tremely important that consideration of the quickrelease fastener selection should be given at the

earliest possible opportunity and this foresight will

undoubtedly result in a correct selection of the

fastener for the particular application.

Fixed length rotary stud fasteners are usually

based on some form of helical cam or bayonet prin-

ciple, where the cams are either machined into

the fastener body or as excrescence swaged fromthe stud exterior. The cams engage with matingparts of the anchor member and as the stud is man-ually rotated the smooth action of drawing the parts

together is achieved. An example of quick release

fastener employing the helical cam principle is

shown in Fig. 2. The illustration shows the stan-

dard Dzus fastener assembly and this principle

may be employed in a variety of fastener types.

Fig. 2. The standard Dzus fastener assembly.

The function of the spring element is to obviatebacklash in the mechanism and to provide a tensionor force which finally clamps the fastener materialstogether. This clamping force can vary from a

few pounds up to about a maximum of 100 lb. High-er initial figures are not practically achieved withsuch designs.

Other designs of quick release fastener differ inso much as the cam is formed with spiral ramps in

the anchor member itself, the stud being providedwith projecting pins which provide the engaging ele-

ment. In the aforementioned design the cam actionhas to be supplemented with a spring compensator-or resilient element, which is either incorporatedin the anchor member or fitted under the fastenerhead built-in unit. An example of this type of fast-

ener is the Oddie quick release fastener and, in

this particular type of fastener, the resilient ele-ment is provided by a rubber washer fitted underthe head of the stud member. The Camloc fast-

ener is an example of quick release fasteners em-ploying the spring compensator mechanism fitted

under the^head of the stud and the projecting pinson the stud itself engage on the spiral ramps of theanchor member of the receptacle.

APPLICATIONS

As previously indicated in this Chapter the applica-tions for which quick release fasteners are nowused range from aircraft fasteners, for which thequick release fastener was originally designed, to

applications is such industries as tne automotive,electronics, lighting, machine tool, agriculturalmachinery and other industries allied to the engi-neering field. In the building industry there is anincreasing fastener demand for trunking - accesspanelling and suspended ceiling access traps.

The object of using quick release fasteners in allthese industries is generally the same, i. e. to pro-vide means of quick access for servicing purposes.As the reader will realise this is particularly im-portant in the aircraft industry and quick releasefasteners have been employed for some 30 yearsto fasten cowling panels, for instrumentation onthe flight decks and also on the galley equipmentinside the aircraft. A typical application in themotor industry would be to fasten radiator grilleson commercial vehicles and to fasten the floor ofthe car boot where the spare wheel is housed be-neath the luggage compartment. Quick releasefasteners are also widely used on agricultural trac-tors as hood fasteners and there are many otherinstances of the use of quick release fastenerssaving many valuable man hours. Figs. 3 and 4show two typical applications; Fig. 3 shows panelfasteners in operation on microwave equipment;Fig. 4 a bank of switchgear cubicles incorporatingquick release fasteners for cover removal.

The increasing use of this type of fastener has ledto the development of a wide range of head styles.Originally the fasteners were designed for use witha screw driver but fasteners are now generallyavailable for hand operation by means of a wing invarying forms, a ring or with a knurled head. In

addition it is becoming increasingly necessary tooffer such devices with a head style which will pre-vent unauthorised entry. This can be achieved bythe use of what may be termed a tamper proof fast-ener, i. e. having a head style operable only witha special key, or it may be achieved by means ofa quick release fastener incorporating a key lock-ing mechanism. The introduction of these vary-ing head styles leads to a rather complex produc-tion and stock holding problem but the need for suchvariations in the basic fastener is now well estab-lished.

Fig .3. The Dzus panel fastener in operation onmicrowave equipment . y

Fig. 4. The standard Dzus fastener in use onswitchgear cubicles.

MATERIALS

The normal quick release fastening device has a

stud produced from carbon steel and heat treated

whilst the receptacle or spring component is pro-

duced from some form of spring steel. The speci-

fication of the stud will normally be produced fromthe group of steels having 55 ton/sq. in. tensile

strength as typical. However, quick release fasten-

ers are available in other alloy steels and wherehigh stressed fasteners are required a toughertype, having 75 ton/sq. in. tensile strength, maybeused. The call for stainless steel fasteners is alsoincreasing and, in addition, fasteners may be pro-duced from phosphorus bronze or brass. The nor-mal fastener material will have a hardness rangeof 262-311 HB, although this may vary dependentupon the make and type of fastener.

This broadly covers the materials from which metalfasteners are produced, although at this stage it

should be stated that a relatively new departure in

the quick release field is to produce fasteners fromplastics. This aspect of quick release fasteners

will be covered more fully later.

It is quite usual to provide some form of protective

finish to the metal parts of most fasteners and this

is generally cadmium or zinc plating with, perhaps,chromate passivation. Other forms of decorative

or functional finishes can be supplied and the mostusual of these is chromium plating.

The selection the fastener finish will, of course,

depend to some extent on the actual application.

Where appearance is important chromium plating

offers an obvious advantage. Where the fastenerwill be subject to weather conditions it is import-ant that the appropriate grade of plating is stated.

The cost of chromium plating can add considerablyto the. cost of the fastener but where the fastener

is visible and is incorporated on an expensive pieceof equipment the cost of this finish may well bejustified. It is perhaps more usual for the headof the fastener to be painted once fitted to the cus-

tomers equipment, thereby blending with the gener-al appearance of the equipment. In this case a

cadmium or zinc plate with chromium passivation

is the most suitable, as this finish provides a goodkey for paint. Furthermore, this finish is relatively

inexpensive and, provided that the correct thick-

ness of plating is applied, will give satisfactory

service under adverse weather conditions. Ofcourse where corrosive atmosphere is likely to beencountered, it is more usual for the customer to

specify stainless steel parts that require no after

treatment.

PRICES

It will be obvious to the reader that as this form of

fastening device offers advantages over permanent,more conventional fasteners, they will be rather

more costly than, say, a screw or bolt and nut.

Furthermore, not only will the piece part cost behigher, but the cost of installation -is likely to behigher. Much has been done recently in an attempt

to reduce the cost of installation. In certain cases

the spring element of the assembly may now be

spot welded or clipped on, whilst the stud itself

can be retained in the unlocked panel by means of

retaining devices which can be hand fitted. A great

deal can also be done by customers themselves in

providing the correct form of tooling (for mount-ing holes, etc. ) where a production run justifies

the initial tool cost.

It is extremely difficult to be specific about the

cost of a quick release fastener assembly, bearingin mind the various sizes and types of fastener

available, and only a rough guide can be given for

the potential user. In its simplest form, the metalquick release fastener assembly may cost as little

as 5d. per assembly when called up in large quanti-

ties. Fastener size, fastener quantity, head style

and finish all play important parts in determiningthe eventual cost of a fastener assembly. In its

most sophisticated form, a complete fastener as-

sembly may cost as much as 30s. each.

Even at the highest level the cost of quick release

fasteners may well be justified by the function they

perform and the eventual time they will save. Asmachinery and equipment becomes more expensiveand sophisticated so the cost of servicing and downtime through machine failure increases. Designersand engineers are increasingly aware of this factor

and are, therefore, able to justify the initially high-er cost of quick release fasteners.

It should perhaps be emphasised at this point that

the main benefits of quick release fasteners, undernormal circumstances, accrue not to the manufac-turer of the equipment in which they are installed

but in fact to the manufacturer's own customers.Thus, forms of quick release device often have asales appeal of their own and have been, on a num-ber of occasions, used as a selling feature for the

end product.

As quick release fasteners are used in such a widerange of industries and on so many different types

of machinery or equipment it will be obvious that

the manufacturer of quick release fasteners re-ceives orders varying in quantity from only a fewto hundreds of thousands. As in many other in-

dustries this creates a number of commercial pro-blems, but in general even very small orders will

be accepted. As in other fields, it is usual to fix

a minimum order charge and this will vary frommanufacturer to manufacturer. In view of the fact

that the commodity is a relatively low cost itemthe minimum charge may be of the order of £1

.

This sum will normally cover supply of fifteen to

twenty standard assemblies or perhaps as few as

ten assemblies where the fastener has some spec-ial feature such as a wing head.

The figure of £1 will normally only apply to stocked

fasteners. A fairly large percentage of fastenersproduced are designed for special applications, andin these circumstances the minimum cost for spec-ials can be higher. In these cases there will usual-

ly be setting-up charges and the minimum ordercharge is likely to be £2 at the lowest, and may

Fig. 5. The Dzus Dart assembly manufactured inacetal copolymer.

even be as high as £5. It should be emphasisedthat the figures quoted in this section are intendedas a guide only and the policy on this particularmatter will obviously vary from manufacturer tomanufacturer.

In addition to the prices discussed above and thevarious guides given on ordering quantities it shouldbe mentioned that under certain circumstances it

will be necessary to make some charge for toolswhere a special fastener assembly is required.Obviously such a charge will only occur where re-latively large quantities are required and tool costscan be negotiated with the individual manufacturershould the occasion arise.

FUTURE TRENDSRecently developed plastics have made possiblefastening devices which were not feasible ten yearsago, and although there are still some limitationsimposed by this material, for example reducedload capabilities, there are now a number of plas-tics quick release fasteners available. The prin-ciple of this new type of fastener is usually basedupon that of the metal fastener, i. e. the fastener studhas projecting members which engage on ramps inthe anchor member itself. Plastics fastenersdiffer from metal fasteners in that the resilientelement is supplied by the characteristics of theplastics, thereby obviating any spring member ofthe assembly. A typical assembly usually cdm-prises the same three basic components, i. e. studretainer and receptacle.

Examples of this type of fastener are the GKNRotolock fastener, the Dzus Dart fastener (Fig. 5)and, in addition, some of the Oddie fasteners areavailable in a combination of metal and plastics.The normal plastics materials used for quick re-lease fasteners belong to the acetal homopolymeror copolymer family.

There are many other trends in the quick releasefield and perhaps the most sgnificant of these isthe attempt to produce a satisfactorily variablegrip fastener. This fastener would retain the vir-tues of quick release and at the same time obviatethe necessity to change fastener lengths with varia-tions in material thicknesses.

If everyad in this book

there d still notbe room for all

the new DzusFasteners.

So we printed ourown book.

Once, there was only one kind of Dzus fastener.The kind everyone knows. The quarter-turn-and-click kind.

Now there's a bookful. Pawl latches. Universalthreaded fasteners. Ejecting fasteners. Panel fast-eners. And more besides.

You need our book like you need the phone book.Right there beside you.So send the coupon.

Send me your big D4 Dzus fastener catalogue.

NameCompanyAddress

DEFH/69

WADzus Fastener Europe Limited

Farnham Trading Estate Farnham Surrey Telephone 4422

96

NOTE S

97

14

Rivets - blind (metal and plastics)

by J.S. Sanders, B.Eng. (Avdel Ltd.)

Blind rivets are so called because they are design-ed to be installed from one side of the work only bya single operator. They find application not onlyfor truly blind situations where access to the rearof the rivet is impossible but also where the workis of such size or shape as to make rear accessat least inconvenient and require a second operator.

DESIRABLE PROPERTIES OF RIVETS

Shear strength. The ability to resist applied shearloads.

Tension strength.

tensile loads.The ability to resist applied

Clench. The ability of a rivet to draw the joint

members tightly together and close any small gapspresent before the rivets are installed. Althoughusually associated with another property known aspretension, it should be distinguished from it.

Pretension. The ability to develop and maintain atensile load in the rivet and hence a compressiveforce on the joint members. This property is bene-ficial in several ways. Firstly, it improves theshear strength of a joint by producing a high fric-

tional resistance at the interfaces of the joint mem-bers. Equally important is the increased resist-ance of the joint to alternating stresses (fatigue).

Ideally, the static tensile stress induced in eachrivet by pretension should exceed the maximumtensile stress value in the alternating stress cycle.If this condition is obtained, the rivets themselvesare not subject to tensile alternating stresses andthe effects of fatigue are avoided.

Grip range . The variation in total joint thicknessin which a rivet can be satisfactorily installed. Awide grip-range is beneficial to the user since it

reduces the number of basio rivet lengths he needsto stock and also reduces the chance of error in

assemblies where more than one basic length ofrivet would otherwise have to be used.

Hole-Fill .. The ability of a rivet to accommodate

its own tolerances and those of the hole, and fill

the clearance between the rivet shank and the hole.Good hole-fill promotes uniform distribution ofshear load between a group of rivets and thus pro-duces a joint with improved proof shear strength,i.e., a higher load may be applied to the joint be-fore a permanent set is produced. It is very de-sirable that the hole filling operation occurs after

the tail expansion and clench phases of the instal-

lation cycle. This enables the very best clenchto take place before the sheets are jammed againstthe expanded rivet shank.

Avoidance of external forces. To obviate the riskof damage to fragile structures, it is importantthat the installation forces and their reactions arecontained in the rivet and its associated installa-tion tool. Blind rivet systems are usually, but notinvariably, designed to achieve this requirement.

Minimum rear protrusion. When used in blind 'box'

sections of limited depth it is important that therivet can be properly seated before the installationcycle begins. It is therefore essential that a blindrivet protrudes by the least possible length fromthe rear of the joint members before installation.

RIVET TYPES -DESCRIPTION

GENERAL

All blind rivets employ a tubular rivet body in someform. The means of expanding the blind side tail

is a convenient method of classification.

Group A. By pulling a stem or mandrel into thehollow body. Virtually, all rivets suitable for air-craft applications occur in this group. This canbe subdivided into:

1. Pull-through.

2. Break-head or stem.3. Self-plugging break-stem.4. Self-plugging lock- stem.5. Tail splitting break stem.6. Screwed stem.

Group B . By pushing a stem or mandrel into thehollow body.

Group C. By detonating an explosive charge withinthe hollow body.

Group A

(1) Mandrel pull-through type. In this rivet, the

bore is reduced in diameter at the tail in the formof a taper such that when a mandrel with an enlarg-ed head is pulled through, the shank is expandedto form the blind tail (Fig. 1). The mandrel is ef-

fectively part of the installation tool and is capableof expanding a large number of rivets. The tool

itself, which may be manually or power operated,contains a magazine of rivets and pulls the mandrelthrough the rivet while reacting on the rivet head.

98

MATERIALSSteel, Monel, Aluminium Alloy, Copper,Pure Aluminium—for in situ anodising after

setting.

BASIC TYPESStandard OpenSealed (pressure tight up to 500 p.s.i.)

Grooved (for soft panels, timber, etc.)

VARIATIONS•$%" to 3" diameters—in a wide range of

lengths to suit any specific application—thussaving cost on excessive metal in oversize

rivets.

Available with Clips — Washers — LargeHeads. Long Mandrels — for use in appli-

cations with awkward access.

SERVICESWe have tools for all services: mechanical,hydraulic, pneumatic — and the onlyelectric blind riveting tool on the market.

Special corner heads and extension nose-pieces are available for applications in-

accessible to ordinary blind riveting tools.

that secure productivity

For industrial fasteners

talk to TUCKERSCatalogue and advice from—

GEO. TUCKER EYELET COMPANY LIMITED WALSALL ROAD, BIRMINGHAM 22B TEL: 021 -356 4811

99

Fig.1 . Mandrel pull-through rivet (Chobertsystem)

.

(2) Break-head or break-stem type. In this type

the hollow rivet is assembled with a headed stem.This stem is formed with a reduced neck or break-notch and projects from the head end of the rivet

to enable it to be gripped by the installation tool.

In operation the tool is engaged with the rivet stemand, by means of suitable jaws, grips and pulls

the stem while reacting on the rivet head. Therivet tail is deformed to produce an enlarged blind

tail. On completion of the cycle the stem breaksat the weakened break-notch and is discarded.

The position of the break-notch determines whetherthe stem head is retained to plug the tail end of the

rivet bore (break- stem) or whether it falls awaywhen the stem is discarded (break-head). The de-

sign of the stem head determines the type of blind

ra ra

Fig. 2. Breakstem rivet ('Pop').

S S

B ,§9

(FORM ASSUMED BY SAME RIVET INDIFFERENT SHEET THICKNESSES)

Fig. 3. Avex rivet.

of higher strength but limited ductility. However,hole fill tends to be incomplete being usually limit-ed to the tail portion.

In the 'Avex' rivet the tail deformation is severe,demanding the use of high ductility material withassociated relatively low strength. Variations insheet thickness are accommodated by an automaticadjustment of the number of tail folds. The tailform also permits good clench action to occur andthe compressive axial forces on the rivet body pro-duce good hole- fill after clench is complete.

Another variation of the 'Pop' rivet (known as the'Imex') is designed for applications where a sealedbore rivet is essential. Here the rivet body is

formed hollow but it is not pierced at the tail (seeFig. 4). In manufacture, the stem is inserted into

Fig. 4. Imex rivet.

Fig. 5. Self-pluggingbreak-stem rivet

(Avdel).

the rivet bore from the head end and the rivet shankis then closed tightly round it. In operation it is

similar to the other 'Pop' types.

(3) Self-plugging break-stem type. This is a twopiece rivet primarily designed for aircraft use. It

is similar to the previous type in that the stembreaks at a predetermined load after the tail hasbeen formed but differs in that the stem is arrang-ed to fill the whole length of the rivet so as to ob-tain maximum shear strength from the materialsemployed. After installation the broken stem is

left protruding from the rivet head by an amountwhich varies with the joint thickness. This excessstem is usually trimmed off flush with the rivethead (Fig. 5). The stem is retained within the rivetby interference forces between the stem and rivetbore.

tail produced. In the 'Pop' rivet the stem headenters the rivet bore which assumes an enlargedtubular form (Fig. 2). In the 'Avex' rivet the stemhead is largely prevented from entering the rivetbore, the tail being thereby folded and compressed(Fig. 3).

In the 'Pop' rivet the amount of tail deformation is

only moderate, allowing the use of rivet materials

(4) Self-plugging lock stem type (aircraft). Thelimitations of the two-piece self- plugging rivetdescribed above are mainly overcome by this fam-ily of rivets which usually consist of three com-ponents: rivet body, stem and locking ring. OninstaUation, the stem is drawn into the body form-ing the tail and plugging the bore as before, but it

is arranged that the stem breaks flush with therivet head regardless of joint thickness. This feat-

100

Fig. 6. Cherrylock rivet.

Fig. 7.

rivet

.

Bulbed Cherrylock

ure is essential to the function of the locking sys-

tem in which a locking collar is forced into suitably

formed recesses in both rivet head and stem. Thusthe stem is subject to a positive mechanical lock

in addition to the purely frictional retention of the

previous type. The time-consuming stem-trim-ming operation is also avoided. The means of ac-

commodating joint thickness variation while main-taining a flush stem break, demands special tech-

niques. Three systems to achieve this are in cur-rent use. In the first, the plugging portion of the

stem, after forming the rivet tail, is reduced in

diameter and elongated as it is drawn into the rivet(Fig. 6).

In the second, the stem head is provided with ashearable ring which is displaced axially a vari-able amount depending on the joint thickness (Fig. 7).

In the third, the rivet tail is designed to fold andcollapse in a controlled manner, the position ofthe fold always being adjacent to the rear of thejoint regardless of thickness (Fig. 8).

(5) Tail splitting break-stem type. Splitting of arivet tail is normally a defect to be avoided or keptto an absolute minimum. In this type of fastener,consisting of two pieces, the tail is deliberatelysplit to obtain a very large tail contact area withthe sheet. This is accomplished by forming aseries of angular projections on the stem head.

On installation, the tail splits into a number of

regular 'petals' which curl round to touch the blind

side of the joint. The stem is also provided with a

series of rolled grooves and the body with a large

diameter head from which projects an integral

sleeve portion. After the 'petal' tail is formed, but

before the stem fractures, the tool, which is pro-

vided with a suitable 'anvile' nose, swages the pro-

jecting collar material radially inward into the stemgrooves providing a positive stem lock. This rivet

is known appropriately as the Daisy (Fig. 9;.

(6) Screwed stem type (Jo-Bolt fastener). In all

previously discussed types the rivet body has of

necessity been made from deformable materialsince the tail has to be formed from it. This fac-tor places inevitable limitations on its strength.

In this type the tail forming member is separatedfrom the body and takes the form of a loose sleeveof deformable material, allowing both body andstem to be made from very high strength materialfor maximum shear strength. The body is thread-ed internally and is tapered at the tail end. Thestem is threaded externally and provided with ashear- neck and driving flais to enable it to be dri-ven by a rotating tool. The body and stem are as-sembled with the cylindrical sleeve as shown in

Fig. 10. Installation is effected by applying a rot-

ary tool which turns the stem while keeping thebody stationary. As the stem moves axially into

the body the sleeve is forced over the tapered endof the body expanding it to form the tail member.

Group B — Stem push types

Because of the unavoidable load applied to the workduring its installation, this type has found little

favour in the metal rivet field. Furthermore, sincethe mandrel is driven in the direction head to tail,

the development of clench and pretension forcesis rather difficult. However, a notable exampleof this type is available in plastics (Fastex). It con-sists of a hollow rivet with a parallel stem mould-ed integrally with it and attached at the head by ashort shear section. The rivet shank is mouldedas a number of prongs splayed out towards the tail.

101

Fig. 10. Screwedstem rivet

(Jo-Bolt).

In operation, the prongs are first closed inwardsas they are inserted into the hole. Installation is

completed using a tool which supports the project-ing stem radially while driving it into the rivet.

The stem is sheared from its attachment and fills

the rivet bore thus expanding the portion of prongswhich project at the rear forming a blind tail (seeFig. 11).

Group C - Explosive rivets

In this type of rivet a small controlled explosivecharge is packed into the hollow bore which is thensealed at both ends. On installation, the charge is

(>> 1 o

Fig .11. Stem-pushrivet (Fastex Rocut).

Fig. 12. Explosiverivet.

usually detonated by the application of heat by some

The tail of the rivet is expanded to a bulbous formand the rivet shank, enclosed by the joint material,expands to fill the hole clearance (Fig. 12).

Fig. 9. Huck 'Daisy' rivet

(front of installation tool

shown in views 2, 3 & 4).

Explosive rivets possess relatively low clench andpretension porperties, and strength is somewhatlimited by the fact that the bore remains perman-ently unfilled.

COMPARISON OK TYPES ANDAPPLICATION SUITABILITY

Pull—through types

This rivet can be manufactured in a wide range ofmaterials provided they possess adequate ductility.

These include aluminium alloys, brass, monel(copper-nickel alloy) and steel. Qualities suitablefor both aircraft and commercial purposes in dia-meters from

,-fe in. to i in. are available. The griprange is normally limited to Ain. which is averagefor a blind fastener. Its basic design can be adapt-ed for unusual or specialised duties. For instance,when provided with external grooves, for rivetingwood or plastics. The shear strength of this rivetis good, particularly when made from steel, monelor high strength aluminium alloy. Typical appli-

cations are illustrated in Figs. 13-16.

The shear strength can be further increased byfilling the bore with interference- fitted pins. It

should be noted that while the rivet can be installedwithout applying loads to the work, the same doesnot apply to the pins which are driven by a hammer.

The tail expansion of this rivet is rather limitedby virtue of its means of operation. This featureis of little consequence in most applications ofreasonable thickness but may cause difficulty injoints of very thin material.

Clench and pretension are very limited in this fast-ener but hole fill is good, being obtained by a con-trolled expansion of the parallel section of rivetbore. This expansion must not be overdone or

These rivets were used for a time in the secondWorld War. They were fired by application of ahot iron. Owing to some uncertainty in operationand the element of danger in manufacture and stor-age, they lapsed into disfavour. Recently, theyhave been reintroduced as a hopper- fed repetitionsystem for which they are very suitable. The rivetis rapidly heated electrically by a current obtainedfrom contacts in the tool. The operational reliabi-lity of this type has been much improved and offerssome advantages over other repetition systems.These include simplicity and compactness in de-sign of the installation tool and ability to reachvery difficult situations.

Fig. 13. Blind rivet-

ing application on atubular chair. (Bycourtesy of Avdel Ltd.).

102

Fig. 14. Riveting commercial vehicles panels

using a pneumatic magazine loaded placing tool

(By courtesy of Avdel Ltd.).

Fig. 16. Panelling being attached to hangardoors (By courtesy of Avdel Ltd.).

Fig.15. Lighting channel attachments being

placed on site with a hand operated magazineloaded placing tool (By courtesy of Avdel Ltd.).

there is a possibility of introducing defects knownas sheet separation and head retraction. In the

first, an excess of rivet body material is forced,

between the joint members, thus driving themapart. In the second, the excess of material ap-

pears as an axial extension of the rivet shank sothat the rivet head is lifted off the sheet as the

mandrel is withdrawn. The function of this rivet

is not affected by the use of sealants in the joint

construction.

As regards installation, this rivet is very suitable

for use in magazine loaded tools designed for rapid

repetition riveting. For this purpose the rivets

are packed end-to-end in 'pods' so that reloading

the tools is simple and rapid. Tools are available

either pneumatically power operated for 'factory'

use or in rotary manual form for use on 'site' workwhere pneumatic power may not be available. Forreaching difficult situations, a hand plier tool is

available for 'single- shot' riveting.

Mention should be made of the desirability of using

riveting clamps, particularly for high quality work.

These are available in types to suit work of vary-

ing stiffness and thickness. A simple type is shownin Fig. 17. The central claw is pushed through the

hole and hooked behind the rear sheet. The nut is

then tightened to clamp the joint members tightly

together. Clamps are normally applied to alter-

nate holes while the vacant ones are riveted. Theclamps are then removed and riveting completed.

A typical view of clamps in use appears in Fig. 18.

Break—stem types

Good clench and hole -fill, a large tail and fairly

low strength characterise the 'Avex' rivet. It is

103

Fig.18. Riveting clamps in use prior to rivet-ing an aircraft component (By courtesy ofAvdel Ltd.).

available only in aluminium alloy in diameters fromi in. to | in. It has a wide grip range (Sin. ). It is

very suited to thin sheet applications but is equallysatisfactory at the thick end of its grip range. It

can be easily removed for repair work by drillingoff the rivet head. The hole-fill feature holds therivet against rotation whilst drilling.

The 'Avex' rivet is comparatively insensitive tohole size and will accommodate the irregular andoversize holes often produced by unskilled labour.

Break- stem rivets of all types find wide use in lowand medium strength applications which includevehicle bodies, garage doors, wall cladding andducting (Figs. 19 and 20).

Rather better shear strength is obtainable fromthe 'Pop' rivet which is available in a wider selec-tion of materials including aluminium alloy, steel,monel, stainless steel and copper. It is availablein diameters from k to 1 in. and a grip range whichvaries from ito i in. approximately depending ondiameter.

Both types may be rapidly and efficiently installedby power tools usually of the pneumatic -hydraulictype. Hand pliers are available in many formsand may be employed for difficult-access positions

or small volume work. The use of sealants doesnot affect these types.

Self-plugging break-stem

This is a high strength rivet available in aluminiumalloys, corrosion resisting steel, and titaniumalloy in diameters from * to ft in. The grip rangeis of the order of Ain. It is usually limited to air-craft use, but has been employed on commercialprojects where special requirements have to bemet or arduous environments withstood. For in-stance, corrosion resisting steel rivets have beenapplied to food machinery and chemical plant aswell as to high speed aircraft. Titanium rivetshave solved fastener problems on atomic reactorcomponents where erosion from high intensity radi-ation and elevated temperatures is severe. Theyalso find an important duty in advanced aircraftstructures because of their high strength to weightratio.

Lock-stem types

These find applications almost exclusively in theaerospace industry. They are designed to meetthe stringent requirements laid down in US stand-ards. The 'Cherrylock 2000 Rivets', 'CherrylockBulbed Rivets' and 'Huck Blind Bolts' are all install-ed by a special tool incorporating a 'shifting head'.This is shown in operation in Fig. 21. It will benoted that the reactive load is applied to the rivethead in the initial stages of installation. When thestem has reached its final position in the rivet, theshifting head transfers the reactive load from therivet head to the locking collar which is thereforedriven home into its recess. The stem is finallyfractured flush with the rivet head to complete theinstallation cycle.

The 'Cherrylock 2000 Rivet' is available in severalaluminium alloys, monel and precipitation harden-ing steel. The relatively small blind tail is a dis-advantage in very thin sheet but in all conditionsthis rivet has good clench and hole fills well.

The bulbed version has a much larger blind tail,presents a much larger bearing area to the blindside of the joint and therefore is very suitable forthin sheet applications. Hole filling tends to be

Fig. 19. Avex rivetsbeing placed into com-mercial vehicle panel-ling by a hydro-pneu-matic hand tool (Bycourtesy of Avdel Ltd

. )

.

Fig.20. Fan rotorblades attached byAvex rivets (By court-esy of Avdel Ltd

.)

.

104

JAWS

JAW HOUSING-

NOSE CAS1NG-

Fig.21 . OpeTation of shifting head in conjunction

with lock-stem rivet (Huck blind bolt).

less complete however since this property depends

in this case on axial compression of the rivet with-

in the hole. Bulbed rivets are available in alum-

inium and monel.

The "Huck Blind Bolt' is available in alloy steel in

diameters from &to i in. with grip range of A in.

'Huck Blind Rivets' based on the same principle are

available in aluminium alloy, monel and precipita-

tion-hardening stainless steel in diameters from

*to ft in. The grip range of the rivets is rather

limited at approximately a in.

Tail—splitting break-stem

'Jo- Bolts' are usually manufactured with the body

and stem in high tensile low alloy steel or with an

aluminium alloy body and low alloy steel stem both

in conjunction with a collar in 18-8 stainless steel.

For corrosive conditions, 'Jo- Bolts' have also been

made in limited quantities in martensitic stainless

steel, again with an 18-8 stainless steel sleeve.

Stem-push types (drive-pin rivets)

The drive- pin rivet is currently limited to the plas-

tics version. Metal types are likely to be intro-

duced in this country shortly. Plastics rivets are

available in a variety of materials, the most pop-

ular probably being nylon. They are currentlyused in situations where high strength is unneces-

sary and where freedom from corrosion, chemical

inertness or electrical insulation is a vital factor.

Typical applications are therefore found in internal

fitments on refrigerators, trim and accessory at-

tachment on motor vehicles, and panel and com-ponent assembly on electronic equipment. Nylon

has the property of absorbing appreciable amountsof moisture from its environment which has the

effect of lowering its electrical insulation and pro-

ducing dimensional instability. Where these fac-

tors are important, other plastics may be chosen.

Acetal resins, for instance, offer superior dimen-

sional stability and reduced moisture absorption.

Polystyrene and polyethylene offer superior insula-

tion properties. Polyethylene is much more flex-

ible than polystyrene which tends to be brittle.

The split tail rivet is specifically designed for

riveting thin sheet members together. Its large

bearing areas both on the front and blind sides en-

sure wide distribution of clamping loads. This

enables not only metal to metal joints to be madebut also between plastics, rubber or plywood and

metal. A synthetic rubber washer can be added

under the head of the rivet to weatherproof the

joint. It is available either in aluminium alloy or

steel in one diameter only (&in. ). The grip range

is large at approximately | in.

Typical applications include attachment of corru-

gated roof and wall cladding, lining of containers

with plastics foam sheeting and ducting.

Screwed stem type

This fastener is most often employed in aircraft

construction. It offers excellent shear strength and

clench and has good tension properties. No hole-

fill need be expected since the body and stem are

in high- strength alloys and remain undeformed.

For best joint strength, therefore, good quality

close tolerance holes are essential. A typical

structural joint is shown in Fig. 22.

While many blind rivets will tolerate conditions

slightly beyond the recommended grip limits, this

fastener is very sensitive to this kind of error.

Problems are liable to arise if careful considera-

tion is not paid to this matter. The lavish use of

sealants may also cause difficulty and interfere

with the proper expansion of the sleeve member.

MATERIAL AND FINISH SELECTION

The choice of material for rivets is governed by

strength, corrosion, environment and cost con-

siderations.

The strength properties of a rivet will de dependent

to some extent on the strength of the materials it is

intended to join. It is a usual, though not invari-

able, rule to select a rivet of somewhat higher

strength than that of the sheet. The most economicdesign is often the one where rivet and joint mat-erials have similar ultimate strength.

Fig. 22. Screw stem rivets (Jo-Bolts) being

placed into an aircraft wing structure (By court-

esy of Avdel Ltd . )

.

105

Compatibility of rivet and sheet from the corrosionaspect must also govern the choice. It is obviouslyof little satisfaction to a consumer of a riveted pro-duct to have the rivets in a perfect state of preser-vation while the adjacent joint material is severelycorroded as a result of electrochemical action. Incases where a rivet is desirable from strength con-siderations but is incompatible with the joint mat-erial, a solution to the difficulty can often be ob-tained by plating the rivet with a suitable metal.This should be chosen to have an electrode poten-tial intermediate between those of rivet and jointmaterial. For instance, if it is desired to rivetaluminium or magnesium alloy sheet with uncoatedstainless steel rivets, we have an unsatisfactorycombination from the corrosion aspect due to thelarge difference in electrode potential between rivetand sheet. Plating the rivet with cadmium provid-es a zone of intermediate potential and the corro-sion tendency is reduced to an acceptable level.Cadmium plating is, however, usually limited toaircraft and special applications due to its relative-ly high cost. Zinc plating is usually applied torivets for commercial use where necessary fromthe corrosion aspect, as its basic cost is of the ord-er of Aith of that of cadmium and for many environ-ments offers results almost as good as cadmium.For fasteners subjected to elevated service temp-eratures, silver plating is employed in place ofcadmium due to the low melting-point limitation ofthe latter.

FUTURE TRENDS

In the commercial field, future development islikely to be directed mainly towards the means ofinstallation. Detail refinements in rivet designare, of course, continuously being made but 'major-breakthrough' advances involving completely new

principles are unlikely. This is mainly becauseonly a limited number of basic blind rivet princi-ples are possible, and these have already beenwell explored.

There is, however, wide scope for improvementin installation tools. Faster, more efficient eco-nomically designed tools will be needed in the driveto increase productivity, improve operator com-fort and reduce operator fatigue.

Still more advanced is the continuous hopper-fedriveting machine several versions of which haverecently appeared. This concept is capable ofbeing extended to a completely automatic assemblysystem dispensing with the human operator entirely.

In the aircraft field where cost is less of a con-sideration, the demands of the aerospace industrywill require the exploitation of very expensive andsophisticated materials to satisfy the very severestructural and environmental conditions. Thesematerials will probably include precipitation hard-ening stainless steels, the 'multiphase' alloys,titanium alloys, beryllium alloys and perhaps evenceramics.

As the reader may have noted, the perfect blindrivet with all desirable features embodied in asingle design, has so far eluded inventors and re-mains to be developed.

ACKNOWLEDGMENT

The author wishes to thank his colleagues, F. A.Summerlin (Chief Engineer, Avdel Ltd) and G. R.Russell (Standards Engineer, Avdel Ltd) for theirassistance in the preparation of this Chapter.

106

much more thanjustfasterfasteningAvdel offers you a major breakthrough in fastening—in

production cost, in time, in quality.

Avdel—the most sophisticated advance in industrial fastening

techniques in the last 1 00 years. Yet simple to incorporate in any

production system in industrial fabrication or mass production

assembly. And simple to operate. Because Avdel systems can be

operated with 1 00% consistent quality—even by unskilled, semi-

skilled and female labour.

The increased speed and quality, the decreased cost inherent

in Avdel systems are made possible by the use of brilliantly simple

tools that eliminate operator errors. Write to us for further details

on any industrial fastening system. Avdel industrial fastening

systems.

INDUSTRIAL FASTENING SYSTEMSAvdel Limited. Welwyn Garden City. Hertfordshire.

Telephone : Welwyn Garden 281 61 Telex : 24254 Cables : Avidev. Welwyn Garden.

107

15

Rivets - solid and tubular

by J.M.A. Paterson, M.A. , J.P. (The Bifurcated and Tubular Rivet Co. Ltd.)

Firstly, we should consider the various types of

rivets which are available to industry today.on the bulkiness of the component and the size ofthe rivet to be set.

2.

3.

4.

5.

SOLID RIVETS

The solid rivet has been in use for many hundredsof years. In this country, a standard range of theserivets is covered by BS641:1951, which coversrivets from £in. diameter. The rivets listed inthis Standard are sub-divided by head styles, asfollows

:

1. Snap or round head rivets

Pan headMushroom headFlat headFour types of countersunk head rivets, with

angles of countersink ranging from 60° to 140°.In addition there is a table which governs the di-mensions of countersunk head reaper rivets.

The Standard covers rivets made from mild steel,

copper, brass and a range of aluminium alloys andpure aluminium as specified in BS1473:1955.

Components to be riveted with solid rivets requirea hole to be punched or drilled, prior to the rivetbeing inserted.

With regard to types of equipment for setting solidrivets, the majority require the rivet firstly to beinserted by hand, and then clinched by one of thefollowing methods:

a. A hammer and snapb. A portable pneumatic percussion tool

c. A portable pneumatic or hydro pneumaticsqueeze riveter

d. Under a light press fitted with suitably profiledrivet snapse. A hand feed bench riveting machinef. An automatic feed rivet setting machine.

Generally speaking, it is only possible to use meth-od (f ) provided the component can be taken to themachine. The choice of riveting equipment depends

The advantages of using solid rivets are (i) thatthey are cheap to manufacture, and (ii) that it is

possible to use one length of rivet for a fairly widevariation in the thickness of the components to beriveted, as any excess metal can be squeezed outto form a larger or smaller clinch as the case maybe. The disadvantages are (i) that, without care,one is apt to get a rather untidy clinch, (ii) con-siderable force is required to set the rivet, and(iii) except in those cases when an automatic feedmachine is used, the time cycle to insert and setone of these rivets is considerably in excess ofthat when tubular, semi-tubular or blind rivetsare used.

Probably the greatest user of solid rivets today isthe aircraft industry, where speed of riveting isnot of paramount importance, but where a goodfinish is required, certainly as far as the outsideskin is concerned.

When setting solid rivets, the following pointsshould be borne in mind to ensure the best results:

a. Rivet support - for good results, rivets shouldbe well supported by material of equivalent strength.b. Hole clearance - keep to the absolute minimumto avoid sloppiness (which results in a smaller.clinch and poor finish). The recommended clearan-ces, where condition - permit, are shown in Table 1.

c. Rivet length - ensure that the rivet is of correctlength for thickness of work and form of clinch re-quired. Snap clinch rivets are most commonly usedand the correct protrusion for these is li times thediameter.d. Rivet clinch form - the selection of rivet clinchcan determine the size of the riveter. On alumin-ium alloy, for example, taking the snap clinch asa factor of 2, the relative squeezing pressures re-quired for alternative clinches are:

Flat clinch 1

North American cone clinch 1.

1

Rivet diameter Hole diameter Rivet diameter Hole diameter Table 1 . Recommendedclearances

.

ins

.

mm. ins

.

mm. ins

.

mm. ins

.

mm

.

iM 2.38 0.096 2.43 i 6.35 0.257 6.52i 3.17 0.128 3.25 i.

16 7.93 0.316 8.02a.M 3.96 0.159 4.03 1 9.52 0.386 9.80ft 4.76 0.191 4.85 1

7 12.70 0.516 13.107 5.55 0.221 5.61

108

Halfthe television sets in Britain havea built in commercial forB&TR

This special purpose

machine, widely used in

the electrical industries,

is primarily intendedfor

setting small electric

contacts.

Sn>

It doesn't show on the screen, ofcourse. But the set reliability

you take for granted owes a good deal to the assembly andfastening methods devised by B & TR in collaboration withleading television set manufacturers.

Throughout industry, you'll find the experience ofthe

Bifurcated and Tubular Rivet Company making for moreefficient, more economical and quicker fastening and assembly

on every kind ofjob from motor cars to micro-switches. Ifyourproduction process means fastening one thing to another, youcould benefit from B & TR's skill and experience. They don't

simply make rivets— they design and manufacture complete

rivet setting systems tailored to give you the fastest, mostefficient assembly or fastening method for your especial needs.

They've been doing it for years : the experience they've built

up is yours for the asking.

Write or 'phone for technical

literature, or detail your problemand let us devise a solution.

THE BIFURCATED AND TUBULAR RIVET CO. LTD.Aylesbury Bucks Telephone: Aylesbury 5911 Telex: 83210

Countersunk 60° clinch 1. 7

Pan clinch 1. 8

Thus, to form a snap clinch requires twice the loadof a flat clinch.

e. Rivet snaps - take care to provide snaps wellfinished to the correct form.

TUBULAR RIVETS

Solid rivets only were available until 1874, whenan American, Mellen Bray, patented the soliddrilled tubular rivet. This was, to all intents andpurposes, a solid rivet which had a hole drilledup the centre of its shank (see Fig. 1). The idea

SOLID DRILLED TUBULAR RIVET

Fig.1

.

was to produce a rivet which was self piercingthrough leather and similar materials. The rivetwas driven straight through the leather and clinch-ed in one operation, the slug of the material beingriveted being retained in the bottom of the hole.

This speeded up the operation very considerably,compared with the use of solid rivets, where first

a hole had to be punched in the material, the rivetinserted, the work turned over, and a washer plac-ed over the projecting portion of the shank, whichwas then clinched by means of a hammer and snap.

Today, the principal use of tubular rivets is for

riveting components which are apt to vary in thick-

ness, or when the rivet is unsupported by the com-ponent, and is, therefore, apt to buckle when beingset. An example of the former case, and whereprobably the greatest number of tubular rivets is

used today, is in the riveting of friction linings to

brake shoes, where the brake shoe is apt to varyin thickness from end to end. The rivet accommo-dates this by the formation of a larger or smallerroll when forming the clinch.

An example of the latter application is in the as-sembly of folding tubular furniture, where the tubu-lar components have to swivel one on the other.Here a rivet is required which can be set to givea sufficiently large clinch without the setting forcecausing the rivet to buckle and lock the componentstogether.

Standard ranges of tubular rivets are covered bytwo British Standard Specifications. Part II of

BS1855:1952 covers the dimensions of oval head,flat countersunk head and flat countersunk bevelhead solid drilled tubular rivets, with shank dia-meters &in. and No. 9|- gauge. Tubular rivets

Fig. 2.

TJ @D^(a) A SOLID DRILLED (b) CROSS SECTION

TUBULAR RIVET OF MATERIAL ANDCLINCHED RIVET

Cc) PLAN VIEW OFTHE CLINCHSHOWING ROSE-CUT ROLL

H(a) A SOLIO DRILLED (b) GROSS-SECTION (c) PLAN VIEW OF

TUBULAR RIVET OF MATERIAL AND CAP IN POSITIONWITH AN'IDEAL'CAP RIVET SET IN ANBEFORE SETTING 'IDEAL'CAP

Fig. 3.

used for the attachment of friction linings are cov-ered by BS3575 : 1963. In addition to specifyingthe dimensions, materials and recommended holesizes for the rivets, it also specifies the correctrivet hole sizes for the components being rivetedtogether.

Equipment used for setting tubular rivets is thesame as listed for solid rivets, but due to the factthat the shank of the rivet is now hollow, consider-ably less power is required to form a satisfactoryclinch. For this reason, equipment which is con-siderably lighter and, therefore, cheaper, can beemployed. Another advantage of having a tubularshank is that it is possible to design a setting toolor snap which can locate in the hole and roll theclinch into a uniform shape, thus avoiding the dis-tortion often experienced with solid rivets.

When setting a tubular rivet, the clinch can beformed either into a plain roll or rose- cut, by em-ploying a suitably profiled anvil (see Fig. 2). Wherea particularly smooth finish is required, it is alsopossible to set the tubular rivet into a cap (Fig. 3).

Naturally a tubular rivet is considerably more- ex-pensive than a solid rivet, due to the drilling oper-ation which has to be performed, but this is usuallyoffset by the increased speed of riveting, coupledwith the fact that the resultant clinch is neater.

BIFURCATED RIVETS

This rivet was first produced and patented in theUSA in 1889 by Jacob J. Unbehend. It is producedby cutting a tapered section out of the centre of theshank of a solid rivet (see Fig. 4a). It is princi-pally used where the rivet is able to penetrate thematerials to be riveted together, and unlike theoriginal use of the tubular rivet, it can pierce thecomponents without removing any of the material,thereby unimpairing its strength. It can be driventhrough the material using a hammer, while hold-ing the rivet with a specially formed wire clip.When the prongs of the rivet have pierced the mat-erial, they are clinched by hitting them with a ham-mer, while the head of the rivet is supported onthe hard surface. The normal method of settingthese rivets, however, is to use a hand or auto-matic feed rivet setting machine, which drives the

Fig. 4.

I a d

'*' oiSft^ ATED 0>) CROSS-SECTION (c) PLAN VIEW OF THERIVET OF MATERIAL AND CLINCHCLINCHED RIVET

110

rivet through the work and clinches it in one single

operation, using a specially profiled solid anvil

which turns the prongs of the rivet outwards andbackwards into the face of the material (Fig. 4b).

Though more expensive than a solid rivet, a bifur-

cated rivet is very much cheaper than a drilled

tubular rivet. Its principal use is in the manufac-

ture of travel goods of all types, fibre and leather

articles and the assembly of plywood containers

with terneplate angle pieces on the corners. It is

also used for riveting terneplate handles on to chip

baskets, as .a normal bifurcated rivet can easily

penetrate this.material.

f roi(a) A BIFURCATED 0>) CROSS-SECTION

RIVET WITH AN OF MATERIAL'IDEAL' CAP AN° RIVET SETBEFORE SETTING IN AN 'IDEAL' CAP

(c) PLAN VIEW OF CAPIN POSITION

Fig. 5.

If a particularly good finish is required on the side

of the clinch, the rivet can also be set into a cap,

as with the tubular rivet (see Fig. 5).

The standard range of bifurcated rivets is listed in

BS1855:1952, Part 1, covering rivets from No. 3

gauge to No. 16 gauge with oval, flat countersunk

and flat countersunk bevel heads.

SEMI-TUBULAR RIVETS

This rivet was first introduced by the Tubular Rivet

and Stud Co. of America around 1929, when the

manufacture of light metal parts began to develop

in a large way, and mass production techniques be-

gan to extend to all types of industry. Since that

time, the use of the semi-tubular rivet has beenextended to the assembly of components made of

plastics, ceramics and other materials which canbe produced by moulding or die casting. Since the

holes can be drilled, punched or moulded in the

material before riveting, and the thickness of the

components can be kept to fairly close limits,

there is no need to drill such a deep hole as in

the solid drilled tubular rivet. Semi-tubular rivets

are usually manufactured with one or two types of

tapered hole, the depth of hole varying from 80 to

100 per cent of the shank diameter, according to

requirements.

It will be seen from Fig. 6 that when the rivet is

clinched, the tubular portion is rolled back, leav-

ing a solid shank to give maximum shear strength,

similar to that obtainable with a solid rivet. Whensetting the rivet, in addition to rolling back the

tubular portion, the solid shank of the rivet is made

tr

Fig. 6.

(a) SEMI-TUBULAR (b) CROSS-SECTIONRIVET OF MATERIAL AND

CLINCHED RIVET(c) PLAN VIEW OF

CLINCH

to swell and thus completely fill the hole in the

components being joined together.

Semi-tubular rivets can be set by any of the meth-

ods previously listed for setting solid rivets, but

again much lighter automatic feed equipment can

be used, owing to the fact that the clinching force

required to roll back the tubular portion is muchreduced. Consequently, the semi-tubular rivet is

very suitable for setting by means of an automatic

feed rivet setting machine, where very high speeds

of assembly can be obtained. On straightforward

work an operator can set as many as 3000 rivets

an hoUr. The standard range of semi-tubular rivets

is covered by BS1855:1952, Part III, which gives

Table 3

1

•

\_x\\\W V//////s,

H

\

CLINCHING FORCE (LB.)

Rivet Rivet

gauge Steel Brass Copper Aluminium 2.69 gauge

18 220 160 120 15 18

17 280 240 175 27 17

A 365 330 233 40 160 &16 405 375 263 50 16

15 500 460 320 70 15

141 580 530 375 90 141

14 705 640 445 120 14

13 800 713 500 145 330 13

12 1040 940 645 225 12

11 1310 1180 810 322 11

i 1420 1285 880 363 599 i

10 1510 1365 940 400 10

91 1860 1680 1140 545 91

9 2140 1940 1320 670 890 9

8 2440 2200 1490 792 8

7 2770 2500 1690 940 7

SL10

3170 2860 1930 1103 10

6 3580 3220 2200 1270 1325 6

5 4130 3725 2565 1500 5

4 5000 4540 3180 1895 4

3 5560 5050 3600 2145 2340 3

Note: The above figures are values obtaine d Fromactual tests on un-heat-treated rivets not.es<ceed-ing £ in. length.

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the dimensions of oval and flat countersunk headsemi-tubular rivets, from ,$in. to No. 16 gauge.

Today, machine riveting with semi-tubular rivets

is a very simple process, but there are a few es-sential details which should be fully understood if

good results are to be obtained and high rates of

production maintained. For example, it is essen-

tial to have the correct diameter of hole in the com-ponents to be riveted, and also to use the correct

length of rivet. The diameter of hole is most im-portant, so many designers have the mistaken im-

pression that a hole which will just take the rivet

shank, as for solid riveting, is satisfactory, but

this is definitely not the case. The rivet holes mustbe made large enough to permit the tubular end of

the rivet to roll around when being set and, at the

same time, allow the shank to expand to fill the

hole exactly. If the holes in the components are

made too small, the rivet cannot roll, and the re-

sult is usually one where the rivet is half clinchedand the head stands proud on the other side.

The length of the rivet to be used must be equal to

the total thickness of the components being riveted

together, to which is added a certain riveting allow-

ance, which varies according to the shank diameterand the material from which the rivet is made.

Table 2 gives details of hole sizes and riveting al-lowances for rivets ranging from No. 18 to No. 3

gauge inclusive.

It is sometimes of interest to know the clinching

force required when setting semi-tubular rivets

other than by means of specially designed rivet

setting machines, e. g. a press fitted with profiled

setting tools. Table 3 gives details of clinching

forces in pounds required to set steel, brass, cop-per and aluminium rivets from No. 18 to No. 3

gauge inclusive.

113

16

Screws - machine

by D.S.Thompson (GKN Screws & Fasteners Ltd.)

The machine screw fastener is certainly one of thesimplest and cheapest methods for joining partstogether. Despite the introduction of alternativeand more sophisticated fastening techniques, itsusage is still increasing and somewhere in the reg-ion of 5000 million machine screws are used eachyear in the UK in nearly every type of industry.

GENERAL DESCRIPTION

A machine screw consists of a shank, which is

threaded, and at one end of the shank - a headwhich is equipped with a means of driving. It is

surprising that from this simple design such a widerange of combinations of shank size, thread type,head style and method of driving should have evol-ved. Such combinations run into several thousandsand present an immense variety of problems tomanufacturers and users.

The prime function of a machine screw is that it

should be capable of securing a component in place.

It is not often used to its maximum mechanicalstrength or to perform a multiplicity of functions.

The fundamental requirements are therefore:

a. Inexpensive.b. Easily obtainable.

c. Suitable quality to mate with internal thread.

Table 1 List of British Standards .

BS NO. Title

84

9315803643

57*

450*

1981

3155

4183

1083*

1768

3692

Parallel screw threads of Whitworthform.BA screw threads.Unified screw threads

.

ISO-Metric screw threads.

BA screws, bolts and nuts.Machine screws & machine screwnuts (BSW& BSF).Unified machine screws, machinescrew nuts Qi in. dia. & larger).American machine screws & nuts(size below M in. dia.).

Machine screws & machine screwnuts - Metric screws .

Precision hexagon bolts, screws,nuts (BSW& BSF).Unified precision hexagon bolts,

screws, nuts (UNO & UNF).Dimensions of ISO-Metric precisionhexagon bolts, screws & nuts.

*BS57, BS450 and BS1083 were renderedobsolete in 1966.

d. Capable of being driven easily, safely and ac-curately.

e. Capable of withstanding the environmental con-ditions.

Provided the machine screw can meet these re-quirements it will invariably prove superior eitherin function or cost to other screw fastening systems.Its main disadvantage is that it requires to matewith an internal thread to complete the assembly,and with the large range of thread types in use, mis-match can occur; also tapping is an expensive pro-cess. The development of self-tapping types ofmachine screw now provides, in many cases, amore suitable method of assembly.

British Standards

Screw threads and machine screws are producedto the British Standards shown in Table 1.

FACTORS INFLUISCREW DESIGN

NCING MACHINE

The main factors influencing machine screw de-sign are:

1. Threads.2. Heads.3. Method of driving.

4. Point.

5. Length.

6. Material and mechanical properties.7. Tightening torques and clamping load.

8. Protective and decorative finishes.

9. Availability.

Threads

There are now 6 basic thread types in use in theUK - BSW, BSF,BA,UNC, UNF.ISO-Metric. Othercountries do not suffer from this problem and it is

necessary to rationalise these thread styles in theUK to maintain a reasonable price structure forthese products.

The screw thread is largely attributed to HenryMaudsley and the first attempt at standardisationwas by Whitworth in the middle of the nineteenthcentury. The British Standard Whitworth (BSW)thread form has been predominantly used in theengineering industry in the UK and the metric di-mensioned BA thread form was largely adopted bythe scientific and instrument industries and later

114

Table 2.

ISO-UNIFIED THREAD DIAMETER AND T.P.I. ISO-METRIC THREAD DIA. & PITCH

Diameter inches Equiv

.

T.P.I. T.P.I. DIAMETER INCH. PITCH EQUIV.mm. (Fine) (Coarse) Equiv

.

(mm). T.P.I.

0.060 1 .52 80 —

1 0.072 1 .83 72 642 0.085 2.16 64 56 M2.5 0.098 0.45 56.53 0.098 2.50 56 484 0.112 2.85 48 40 M3. 0.118 0.50 50.86 0.138 3.50 40 328 0.164 4.16 36 32 M4. 0.157 0.70 36.2

10 0.170 4.80 32 24 M5. 0.197 0.80 31 .8

1 0.250 6.35 28 20 M6. 0.236 1 .00 25.4,-1 0.3125 7.94 24 18 M8. 0.315 1 .25 20.31 0.375 9.52 24 16 M10. 0.394 1 .50 17

Table 3

.

BA No. 8. 7. 6. 5. 4. 3. 2. 1 . 0.

Recommended ISO-Metric Size M2. M2.5 M3. M3. M4. M4. M5. M5. M6.

made standard in the electrical industry. In the

USA the American National series thread form wasused, and as trade between the UK, USA and Canadadeveloped the need for a common thread standardwas apparent. A Unified thread form was first pub-lished in 1949 (BS1580) by the three ABC countries.

This standard was based on compromises betweenthese countries and was subject to some criticism

resulting in a modified standard which was issuedin 1953. This standard has now been recognised bythe International Organisation for Standardisation(ISO) as an internationally accepted screw threadstandard based on the inch system and can thus becalled the ISO-inch series. BS1580 was re-issuedin 1962 and now meets the ISO requirements. Forvarious reasons, the Unified series did not replaceexisting BA, BSW, and BSF thread forms for machinescrews except in such industries as the motor tradeand certain manufacturers of consumer durables.In 1965, following recommendations by Industry to

the Government, a change to the Metric system wasannounced and therefore the adoption of an ISO-Met-ric thread form. BS3643 Part II was issued in 1966providing a thread standard for a coarse threadseries in Metric terms and in the same year .the

British Standards for BA, BSW and BSF threadforms were rendered obsolete. It is expected that

by 1970, Industry will have commenced changingspecifications to Metric dimensions and will havecompleted 7S per cent of the changeover by 1975.

Industry will thus be provided with 2 basic threadforms:

ISO - Metric and

ISO - Inch (Unified)

These two standards will provide complete inter-

changeability throughout nearly the entire worldpopulation and all new designs should now be basedon these thread systems. The recommended sizes

for substituting BA threads with ISO- Metric threadsare as shown in Table 3.

Head styles

Logically, head styles fall into two categories:

a. Those that fit flush with mating component (i. e.

countersunk heads) and where the clamping load is

developed against the flank of the head.b. Those that have a flat underhead condition

against which is developed the clamping forces.

Originally machine screws were manufactured byauto-machining methods from a round bar and thus

a cheese head form became most economic to manu-facture. Machine screws larger than M2 or 2-UNare now generally cold forged from wire stock andthus the economic design of the cheese head is nolonger applicable, however, its usage has only

slightly decreased. Tradition dies hard and yet the

cheese head screw shape is the least satisfactory

for cold forging, except for the tapered cheesehead form which is now standard practice for ISO-

Fig. 1. Basic ISO thread profile. This thread

profile is common for ISO-Unified and ISO-Metric]screw threads.

H=0.86603PH/4=0.21651 PH/8=0.10825P

3/8H=0.32476P5/8H=0.54127P

115

4 6 8 10

III I

16 8 UNIFIED SERIESH

rt H—r- -+-M2.5 M3 M4 MS M6 M8 M10 M12

METRIC SERIESSCREW DIAMETER

Fig. 2. Diameter - pitch comparison. ISO-Metric coarse v Unified.

Metric screws. Whilst the cheese head screw is

not recommended, due solely to its large usageit still remains a 'preferred' type for pricing pur-poses.

Countersunk heads. These heads differ in the angleof countersink as follows:

BA, BSW, and BSF screws - 90°/ 92°

ISO-Unified - 80°/82°ISO-Metric - 90°/92°

The basic requirement for countersunk head screwsis that the head should fit into the countersunk holewith as great a degree of flushness as possible. It

is therefore necessary to control the dimensions of

the head of the screw and the countersunk hole with-in prescribed limits.

British Standards specify 'round heads' for BA,BSW and BSF threads and 'pan heads' for BSW andBSF, ISO-Unified and Metric series. The roundhead is composed of 2 radii, whereas the pan headis flat and parallel to the base of the head, beingradiused at the edges. The only British Standardto incorporate both round and pan is BS450 for

BSW/BSF screws, however, the most commonlyused style is round head. Although not incorpora-ted in BS57 for BA screws, pan heads, sometimescalled 'binder heads', are also used. Round headsare also used with ISO-Unified screws.

With the recess head, a combination of round andpan is used for all thread types and is termed apan head (Fig. 7).

Fig. 5. Raised countersunk head .

(a) ROUND > IEAD (b) PAN HEAD

Fig. 6.

Thus, the standard descriptions are as shown inTable 4, and where iwo styles exist the predomi-nant one is underlined. With the rationalisation of

thread types to ISO-Unified and ISO-Metric, thepan head style only will exist.

The maximum or design size of head is controlledby a theoretical diameter to a sharp corner D andthe minimum head angle, i. e. 90°. The minimumhead size is controlled by a minimum head dia-

meter d, the maximum head angle, i. e. 92°, and aflushness tolerance. Fig. 3 shows the maximumand minimum metal conditions that can exist. TheThe edge of the head may be flat or rounded as shownin Fig. 4. The flat portion is referred to as theland and is required for cold forged heads.

A variation of countersunk head is the 'raised coun-tersunk head' (Fig. 5). This is sometimes called an'instrument head' and has an improved appearanceand greater slot depth or recess.

Round and pan heads . These are the remainingcommon head styles. Confusion again exists onterminology and the following notes will clarify thestandard description. For slotted heads, the

FLAT EDGE ROUNDEDEDGE

FLUSHNESSTOLERANCE

(MAX.)

Fig .3.

MAXIMUMCONDITION

MINIMUMCONDITION

^ VFig. 4.

Slotted or recessed screws. The slotted machinescrew suffers several disadvantages:

1. A multitude of slot widths, depths and lengths,

requiring many sizes of screwdrivers.2. Usually has 'burrs' present, which can disruptautomatic assembly.3. Screw head can suffer considerable damageduring driving.

4. Screwdriver blade can slip out of head thusdamaging surrounding surfaces.5. Difficulty in transmitting high driving torques.

All these problems are overcome by using a recessscrew head. Although these are more costly, dueto increased tool cosis, the increases in produc-tivity and reduction in damage can offset this ini-

tial extra high cost.

fable 4

.

THREADHEAD STYLE

SLOTTED. RECESSBA Round Pan

BSW/F Round . Pan Pan

UN Pan Pan

ISO-M Pan Pan

Fig. 7. Recess pan headprofile

.

Fig. 8. Section throughthe Pozidriv recessform head. r7

XT

The slot and the recess form the main methods of

internally-wrenching machine screws. External

wrenching is normally achieved by spanner or hexa-

gon power tool in conjunction with a hexagon head

screw.

Hexagon heads. Variations of hexagon head machinescrews are shown in Fig. 9. The type of hexagonhead is largely left to negotiation between supplier

and customer. For forged and trimmed hexagonhead machine screws, the washer faced type is

recommended. New techniques of cold forging

have led to the introduction of the indented hexagonhead, although the quality of the hexagon form wasgenerally poor, and more advanced techniques nowbeing exploited may lead to an increased usage of

a plain hexagon head.

Combinations of internal and external wrenching

can be obtained using a slotted hexagon head or

recessed hexagon head.

Other head styles do exist (see Fig. 10), although

they are normally non-preferred types and their

use is limited.

Point styles

Machine screws are generally unpointed. Thethreads are produced by a rolling process and are

Fig. 9.

CXX)trWASHERFACED

PLAIN SINGLE DOUBLE INDENTEDCHAMFERED CHAMFERED

Fig. 10.

DUMUSHROOM RAISED(TRUSS) CHEESE

(FILLISTER)

3

Fig. 11 .

Rolled end

.

Fig. 12.

(a) DIE POINT

(t>) DOG POINT

(C) PINCH POINT

(d)CONE POINT

thus slightly smaller in diameter at the end of the

screw as the last two threads are undersize. Thelast thread slightly 'rolls over' leaving a character-

istic indentation at the end of the screw (Fig. 11),

this is termed a 'rolled end'. For applications re-

quiring easier assembly conditions or where mis-match occurs between the mating holes or for use

with cage nuts, a more positive lead is required on

the screw. There are several versions available

as 'specials' and these are listed below.

The die point type (see Fig. 12a) has a lead angle

produced on the cold forged blank at an inclusive

angle of 40 -45 . After thread rolling, the section

is slightly deformed and results in an approximate70° chamfer point. This is the nearest equivalent

to a 90° chamfer point provided on machine cut

screws.

Table 5. A preferred range of length increments.

ISO-Unified ISO-Metric(inch) (mm.)

*iie 5

i 6

i 8

i 10

i 12

§ 16

i 20

i

1

Then + 1 in. Then + 5 mm

.

117

Table 6. Minimum tensile properties.

Steel 25 ton/sq.in 40 kg/sq.mm 40 hB392 MN/sq.m.

Stainless Steel 37-42 ton/sq . in

.

60 kg/sq .mm 60 hB628 MN/sq.m.

Brass 20 ton/sq.in. 32 kg/sq.mm. 32 hB314 MN/sq.m.

Aluminium Alloy 20 ton/sq. in 32 kg/sq.mm. 32 hB314 MN/sq.m.

i able 7.

MATERIAL FASTENER PRODUCTSRELATED

SPECIFICATIONSTENSILESTRENGTH HARDNESS

0.1% Carbon Steel

Bright Drawn

.

Slotted Machine Screws.Recessed Machine Screws

.

EN2A/1SAE 1008

ton/sq.in.

28 min. 140-200

0.1% Carbon SteelSoft Drawn

.

Recessed Machine Screws.Weld Bolts.

EN2A/1SAE 1008

25 1 20-1 60

18/8 TypeStainless Steel

.

Slotted Machine Screws. AISI 305 40 160-200

Brass. Slotted & Recessed MachineScrews.

BS2873 CZ 108 25 70-120

Aluminium Alloy:-High Strength

.

Slotted & Recessed MachineScrews

.

BS1475 HG 15 OD 18 60-100

CorrosionResistant.

Slotted & Recessed MachineScrews

.

BS1475 NG 6 OD 20 60-110

A type of lead point often used to prevent crossthreading is the dog point shown in Fig. 12b. Thepinch point (Fig. 12c) is virtually equivalent to theconventional cone point except that the includedangle is 60 . It is produced by a press process,more economic than machining, and is used forlocating hole positions.

The full cone point (Fig. 12d) is produced by machin-ing, which is more expensive than the method usedfor pinch pointing.

Pointing is usually charged as a list extra to astandard screw , for example, the following addi-tional costs are usually charged on \ in. diametermachine screw:

Die point 2s. lid per 1000 extra.Pinch point 8s. 4d. per 1000 extra.Cone point 10s. 5d. per 1000 extra.

Length of machine screws

The nominal lengths of machine screws are subjectto tolerances stated in the appropriate Standard.Tolerance practice is not standard and is as follows:

ISO- Unified screws Unilateral tolerance - Minusvalue.

ISO- Metric screws Bilateral tolerance.BA screws Unilateral tolerance - Plus

value.

BSW & BSF screws Unilateral tolerance - Minusvalue

.

BS4183 for ISO-Metric machine screws makes aserious attempt to restrict the choice of lengths.A similar system is intended for the revision ofBS3155 and BS1981 for Unified machine screws.Table 5 shows a preferred range of length incre-ments in millimetre and inch dimensions.

With some justification it can be claimed that

lengths smaller than 5 mm. will be required forMetric screws and thus lengths of 3 mm. and 4 mm.would also be standard.

It is to everyones advantage that screws are de-signed around these preferred lengths. Many in-dustries still specify fasteners in lengths of & of aninch and k of an inch and, through thoughtless de-sign, pay the penalty of high prices and difficultprocurement.

Mechanical properties and materials

Cold forged machine screws are generally manu-factured in either steel, stainless steel, brass oraluminium alloy, conforming to the minimum ten-sile properties shown in Table 6.

Cold forging steels do not exactly conform to Enspecifications, however. Table 7 briefly lists thematerials used, typical mechanical properties andrelated specifications.

118

Table 8. Recommended tightening torque ratios for machine screws.

16"

3_16

BSF

16"

316"

5_

16"

10—8—

_5_

16'

10-

BSW BA UNF UNC

0.03

0.03

0.01- -

3—2

1SO-M

Steel machine screws are cold forged from wire,

which itself is subject to several drawing passes

to achieve the smaller diameters. Each draw will

work harden the material and the screw manufac-

turing process of cold forging and thread rolling

will further work harden the material. The final

product will therefore often possess appreciably

higher mechanical properties than the minimumtensile quoted, rising to 40-50 ton/sq. in. for

small diameter screws. Mild steel slotted machinescrews need not be stress relieved after manufac-

ture and cannot have their strength properties in-

creased by hardening and tempering. Some recess

screws are stress relieved to reduce the high stres-

ses induced immediately beneath the recess during

forging. Such stress relieving is performed after

the cold forging stage and prior to thread rolling

and the temperature should not exceed 550°C. It

is recommended practice that stress removal is

achieved by stress relieving of machine screws

rather than annealing.

Tightening torques and breaking loads

In order to obtain satisfactory application of

machine screws, tightening torques should be ac-

30360

35 L-B.FT.400 LB.IN.

TIGHTENING TORQUE

curately controlled. This is fully appreciated with

high tensile products but is often, and wrongly,

considered less important on mild steel items. It

is perhaps to the credit of machine screw manu-facturers that their products behave as well as

they do with such abuse. The essential feature of

controlling the tightening torque is to ensure that

a suitable clamping load is established on the mem-bers - insufficient and the assembly can be left

loose or will work loose, too much and the induced

tension will rise beyond the elastic limit perman-ently stretching the screw. Problems exist not

only in deciding the correct tightening torque for

a particular application but also in ensuring that

it is in fact being applied. Under hand-assemblyconditions no control is possible unless special

torque drivers are used, and here again setting

these to a predetermined torque and maintaining

it is not easy. Perhaps this problem will eventu-

ally be solved by the manufacturers of screw driv-

ing tools. Many factors can affect the establish-

ment of the correct tightening torque, these are:

1, Dimensions of male and female components

within tolerance band.

119

2. Surface condition of components, i. e. oily, dry,scaly, roughness or smoothness of thread.3. Electroplated deposit and other surface coat-ings.

4. Underhead friction - dependent upon joint com-ponent materials.5. The length of thread engagement.6. The material and yield strength of the machinescrews used.

Table 8 shows recommended tightening torques forvarious diameters and tensile strengths of machinescrews. These figures were obtained using selfcolour nut - bolt - washer assemblies lightly oiledand therefore only provide a guide which needs tobe adjusted to suit specific application conditions.

Clamping load

For a threaded joint tightened to the yield point ofa fastener, the clamping load will be about 70-80per cent of the normal proof load of the fastener asobtained under pure tension. This is for normallubricated threads where \i is say 0. 2 to 0. 15.

If the co- efficient of friction ((X) is reduced to 0.

1

this figure is increased to about 90 per cent witha high u figure, i. e. for dry, unlubricated threads,the figure may reduce to 50 per cent, thus the re-lationship between tightening torque to induced ten-sion and thus to clamping load is very dependentupon surface conditions. Some electroplated de-posits, e.g. cadmium, reduce the co- efficient offriction from the self-colour condition.

Applied tightening torque is utilised in the threefollowing was:

1. 10 per cent to drive the mating thread helicesover each other against the action of the axial loadJo which they are inclined and hence induce tensioninto the bolt.

2. 40 per cent to overcome thread friction.3. 50 per cent to overcome friction between thebearing face of screw and nut.

Due to the importance of friction conditions, thefollowing simple formula can be used:

Torque T = A. Po. D. Where Po = axial loadD = Basic majorthread diameter.

This formula is only accurate to about ± 20 percent and where more accurate calculation is re-quired direct measurements should be made forthe particular assembly conditions. Because ofpractical difficulties in applying exact tighteningtorques, locking washers are often used. Whilstfrequently preventing unscrewing, such washerscan often result in loss of tightness during servicethrough bedding down. The best method of main-taining the stiffness of a threaded joint in generalis by adequate pretightening and provision of goodbearing surfaces.

Protective and decorative finishes

Most finishes can be applied to machine screwsand the important aspect is to ensure that somecorrosion protection is provided for without caus-ing thread form interference. Machine screws arestocked to pre-plating limits, which, for Unifiedthreads to class 2A, has an allowance of practically0. 001 in. , and for ISO-Metric threads to tolerancegrade 6g, an allowance of approximately 0. 020-0. 030 mm. dependent upon diameter in the rangeM2.5 - M12. The maximum deposit thicknessthat can be accommodated on self-colour machinescrews is a function of the thread angle. Fig. 13shows the effect of electroplating a screw thread.

A

Fig.13. Screwthread with elec-troplated deposit.

AC represents deposit thickness.AB represents increase on | the machine screwdiameter.

ACgg- = cos BAC = 0. 5 for 60° thread form,

.". AB - 2 x AC

Thread diameter increase = 4 x deposit thick-ness. Thus, for a total screw thread allowance of°- °°1 in. . the maximum deposit thickness wouldbe

J =0. 00025 in. This value will vary slight-ly for different thread diameters (Table 9) due todifferent thread forms.

BS3382 provides an electroplating standard forthreaded components and Table 10 shows the maxi-mum deposit that can be accommodated on screwdiameters without making special allowances onthe thread form.

Plating thickness

It should be noted that deposit thickness is mea-sured in terms of Average Batch Thickness, notlocal thickness which is impractical to measure onmachine screws. The normal method for deter-mining Average Batch Thickness is by the 'Stripand Weigh' technique (BS3382 Appendix B).

Table 9. Depos i I thickness factors .

Thread Form. Factors.

ISO-UnifiedISO-MetricBABSWBSF

44

5

4.34.3

120

Table 10. Plating Thickness.

BASICMAJORDIAMETER(mm)

BATCH AVERAGETHICKNESS (mm)

BASICMAJORDIAMETER(in)

BATCH AVERAGETHICKNESS (in)

Minimum Maximum Minimum Maximum

1 .52-3.203.20-6.356.35-12.7012.70-19.05

0.00380.00510.0064.0076

0.00510.00640.00760.0089

0.060-0.1260.126-0.2500.250-0.5000.500-0.750

0.000150.000200.000250.00030

0.000200.000250.000300.00035

Table 1 1 .

FEATURE ZINC CADMIUM

Cost Zinc deposits considerably

cheaper than cadmium .

Expensive

Toxic ity Not recommended with food

and beverages.

Strongly toxic, particularly if

vapourised at welds

.

Solderability Special care, and possibly

low antimony solders neededGood, preferably not

passivated

.

Appearance Brightness not usually re-

tained as long as cadmium .

Matches against aluminium.

Better than zinc . White coi

—

rosion products formed are

not voluminous

.

Thread lubrication Increases friction. Reduces friction.

Upper temperature limits

For service, -re. significant

change in appearance.

200°C 250°C

-re . subsequent room temp-erature corrosion resistance.

250°CNo embrittlement occurs in

excess of 300°C

350°CNo embrittlement of standard

fasteners at above or below

the melting point of cadmium(321 °C). Embrittlement only

reported in fasteners with

tensile yield strengths in ex-

cess of 95 ton/sq . in

.

Hardness Hv 40 to 60 12 to 22

Contact with other metals Similar characteristics. Contact with cathodic metals and alloys, for

example copper , nickel and stainless steel , will increase the attack

on the coating when wet. This can be minimised by suitable insulating

washers and jointing compounds . Cadmium corrosion products are

less detrimental to appearance than the voluminous white corrosion

products of zinc

.

The deposit thicknesses shown in Table 10 are

those which will be obtained if plating is specified

to BS3382 Parts 1-4. This gives a guarantee of

minimum plating performance on standard threads.

'Commercial plating 1

, which is essentially just a

colour finish, provides no guaranteed minimumdeposit thickness and may give coatings as little as

0. 00001 in. BS3382 should be specified in pre-

ference to other British Standards for similar de-

posit thickness as it is specifically designed for

threaded parts.

For greater corrosion resistance, deposits thicker

than those specified in BS3382 Parts 1-4 are nece-

ssary. To minimise thread interference on as-

sembly when these thicker deposits are present, it

is necessary to manufacture threaded componentswith special allowances. BS3382 Part 7 provides

the information on these allowances.

Greater corrosion resistance wihtout special threadallowances becoming necessary can also be achiev-ed by either selectively plating those parts of fast-

eners which must have a thick deposit, usually

the heads, and plating the threads with a thinner

acceptable deposit, or by selecting an appropriatecorrosion resistant alloy such as austenitic stain-

less steel.

121

Table 12. A typical price list for steel slotted screws, round and pan heads, Whit BSF BA UNCand UNF threads.

ROUND

DIAMETER

& WHIT.2 BA10 UN

LENGTH

*

i

I

i

PRICES SHILLINGS PER 1000

'AA' AND A LIST PRICES FORPOPULAR SIZES

Head styles available

at 'AA' or A prices

WHIT

© P3

) p

© p©©R

BA

R P©P©P©P©©P©P©R

BSF UNC

P

R P

R PRR P

UNF

RRRRRRRRR

UNDER4,000

704848485050545660

4000ANDOVER

('AA 1

}

22222223232526

A

35

2424

242525272830

B LIST PRICES

UNDER 4,000 100,00C|

4,000 TO AND99,999 OVER

105

7272

727575

81

8490

70 3548 2448 2448 2450 2550 2554 2756 2860 30

Degree of protection from corrosion. It is not pos-sible in this Chapter to give details of all the fac--

tors determining the choice and thickness of a part-icular plated deposit, however, the following pointsshould be noted.

1. For sacrificial deposits, such as zinc and cad-mium, the rust free life is approximately propor-tional to a deposit thickness.

2. Passivation of zinc or cadmium deposits willincrease their rust free life.

3. Zinc plating is superior to sherardising, thick-ness for thickness, and is more suitable for smallthread diameters.4. Nickel deposits which are chromium plated haveimproved appearance and corrosion resistance.5. Corrosion protection in excess of five years inall but mild environments is difficult to guaranteeby electroplated deposits and austenitic stainlesssteel should be considered as an alternative.6. Deposits such as zinc, cadmium and nickel aremore suitable for recess finishes.

It is always recommended that specification depo-sits are used whenever coatings are required to bemore than just a decorative finish.

The effectiveness of a deposit is often measuredin terms of its performance in Salt Spray tests.

Whilst not particularly related to service condi-tions these tests either state a minimum time to

the first appearance of rust or the minimum timefor the first appearance of white corrosion pro-ducts. The latter is a test of the supplementarypassivation finish, whilst the former is a rathercrude test of coating thickness. Typical minimumspecification performances in a neutral 5 per centNacl Salt Spray test of 95°F (ASTM B117 test) are:

Zinc. Deposit thickness 0. 0002 in. 24 hours be-fore first rusting.

Zinc. Deposit thickness 0. 0005 in. 96 hours be-fore first rusting.

Zinc and passivation. Deposit thickness 0. 00035in.96 hours before first rusting.

Zinc and passivation. Deposit thickness 0. 00035in.72 hours before first white corrosion products.

Under these test conditions cadmium is alwayssuperior to zinc in the time to first rusting. How-ever, in service in industrial and urban atmos-pheres, cadmium performs less satisfactory thanzinc, thickness for thickness, and hence the ac-celerated Salt Spray tests are not totally reliable.

The following British Standard Specifications areconcerned with electroplated deposits.

BS3382 Part 1 Electroplated coatings on threadedcomponents. Cadmium plating.

BS3382 Part 2 Electroplated coatings on threadedcomponents. Zinc plating.

BS3382 Part 3 Electroplated coatings on threadedcomponents. Nickel or nickel chrom-ium on steel.

BS3382 Part 4 Electroplated coatings on threadedcomponents. Nickel or nickel chrom-ium on copper or copper alloy com-ponents.

BS3382 Part 7 Electroplated coatings on threadedcomponents. Thicker deposits.

BS1706 Electroplated coatings of zinc andcadmium on steel.

BS1224 Electroplated coatings of nickel andchromium.

BS1872 Electroplated coatings of tin.

AVAILABILITY

The basic factors of machine screw design havenow been covered, these are: thread, head, point,

length, material, strength and finish, the final fac-tor affecting a specification is availability.

From the 7 basic parameters listed above manycombinations arise. In recent years, most largemanufacturers have produced new pricing policiesbased on low prices for a preferred range of fast-

eners with price penalties for non-preferred sizes.

122

Diameter M1 , M1 .2, M1 .6, M2 M2.5, M3 , M4, M5, M6, M8 , M10.

Length (mm) 5 6 8 10 12 16 20 25 30 35 40 45 50 etc.

Table 1 3 . PreferredMetric sizes.

A safety device enables a non-preferred size to be

brought back to the same low price if orders of a

sufficient quantity are placed.

Table 12 illustrates a typical preferred and non-

preferred pricing scheme.

This type of price list should be studied by design-

ers to ensure that price penalties are not being

borne unnecessarily. The list is used as follows:

lows:

1. The general description is stated at the top,

i. e. steel slotted screws, round and pan head,

threads BSW, BSF, BA, UNC and UNF.2. The first left hand column shows the diameter

within the thread range, i.e. 3/16 BSF, 2 BA, and

10 UN.3. The second column lists length increments.

4. The remaining columns are divided into two,

'AA' and 'A' sizes and 'B' sizes. 'AA' sizes are

the most commonly used and have the lowest price.

'A' sizes are preferred and have a low price. 'B'

sizes are non-preferred and are priced against or-

der quantity.

5. The indication for 'AA', 'A' or 'B' is found

under the column heading for the thread types, i. e.

Whit, BA, BSF, UNC and UNF. If the letter Rfor round or P for pan head appears against the

length for the thread type, then that head styles is

an 'AA' or 'A' size. If the letter is 'circled' it is

an 'AA' size, if not an 'A' size , or if not markedthe item is a 'B' size. Thus i in x &in. Whitsteel round E) is shown as 'circled' and is there-fore priced from the column headed 'AA'. For the

same size with a pan head, it is shown without acircle ahd is therefore priced from column headed'A'. If the same style of screws has BSF threads,

there is no mark and they would be priced fromthe 'B' list which would make such a screw twiceas expensive unless the quantity was in excess of

100, 000. Thus sensible screw design will savecosts.

It is immediately obvious that the particular price

list shown does not yet include ISO-Metric and doesnot include Unified items as 'AA' sizes. As usageof BA, BSW, and BSF screws declines, predomi-nance will be shown for the ISO thread system.

METRICATION

No article on fasteners would be complete without

some mention of metrication. Following inter-

national agreement on two common thread systemsISO-Unified (inch) and ISO-Metric, recommenda-tions R261 and R262 resulted in the publication of

BS3643 providing a metric thread series. In 1967,

BS4183 'Specification for machine screws and ma-chine screw nuts - metric series' was published,

and leading manufacturers are now carrying astock range of ISO-Metric machine screws to this

standard. The theory of preferred sizes is incor-

porated in the British Standard and the sizes shown

in Table 13 should be utilised.

The strength classification for steel machine screws

is grade 4. 8 (40 kg. /sq. mm. ) and the tolerance

grade for screws is 6g (medium fit). 95 per cent

of metric screw usage in Europe is with coarse

threads and thus machine screws are stocked only

with the coarse thread series.

It is anticipated that the usage of BA, BSW, BSFthreads will decline from 1970 onwards and, by

1975, 75 per cent of procurement will be for either

ISO-Metric or ISO-Unified screws. All new de-

signs should now be based on one of these two

thread systems and with immediate effect the use

of BSF threads should be totally discouraged.

Prices for ISO-Metric fasteners are comparablewith imperial equivalents.

FUTURE DEVELOPMENTS

With such a basic product as a machine screw, few

startling developments are likely to occur in the

immediate future. Improvements to quality are

most important to assist automatic assembly

methods and the Pozidriv recess represents animportant development in this respect. Additional

features can be provided on machine screws to im-

prove their usefulness such as:

1. Paint removal - the inclusion of flats or groovesat the end of the screw to clear paint from tappedholes.

2. Locking - the incorporation of stiff elements in

the threads or teeth under the screw head to pre-vent untightening.

3. Earthing screws - the use of pips or teeth undera screw head to ensure electrical contact betweenthe screw and mating member.4. Screwdriving methods - variation in the form of

recesses may arise although it is felt that industry

cannot afford a multiplicity of alternative screw-driving systems. More sophisticated methods of

automatic screw driving, possibly of the cartridgeloaded type, are being required by the larger screw .

using industries.

5. Materials - machine screws are available in

nylon, and other materials within the plastics fam-ily may prove suitable. The use of impregnatedsteel or preplated steel may be introduced to assist

corrosion problems.

The future of the machine screw industry is still

one of expansion and the main rewards to pur-chasers will result from greater concentration onstandardisation and rationalisation. 'Specials' are

costly and difficult to procure quickly, often result-ing in service problems at a later stage. The UK,with even only two thread systems, will be at a

disadvantage to foreign competitors who may con-centrate either on Metric or on Unified only and acompetitive situation can only be maintained bysuperiority of production methods and large scale

production.

17

Screws - self tapping etc.

by T . E . Harris

A class of screws exists which can be describedunder the general heading of this Chapter. Theyare (1) of the types which can form or cut a threadin a hole already prepared, or (2) of the typeswhich drill or pierce their own holes before form-ing the thread.

The first type are self tapping screws which can.befurther sub- divided into thread forming screws andthread cutting screws.

THREAD FORMING SCREWS

This category of self tapping screw, as the nameimplies, are not provided with cutting edges to tap

the thread in the metal, plastics or other type of

material being fixed, but rather to form a matingthread by a thread rolling or swaging action.

No pre-tapped holes are necessary in the materialso that costly tapping operations and the equipmentinvolved are no longer required. The principal

advantage, therefore, of a self tapping screw is the

low in place cost of the fastening.

Thread forming screws have the advantage of high

strength when compared with machine screws andfigures are normally in excess of 50 ton/sq. in. asa result of the case hardening treatment normallyapplied to the screw. This class of screw, as a

result of forming its own closely mating thread, bydisplacing or forming the material in the wall of

the pilot hole, gives a perfect fit between the maleand female threads which cannot be achieved in a

normally tapped hole with a mating machine screw.It is obvious that this results in a joint which pos-sesses a greater resistance to failure due to vibra-

tion or shock loads.

The thread forming screws and the thread cutting

screws described below are all referred to in

BS4174:1967 which is a specification for self tappingscrews and metallic drive screws. In this Standarddetails can be found of hole sizes for various thick-

nesses and types of materials into which the screwis to be driven. It should be remembered, how-ever, that these hole sizes are only recommended

for guidance and particular conditions affect the

performance of the screw, and these particular

conditions may require different hole sizes fromthose recommended. For example, harder mat-erials normally require slightly larger holes andconversely for softer materials.

•A' Type

This gimlet pointed screw (Fig. 1) is one of themost widely used types of thread forming screwand is primarily designed for use in thin metals. It

has a 60° thread form based on the Unified threadtype, but is widely spaced with pitches approxi-mately double the equivalent diameter UNC threadseries.

Certain shorter lengths of screws have finer pit-

ches, which are in fact the same as those of the 'B'

type screw described below. The standard sizes

available are from Number 4, with a maximummajor diameter of 0. 114 in. , to Number 24 with a

maximum major diameter of 0. 390 in.

'B' Type (or 'Z' Type)

This type of screw also has widely spaced threadswhich are slightly finer than those of the normallengths of 'A' type screws. The principal difference

between the two types of screw is the blunt but

slightly tapered point of the 'B' type screw, as illu-

strated in Fig. 2.

(^ fymmm

Fig. 2. 'B' Type screw.Fig.3. 'U' Type screw.

124

"U" Type

This is a type of thread forming screw which is

usually termed a metallic drive screw (Fig. 3).

The screw has multiple threads with a long helix

angle, so that rapid advance into the material can

be achieved. As Fig. 3 illustrates, there is no slot

provided in the head of the screw and application is

by hammer driving rather than a turning movement.

Whereas 'A' and 'B' type screws are primarily in-

tended for use in light sheet metal, fibre reinforced

resins, resin impregnated plywood and similar

materials, 'U' type screws are designed for light

alloy diecasting,.cast iron, brass, and plastics, as

well as thick steel sheets. The maximum thickness

of the materials into which the 'U' type screw can be

driven, should be not greater than the diameter of

the screw.

THREAD CUTTING SCREWS

The screws in this group are provided with cutting

edges and chip flutes so that they produce a mating

thread by removing material from the sidewall of

the hole in the component material.

The very high bursting forces experienced whenusing thread forming screws, sometimes neces-

sitate the selection of a thread cutting screw, which

removes some of the material and considerably

lowers the bursting stresses in the component. In

certain applications a lower drive torque is parti-

cularly desirable and in this case selection of a

thread cutting screw is recommended. There are

several types of thread cutting screws in service

and the main ones are described below.

•T'Type

Fig. 4 illustrates this type of screw, which is of the

Unified machine screw type of thread, but with a

blunt, slightly tapered point. The screw is provided

with one or more flutes and cutting edges extending

from the point a short distance along the shank of

the screw.

Fig. 4.

'T' Type screw.

Fig. 5.

'BT' Type screw.

Fig. 6.

'D' Type screw.

.

These screws are designed for use in materials

such as cast zinc and aluminium, sheet aluminium,

sheet brass, lead diecastings, sheet steel, stain-

less steel and cast iron.

They are available in coarse and fine thread pitch

series, the fine thread series being recommended

for the thinner materials, and the coarse threads

for weaker materials. With weak materials a

greater thread depth is necessary in order to ach-

ieve the same degree of stripping strength.

*BT* Type

The form of these screws is similar to the 'B' type

thread forming screw, as can be seen from Fig. 5,

but in this case the thread cutting action is achieved

by the provision of a single cutting flute extending

from the point a short distance along the shank.

They are designed for use in plastics, diecastings,

asbestos and other similar type compositions.

'D 1 Type

As with the >T' type screw, 'D' type screws (Fig. 6)

have threads of Unified form, but have one slot to

form a cutting edge from the point for a short dis-

tance along the shank. The low driving torque

found with these screws is a result of the cutting

edge being formed radially to the screw centre

line. These screws are ideal for low strength

materials, plastics, brittle metals and for re-

threading pre-tapped holes which have been clogged

after tapping, for example by painting operation

being performed on the component.

'Y' Type

The 'Y' type screw (Fig. 7) has widely spaced threads

with a blunt tapering point similar to the 'BT' type

screw. The screw is provided with multiple cut-

ting flutes extending from the point to the head,

making it suitable for use in brittle plastics and

diecastings. It can be used with extremely long

thread engagement especially in blind holes and is

unique among self tapping screws in this respect.

USE OF SELF TAPPING SCREWS

The following are four alternative combinations of

fixing conditions which can occur when using 'A'

and 'B' type screws. There are, of course, other

special combinations which can occur.

125

1. Holes drilled or punched in both sheets, asillustrated in Fig. 8.

2. Holes in both sheets pierced and plunged to givea stronger joint (Fig. 9).

3. Clearance hole in second panel with a piercedand plunged hole in the first panel (Fig. 10).4. Clearance hole in second panel with an extrudedhole in the first panel (Fig. 11).

For all other types of screw it is more usual toprovide a clearance hole in the second panel withthe correct tapping hole size in the first panel,casting or moulding.

TORQUE FIGURES

It is essential that the self tapping screw remainsin tension and initially that the correct tension isapplied. This- can be controlled by the correct se-lection of application torque for the screw, withthe particular set of conditions involved and can•only be accomplished by carefully testing the as-sembly under actual conditions to find the tappingtorque and the stripping torque. Subsequently asafety factor is applied to the minimum strippingtorque value found from testing, to arrive at a suit-able application torque. Fig. 12 shows the type ofgraph that can be obtained by testing an assembly inthis way.

Fig. 12.Torque spreadagainst holediameter.

HOLE DiAMETEK •

It is -evident from this graph that there is a biggerspread of stripping torque than of tapping torque forall diameters. It is also important to realise thatwith coarser pitch screws in thinner materials thedifference between tapping torque and strippingtorque becomes less, so that accurate setting ofapplication torque becomes far more critical.

USE OF SCREWS IN PLASTICS

With plastics materials it is also very importantthat the correct tightening is applied. It is general-ly found that the softer the plastics the nearer arethe two values of tapping torque and stripping tor-que, making the selection of the correct. applicationtorque far more critical.

The most secure method of mating a screw threadin a plastics article is to mould in a nut insert, butthis is prohibitive in cost of insert and additionalmoulding cost. A cheaper method is to tap a femalemachine screw thread in a hole moulded into thearticle. Taking the analysis one stage further, ifself tapping screws are used in the plain hole aneven cheaper assembly results, as well as offeringthe benefit of a snug fit between the screw and itsmating thread. This snug fit gives a vibration resis-tant joint since the screw has formed an exact threadwith a frictional grip being exerted by the threadflanks, on to the screw. A self tapping screw foruse in plastics materials should possess the follow-ing properties:

a. Low driving or tapping torque to form a threadin the plastics.

b. High stripping torque, i. e. torque to shear thethread from the plastics during driving.c. High pull out strength in tension.d. It should generate low radial forces duringscrewing, to avoid bursting the plastics.

Thread cutting screws offer obvious advantagesover thread forming screws in the first require-ment, because the driving torque is lowered by thecutting action of the fluted screw. The greatestadvantage in previously discussed screw types iswith the >Y« and 'BT' types, especially the latter, asthese types have the coarse pitch thread whichgives high ratios of pull out strength and strippingtorque to driving torque. Hole sizes recommendedfor different plastics for 'T' and 'B> type screws areto be found in BS4174:1967, but for some reason notable of hole sizes for 'Y' type screws is included.It is felt that the table for 'BT' type screws can beused as a guide for 'Y' type screws.

Hole sizes for plastics other than the listed ones,cellulose acetate and nitrate, acrylic and polysty-renes, must be arrived at by experiment and varywith hardness and bursting tendencies of the mater-ials. The hole sizes listed for the above mentionedmaterials can be used as a starting point if onetakes into account the similarity between the pla-stics being used and one of those listed.

It is usual to provide a counterbore or countersinkin the plastics to reduce or eliminate the tendency

126

to chip around the holes, which occurs in harder

plastics when no counterbore is provided.

RECENT DEVELOPMENTS

The foregoing comments apply to screw designs

which have been produced for many years and it is

not surprising that in such a vast market as that

existing for this type of product there have been

new developments in recent years. Special thread

forms have been designed to give greatly improvedperformance in the role of thread forming or thread

cutting screws.

The 'Hi-Lo' screw thread

The 'Hi-Lo 1 has been designed with the above men-tioned requirements for plastics in mind.

Fig. 13 illustrates the form of the thread, which is

double start, with one thread being a high thread

about 1. 5 times the height of an 'A' type or 'B'

type thread, as shown. The low thread is approxi-

mately 80 per cent of the height of the 'A' type or

'B' type thread.

The 'Hi-Lo' screw provides greater thread engage-

ment than conventional self tapping screws with a

corresponding increase in pull out strength. Fig. 14

illustrates this feature, as well as the increased

volume of material contained between the threads of

the 'Hi-lo' screw. This increased volume of mat-

erial gives both improved pull out strength and

Stripping torque. The high thread is designed with

a 30° thread angle to minimise radial forces pro-

duced during driving to approximately half the value

found with conventional thread forms. Fig. 15 com-pares the force diagrams of the two thread forms.

The main purpose of the low thread is to provide

IT-2P/10 APPROX

AMERICAN NATIONAL MACHINESCREW THREAD FORM

(COARSE & FINE)

—1 H-P/8 APPROX

AMERICAN NATIONAL SPACEDTHREAD FORM

(TYPES 'A' & 'B')

Fig. 13.

APPROX

HI-LO THREAD (DOUBLE LEAD)

P - THREAD PITCHH - THREAD HEIGHTT - THREAD THICKNESSL - THREAD LEAD

(ONE REV .

)

GRIP OF TYPE 'B' THREAD

INCREASED GRIP OF .HI-LO THREAD!

Fig. 14.

Comparisonof contained

material

volume

.

HI-LO TYPE "B"

I

F

R1

R2

I 7—

"~^> f (^>

^-—"""l 8' )\ 92

r-"^ > 30° -/ \ 60°

F = TOTAL CLAMPING FORCER = RADIAL (BURSTING) FORCE6 INCLUDED THREAD ANGLE

Fig.15. Comparative radial pressures.

HI-LO DRIVE TORQUE^

HI-LO STRIP TORQUE

TYPE B STRIP TORQUE

TYPE BDR1VE TORQUE

\ r + +0.112 0.120 0.128 0.136 0.144

HOLE SIZE - INCHES

Fig.16.

stability during the driving of the screw whichotherwise would have a tendency to tilt.

Fig. 16 shows that one important property of the

screw is its low driving torque, and being a twostart thread the speed of application is faster than

with conventional thread forms.

We have seen that the 'Hi-Lo' screw has all the

requirements mentioned as those of a self tapping

screw for plastics and shows improvement overconventional threads by (a) lower driving toraue,

(b) higher stripping torque, (c) greater pull out

strength in tension and (d) reduced radial pressure.

The Taptite screw

The 'Taptite' screw (Fig. 17) has a tri-lobed thread

structure which enables it to virtually 'roll' a thread

in a prepared hole, compared with the cutting act-

ion of screws of type 'T', 'BT', 'D' or 'Y'.

The principal advantages of 'Taptite' screws are

as a direct result of this forming action, which

gives an uninterrupted grain flow within the mat-

erial, compacting and burnishing a female thread

into close fitting contact with the screw. As a con-

sequence of this, a stronger joint is obtained com-

127

pared with a simple machine screw into a tappedhole, with the resulting firmness of fit enablingthe joint to resist vibration under which a machinescrew in a mating tapped hole would shake loose.

The higher stripping torque obtained with a 'Tap-tite' screw can be av. • ibuted to the thread formingaction and the strength of the screw compared withmachine screws. Tint; strength emanates from thecase hardening treatment after manufacture of thescrew, which consists of a controlled treatment togive a 0. 004-0. 006 in. case and a toughened core.Minimum torsional strength figures for varioussizes are shown in Table 1. Recommended holesizes are also shown. The stripping torque to driv-ing torque ratio with 'Taptite' is considerably higherthan conventional types of self tapping screws, andenables higher tightening torque figures to be used,with more likelihood of correctly tightened joints.The performance is increased even more by the useof extruded holes in Hun sheet metals to give an in-creased length of thread engagement. The greatestsuccess occurs when the material is thinned downby between 40 and 50 per cent of its basic thickness.

Table. 1 . Taptite, torsional strength values and holt sizes

.

SCREW SIZE MINIMUMTORSIONAL

MATERIALTHICKNESS

HOLE SIZEMILD STEEL ALUMINIUM SHEET

STRENGTH SHEET (in.) ALUM. & ZINC DIE(lb. /in.) (in.) CASTING (in.)

4-40 16 0.048 0.098 0.098UNC 0.064 0. 102 0.100

0.125 0. 104 0.1020.250 - 0.102

6-32 28 0.048 _ 0.118UNC 0.080 0.122 0.122

0.187 0.122 0.1220.250 0.126 0.1260.275 - 0.126

8-32 52 0.080 0.146 0.146UNC 0.187 0.150 0.150

0.250 0.154 0.1500.375 - 0.154

10-24 70 0.080 0.165 0.165UNC 0.187 0.173 0.165

0.250 0.177 0.1690.375 - 0.173

10-32 92 0.080 0.173 0.173UNF 0.187 0.177 0.173

• 0.250 0.181 0.1770.375 - 0.181

i in . - 20 176 0.125 0.221 0.217UNC 0.187 0.221 0.221

0.250 0.228 0.2210.375 0.236 0.2280.500 0.236 0.228

ft in. - 1

8

380 0.125 0.280 0.280UNC 0.187 0.280 0.280

0.250 0.287 0.2840.375 0.291 0.2870.500 0.291 0.287

| in. - 16 700 0.187 0.343 0.339UNC 0.250 0.350 0.343

0.375 0.354 0.3500.500 0.354 0.350

128

Table. 2. Taptite , extruded hole diameter (inches).

SCREW SIZE

6-32 UNC

8-32 UNC

10-24 UNC

10-32 UNF

iin.-20 UNC

ft in. -

18 UNC

lin. -

16 UNC

MATERIAL THICKNESS (in.)

0.02 0.03 0.04 0.06 0.09 0.13 0.16 0.19 0.22 0.25 0.31 0.38

0.116 0.117 0.118 0.119 0.1220.119 0.119 0.121 0.122 0.125

0.142 0.143 0.143 0.144 0.146 0.149

0.145 0.146 0.146 0.147 0.149 0.152

0.160 0.161 0.162 0.163 0.166 0.169

0.164 0.165 0.166 0.167 0.170 0.173

0.167 0.168 0.169 0.170 0.172 0.174

0.170 0.171 0.172 0.173 0.175 0.177

0.215 0.217 0.220 0.222 0.224 0.227 0.229 0.231

0.219 0.221 0.224 0.226 0.228 0.231 0.233 0.235

0.271 0.272 0.275 0.277 0.279 0.281 0.284

0.275 0.276 0.279 0.281 0.283 0.285 0.288

0.332 0.334 0.336 0.337 0.339 0.342 0.3450.336 0.338 0.340 0.341 0.343 0.346 0.349

Recommended extruded hole sizes are shown in

Table 2. Accumulation of chips which occur with

thread cutting screws is not a problem with the

'Taptite' screw which is consequently ideally suited

for use in blind holes.

The torque characteristics of the screw are impro-

ved by the finish coating which consists of treating

the screw with a dry wax film after plating. The

wax assists in lubrication of the thread surfaces

during driving, thus preventing galling or seizing o

the threads.

The screw is available in many different head styles

and shank lengths.

^SELF DRILLING OR PIERCING SCREWS

The second category of screws to be examined is

that of self drilling or piercing screws, and three

types have been selected for this purpose:

1. "Shakeproof Type 17 screw.

2. 'Spat System' screw.

3. 'Teks' screw.

When considering this category of screws it is of

paramount importance to study the cost of providing

a fastener hole. Basic methods of providing holes

include: (1) punching, (2) drilling, (3) piercing and

(4) moulding. For the purpose of this Chapter,

punching refers to a hole provided by the use of a

punch and die, whilst piercing is the use of handtools to puncture a hole without removing metal.

Punched holes can be very expensive if one con-

siders the cost and maintenance of expensive dies,

but can also be quite inexpensive if many holes are

punched during one pressing operation, especially

if these holes are not distorted by subsequent form-ing operations .

Drilled holes can be very accurate and clean but

can also be the most expensive method of providing

a hole.

Piercing is generally the most expensive method

of providing a hole because it is not normally auto-

mated. It normally involves the disproportionate

combination of cheap tools (a hammer and awl) and

high labour costs.

Moulded holes can be provided in die castings or

mouldings of thermoplastic or thermosetting plas-

tics materials, fairly easily and cheaply. Problemscan occur with holes required at angles to the gen-

eral directional layout of the moulding, which neces-

sitates the use of more costly multi opening dies.

Thus, it can be seen that costs of providing holes

vary considerably and studies have shown that in

general it can be stated that the making of fastener

holes in a separate stage of manufacturing is an

expensive operation.

It is here that the self drilling or piercing fastener

comes into its own and should be studied in com-parison with other fasteners on the very important

basis of installed cost and not. as'is too often the

case, on the basis of actual fastener purchase price.

They completely eliminate the cost of fastener holes.

Type 1 7 screw

The Type 17 self drilling screw consists of the spac-

ed thread with a gimlet point and a sharp, off cen-

tre, slot as illustrated in Fig. 18. It has advant-

Fig.18.Type 1 7 screw

.

129

Fig. 19. Spat system gun.

ages over other screws when used in wood or plas-tics, dispensing with the need for pilot holes anddecreasing assembly time.

The Type 17 screw is used for mounting gypsumboard to metal studding for internal wall construc-tion in modern buildings. In this application bugleheaded screws are used to sit snugly just below theouter surface of the board.

The screwdriver used must be provided with a'depth- setting' clutch which can be set to automatic-ally cut out when the top of the screw head is drivento a predetermined distance under the outer surfaceof the board.

'Spat System" screw

The 'Spat (self piercing and tapping) System' hasbeen fairly recently developed, coupling the use ofa special self piercing and tapping screw with aspecial gun. The gun is dual purpose, providinga high energy impact to pierce the sheet metal withthe point of the screw and then providing the rota-tion necessary to drive the screw into the lockedposition, at up to 500 rev. /min. It operates offstandard air line pressure of 80 lb. /sq. in. and is

provided with an adjustable clutch which allowstorque setting for different screw sizes and appli-cation conditions. Fig. 19 illustrates the gun anda 'Spat' screw being applied to the kick strap on anautomobile door surround.

The 'Spat' screw is illustrated in Fig. 20 as a coarsepitch, dual start thread with a special point; thepoint consists of four planes meeting at a designed

Fig. 20. Spatsystem screw.

mjui5aa

2

SCREW SIZE: NO 8

._. SPAT SYSTEM STRIP-PING TORQUESPAT SYSTEM TIGHT-ENING TORQUE'A' TYPE STRIPPINGTORQUE

METAL GAUGE

Fig. 21 . Screw size No. 8.

angle to give the most effective piercing action.The piercing action produces a plunged hole withgreater effective panel thickness and, as a conse-quence, 30 per cent greater pull-out strength whencompared with the equivalent self tapping screw.

With the piercing and tapping action of 'Spat Sys-tem' screws there is no problem of unwanted swarfinterfering with mechanisms.

The dual start thread gives balanced driving and afaster screwing action than that experienced withself tapping screws.

Fewer fasteners or -smaller fasteners can be usedbecause of the higher strength of 'Spat System'screws so that "installed cost' is lower. Fig. 21shows a comparison of a No 8 'Spat System' screwwith a No 8 'A' type self tapping screw. A limitationof the screw is that It is unsuitable for the thickermetals because of difficulty with the piercing action.

The 'Spat System' screw is available in a variety ofsizes, lengths and head styles.

1_

S A

L - SCREW LENGTHA - MIN. THREAO LENG I -I

B - DRILL POINT LENGTHD - DRILL POINT DIAML~FR

^H

Fig. 22. Teks screw.

130

NOTE: THE DRILL POINT MUST CLEAR THE SHEET BEFORE THETHREAD ENGAGES TO AVOID POINT BREAKDOWN

SCREWADVANCES0.055 IN PERREVOLUTIONWHENTHREADED

TOTAL THICKNESSTO BE DRILLED

THICKNESS OFSHEET NO 2

DRILL POINT ADVANCES 0.005 IN PER REV WHEN CUTTING

Fig. 23. No. 8-1 8 Teks Fastener.

'Teks' self drilling fasteners

'Teks' is a self drilling screw which possesses a

true drilling action by virtue of its drill point des-ign, except that, unlike a drill, no compromise is

necessary in its design to give optimum performancebetween drill life and speed. With 'Teks', which are

normally required to drill only one hole, optimumdrill speeds are the criterian and consequently the

'Teks' screw drills faster than conventional drills.

Fig. 22 illustrates the general configuration of the•Teks' screw, which can be manufactured in coarsepitch types of thread as 'A' or 'B' type screws orin standard machine screw threads of Unified form.

The screws are applied using electric or pneumatichand power tools fitted with a standard adjustable

torque limiting clutch device. The most desirablerunning speed is between 2000 and 2500 rev. /min.

,

and the average axial pressure applied by the oper-ator is of the order of 25-30 lb.

In the selection of the correct 'Teks' for any parti-

cular application great care has to be taken to en-sure that the point length is sufficient to permitbreakthrough of the leading edge of the drill point,

before the thread engages. Fig. 23 illustrates this

clearly; when drilling the screw advances at appro-ximately 0. 005 in. per revolution and with, for

example, a No 8-18 thread, the screw advances at

0. 055 in. per revolution when the thread starts to

engage. It is obvious that such a rapid advance .and

a chip thickness of 0. 055 in. would cause the pointto burn and the screw to seize up. For this reasonalso, 'Teks' cannot be used in blind hole applica-

tions. Once the correct selection has been made noproblems in driving should occur and the total dri-ving time is normally less than 5 seconds.

'Teks' have been designed so that the stripping or

breaking torque is greatly in excess of the driving

torque for all conditions likely to be met in prac-

-50' BREAKING TORQUE-45' STRIPPING TORQUE

PEAK OF THREADCUTTING TORQUE'DRILL POINTBREAKTHROUGH

KDRIVING START

PEAK OF TIGHTENINGTORQUECLUTCHING OUT OF GUN

Fig.24. Teks driving torque profile.

tice. Fig. 24 illustrates the torque values obtained

when driving a No 8 - 18 'Teks' into a 0. 094 in.

thick steel sheet. It can be clearly seen that the

'Teks' gives a large safety margin between the

maximum applied torque and the stripping and break-

ing torque figures.

With the correct point length 'Teks' screws can drill

through steel plate up to iiin. thick; this is a rareadvantage in this type of fastener.

The 'Teks' screw is available in many sizes, lengths,

head styles and finishes with the normal standards

being Nos. 6, 8, 10, 12 and \ in. with a maximumpanel range of 0.090 in. in the No. 6 and up to

0. 250 in. in the | in. size.

The screw offers the advantage of low 'installed-

cost' combined with a good quality high strength

application.

CONCLUSIONS

All the fasteners described in this Chapter are the

optimum under certain conditions: 'A' and 'B'

screws, where the provision of a hole is cheap and

no problems of alignment exist, 'U' type screws,where holes can be provided in light alloy diecast-

ings, cast iron, brass, plastics and thicker sheet

metals, and thread cutting screws of 'T', 'BT',

'D' or 'Y' type, where low driving torque figures

are required in plastics, diecastings, fibre rein-

forced plastics, etc. The 'Hi-Lo' screw can bespecified for more critical applications, wherevery high pull-out loads are required with low driv-

ing torque and bursting stresses, as in softer plas-

tics applications.

The 'Taptite' screw gives considerable strength ad-

vantages compared with machine screws in diecast-

ings and extruded holes.

Finally, the self drilling or piercing generation of

fasteners gives low 'installed- cost' compare with all

other systems, with the 'Spat System 1 showing ad-

vantages in thin metals compared with 'A' type

screws.

The 'Teks' self drilling fastener gives low 'installed-

cost' and can be used in a large variety and thick-ness of materials.

ACKNOWLEDGMENTS

G.K.N. Screws & Fasteners, Linread Limited,ITW Limited, Barber & Colman Limited.

131

18

Screws - set

by Dennis Troop and Barbara Shorter (Unbrako Ltd.)

.

A set screw is essentially a semi-permanent fasten-

er. Its purpose is to hold a collar, sleeve or gearon a shaft against torsional or axial forces. In con-trast to other fastening devices, the set screw is

primarily a compression device. It produces a

strong clamping action which resists relative mo-tion between assembled parts through the forces

that are developed by the screw point on tightening.

Selection of the proper set screw will depend uponfinding the best combination of form, size and point

style to provide the required holding power.

Basically set screws can be divided into two cate-

gories, by their forms and by the style of point, as

required by British Standards 2470; 4168; 768; 4219

and 451. Basic forms and point types are displayedin Fig. 1.

Form selection is based upon factors other thantightening: for instance, the selection of the typeof driver. The square head screw may be tightened

much more, but obviously in many considerationsits protruding head :s a major disadvantage. Otherconsiderations such as compactness, weight saving,

safety and appearances may dictate the choice of

screw that is used.

SIZE SELECTION

The selection of size will, of course, be determinedby the holding power required. Fig. 2 shows a typi-

Fig.1 . Set screw types and standard points.

STANDARD HEAD FORMS STANDARD POINTS

i lEXAGON SOCKET SLOTTED HEADLESS

(e) Cup. By far the most widely used. Forquick, permanent location of gears, collars,and pulleys on shafts, when cutting-in actionof point is not objectionable. Heat-treatedscrews of Rockwell C 45 hardness or greatercan be used on shafts with surface hardnessup to Rockwell C 35 without deforming the point.

(f) Flat. Used when frequent resetting of onemachine part in relation to another is required

.

Flat points cause little damage to the partagainst which the point bears , so are partic-ularly suited for use against hardened steel

shafts. Can also be used as adjusting screwsfor fine linear adjustments. Here, a flat is

usually ground on the shaft for better point

contact. Also preferred where walls are thin

or threaded member is a soft metal

.

(g) Cone . Used where permanent location of

parts is required. Because of penetration, it

develops greatest axial and torsional holdingpower when it bears against material of Rock-well C 15 hardness or greater. Usually spottedin a hole to half its length, so that penetration

(e) (a)

is deep enough to develop ample shear strengthacross cone section

.

(h) Half Dog. Normally applied where perm-anent location of one part in relation to anotheris desired , spotted in a hole drilled in the

shaft . Drilled hole must match the point dia-

meter to prevent side play: holding power is

shear strength of point. Occasionally used in

place of dowels, and where end of thread mustbe protected . Recommended for use with

hardened members and on hollow tubing , pro-vided some locking device holds screw in place.

(i) Oval. Used when frequent adjustment is

necessary without excessive deformation of

part against which it bears . Also used for

seating against angular surfaces. CircularU-grooves or axial V-grooves are sometimesprovided in the shaft :o allow rotational orlongitudinal adjustment. In other applications,

shaft is spotted to receive the point. However,has the lowest axial or torsional holding power.

(j) Full Dog. Same as half dog except for a

longer point.

132

FASTENINGS

Macnays of Middlesbroughhave in stock the widest

selection of Bolts, Set-

screws, Machine Screws,

Socket Screws, Nuts andStuds in the United King-

dom, including new I.S.O.

metric standards. Delivery

from Stock can save Capital

Outlay, Storage Space,Handling Costs and Spot

Losses. Free weekly deliv-

eries throughout thecountry.

MACNAYSSend for the "Guide to

Stock Range and Spec-ifications" booklet to:

—

MACNAYS LTD48-50 West Street

MiddlesbroughTeesside

Tel : M idd lesbrough 48144

Spring LockWashers

Standards Ex Stock We offer immediate service to

stockists and users. All types and materials available.

Specials As the largest U.K. manufacturer, we have

unrivalled experience in the design and production of

special purpose Spring Lock Washers for any application.

And don't forget Morlock for Brazing Preforms, Snap

Rings, Crinkle Washers, Disc Springs and all types of

Light Spring Presswork.

Write for full technical information and a list of Lock

Washer Stockists to:-

MORLOCKMorlock Industries Limited

P.O. Box 2, Wombourn, Nr Wolverhampton, Staffs.

Telephone : Wombourn 2431 -4 Telex : 33276

133

Fig. 2. Shaft

and collar as-sembly showsforces devel-

oped in typical

set-screw in-

stallation.

cal shaft and collar assembly in which force F de-veloped by the cup-face on the shaft, due to tighten-ing, produces an equal reaction, force F,. Thisclamping action results in two frictional forces.One occurs between the shaft and collar (F

? ) andthe other between the shaft and point. These forcesprovide most of the resistance to relative axial andtorsional movement of parts.

Some additional resistance is contributed by pointpenetration. Cup point and cone point set screwsare used without a spotting hole. In these casesthey penetrate the shaft more than oval point of flat

point set screws because of their small face area.

The total static holding power of the cup point setscrew as shown in Fig. 2 is a function of the twofriction forces and the point penetration resistance,and can be used as a single effective force actingtangentially at the surface of the shaft. The magni-tude of the single force equals the axial holding pow-er of the set screw, or the resistance of the assem-bly to relative movement along the longitudinal axisof the shaft. Torsional holding power is determinedby multiplying the axial holding power by shaft rad-ius. Axial holding power is generally specified asa tangential force in lb. , since design considera-tions may cause different sizes of shaft to be usedwith a particular size of set screw.

In selecting a particular hollow set screw, engin-eers are often guided by an old rule: set screw dia-meter should be roughly equal to half the shaft dia-

meter. While the old rule is not without merit, its

range of usefulness is- limited. Table 1 has beendeveloped from experimental data and can be usedas a more scientific guide to size selection.

The holding powers, as indicated in Table 1, areultimate strength and should be coupled with spe-cific safety factors appropriate to the given appli-cation and load conditions. A safety factor of 1. 5

to 2. under static load conditions and 4. to 8.0under various dynamic situations should bring goodresults.

Table 1 was developed for a specific set screw formand point style, but these values can be modifiedby percentage factors to provide design data foralmost any other form and point style.

There are a number of other considerations involv-ed in selecting the optimum set screw size for anyrequirement. These will include seating torque,point style, relative hardness, flat on shaft, length

of thread engagement, thread type, type of driver,number of set screws and plating.

Each of these factors are analysed below:

Seating torque

Extensive tests have shown that torsional holdingpower is almost directly proportional to the seat-ing torques of cup, flat and oval point set screws.The graph in Fig. 3 shows a typical plot of this

characteristic. An increase of 50 per cent in theseating torque will also increase the holding powerof the set screw by 50 per cent, obviously withinthe strength limits of the assembly. For example,the torsional holding power of a one inch diameterset screw seated at 7000 lb. in. , on a one inch shaft,

as shown in Table 1, would be 3500 lb. in. , or onehalf of the tabulated value.

Point style

A hollow set screw point is capable of contributingas much as 15 per cent of the total holding powerwhich it accomplishes by its penetration. A conepoint set screw, which contains neither a spottinghole nor a pre-drilled hole in the shaft, gives thegreatest increase in holding power because of its

deeper penetration. The oval point, because of its

lesser contact area, gives the smaller increase.

At the index where the cup point is taken as one,the holding power values from Table 1 should bemultiplied by 1. 07 for cone point. It would be mul-tiplied by 0. 92 for dog points or flat points, and by0. 90 for oval points. These values assume the pointof the screw is not specially reset into the shaftand that the penetration is the sole result of tighten-ing. A dog point, for example, seated in a holedrilled in a shaft acts only as a pin. In this casethe holding power must be determined by the shearstrength of the screw material.

Relative hardness

In some cases, hardness will be an important fac-

tor in set screw selection. An example is whenthere is less than 10 Rockwell C-scale points dif-

ference between the set screw point and the shaf-

Fig.3. Torsional holding power is almost direc-tly proportional to tightening torque. Set screwused to obtain plot was ,-| in. knurled cup-pointtype seated in one inch diameter shaft with hard-ness Rockwell C-15.

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30 4bSHAFT HARDNESS (ROCKWELL C)

Fig. 4. Considerable loss in holding power isexperienced when the difference in hardnessbetween shaft and screw is less than 10 Rock-well C points. Set screw used to obtain plotwas ,-$ in. knurled cup-point type seated with165 lb. in. against one inch diameter shaft withhardness as indicated

.

ting. The graph in Fig. 4 shows a typical plot. Asillustrated, there is a slight gradual decrease inholding power, actually about 6 per cent with in-creasing shaft hardness up to 10 Rockwell pointsbelow the hardness of the screw (Rockwell C 50).At that point a loss of about 15 per cent holdingpower is experienced. This 15 per cent loss re-presents the amount of holding power contributedby penetration of the point. Consequently, becausethe hardness affects the ability of the screw to pen-etrate, the lack of holding power is a function oflack of penetration.

Fig. 4 is based upon a relatively hard Rockwell C50 screw point. Here the 10 Rockwell point differ-ential can be applied generally. From this we ob-serve that a screw hardness of Rockwell C 45, a15-20 per cent loss in holding power should be ex-pected if the shaft hardness is Rockwell C 35 orgreater.

Flat on shaft

Only about 6 per cent more torsional holding powercan be expected when the screw seats on a flat sur-face. Flatting does little to prevent the 0. 01 in.

relative movement which is ordinarily consideredas a criterion of failure. The axial holding powerwill be the same.

Length of thread engagement

Assuming that there is sufficient engagement toprevent stripping in the tightening process, thelength of thread engagement has no noticeable effecton axial and torsional holding power. The lengthof engagement depends upon such factors as theamount of applied load, the type of material, typeof thread and screw diameter. In most uses, theminimum length of engagement recommended is

the diameter of the set screw itself.

Ordinarily this will permit the development of re-commended seating torques without danger of threadstripping. The tabulated values for seating torquewere developed with the assumption that the engage-ment length was long enough to prevent stripping.

Thread type

Experimental work indicates that there is no differ-ence in the performance of coarse and fine

sthreads

of the same class of fit. Consequently the valuestabulated in Table 1 apply to either thread type.

Type of driver

The values tabulated in Table 1 are for socket typeset screws. However, they apply equally well to

slotted and square head set screws provided theindicated seating torque is developed. Whilst theshape of the driver itself has no direct bearing onthe holding power, it does have an effect on theamount of seating torque which can be attained.

For the slotted set screw, the maximum seatingtorque is that which can be developed by a screwdriver. Deformation of the screw slot occurs at

a torque value much less than a torque which wouldstrip the threads.

The maximum torque which can be applied tq sock-et or spline-head set screws is also lower than

that which would strip the threads, but it is higherthan that which can be developed by the driver.Consequently the torque which can be applied, is

a function of the driver. Conversely square headset screws can be tightened with a wrench until thethreads strip or the screw fails in torsional shear.Table 2 lists typical recommended installation tor-ques for square head set screws.

Number of set screws

Two set screws will give more holding power thanone, but not necessarily twice as much. The hold-ing power is approximately doubled when the sec-ond screw is installed in an axial line with the first.

It is only about 30 per cent greater when the screwsare diametrically opposed. The tabulated torsionaland/or axial holding powers (Table 2) can be mul-tiplied by from 1. 30 r.o 2. 00 depending upon theangle between the two screws. The graph in Fig. 5

shows how much to compensate for any angle bet-

ween. When the design calls for the two screwsto be installed on the same circumferential line,

an optimum displacement of 60° is recommendedas the best compromise between maximum holdingpower and minimum metal between tapped holes.This displacement gives 1. 75 times the holdingpower of one screw alone.

"able 2

.

Recommended TighteningScrew Size Torques

lb. /in

i 2121 4201 828ft 1 ,344i 2,100i 4,248i 7,704

Recommended tightening torque for square head.

SCKS-^S .« (Dfi)

Fig. 5. Angle between two set screws has a

straight-line effect on torsional holding power.

Plating

A soft plating, such as cadmium or zinc, will in-

crease the holding power by 5 to 10 per cent for

the same tightening torque. The plating acts as a

lubricant and less of the applied tightening energy

is dissipated in friction at the mating threads. Acomparable increase can be achieved by plating

the female tapped member or by using a thread

lubricant.

Set screws can also be plated purely for anti- cor-

rosion purposes or for decoration.

SCREW RETENTION

There is a significant difference in the perform-ance of set screws and a nut and bolt assembly,

based upon their different functions. When a set

screw point disengages, the parts it has fastened

will normally separate. The nut and bolt assemblywill hold parts together for some time in spite of

becoming loose. The dog point set screw seated in

a drilled hole will hold parts together, but evenhere failure will follow rapidly after initial loosen-

ing. Seating torque is essential to secure retention

of the set screw.

Referring again to Fig. 2, the shaft and collar as-

sembly, as the screw is tightened, the pressure onthe point forces the screw back against the flanks

of the thread in the tapped hole, where friction is

developed. It is this friction, plus the friction at

the point of contact of the screw and the shaft, that

hold the screw in place. The cup point is highly

efficient because of high point -to -surface friction.

A number of screw designs have special locking

features such as ratchet -like teeth on the face of

the point surface (knurled).

The diameter of the set screw is also a consider-ation in developing vibrational holding power. How-

ever, it is difficult to develop an efficient quantitive

analysis of the set screw's capabilities in this re-

spect. Frequently a size or two larger set screw,

or an additional locking feature may be the solution

in applications where other means have failed to

develop satisfactory vibrational holding power. Thelarger screw permits higher seating torque and

consequently develops greater clamping forces andhigher resistance to loosening.

MATERIALS

The statements in this Chapter apply to screwsmade from alloy steel, but set screws are also ob-tainable in many other materials, including stain-

less steel or brass.

PRICING

The pricing of set screws is based on the samevariety of considerations - design features, tooling

costs, the number of operations to completion - the

same factors that govern the pricing of any compo-nent. Prices are usually quoted per 100. As anindication small sizes of hexagon socket set screwssuch as 8 BA have a basic price of 25s. per 100,

5 in. diameter are priced 20s. per 100 and f in.

diameter axe priced 50s. per 100. Very large

sizes, one inch diameter, are priced between 350s.

and 550s. per 100. Quantity of course, plays a

considerable part in the pricing of set screws, andfor this reason a system of quantity and single type

discounts is often employed. For example, socket

and set screws of % in. diameter and smaller maybe subject to discounts ranging from 5 per cent

to 20 per cent and more, for ordered quantities

from 5000 to 99, 000 and more.

Order quantities and specials

If a customer orders direct from the manufacturersnormally a quantity of 10, 000 would be the likely

minimum economical quantity. However, set

screws, like other fasteners, are available fromengineers' suppliers who are geared up to supply

any quantity from a few off to thousands.

A minimum order for a special may be consideredat 250, but a customer would find that a more econ-

omical order number would be for 1000. For ex-

ample, one small special set screw (\ in. BSWx i in. ) would be priced 507s. 6d. per 100 for one

hundred only, 181s. a hundred for 500 quantity and101s. a hundred for 1000 quantity.

137

19

Screws-wood

by J.M. Humphrey, C.Eng. .M.I.Mech.E. (G.K.N. Screws & Fasteners Ltd.)

Basically, there are two types of wood screw: the

conventional wood screw and the wood screw thread-

ed to head.

1. The conventional wood scre.w has a head, a

length of plain shank and a threaded portion term-inating in a gimlet point (Fig. la). The thread formand gimlet point have been developed over the yearsto give good holding power and easy entry whendriven into wood. At least 60 per cent of the over-

all length of the wood screw is threaded, the plain

shank, between thread and head, acting as a dowelin the wood when attaching thin components.

2. The wood screw, threaded to head, can haveeither a single or two start thread running the full

length of the screw from the head and terminating

in a gimlet point (Fig. lb). Screws over one inch

long may have a relieved shank, i.e. the diameterof shank is less than the outside diameter of the

thread but is greater than the core diameter.

The above types of wood screw are available in

three head styles: countersunk, round and raisedcountersunk.

Wood screws are driven by engaging a driver in

the slot, or recess, in the screw head. The re-cessed head wood screw, as mentioned in BS1210'Specification for wood screws', offers many ad-

vantages over its slotted counterpart when drivenusing either hand, spiral ratchet or power drivers.

MAIN ADVANTAGES

Wood screws are superior to nails and staples

where firm joints, between wood and wood, arerequired and for attaching metal components, e.g.

hinges, brackets, latches, locks, decorative trim,etc. , to wood.

Wood screws can be removed, and re-tightened,if subsequent adjustment to the assembly is neces-sary. Any adjustment is not possible in the caseof nailed joints particularly if the nails or stapleshave been clinched. Unclinched nails and staplescan be subsequently removed but not without caus-ing damage to the surface around the head of thefixing.

The wood screw thread, as the screw head is rotat-ed, draws the wood screw down into the wood andcreates a clamping force between the surface ofthe wood and the screw head which, assisted byfriction between the mating surfaces of the com-ponent and the wood, really grips the attachment.

Wood screws can carry higher axial loads, i. e.

loads tending to withdraw the screw from the wood,than nails of a similar diameter inserted at right

angles to the wood grain.

Basic resistance to withdrawal in soft woods of:

10 sg. (0. 192 in. dia. ) wood screw = 80 lb. perinch of thread penetration

6 swg. (0. 192 in. dia. ) round nail = 25 lb. per inchof penetration6 swg. (0. 192 in. dia. ) ringed-shank nail = 37 lb.

per inch of penetration

Note. When large changes in moisture content,after nailing, are expected the loads quoted fornails must be divided by four.

If loads in single shear have to be carried then thelength of nail required is much longer, diameterfor diameter, than the length of the wood screwrequired, i. e. 10 sg. wood screw requires a pen-etration depth of II in. and the 6 swg. round naila penetration depth of 2j in.

If glued joints are required in an assembly, e.g.

fixing plywood sheet to a wood framework, thenwood screws, because of their clamping action,

will pull the mating glued surfaces together anda stronger joint will be obtained than if the gluedjoints were nailed only. Wood screws are partic-ularly useful, where glued joints are required, forsupplying temporary joint strength whilst an art-icle, under construction, is proceeding through asequence of operations, since the glued joint itselfmight take 24 hours to set or cure.

Wood screws are generally used in high class join-

ery and cabinet work, nails are primarily used in

heavy construction and rough work, e.g. fences,sheds, pallets, roof structures, etc.

138

Wood screws are preferable to nails for vehicle

body or caravan construction when the framework

is liable to flex or bend in use.

MAIN DISADVANTAGES

Wood screws cost more than nails or staples and

are more costly to apply.

TYPICAL APPLICATIONS

The three basic head styles on wood screws have

particular uses:

The countersunk head wood screw (Fig. 2a) is prim-

arily used for fixing wood to wood and for fastening

metal to wood. The countersunk head is drawn, as

the screw is tightened, directly into the surface of

the wood attachment, if soft, or into pre-drilled

countersunk clearance holes, if the wood is hard

or the attachment is metal. This leaves the surface

of the attachment, or fixture, completely smooth,

e. g. the inside faces on the leaves of a butt hinge

are free to close up flush with one another. The

countersunk head wood screw is the most commonly

used of the three head styles.

The round head wood screw (Fig. 2b) is primarily

for fixing metal components to wood, e. g. metal

shelf brackets, gate latches and rough ironmongerypossessing punched clearance holes only.

The raised countersunk head wood screw (Fig. 2c)

is used for fixing costly attachments to wood and

which, from time to time, have to be removed for

adjustment or repair, e.g. wood strips retaining

glass panelling, wood panelling and high class

architectural ironmongery.

The screwdriver blade, engaging in the slot of a

raised countersunk head does not come into contact

with the expensively finished surface on the fixture

when the screw is finally tightened. These screws

can also, of course, be removed without causing

any damage to the surface close to the screw head.

If these screws are to be removed regularly to re-

lease a wood attachment, e. g. ammunition box lids,

wooden access panels, etc. , then to ensure protec-

tion for the wood itself, as distinct from its sur-

face, screw cups should be let into the surface or,

alternatively, surface screw cups can be used.

Other head styles which have particular uses are:

Clutch head wood screw (Fig. 3a). This head style,

since it is non-removable, is thief proof. When

driving these wood screws into wood, i. e. turning

the screws in a clock-wise direction, the screw-

driver blade makes contact with the walls in the

slot. However, the wood screws cannot be un-

screwed, i. e. turned anti-clockwise, as the slot

walls have been removed.

Note. Test the application with a conventional head

first before attempting to drive the clutch headwood screw!

Laying-in screw (Fig .3b). These screws are de-

signed to receive a moulded, or cast head, e. g.

cast door knobs and screws with special decorative

heads usually brass or cast iron.

Headless screw (Fig.3c). The shank of the screwcan act as a moving part, or stop, in a mating slot

machined in a wood component.

Mirror screw (Fig. 3d). The chromium plated

brass dome top, screws into a tapped hole in the

top of a countersunk head wood screw.

Plastidome tops and retaining washers (Fig .3e)

.

These moulded plastics tops provide a decorative,

or protective, cap for use with wood screws and

are available in various colours.

Recessed head wood screws

The recess wood screw (Fig. 4), as mentioned in

BS1210, offers many advantages over its slotted

counterpart.

Am =8# 4Fig. 4.

The resulting benefits are:

1. Minimum damage to recess during driving,

which facilitates full tightening and lessens the.

hazard of loose screws in an assembly. Higher

torques can be applied and tighter joints obtained

than with traditional slotted head screws.

2. Reduced damage to work surfaces since the

driver will not slip out accidentally and damageexpensive finishes as present in top- grade architec-

tural ironmongery and pre-finished wood surfaces.

139

3. Reduced operator fatigue because negligibleend-load is required to keep the driver in the re-cess.

4. The fit of recess drivers makes for easieralignment and greater control in driving the screwsin near inaccessible positions; the screw and driv-

er behave as a single tool. Once the screw is pos-itioned on the driver only one hand is required to

drive the screw home. The remaining hand is thenfreed to hold the components being assembled or

to maintain the operator's balance if standing on aladder or scaffolding. Again, this is not possible

with the driver blade and slot combination.

WOOD SCREWS - CONVENTIONALTHREAD

The general applications of this type of wood screwhave been discussed under the uses of its different

head styles.

of stops and starts, have also to overcome static

friction, i. e. the resistance to start the screwmoving. Pilot holes and thread lubrication aretherefore more often required when screws arehand driven.

Conventional wood screws used with Fibre or plas-tics wall plugs

The plain shank on a wood screw should never enterthe wall plug. Obviously, if one attempts to burythe shank of the wood screw into a wall plug, sincethere is not the available space between the shankand drilled hole for ihe plug to expand, the woodscrew will sieze or lock and further turning willbreak the wood screw at the thread shank junction.

Therefore, where there is a surplus length of shankof the screw after passing through the article to befixed and which would otherwise enter the wall plug,the latter should be sunk that much below the sur-face.

Experience over the years has shown that the fol-

lowing summary of conditions contributes to opti-

mum wood screw performance.

Pilot hole size

In soft woods the diameter of the pilot hole is im-portant and should be approximately 70 per cent

of the core diameter of the screw.

In hard woods the diameter of the pilot hole should

be about 90 per cent of the core diameter of the

screw.

Lubrication

Lubrication such as soap, tallow, beeswax or lano-

lin may be used, when necessary, for easy inser-tion of the wood screw without any great loss in

holding power.

Holding power

For screws subjected to lateral loads the screwthread penetration depth should never be less thanfour diameters and should preferably be equivalent

to seven times the screw shank diameter. Woodscrews should never be loaded in tension, tendingto cause withdrawal, if driven into the end grainof timber.

Design of timber structures

The permissible loadings on wood screws in hardand soft timbers, their correct spacings, etc. , arefully covered in the British Standard Code of Prac-tice CP112:1967 'The structural use of timber'.

Power-driving wood screw into timber

It is possible to drive wood screws with powertools into timber which, if they were hand driven,would break; power driven screws have only to

overcome prevailing frictional resistance. Handdriven screws, since they are driven in a series

WOOD SCREWS - THREADED TO HEAD

The most popular type of screw in this range hasa two start thread with parallel core diameter ter-minating in a gimlet point. The screw is partic-ularly suitable for use in low density chipboard,block board and soft woods.

These screws can be driven home in half the time.Splitting of the wood or board is minimised; thecore diameter/outside diameter possessing paral-lel contours except, of course, the gimlet point.The gimlet point functions like a drill point in thatthere are two diametrically opposed threads engag-ing in wood, at the commencement of entry, whichgive symmetrical loading conditions. In fact, thesescrews can be driven without the assistance of pilotholes straight into soft woods at 60° to the surface.

There are two types of two start threaded woodscrews; the shorter screws which are threaded tohead and the longer ones, over § in. long, whichare threaded at least 75 per cent of their overall

length (Fig. 5). The latter have a relieved shank,the diameter of which is less than the outside dia-meter of the thread, between the thread and head.

The screws which are threaded to head are partic-ularly suitable for fixing thin attachments to chip-board. They offer 25 per cent greater holdingpower, because of the extra threaded length, thanthe equivalent conventional wood screw. This in-

crease can be even greater in sandwich type chip-boards when the extra threads beneath the head can

140

Table 1 . Availability of wood screws.

Material RelevantSpecifications

Recessor Slot

HeadStyle

Range of

GaugesRange of lengths

according to

diameter

Conventional Wood Screws s.g. in.

Steel BS1210 Slot Csk.RoundRsd. Csk.

0-200-142-12

i -6* -3*6 -2

Steel BS1210 Recess Csk.RoundRsd. Csk.

3-143-123-10

i - 34I -2i -2

Brass BS1210 Slot Csk.RoundRsd. Csk.

0-201 - 16

2-12

4 -44-3i - 24

Brass BS1210 Recess Csk.RoundRsd. Csk.

3-123-84-8

1 -2i - 14

1 - 14

Stainless Steel

18/8 Austenitic

BS970 EN 58 Slot Csk.RoundRsd. Csk.

2-182-142-16

i - 41 -3i-24

Stainless Steel

18/8 Austenitic

BS970 EN 58 Recess Csk.RoundRsd. Csk.

4-124-124-10

4-24-24 -14

Aluminium Alloy BS1473 HB 15 Slot Csk.RoundRsd. Csk.

3-124-124-10

1 -34-144-14

Aluminium Alloy BS1473 HB 15 Recess Csk.RoundRsd. Csk.

Made to order

Silicon Bronze BS2873 CS 101 Slot Csk.RoundRsd. Csk.

4-205-184-8

4 -5i -2J - 14

Monel 500(Trade name -

Henry Wiggin & Co.Ltd.)

BS3075 Slot Csk.RoundRsd. Csk.

Ms»de to order

Wood Screws threaced to head

Steel Recess Csk.RoundRsd. Csk.

4-144-104-10

8-24i -21 -2

engage in the denser layer of wood chips near the

board's surface. In this type of application it is

possible to replace conventional wood screws with

shorter wood screws possessing two start threads.

The longer screw, with the relieved shank, can be

driven more deeply into thin sections of soft wood,

or the edge of chipboard, than conventional screws

of the same size, before splitting occurs.

However, screws driven into the edge of chipboard,

or into wood end grain, should the necessity arise,

may require pilot holes so that long screws maybe used without causing splitting.

Pilot holes in chipboard should be between 60 and

90 per cent of the thread core diameter dependent

upon board density. The larger pilot hole is neces-

sary if mimimum disruption of the surface layer

of wood chips is essential; all screws inserted

without pre-drilled pilot holes, tend to lift the sur-

face chips, and disrupt those beneath, so causing

a loss in holding power.

Wood screws threaded to head do not require double

drilling for shank and thread as does the conven-

tional wood screw, when fixing thin attachments to

laminated chipboards.

However, these wood screws require a clearance

hole in the attachment to ensure that the attachment,

on final tightening, is pulled down on to the surface

into which the screw is driven. Otherwise, the

head will pull down and lock against the thread formgenerated in the attachments.

141

Table 2 .

Steel wood screws Referencenote Nos.

RelevantBritish

Standards

Protective and

1

1

BS1706

BS1706

BS1224

BS1224

decorative finishes

Bright zinc plated

(electro galvanised)Bright cadmiumplated

Nickel plated (bright

or dull)

Nickel chromiumplated

Decorative finishes

2

2

2 & 3

4 BS729

Copper plated

Brass plated

BronzedBluedJapannedBerlin blackedSherardised

Brass screws

BS1224

BS1224

decorative finishes

Nickel-chromiumplated

Nickel plated (dull

or bright)

Decorative finishes

2 & 3Bronzed

Aluminium wood

2 BS1615

screws

Protective finishes

Anodised andlanolin dipped—Protective and

2 BS1615

BS1615

decorative finishes

Anodissd and dyed(colour anodised)Bright anodised

Note 1 The protective value of zinc and cadmiumand their receptivity for paint or lacquercan be increased by supplementary pas-sivation treatment.

Note 2 The durability of appearance and protec-tion of many finishes can be improved byapplication of lacquer or wax.

Note 3 'Bronzed' covers many decorative finish-es applied to brass or copper plated sui

—

faces, e.g. florentine; bronze metalantique; copper oxidised; steel bronzed;antique coppered; antique brassed . Cor-rosion resistance can be conferred byspecifying an adequate coating beforebronzing

.

Note 4 Sherardised screws tend to develop arusty colour and stain if not painted priorto weathering.

Wood screws, threaded to head, are particularlysuitable, when used in conjunction with plastics orfibre wall plugs, for fixing attachments to glazedtiling in bathrooms; the plugs can grip into the tile,

as well as the brickwork beyond, without tendingto burst the tile when the screw is driven home.

MATERIAL, SELECTION ANDSPECIFICATION

Wood screw materials

The range of sizes and gauges available in variousmaterials is listed in Table 1.

Selecting the correct material

The material selection is primarily based on achoice bearing in mind the corrosion aspects ofthe application, and the physical and chemical pro-perties of the material from which the attachmentis made, the physical strength of the wood screwbeing much stronger than the wood into which it

is driven. However, wood screws, threaded tohead, are generally available in steel only.

Selecting the correct finish

The selection of wood screws for particular appli-cations should be based upon the 'in-place 1 cost.The fastener may cost more initially, but costlyreplacement action, due to rusting, for instance,will be eliminated. The quality of plated coatingdepends largely upon the thickness of the depositbut one should be careful to discriminate betweenprotective and decorative finishes, e.g. chrom-ium plated brass screws are corrosion resistantwhilst bronzed steel screws are decorative only.Protective finishes for wood screws are listed in

Table 2.

PRICES

These are dependent upon the type and size of thescrew and material. To keep costs down it is bet-ter to select wood screws which are classified as'preferred sizes', these are much cheaper thanthe 'non-preferred sizes'. BS1210 lists the cate-gory of each size and type of wood screw.

FUTURE TRENDS

The recessed head wood screw will, according tocurrent trends, gradually supersede the slottedhead wood screw.

Metrication will not directly affect wood screws inthe forseeable future, since they are already ac-cepted internationally. Metric conversion tables inBS1210 show the range of wood screws availableboth in inch and metric units.

142

Table 3 . Principal dimensions of wood screws .

SLOTTED HEADS V __j .^C

B

90 Al\»\V\%' V\»A\»

COUNTERSUNK HEAD

RECESS HEADS

90°I

A l\v\\n\M\»\v\i»i

COUNTERSUNK HEAD

If*

ROUND HEAD

RAISED HEAD ROUND HEAD

Nom . Size Number of

Threadsper inch

Countersunk & Raised Heads Round Heads Slot Width Recess&

DriverNumber

A B C D E H

S.G. Dec. Max. Max. Approx. Max. Max. Min.

0.060 30 0.120 0.035 0.O20 0.116 0.045 0.016 -

1 0.070 28 0.140 0.041 0.O23 0.140 0.053 0.021 -

2 0.082 26 0.164 0.048 0.O27 0.164 0.062 0.026 -

3 0.094 24 0.188 0.055 0.031 0.189 0.071 0.030 1

4 0.108 22 0.216 0.064 0.036 0.215 0.081 0.032 1

5 0.122 20 0.244 0.073 0.O41 0.241 0.090 0.035 2

6 0.136 18 0.272 0.082 0.O45 0.267 0.100 0.040 2

7 0.150 16 0.300 0.091 0.O50 0.293 0.109 0.040 2

8 0.164 14 0.328 0.100 0.O55 0.319 0.118 0.045 2

9 0.178 12 0.356 0.109 0.059 0.345 0.127 0.045 2

10 0.192 12 0.384 0.117 0.064 0.372 0.136 0.050 2

12 0.220 10 0.440 0.135 0.073 0.424 0.154 0.055 3

14 0.248 9 0.496 0.153 0.O83 0.476 0.171 0.065 3

16 0.276 8 0.524 0.170 0.092 0.529 0.190 0.065 3

18 0.304 V4 0.608 0.188 0.101 0.580 0.207 0.075 -

20 0.332 7 0.664 0.205 0.111 0.632 0.226 0.075 -

24 0.388 6 0.776 0.241 0.129 0.743 0.265 0.085 -

28 0.444 5% 0.888 0.276 - - - 0.095 -

32 .500 5 1 .000 0.310 — — — 0.095 -

143

20

Spring steel fasteners

by H.D. Browne (Firth Cleveland Ltd.)

Spring steel fasteners were originally developedin the United States in the early twenties. The in-

ventor was A. H. Tinnerman, President of a corp-oration which pioneered the development of sheetmetal cookers. Problems associated with the in-

troduction of vitreous enamelled sheet metal fabri-cation gave rise to the invention of the first springsteel nut, which they called the 'Speed Nut' (Fig. 1).

These arched spring steel nuts gave the necessaryresilience to prevent cracking of the vitreous enam-elled panels during transit and yet locked the screwunder firm spring tension. Ordinary nuts had to be

tightened very securely to ensure that they remain-ed locked, whereas the new spring steel nuts ach-ieved their locked position at a much lower tighten-

ing torque.

This principle is still used today in speed nutsmanufactured in Great Britain under Tinnerman'slicence and extensively employed in a wide varietyof industries. Many other variants of the rangeare now available but they all employ a similarprinciple to that invented by Tinnerman.

In spite of the success of spring steel fasteners in

the United States it was not until after the war that

they became available in the UK, although theywere manufactured in a small way, mainly for spec-

ific military applications, during the war.

MATERIALS

Spring steel fasteners are usually made from closeannealed carbon steel strip and after being formedthey are heat treated - hardened and tempered - to

give them their characteristic resilience and tough-ness. The hardness figure varies, being adjustedwithin a range which will suit the duty of the partic-

ARCHEO PRONGS COMPENSATING THREAD LOCK

ARCHED BASE

Fig. 2.

SELF-ENERGISINGSPRING LOCK

Fig. 3.

ular fastener. Spring steel fasteners are usually .

made on multi-stage progression tools on high speedpower presses, or four- slide presses.

THREADED FA; ENERS

These fasteners have been discussed in greaterdepth in previous Chapters but are further mention-ed in this Chapter due to their importance as afastening medium.

Spring steel fasteners can be divided into two maincategories: those that receive a threaded member,such as a screw or a bolt, and others for non-threaded members.

The basic threaded member has a double lockingaction provided firstly by the arched base and sec-ondly by the arched prongs, the principle is demon-strated in Fig. 2. Fig. 3 shows the nut tighteneddown and locked by tiie self- energising spring lockof the base and the compensating thread lock, asthe arched prongs engage the thread. This type,which is normally available in rectangular or cir-

cular form, offers several advantages over con-ventional fasteners.

1. It is self-locking and thus eliminates locking

washers.2. Its relatively large surface distributes the

load over a greater area.

3. It is locked at a much lower torque than con-ventional nuts and it is this resilience which hasgreat advantages when assembling glass, plastics

or vitreous enamelled components because it pre-vents cracking or grazing, due to overtighteningor to shock in transit.

4. It is much quicker to use than conventional nutsparticularly if used with the coarse pitch screwswhich are specially designed for the purpose.

The conventional sheet metal screw or self-tappingscrew can, however, ac used with this type of fast-

ener and they are usually made to suit a wide vari-

144

ety of threads from 6 BA up to & in. Whitworth,

UNC or ACME.

However, the finer the pitch of the screw, the thin-

ner the material of the nut must be and thus BAsizes should be used for light duties only. If a

stronger nut is required, then a Whitworth or sheet

metal screw type should be chosen, or for very

heavy duties those designed for use with an ACMEbolt. Such bolts can achieve an ultimate tensile

loading of over 2000 lb. , whereas the 6 BA at the

other end of the scale will give a tensile load of

about 95 lb. only.

One of the most useful features of spring steel fast-

eners is their ability to overcome problems of

blind assembly, and the 'U' type nut (Fig. 4) is one

of the many spring steel fasteners which enable

very substantial savings in production cost to be

made, for the following reasons:

1. They can be assembled to the panels by hand,

by unskilled operators. No welding, no riveting

or staking, and no special tools being required.

2. They remain captive to the panel, anchored

by means of a sheared tongue on the lower leg,

which drops into the mounting hole, and while hold-

ing the nut in the screw receiving position, allows

a certain degree of 'float' to facilitate speedy as-

sembly.3. This type of fastener can be fitted before or

after the panels have been painted because there

is no danger of clogging during the spraying opera-

tion.

4. If the panels are to be vitreous enamelled,

there is no problem of masking threads, or re-

tapping after enamelling. The nuts are merelyslipped on to the panels at any convenient point on

the production line after the enamelling process.

5. On finally inserting and tightening the screwthis type of fastener is securely locked and elimi-

nates the need for any other form of locking such

as special washers of various types. Furthermore,

the lock has been achieved at a much lower tighten-

ing torque than when using ordinary threaded fast-

eners, so minimising the danger of cracking the

vitreous enamelled surface.

6. If conventional fasteners are used and the

thread is stripped or crossed or found to be faulty

in some way, the cost of rectification may be quite

considerable, involving side-tracking the compon-ent to have the fastener drilled out and replaced

or, in extreme cases, the whole assembly mayhave to be scrapped. This is not necessary whenthe 'U' nut is used because it can be quite easily

removed without damage to the panel and replaced

just as easily without any disruption of the produc-

tion line.

7. Spring steel fasteners are usually lighter than

conventional fastenings and in certain applications

- such as aircraft, for example - this factor maybe very important.

The 'U' type spring steel fastener is today verywidely used in the major mass-producing indust-

ries but there are also hundreds of other variations.

Ezzzazg

Fig. 5. The expansion nut,

Fig. 6. The heavy duty latching type nut.

Fig. 5 illustrates an expansion nut. This is used in

a square hole when the fastening position is remotefrom the edge of the panel. As the screw is insert-

ed it expands the body of the nut, thus holding it

firmly to the panel. This type of fastener offers

all the advantages of the 'U' type with the exception

of the floating feature which is sometimes neither

necessary nor desirable.

Fig. 6 shows a latching type nut which is usually

used with ACME threaded bolts and produces a

very heavy duty fastener. A typical example of

an application is its use to fasten the top half of

a commercial vehicle cab to the lower half. Thisenables the overall height of the vehicle to be con-siderably reduced for shipping, and the simple,

virtually foolproof, fastener enables unskilled

labour to reassemble the cab on arrival both quick-

ly and easily.

145

Fig. 7. Assembly of the caged nut.

SSZ33

Fig. 8. The 'J' nut.

Fig. 7 is also used in' a square hole and replacescostly welded cage nuts and fastenings of a similartype. It is installed into the panel by hand whereit remains captive. The full threaded nut in the

cage - a nut of full depth - floats slightly to over-come the problem of misaligned holes. The fast-

ener is fitted after the finishing process at anyconvenient point on the production line and is avail-

able in three sizes of cage, covering threads from6 BA up to | BSF/BSW or Unified threads.

Similar in concept to the 'U' nut, but with a shorterleg designed to snap into a clearance hole, is the'J' type (Fig. 8) which is easily started over theedge of a panel and pressed into position with thethumb. A typical application for the 'J' nut is thereplacement of reinforcing rings and blind busheson headlight assemblies in the automobile industrywhere, clipped into screw receiving positions onthe wing aperture, tiie short leg on the front sideof the nut ensures a good seal between gasket andwing, thus precluding mud leakage (Fig. 9).

These are just a few of the spring steel fastenerswhich are available today for use with threadedmembers. The full range includes specially de-signed nuts to allow tightening by spanner; self

Fig. 9. A typical application of the 'J' nut.

retaining types, such as those already described,with two spring arms lo provide sufficient tensionfor the nut to slide along the moulding channel andhold it in place; weld nuts; multi-impression nutswhich are particularly useful for applications suchas securing hinges to domestic appliances and foroffice furniture; locking nuts which permit fineadjustment of the screw while maintaining a cons-tant torque and vibration-proof locking and whichare often used as trimmer nuts and movement con-trol on push-button switches; angle nuts, designedto overcome the problem of attaching back panelsto cabinets and which can replace a flange or formthe corners of a complete assembly; latching nuts- captive heavy duty nuts such as those alreadydescribed and which are positioned from one sideof the application and can have an ultimate tensileloading of well over half a ton; and a variety ofwood anchor nuts for wooden structures and as-semblies; captive nuts; expansion nuts as alreadydescribed; quick release nuts designed to speedup assembly when it is necessary to run nuts downa length of 2 BA studding; clamping nuts, suitablefor use as terminal nuts to make electrical con-nections; beading nuis, designed to positively secureradio and television cabinet backs; and a wide vari-ety of caged nuts.

NON-THREADED FASTENERS

Of those spring steel listeners not associated withthreaded members the most widely used is the push-

146

on fix. This takes many forms but the basic rec-

tangular or round type uses the same arched based

principle as the plate type of threaded fastener

(Fig. 10).

These fixes, unlike the nuts, are not pitched to

follow the helix of a thread; the two sheared armsare of equal height. As the push- on fix is forced

over the plain stud the fixing legs bite into the sur-

face and, on finally depressing the arched base,

which of course reacts as soon as the pressure is

released, the fixing legs are given a strong upward

and inward pressure which firmly holds the fast-

ener in position. This reaction has the effect of

drawing the assembly together, thus removing any

possibility of rattle because it is held under spring

tension.

Various types of these fixes are used for a wide

variety of applications - from fixing decorative

trim to domestic appliances to retaining intricate

electrical components in computers and other elec-

tronic devices. They are extremely cheap and easy

to use and help to simplify many design problems.

However, it is very important when using push-on

fixes to ensure that the tolerance on the stud dia-

meter is held to within reasonable limits and to

get the best result from this type of fastener a tole-

rance of + 0.002 in. - 0.003 in. is recommended.There are special parts made for fixing chromiumplated studs but it is usual to mask the studs during

the plating process, thus avoiding an excessively

hard surface.

Push- on fixes take many forms, from the simple

sheared type, still widely used, to the more mod-

Fig. 10. The push-on fix.

Fig.1 1 . The plastics capped push-on Fix.

ern blanked types; the multi-pronged type; fixes

for rectangular studs; and plastics capped types

(Fig. 11).

The plastics capped fix was originally used to sec-

ure glass fibre insulation to the bulkheads in naval

vessels, being pushed over studs which were pro-

jection welded to the bulkhead and protruded through

the neoprene covered glass fibre blanket. This pat-

ented capped fix is now used on washing machines

to hold hinges to spin drier lids, on toys to hold

wheels and in many other applications.

Fig. 12. The tubular type

.

To fix these type of fasteners it is, of course, nec-

essary to have access to the back of the panel. If,

however, assembly is possible from one side only,

the tubular type (Fig, 12) is used. This consists

of a small spring steel split tube which is pushedon to a hole in the panel where it remains captive

and ready to receive the studs of the component to

be fixed.

These fasteners are made in two main types, lock-

ing and removable. The former are used for ap-

Fig.13. The elongated tubular clip.

147

plications not likely to be dismantled and the latterare extremely useful for applications which requireto be dismantled from time to time for maintenance.

A problem associated with this type of fastener is

the difficulty of holding accurately the dimensionsbetween hole centres, or pin centres, when usingmore than one stud, but an elongated tubular clip(Fig. 13) has been developed which allows a usefultolerance between centres.

The fixing of knobs to shafts by means of springclips is a long established assembly method, origin-ally developed in the radio industry and now widelyused for the assembly of electric irons, cookercontrols, thermostats - in fact anything which hasknobs. There are several types of fastener avail-

able among them being the 'D' shaped device for

thermosetting materials (Fig. 14) and the compres-sion ring type for thermoplastics. There is alsothe leaf type and others covering a wide range of

knob styling and shaft diameters.

One widely used on car push-pull turn controls is

shown in Fig. 15. It engages with a stud which in

turn is located in a hole in the knob. This permitsa push-pull action for light or choke controls, forexample, and/ or a turning movement for wind-screen washers. To remove, the stud is pushedout of engagement with the knob by means of a

small peg and the knob withdrawn from the shaft.

This is a typical example of a special developmentto meet specific applications which at the timewere not covered by the standard range available.

All these knob clips offer considerable advantagesover the grub screw method, the main one being

Fig. 15.

that they do not become loose and fall out. Themoulding of the knobs is simplified, no brass in-

serts or tapped holes or trapped nuts, no split toolsor side drilling to provide holes, and no sealingof grub screw holes to avoid electric shock fromlive shafts, being necessary.

The use of spring steel fasteners, selected fromthe wide range of standard parts available, hassteadily grown throughout the years particularlyin the automotive and domestic appliance industries,but in the radio and television industries, while

they too have extended the use of standard parts,it has been necessary to develop many special partsto meet their particular requirements. An exampleis the coil former supports shown in Fig. 16. Thisis designed to accept a plain wire wound tube con-taining a free dust iron core, the core being adjust-ed by means of a threaded brass stem which in turnis engaged in a helix formed in the base of the fast-ener. When the whole assembly is mounted to achassis it is possible io adjust the core as requir-ed, the threaded stem being locked at the desiredsetting by means of the two small arms protruding

from the base which exert a pressure against thecrest of the threads.

There is also a similar part available, designedfor internally threaded formers and threaded coreswhich requires no brass threaded adjusting stem,the dust iron core being adjusted by means of theinternal threaded tube, screw driver access beingthrough the base of the clip.

Other parts have been produced which are partic-ularly applicable to printed circuits, these partsbeing available in hot tin dip finish, enabling theuse of spring steel where previously phosphorbronze or beryllium copper was used.

Another type of specially developed fastener, shownin Fig. 17, is used to fix the screening cans to aprinted circuit chassis, the three-fingered claweffect gripping the edge of the square aluminiumcan and producing a very good electrical contact.The tongue passes through the printed circuit chas-sis and is soldered automatically in a solder bath.Other types of can fixing clips are available, such

148

as those which are attached to the side of the alum-inium can and sprung into the chassis.

Illustrating the versatility of the spring steel fast-

ener, those shown in Fig. 18 are now widely usedby paint manufacturers who find it necessary to

have an additional means of securing the lids of

their cans to avoid accidental spilling should the

can be dropped during transit. Previously this wasaccomplished by soldering, but there are now manytypes of clips available to suit almost any type of

closure. The one illustrated is designed for the

standard type of Metal Box Company's paint can,

used by a very large part of the paint industry.

Another rather specialised fastening is shown in

Fig. 19, it was specially designed to secure the

glass fibre sealing tubes which are widely used on

the inside surface of cooker doors. This clip is

opened by means of a special tool, the glass fibre

tube is then inserted and upon release the clip firm-

ly holds the tube and is ready to be pushed through

the clip receiving holes prepared in the door liner.

The fitting of the clip to the tube is usually done

by means of a specially prepared jig, 10 or 12

clips being fitted to the glass fibre tube in one op-

eration. This method has the additional advantage

that replacement tubes can be provided with pre-

assembled clips, thus considerably facilitating

servicing.

In recent years the building industry has been turn-

ing more and more to the use of spring steel fast-

eners and a typical application is in the erection

of suspended ceilings. The fastener shown in Fig.

20 was developed to engage with the bulb of a Tsection extrusion and to support at its lower end

M//l//t/lli

Fig. 20.

Fig. 21.

a glass fibre or polystyrene foam tile, held bymeans of the pointed legs which are forced into

the edge of the tiles.

The clip shown in Fig. 21 makes a different systempossible. Here the tiles or ceiling boards are sup-

ported on the T section and held down by the armsof the clip, which is pushed over the vertical leg

of the T.

SPECIFYINGFASTENERS

SPRING STEEL

There are many other types of fastener available

and, besides this enormous range, manufacturers

offer a development service which is available if

a ready-made solution to a fastening problem can-not be found. But since the development of specialscan be expensive it is in the designer's interestto call in the fastening specialist at drawing-boardstage when it may be possible, by slight adapta-tions of design, to employ selections from the manystandard parts available at considerable saving.

149

21

Washers

byR.M. Billington, M. Inst. M.S.M. (Mor-lock Industries Ltd.)

There are various applications where the use of themany types of washers available today adds in someway to the efficiency of the joint or bolted assembly.Washers are normally used under the head of abolt or the nut end of a bolt assembly in order to

distribute load, act as a thrust surface, provide alocking or sealing action or, in some cases, to in-

dicate the preload developed' in a bolted assembly.

To assist the design engineer in selecting the besttype of washer for any particular application, it is

intended to cover the whole range of washers avail-able in separate groups: (a) plain and tapered wash-ers, (b) lock washers, (c) seal washers and (d)

load indicating washers.

PLAIN WASHERS

Up until 1961, standard metal washers for generalengineering purposes had been covered by the re-levant British Standard appertaining to nuts andbolts but, with the publication of BS341 0:1 961, anew standard for all flat washers was introduced.BS3410:1961 covers flat, square and tapered wash-ers to suit both British and American threads, inbright and black metals.

Tables 1 to 5 in the British Standard cover fivedifferent standards of flat bright metal washers,embracing the old halfpenny and penny styles, invarying gauges of material. It is not intended toelaborate on the sizes available but these can beascertained from the British Standard, which isreadily available from the B. S. Institution.

In addition to comparable ranges of round washersin black metal there is also reference to squarewashers with round holes and round washers withsquare holes, specifically designed for use withcup head bolts in wood to metal applications. Fin-ally there are taper washers, which are availablein square or D-form and which are used to compen-sate for taper, in steel sections of 3°, 5° or 8°angles.

With the exception of taper washers, standardmetal washers are normally designed into an as-sembly to distribute the load and, in addition toconsidering the thickness and outside diameter ofthe washer, it is imperative that the designer con-sider the best finish for each application. Finish is

sometimes treated too lightly particularly in crit-ical joints where tightening torques are specified

It can be easily demonstrated that for a given tight-

ening torque applied to a nut and bolt assemblyonly approximately 10 per cent of the total torqueapplied goes into the loading of the bolt and themajority of effort is absorbed by overcoming boththread and interface friction. Interface friction is

naturally affected by the protective finish used onthe washer. If zinc plating is used the interfacefriction increases and as a result less is used in

(a) ROUND FLAT WASHER (b) ROUND FLAT WASHER,WITH 30° CHAMFER

'-5&tf- ^Oe(c) SQUARE FLAT WASHER (d) TAPER SQUARE WASHER

Fig.1 .

loading the bolt. In the case of cadmium platedwashers, the reverse is true and unless the lowercoefficient of friction is taken into considerationwhen designing the joint there is a danger of over-loading the bolt.

It is important to remember that once a joint hasbeen designed, the washer finish should not bechanged without reference, as an alteration in re-commended tightening torque may be necessary.

LOCK WASHERS

Lock washers can be best classified into threetypes, i. e. tab washers, spring washers and toothlock washers. The former can be produced in anymaterial whilst the other two are normally suppliedin spring steel, phospor bronze or stainless steel.

There is no general purpose standard for tab wash-ers, which are generally being replaced by othertypes of lock washer requiring less investment intooling costs, but a standard covering straight,right-angled and left-angled tab washers does existin aircraft quality - SP 41 to SP 45 (BSF) and SP107 - 109 (UNF). Designers requiring tab washers

150

(a) SINGLE COIL FLAT SECTION' SPRING WASHER

fb) SINGLE COIL SQUARESECTION SPRINGWASHER

(c) SINGLE COIL GIRDER SECTIONSPRING WASHER

(d) DOUBLE COIL SPRINGWASHER

(e) SINGLE COIL 'POSITIVE' TYPE SPRING WASHER

Fig .8.

are recommended to consult this standard, covering

from 2 BA - 1 in. diameters before designing spe-cial tab washers, which may entail a comparatively

high tooling cost.

Helical spring lock washers are the most commonlyused and fulfil the dual function of compensating for

loss of tension and developed looseness in a bolted

assembly, whilst also acting as a thrust surface to

facilitate assembly and disassembly of a bolted

fastening, by reducing interface friction.

Helical spring lock washers (Fig. 2) are available

in four basic types:

(a) single coil square section

(b) single coil rectangular section

(c) single coil girder section

and (d) double coil rectangular section

BS1082 and BS2061 are already in existence cover-ing all types manufactured in spring steel - En42 -

and all except the girder section washers in phos-phor bronze. In most cases, the British Standardwashers are suitable for the majority of jobs but

leading manufacturers usually have available at

least one range of cheaper, lighter section washerswhich can prove adequate and more economical.

Girder section steel washers have in the past beenused in an attempt to save material and money.However, the cost of manufacturing this special

section wire has made the finished washer moreexpensive and it is now generally prudent to investi-

gate the use of lighter square section spring wash-ers, which are more readily available and moreeconomical.

The motor car industry has adopted the AmericanStandard for helical spring washers - ASA B. 27. 1-

1965, which offers a wider range of qualities of

rectangular section single spring washers up to and

including bolt sizes of 3 in. diameter. It should be

appreciated that the wire section of a helical spring

washer, after coiling, becomes trapezoidal. That

is to say that the thickness at the inner periphery

is somewhat greater than that at the outer. This

problem is largely overcome with American Stan-

dard Washers by producing them from keystone

wire to compensate accordingly.

It is normal practice to produce rectangular sec-

tion spring washers with the width greater than

the height but because of the occasional necessity

to use helical spring washers with socket screws,

a range of Hi-collar washers is produced where

the rectangular section wire is coiled the reverse.

There is not yet a British Standard for metric heli-

cal spring washers but until one is published the

German DIN Specifications are normally followed.

All of the single spring washers already mentioned

are normally produced so that they will not tangle

or link together, but where double coil washersare used it is necessary to request the special

tangleproof type if required.

Helical spring washers, as previously mentioned,

rely upon the compensating action of their inherent

spring pressure to achieve their locking function.

However, a further factor is sometimes added by

shaping the ends of the washer into barbs which are

forced into both the nut and the parent material.

Whilst the inherent spring pressure keeps the as-

sembly correctly loaded the barbs tend to resist

the loosening of the nut, even in extreme vibration

conditions . This type of helical spring washer is

generally known as "the positive type 1 (Fig. 2e),

and is usually available in steel as a stock item in

sizes upwards of 6 BA.

Although the helical spring washer is the most pop-

ular spring washer used, there are sometimesapplications where a more even, and better con-

trolled, spring pressure is required. In these

cases the designer has a choice of several differ-

ent types of deformed flat washers manufactured

in spring steel. Probably the most simple of these

is the Belleville Washer (see Fig. 3a), which is

©)*(a) BELLEVILLE WASHER OR

DISC SPRING(b)WAVE OR CRINKLE WASHER

Fig .3.

produced in a conical form and derives its lockingproperty from the inherent spring properties of thematerial. Whilst normally used singularly, Belle-ville washers are sometimes combined in series orparallel to produce varying spring characteristics.

A derivation of the Belleville washer is the dishedwasher which is dished internally rather than coned.

Neither of these types of washer are available to a

British Standard although most spring washer manu-facturers are able to offer a comprehensive rangeof their own.

Where spring washers are required to withstand a

comparatively low compression load, the designeris offered either the simple single curved spring

151

washer or the multi-wave, or crinkle washer (Fig.3b). The single curved washer is best suited toapplications requiring a maximum range of deflec-tion using light loads, whilst the multi-wave typeof washer exerts a greater reactive force with asmaller range of deflection.

Multi-wave or crinkle washers are used in mostspring materials and are usually designed for speci-fic applications. However, the electrical and elec-tronic industries have found it necessary to use awasher having a high tensile and. fatigue strength,whilst also offering a high degree of corrosion re-sistance and electrical conductivity. This has re-sulted in the adoption of the beryllium copper crin-kle washer which is covered by BS3401 : 1961 andcaters for sizes from 10 BA to fin. An added bene-fit of all deformed types of spring washer is thattheir design eliminates damage to plated surfacesand consequently reduces the risk of corrosion.

Each of the spring washers covered has its ownparticular advantages to offer and not one can beselected as the best for any particular applicationwithout first considering the following questions:

1. What is the function of this particular springwasher?2. How critical is the application?3. What environmental conditions are likely to beencountered?

4. What are the space limitations?

The remaining type of lock washer is the toothedwasher (Fig. 4),. normally available in the UK aseither 'shakeproof or 'fan disc' type. 'Shakeproofwashers derive their locking function from a corn-

ea) EXTERNAL TOOTH (b) INTERNAL TOOTH (C) INTERNAL/TYPE 'SHAKEPROOF' TYPE 'SHAKEPROOF' EXTERNALWASHER WASHER TOOTH TYPE'SHAKEPROOF'WASHER

(g) COUNTERSUNK'FAN-DISCWASHER

(d) COUNTERSUNK'SHAKEPROOF'WASHER

(o) EXTERNAL'FAN-OISCWASHER

(f) INTERNAL'FAN-DISCWASHER

Fig. 4.

bination of three separate actions - line bite, thehardened tooth material cuts into the face of theworkpiece and nut or bolt head; spring reaction,each tooth acts as a compensating spring; and strutaction, where the teeth individually oppose the ten-dency to loosen by rotation. The teeth of standard'shakeproof' washers- are usually located on eitherthe inside or outside periphery of the washer buta range with both internal and external teeth is

available as is a range of countersunk externaltooth washers, specially designed for use with coun-tersunk screws. 'Shakeproof washers are producedin standard styles in sizes from 10 BA to 1^ in.

diameter and a wide variety of terminals and wash-

er plates having this patented locking feature arealso available.

'Fan disc 'lock washers are a similar type of tooth-ed washer but have the added exclusive feature ofoverlapping teeth, which cannot flatten completelyeven when excessive tightening torques are applied.

All of the previously mentioned lock washers aregenerally accepted as cheaper methods of insuringagainst fastener loosening than the more elaborateforms of stiff nuts or bolts with self-locking fea-tures. However, where mechanised assembly isused the fact that a two-piece fastener is cheaperthan a one-piece fastener with an integral lockingelement is not always ihe prime consdideration.

To assist in mechanical assembly, whilst keepingcos"ts to a mimimum, the designer can considerthe use of combined screw and washer assemblies(see Fig. 5), which ensure that the correct screw

Fig. 5. Represen-tation 'Sems' units

utilising 'shake-proof washers

.

-and washer are used together at any assemblypoint. These units arc available in a range of sizesfrom 6 BA to lin. diameter and usually a com-bination of metal thread screws with either helicalspring or toothed washers. In some cases, parti-cularly on the large diameters, disc spring wash-ers are also used. Pre- assembled nuts and wash-ers are made but are not in general use.

To sum up this section on lock washers, screwsnormally loosen because of either yield in the mat-erial of the fastener, or of the workpiece, or be-cause of improper initial application. Whilst thelatter can only be overcome by better educationand supervision of assembly staff, the former canbe compensated for by either spring action or add-ed interface friction.

SEALING WASHERS

The most simple forms of sealing washer are thosemanufactured as basic plain washers of easily com-pressed material such as rubber, plastics, leatheror, in some cases, soft metals, i. e. aluminium orcopper. There are, however, applications requir-ing a more reliable and efficient seal and in somecases an added locking action. To satisfy thesespecific applications a comprehensive range ofpatented sealing washers is available and furtherdetails of some of them are given below.

*Dubo' sealing washers

The 'Dubo' sealing washer (Fig. 6a) is basically aplain washer of special section manufactured of aspecial nylon material having good flow characteri-stics under pressure. When used under a nut theinner rim of the 'Dubo' washer is forced betweenthe threads of the bolt and nut, whilst also flowinginto the opening of the hole. The outer rim of the

152

(a) 'DUBO' WASHER (b) 'OOWTY' BONDED SEAL

(c) 'SELON' WASHER 'SELOC' WASHERINCORPORATINGEXTERNAL TOOTH•SHAKEPROOF'WASHER

Fig. 6.

(o) 'WEATH-R-SEAL'WASHER

washer simultaneously flows over the flats of the

nut, providing additional locking action to that al-

ready inherent in the spring property of the mat-erial. These washers are available in sizes fromJin. to 1 Jin. inside diameter and offer a compara-tively cheap but efficient seal in temperature rangesfrom -60°C to +200°C in varing load conditions. In

addition, they have excellent insulating properties

and are inflammable.

'Dowty' bonded seals

'Dowty' bonded seals (Fig. 6b) consist of a cadmiumplated steel washer, to which is bonded, under

heat and pressure, a synthetic rubber seal. They

provide a simple, efficient and reliable means for

the face sealing of gases and fluids at low and high

pressures up to 10, 000 lb. /sq. in. The use of bond-

ed seals reduces installation and maintenance costs

and dispenses with groove cutting or special mach-

ining. The presence of the steel washer enables

specific tightening torques to be applied.

'Selon' sealing washers

'Selon' sealing washers (Fig. 6c) are manufactured

in nylon and provide a similar seal to that achieved

with bonded seals, but at much lower working pres-

sures. In additon to a sealing lip on the internal

diameter, there is also a series of location tongues

which enable the 'Selon' washer to be used as a

captive washer for screwed assemblies. Used in

this way the washer ensures concentric location of

the seal. Sizes available range from 2 BA to lii

in. diameter and the properties of .the nylon elimi-

nate damage to the workpieces and the possibility

of electrolytic action between dissimilar metal

work surfaces.

•Seloc' washers

The 'Seloc' washer (Fig. 6d) combines the locking

properties of a 'shakeproof washer with the sealing

properties of the synthetic rubber ring in which it

is encased. The toothed lock washer, which can beeither external or internal tooth type, bites into the

rubber itself, increasing interface friction whilst

not reducing the sealing property of the ring. Ex-ternal tooth type washers are available in sizes

6 BA to \ in. and internal tooth type in the larger

sizes up to 1 in. When installed the 'Seloc' wash-er gives an effective seal in most environmental

conditions.

'Weath-R-Seal' washers

Amongst the very wide range of plastics, nylon

and synthetic rubber washers available for roofing

applications, perhaps the most efficient is the

Weath-R-Seal' type (Fig. 6e)- which is a laminated

compressive washer of a metal backing layer bond-

ed to a neoprene washer. When compressed by

the tightening of the assembled screw, the neoprene

provides a seal around both the outside diameter

of the metal washer and the screw shank. It can

be designed to provide a wide bearing area and the

even distribution of the neoprene provides a posi-

tive, long term protection from leaks.

LOAD INDICATING WASHERS

Load indicating washers are designed to provide a

simple, accurate check that the required pre-load,

or bolt tension, has been achieved in any particu-

lar application. They are usually used with high

strength friction grip bolts and one of their mainfeatures is that they require no elaborate installa-

tion equipment.

The 'preload indicating washer' (Fig. 7a) is avail-

able in sizes from No. 10 to \\ in. diameters, for

use with bolts of 125, 000 lb. /sq. in. and 160, 000

tensile strength. It is, in fact, a four piece as-

sembly, consisting of two concentric steel rings

sandwiched between two close tolerance, hardened

steel washers. The inner ring is smaller in dia-

meter and higher than the outer one by a controlled

amount and a known preload is indicated when the

Fig.7a. The action of the 'preload indicating

washer'

.

Fig .7b. The 'Coronet' load

indicating washer (By court-

esy of Cooper & Turner Ltd.

,

Sheffield.).

153

inner ring is compressed to a point where the outerring can no longer be freely rotated. The 'PLI 1

washer is used where a controlled preload averag-ing 80 per cent is required in the bolt and it is con-sidered accurate to within ± 10 per cent.

Perhaps the most well-known, and widely usedwasher of this type is the 'Coronet' load-indicatingwasher (Fig. 7b), which not only provides an indica-tion of correct tension in a bolt, more accuratelyand reliably than by either the part turn or torquecontrol methods, but also provides a permanentwitness of bolt tension when inspection is neces-

sary at a later stage. The 'Coronet' washer isbasically a flat washer with a number of protru-sions, from 4 to 8 depending on the size and qualityof bolt being used, formed on its upper surface.It is, wherever possible, used under the head ofthe bolt and as the assembly is tightened the pro-trusions are flattened. By gauging the gap betweenthe underside of the bolt head and the top surfaceof the washer a controlled measurement of bolttension can be ascertained. In order to obtain therequired preload in the bolt it is necessary to re-duce this gap to 0. 015 in. A simple feeler gaugeshould be used for this purpose.

154

22

Structural adhesives

by E.B. McMullon and D.T.S.Ilett (Bonded Structures Div., CIBA (A.R.L.) Ltd.)

How do you assemble load- carrying structures

fabricated from sheet metal? The most popular

methods in use today are riveting, and the manyvariations on the welding-brazing- soldering* theme.

Another alternative which is gaining in popularity,

but which is still far less widely used than it de-

serves to be, is structural adhesive bonding. This

is not a new process - it first gained acceptance

for use in aircraft primary structures (the toughest

test of them all) during World War II, a quarter of

a century ago. The technical and economic argu-

ments in favour of bonding are sound and well prov-

en. It is not a difficult technique. In many cases

it offers considerable advantages over the morepopular alternatives mentioned above. So why isn't

it more widely used?

The authors of this Chapter believe that the mainreason, perhaps the only reason, why so manyengineers neglect the possibilities of this process

is the mistaken belief that it is an 'exotic' tech-

ique that only the aircraft manufacturer can afford

tb use. It is true that the most adventurous use of

structural bonding methods is made by companies

in the aerospace industry or with a background of

aerospace experience. But this does not mean that

it is necessarily expensive. Aircraft quality workwill always be relatively costly because of the safe-

guards which must be built in, but in a competitive

market the healthiest manufacturers will be those

who can manufacture cheaply without compromis-ing this quality. We feel it is significant that oneof the healthiest of all European aircraft manu-facturers is the one which is most totally commit-ted to the use of bonding as its main componentassembly method, and is even using adhesives to

improve the mechanical properties of joints which

must be riveted - Fokker, the makers of the out-

standingly successful F. 27 Friendship.

What are the advantages of adhesive bonding? Forstraight-forward metal-to-metal jointing they canbe summarised as follows:

1. Lower production costs, particularly on largearea panels where assembly labour costs are large-

ly independent of size.

2. Reduced weight - the better load distribution

made possible by bonding enables the designer to

use lighter gauge materials.3. Increased stiffness - distribution of the ad-hesive over the whole joint area stabilises the

metal in the vicinitv of the joint.

4. Improved fatigue resistance - no stress con-

centrations (at rivets or spot-welds) or local metal-

lurgical modifications (encountered with brazing or

welding).

5. Smooth external finish.

6. Efficient integral sealing of joints.

7. Protection against galvanic corrosion in joints

between dissimilar metals.

A significant advantage which is not listed above

is the opportunity to explore the merits of sand-

wich construction, which cannot be achieved eco-

nomically by any other assembly process. Realisa-

tion of all these advantages starts at the drawing

board. It requires some understanding of adhesives

and what they can do, with some reorientation of

thinking by everyone associated with the process.

You can take a design intended for riveting and

adapt it for bonding. You may even improve it in

the process, but you won't get the best results that

way. They only come after design, planning, pro-

duction and inspection staff have acquired new habits

of thinking centred on the use of bonding.

WHERE CANBONDING?

WE LEARN ABOUT

A very valuable source of information is the com-pany manufacturing the adhesives. They will ad-

vise on the selection of suitable adhesives, design

techniques, stressing, manufacturing and inspec-

tion methods. They may also be equipped to carry

out pilot or even production assembly runs.

As a modest substitute for such expert advice, oras an armchair preliminary to seeking it, this Chap-

ter presents a brief account of what adhesive bond-

ing may be able to do for you.

WHAT CAN ADHESIVES DO?

A good bond between two pieces of metal will sus-tain high loads in shear or in tension where the

joint is suitably proportioned. A typical shearloading case is shown in Fig. 1; shear strengths

up to 8000 lb. sq. in. can be obtained, which wouldcause failure in the metal in a J in. overlap speci-

men using 16 swg. 2L73 aluminium alloy with theminimum specified UTS of 27 ton/sq. in. This is

the optimum loading condition for an adhesive bond-ed joint.

*See Design Engineering Metals Handbook for in-

formation on these joining methods

.

The adhesive will also sustain a high loading in a

direction normal to the plane of the bond. This

155

Fig.1 . Lap joint withadhesive in shear

.

Fig. 2 . A skin / stringerunder compressive load.

Fig. 3. Metal -metal joint

with adhesive in peel

.

would be impractical with the configuration shownin Fig. 1, but it is frequently encountered in thetype of situation illustrated in Fig. 2. In this casea skin/stringer combination is shown bucklingunder a compressive load, with the bond failing,first in a tensile mode, then continuing at the edgesof the failed section where the bond is subjected tocleavage loads. Excessive distortion in this casewould change the cleavage loading to peel loading,where the tensile load is concentrated along a nar-row line, as shown in more exaggerated form inFig. 3. Adhesives have only moderate resistanceto cleavage loads, and poor resistance to peel loads,and the designer should always strive to avoid thesetypes of loading in bonded structures.

This question of low peel strength is often cited byanti- bonding propagandists as the ultimate proofthat adhesives are useless in a structural context.How much truth is there in this? Consider the al-ternatives. Would you design a riveted, spot-weld-ed, brazed, welded or soldered structure that in-volved peel loading except as a secondary result offailure from some other cause ? Surely the answermust be "No", whatever the method of assemblyused. There is a great deal of loose talk and loosethinking on this subject by those who wish to con-demn or ignore bonding as an engineering technique.All adhesives have some resistance to peel loads,some more than others; none have enough to justifytaking advantage of it in design. Does a high peelstrength make one adhesive better than another?This is a matter of opinion, but our opinion is thatwe would ignore peel strength to gain an advantagein shear strength. Good peel strength will retardthe disintegration of a structure that has failed;

good shear strength will retard the failure - orprevent it.

HOW DOES THE ADHESIVE WORK?

There has been much discussion on this subject,but few agreed conclusions have yet emerged fromit. It is generally agreed that adhesion is a resultof intermolecular forces acting between the adhe-sive and adherend surface molecules. The strengthof the force is dependent on intimate contact be-tween both groups of surface molecules. The bondis made when the adhesive is liquid, and is im-proved if the adhesive has good 'wetting 1 proper-ties. Some materials can generate greater sur-

face forces than others, or produce more 'active'surfaces Metal surfaces are particularly active -

a lucky break for the engineer.

A practical joint consists of a thin layer of adhesivebetween two pieces of adherend material. To trans-mit loads through this joint the adhesive, which wasfluid to give good wetting characteristics, must nowbe modified to create a strong mechanical 'bridge 1

.

This is done by 'curing' the adhesive, which involv-es the transformation from a liquid state to thesolid state. The strength of the cured adhesive to

resist failure within itself when under stress is

called its 'cohesive strength'.

Structural adhesives are broadly divided between'cold curing' and 'hot curing' systems. Cold curingsystems are mostly epoxy based, and the basicresin is usually cured by the addition of a 'hard-ener'. The resin and hardener must be mixed to-gether in appropriate proportions, thus triggeringthe curing reaction. There is then a limited timeavailable for applying the adhesive between mixingand the point where the reaction has proceeded toofar for the adhesive to be worked. After makingthe bond, there is a further period before the ad-hesive has enough strength to withstand handlingor working loads. The time scale depends on thechemical relationship between the resin and thehardener, but at normal room temperatures theoverall time required before the assembly can besafely put to work may be several days. This canbe dramatically reduced by heating the assembly;for example, a typical epoxy system which requires24 hours to cure at room temperature can be curedin 20 minutes at 100°C.

Hot curing systems are those which rely on theapplication of heat to complete the cure, and willnot cure at normal temperatures. All phenolic-based systems and many epoxy-based systems re-act only on heating. There are many variationsin curing requirements; the general run of bondingwork is done with adhesives requiring a curingcycle typified by that for Redux* - 30 minutes at150°C. This is the optimum cure cycle for thissystem to ensure maximum strength and reliability;much more rapid cures are possible where a com-promise is acceptable.

*Registered Trade Mark.

156

When bonding with either hot or cold curing adhes-

ives it is usually necessary to apply pressure to

the assembly, partly to prevent relative movementbetween the adherends, but chiefly to maintain inti-

mate contact between the joint surfaces and the

adhesive whilst it is fluid in the earlier stages of

the curing process. With phenolic-based adhesivesit is essential to apply sufficient pressure to pre-vent 'blistering' arising from the evolution of vola-

tile products of the curing reaction.

WHAT FORM DO ADHESIVES TAKE?

Structural adhesives come in several forms: solu-

tions, liquids or pastes which may be single - ormulti- component, and dry films. One very import-

ant form has already been mentioned - the two-partcold curing adhesive, typical of epoxy resin systems.

This classification covers an enormous range of ad-

hesive systems, many developed for special pur -

poses, which are, perhaps, outside the scope of this

Chapter in the context of engineering structures; they

qualify from the point of view of their high strength

but are mostly used in small scale applications.

Structural applications in which bonding is com-petitive with other assembly methods typically in-

volve relatively large areas and large quantities

of adhesive. Consequently, the most popular sys-

tems are those in which the mixing ratio is not

critical, or which avoid careful mixing altogether.

A 'two-part system' that meets this requirementis the liquid + power adhesive, widely used for

metal-to-metal bonding on an industrial scale. Thisis, in fact, a hot curing vinyl phenolic system. Touse it, the prepared joint surfaces are coated with

the liquid resin, and a thermoplastic powder is

spread over the tacky wet resin. Loose powder is

simply shaken off, leaving a suitable quantity ad-

hering to the resin. It's simple and it works - that

is the adhesive system that was used to hold the

Comets and the Friendship together, among others,

and none of them have fallen apart due to failure of

the adhesive.

This same adhesive system is also available with

the two component parts processed to produce adry film of adhesive sandwich between two easily

removed polythene protective sheets. To apply

this, simply cut the film to size and shape, peel

off the protective covers, and lay it between the

parts to be bonded. It has the virtues of keeping

the proportions of the two parts constant and of

giving an even spread of adhesive, as well as being

extremely simple to use. For this reason, mostnew developments in structural adhesive systemsnow appear in film form.

Another class of adhesives that justifies considera-

tion in a structural context are the heat activated

paste adhesives. These are used widely in sand-wich structures, for splicing segments of honey-

comb core materials and for filling gaps aroundinserts or at panel edges. Particularly interesting

are those which expand due to a controlled foamingaction when heated, and set hard to maintain struc-

tural integrity at these otherwise vulnerable points.

HOW ARE THEY USED?

The process starts with the pre-treatment of the

parts to be bonded - the preparation of the adher-

ends to obtain surfaces of controlled quality for

maximum adhesion strength and reliability. The

usual first stage is degreasing, using suitable vola-

tile solvents which afterwards evaporate to leave

a clean surface. The best method is to use a sol-

vent vapour bath, but this will not remove heavy

grease deposits - these need the more robust atten-

tion of liquid solvent degreasing or an alkaline bath.

Adhesive systems which, are compatible with someprotective greases have been developed, and are

now being used in automobile mass-production as-

sembly processes.

Degreasing is followed by chemical or mechanical

cleaning to expose a fresh active surface. Chemi-

cal processes (pickling or etching) can be closely

controlled to give consistent results. Mechanical

abrasion is a simple and cheap way to prepare oc-

casional bonding jobs, but may be less consistently

reliable. It always creates dust, which, if not

properly extracted, will not help to promote the

cleanliness which is required in the bonding pro-

cess. Chemical processes usually turn out to be

cheaper in the long run. Which process to use de-

pends on the material to be cleaned and the volume

of work being handled - this is a case where it pays

to consult the adhesive manufacturer.

The next stage is to apply the adhesive, which maybe done by brushing, combing, spraying or mach-

ine extrusion for fluid systems, or by cutting and

laying for film adhesives. The parts are then as-

sembled ready for curing. If the adhesive is cold

curing the assembly need only be clamped to keep

the parts in suitable contact, and then set aside

for sufficient time for the adhesive to cure.

Hot curing adhesives require the application of

heat as well as pressure. How is this done? Thepopular choice for the general run of production

bonding is fairly evenly divided between the heated

platen press and the autoclave. Both of these in-

volve capital expenditure, but then, most produc-

tion techniques require some capital investment,

and most involve at least as much as bonding. Forthe cautious, however, it is worth mentioning that

there are other ways of applying heat and pressure,

and some extremely ingenious techniques have been

devised in cases where the job could not be done in

a press or autoclave. Much excellent production

work is done with clamping fixtures in a suitably

controlled-temperature oven, or by using radio-

frequency heating. Of the popular choices mention-

ed, the platen press is ideal where the bulk of the

work is concerned with flat parallel panels, or

assemblies based on flat panels. Where there is

a large turnover of similar flat panels it is worthconsidering the use of a multi- daylight press to

bond several panels simultaneously.

The more versatile autoclave can handle flatworkor curved assemblies with almost equal ease. It

is essentially a large oven with provision for pres-surising the shell and for evacuating a sealed flex-

157

ALUMINIUM ALLOY 2024 - T3'

AFTER !S HOUR ATTEMPERATURE a

-SO $ * i&j 15150

TEMPERATURE °C

10 102 103 104

EXPOSURE AT 150°C - HOURS

Fig. 4. Shear strength against temperature curves. Fig. 5. Ageing characteristics for two high-temperature resistant adhesives.

ible bag placed immediately around the assemblybeing bonded. Heat is usually supplied by livesteam, or may be provided by electric radiant ele-ments. The assembly is mounted on a jig or table,and covered with an impermeable flexible blanket(rubber or very pure aluminium, for instance) andclamped around the edges to form an airtight seal.Flexible tubes allow the inside of this bag to beevacuated or vented to the atmosphere while theshell of the autoclave is pressurised. This createsthe required pressure differential to hold the as-sembly firmly while the adhesive is cured. It alsopermits free escape of any volatile products of thecuring reaction. For rapid cycling of the autoclavesome degree of automatic control is usually incorp-orated; it would be feasible for this to be extendedto enable the autoclave to be completely automatedwhere it could be justified by the volume of work.

SELECTING AN ADHESIVE

The first step is to define the anticipated operatingconditions:

1. General operating temperature range.2. Extremes of temperature.3. Life (a) at the general operating temperaturesand (b) at the extremes of temperature4. Environment.

In that condensed list, the word 'temperature' oc-curs four times - it is the one factor which affectsthe choice of adhesive most critically. This canbe seen from the curves showing the variation ofshear strength with temperature presented in Fig.4.Between them, the adhesive systems shown cancope with the full range of temperatures for whichlight alloy structures would be appropriate, asshown by the equivalent curves for suitably pro-portioned 2024-T3 alloy adherends. Which par-ticular system to use depends on the part of thisrange in which the structure will be most likely tooperate. Since some adhesives are more sensitivethan others to changes in temperature it is alsoimportant to consider the probable extremes oftemperature even if these are only transient ex-posures.

The effect of prolonged exposure at extreme temp-erature is illustrated in Fig. 5, which shows theageing curves for two systems which have initially

similar strength at 150°C. One system retainsits strength up to 30 , 000 hours with no sign of de-terioration; the other has become dangerously de-graded after 1000 hours.

Expected environment is also an important factor.Not all structural adhesive systems are suitable

for use in the continued presence of water or watervapour, for instance. The ability of others to re-sist this type of environment, however, is demon-strated by the continued success of the SRN fam-ily of hovercraft manufactured by the British Hover-craft Corporation, in which adhesive bonding hasplayed an increasingly vital role with each genera-tion. A useful guide to environmental resistanceis provided by the ability of an adhesive system to

meet the requirements of the official specificationssuch as DTD5577 (British), MMM-A-132 (USA),MIL-A-25463 (ASC) (USA), which include a numberof environmental resistance tests. Other proper-ties are also included in these specifications - re-sistance to creep and fatigue, and the various mod-es of loading applicable to honeycomb sandwichstructures.

Such considerations will help the designer to makehis choice of system or systems appropriate to hisneeds. If technical considerations permit, the pro-duction engineer should also have an opportunity to

express his opinion. His co-operation will be need-ed later on. He will be interested in several prac-tical factors: cost of the adhesive; the work involv-ed in preparing it for use; the curing requirementsfor the system; its handling properties; and storagerequirements.

DESIGN CONSIDERATIONS

The design of adhesive bonded joints involves theproportioning of joint geometry to suit the physicalcharacteristics of the adherend materials, the ad-hesive, and the bonding process. These factors in-fluence the allowable stress upon which the designwill be based. For hot cured adhesive systems theallowable stress can be taken as 60 per cent of the

158

DOUBLE LAP JOINT(SYMMETRICAL)

JOINT FACTOR t/l

Fig. 6. Variation of shearstrength with joint factor.

nominal failing stress at the desired operating tem-perature. A severe operating environment, the need

for a long fatigue-free life, or creep considera-tions, may make it necessary to reduce the allow-

able stress still further. Even this assumes that

quality control in production will maintain consis-

tent bond strengths; relaxation of these controls

will necessitate the application of a larger than nor-

mal 'variability factor' at the design stage. Eachdesign team must arrive at its own best compro-mise when determining the allowable stress for

each adhesive.

We have said that all bonded joints should be loadedin shear. In sheet metal fabrications this nearlyalways involves simple overlap joints as the best

working compromise between theoretical and prac-tical requirements. At near-limiting loads there

is a small element of cleavage caused by the asym-metry of the joint, but this would probably exist in

any practical configuration. All technical data

about adhesives relates to the simple overlap joint

as the standard for comparisons of 'shear' strength.

In our own company test data is based on the single

overlap joint made from 16 swg. (0. 064 in.) alum-inium alloy to BS2L. 73, 1 in. wide, with i in. over-lap. The strength data presented in Figs. 4 and 5

was obtained from specimens of this type.

It is important to appreciate that the stress dis-

tribution in the joint is not uniform, and that the

maximum load that can be carried is not, there-

fore, proportional to the overlap. Whilst an. in-

crease in overlap will allow more load to be taken,

the gain is not linear. When considering variations

in joint proportions it is more instructive to talk

in terms of the ratio t/l or 'Joint Factor'. Typicalcurves showing the relation between shear strength

and joint factor, based on. laboratory test results,

are given in Fig. 6. These curves enable the de-

signer to predict, for this adhesive system and

adherend material, the best joint proportions for

a particular stress level.

SANDWICH CONSTRUCTION

The versatility of adhesive bonding as an assemblytechnique enables the designer to explore new waysof using his materials to increase the efficiency

with which they do their work. A striking example

of this is the honeycomb sandwich structure, which

is a practical way of realising the advantages of the

mass distribution of the I-beam in large panel de-

sign. Sheet metal (or other) facing skins are separ-

ated, but structurally connected, by means of a low

density core bonded between them. The skins corre-

spond to the flanges of the I-beam and carry the ten-

sile and compressive stresses. The core corre-

sponds to the web and carries the shear loads and

helps to prevent buckling and wrinkling of the faces.

By varying the skin thicknesses, core density and

panel depth, the designer can achieve a very close

approximation to the optimum distribution of his

material for any purpose, and can produce structur-

es of very high efficiency. Sandwich panels have

been made with skins of thin plywood, decorative

laminate, fibre-glass, etc. , and more convention-

ally from most metals including aluminium alloy,

steel, titanium, copper, etc. Skin thicknesses

may vary from 0. 0025 in. to 0. 25 in. The range

of application of simple sandwich panels is exempli-

fied by the solar- cell support trays for Ariel 3 at

one extreme, to the most heavily loaded deck panels

for the SRN4 hovercraft at the other. Shaped sand-

wich structures are widely used in aircraft workwhere full advantage is taken of their very high

resistance to fatigue, particularly at acoustic fre-

quencies.

Example is always more convincing than precept,

and we are very happy to conclude this brief outline

of bonding technology by quoting what we consider

to be a superb example of integrated design for

bonding at its best. It is particularly satisfying

to be able to say that it is a British design which

uses British adhesives and materials - the SRN4hovercraft. A section of the internal structure of

the buoyancy tank, which is in effect an optimised

flat plate of very large proportions on which the

superstructure is supported, is shown in Fig. 7.

The vertical shear webs are all stiffened by bonded

Z- stringers, and the upper and lower surfaces are

3in. thick bonded honeycomb sandwich panels to

carry the face stresses of what is, in effect, a

large sandwich panel with a rectangular- cell core.

On top of this, the bulkheads, roof beams and roof

plating all make optimum use of metal- to-metalbonding for rapid production and minimum weight.

Examples of the use of structural bonding can befound in every industry ranging from mass-pro-duction of bonded brake and clutch linings through

Fig. 7. Views showing the internal structureof the SRN4 Hovercraft.

"H i i i i r U44XUJ1

11

1 l l "^

Fig. 8.Coupleintroduced

by eccentricloading

.

sandwich construction for bulk containers to theadvanced primary structures for aerospace appli-

tions. We hope that, in the space of a few thousandwords, we have shown that structural bonding is anacceptable technique for general application in in-

dustry, with many technical and economic merits.It is the view of the authors that this techniqueshould be much more widely used than it is; indus-try cannot afford to neglect technical advances fora quarter of a century just because their first field

of application is in aircraft construction.

APPENDIX: ANALYSIS OF THESTRESS DISTRIBUTION IN THESIMPLE LAP JOINT

The predominant stress in the simple lap joint is

shear, but there is a tension component due to thecouple introduced by the eccentric loading (see

Fig. 8).

The adhesive joint may be considered as one in

which an elastic medium - the adhesive - is sand-

wiched between two less elastic pieces of metal;by tensioning the joint a strain gradient is estab-lished. Of necessity ihe stress in the sheet mat-erial is zero at point O, increasing to a maximumat A. The strain differential is therefore greatestacross the section OA, and it is this differentialthat causes the stress in the adhesive to reachmaximum values at each end of the joint. Thispeak of shear stress causes the adhesive to fail

locally, initiating complete failure of the joint.

Since the load transmitted by the adhesive mustbe equal to the load applied through the adherend,the relationship between joint overlap and adherendthickness (for a given adherend/ adhesive combina-tion) is:

Tlw =e tw

where t = the mean shear stress in the adhesiveE = the mean tensile stress in the adher-

ends1 = the length of joint overlapw = the joint width

and t = the adherend thickness

The relationship between mean shear strength andt/1 or 'Joint Factor' may be established from test

results - see Fig. 6.

The Design Engineering Guide to Adhesives, whichgives the properties and principal uses of over 450different engineering adhesives from 50 manufac-turers, has been prepared to aid the designer in

selecting the best adhesive for his application.

A REALLY FIRST CLASSSERVICE EVERY TIME

FOR QUICK DELIVERIES OF RIVETS IN

ALUMINIUM • BRASS COPPER AND ALL

NON-FERROUS METALS, % 2" to %" DIAMETER

CLEVEDON RIVETS & TOOLS LTD.REDDICAP TRADING ESTATE SUTTON COLDFIELD • WARKS.TEL: 021-354 5238 GRAMS: 'CLEVEDON' SUTTON COLDFIELD

160

23

Selected special fasteners

Compiled by A. Griffiths (Consultant Editor)

The actual definition of a fastener has been dis-

cussed in several other Chapters in this handbook.

However, it is worth remembering that a fastener

may be simple or complicated, cheap or inexpen-

sive. For instance, a dressmaker's pin or a paper

clip must be classified as a fixing. At the other

end of the scale a hydraulically operated lock-nut

is also a fastener.

In this Chapter, which is carefully illustrated, re-

ferences have been made of fixing ideas that maynot be covered in the more exact headings of the

preceding Chapters. Undoubtedly there are manyother special fixings that could be included in our

list and we invite readers to submit their ideas

for subsequent incorporation in this Chapter.

Since most special fasteners are known by typical

trade-names, these have been used to aid identifi-

cation. In some instances there may be competi-

tive products of equal merit to those that have been

described.

are moulded from nylon or acetal resins which,

besides being corrosion free and self lubricating,

are also resistant to vibration as well as squeakand rattle proof.

Other advantages of the system are that the twomating fasteners need not be perfectly aligned andthat the device does not wear or lose its holding

power.

At the present this unique system is only available

in a restricted range of sizes, but full information

regarding possible new developments can be obtain-

ed from the manufacturers./

Applications that have already been explored in-

clude the mounting of trim panels in automobiles

and aircraft, access panels for electrical appara-

tus, interchangeable displays and signs, interior

and exterior fixings in caravans and boats.

Manufacturers: Minnesota Mining & Manufacturing

Co. Ltd. New Products Group.

3M Mechanical fastening system

Fig. 1 shows a typical application for this new type

of mechanical fastener. The principle of the fixing

being in the unique design of the identical pin-head

shaped stems on the modules.

These fasteners, which are available in a wide vari-

ety of shapes and fixing, are interchangeable, thus

providing the designer with considerable scope to

achieve the most satisfactory fixing arrangements.The pads may be fixed by a number of methods -

screwing, riveting or adhesives. The fasteners

Fig.1 .

Double sided adhesive tapes

Whilst all readers will accept that cellulose tape

and its many derivatives are in fact fastener med-iums, much use has recently been made of double

sided tapes (Fig. 2).

The carrier for the adhesive film can either be in

the form of thin and mechanically weak tissues or,

alternatively, the sandwich can consist of a strong

flexible member. This part of the 'tape' is often

made from a cellular material which is similar to

foam plastics or rubber.

Fig. 2.

161

Ultrasonic

assembly.

Dawe Sonic Welders

* The new solution to an old problem.

* Weld plastics: join metal to plastic.

* Ultrasonics for quick action.

* Ultrasonics is clean.

* Ultrasonics gives consistent results with less rejects •

more profit.

* Ask for details of latest techniques from:-

Concord Road Western Avenue London W3 Tel : 01 -992 6751^stblmemtsumtted Cab|es . DAWINST LONDON • W3.

Pv>

Fig. 3. Disconnect fast-

ener installation with

extension under wing

.

Engineering application for the foamed tape are

numerous. The material is easy and clean to app-

ly; it also overcomes many of the objections of

applying liquid adhesives. The tape can be pur-chased ready cut to length so that it is economicaland swift to apply. With foamed double sided tape

the main advantage is that a secure fix can be ach-

ieved where the mating surfaces are uneven or un-

dulating.

There are many manufacturers of this tape but weare indebted to Minnesota Mining & Mfg. Co. Ltd.

for the illustration.

Disconnect fasteners

In recent years, with the considerable usage of

uniter plus or multiple pin connectors on a wholehost of electronic devices, it became necessary to

provide a suitable method of both engaging and re-leasing these plug and socket units.

The fastener required for such a purpose had to

be capable of overcoming the insertion and with-

drawal forces (which can be in the order of 30 lb.

or more), provide instant location in guiding the

pins into engagement, be fast acting in closure andrelease, yet resistant to accidental disconnection

under vibration.

The Dzus Universal quadruple thread fastener sys-tem, originally developed for load carrying pur-poses on aircraft cowling panels, was found readilyadaptable to meet these requirements. By adaptation

the quadruple thread stud was housed in a heavyduty aluminium alloy shell and made to engage, closetolerance wise, on to a male spigot projecting froma similarly modified heavy duty receptacle.

In operation the two parts are closely guided to-

gether providing the degree of accuracy and rigid-

ity required to meet the various conditions of ser-

vice and capable of closure and release within ap-proximately one and a half turns of the threadedstud. Alternative head style methods of stud opera-tion are provided, viz. hand operated large wing,

socket recess (for hexagon wrench) and hexagonbolt head style. All too meet varying needs andconditions of access.

These Disconnect Fasteners, as they are called,

became widely used and developments in various

forms have since been applied to additional appli-

cations. This involved further tailored specials

for securing panel modules, printing circuit trays

etc. , all of which incorporated jack-plug connectors

for making circuit contact on attachment of the mod-ule unit.

The applications now range from regular connector

socket fixings in electronics to ground control cub-icle modules used in guidance systems for civil

and military aircraft.

From the foregoing a good example can be- seen of

a fastener, originally designed for a conventional

purpose, being developed into a special, the result

of which has provided a service in a field which,hitherto, had not been intended.

Quarter turn fastener

Although there are many types of efficient quarterturn fasteners, the Vibrex type is shown in detail

in view of its particular properties. Fig. 4 showsthe fastener in the locked and open position. Fig. 5

shows the variety of heads which can be applied to

suit particular applications. This type of quarter-

Fig. 4. The fastener in the open position (left)

and locked position (right).

Fig. 5.

turn fastener can be assembled without special

tools, riveting or spot welding operations. Onlytwo round holes are needed - one in the removablepanel and one in the base. Since the fastener de-

pends on the unique action of the rubber it is bothfirm and flexible in operation. It discourages ratt-

les and is particularly suitable for fixing glass andplastics panels.

Manufacturers: Silentbloc Ltd.

Cold forged fasteners

Whilst this Chapter has been devoted to interesting

fastening systems, some mention must be made to

Fig. 6.

advanced manufacturing techniques in metallic fast-

ener production. Sintered and cold forged compo-nents are of particular interest.

Fig. 6 shows a switch button which would normallybe turned from the solid. This actual example hasbeen forged by the GKN Dynoflow method and the

only secondary operation needed, to give the part

an excellent surface finish is to submit it to mildbarrelling. In this case the material is an alumi-nium alloy and the material saving alone is 75 percent.

Fig. 7 shows a special screw made by the sameprocess but from mild steel.

Manufacturers: GKN Screws and Fasteners Ltd.

Liquid thread locking

Many screw threads can be effectively locked byapplying a liquid sealant which remains fluid whenin contact with air - but when placed between metalsurfaces cures automatically. One brand, knownas Loctite, sets without shrinking into an extreme-ly tough, impervious and non-toxic solid. Thejoint is not affected by vibration but the bond can bebroken by using a spanner.

In addition to locking nuts, bolts and threadedstuds, the solutions can also be used for fitting

bearings retaining components in the correctly as-sembled position and also as a locking/sealing med-ium on pipework.

Manufacturers: Douglas Kane Group Ltd.

Fig. 8. 1 .Thread locking; 2. Bearing fitting;

3. Parts retaining; 4. Pipe and tube sealing.

Self locking inserts

Fig. 9. shows a self locking, threaded insert. Theunit is made from a corrosion resistant steel andthe locking element from a suitable thermoplastic.

To install the insert the holes must be tapped witha regular tap and the device installed by using a sim-ple applicator. The locking element works by pre-venting the inset from disengaging from the tappedhole at the same time securing the mating bolt.

Manufacturers: Long-Lok Ltd.

High torque heads for screws

Many different head designs for screws have beendeveloped to facilitate the speedy assembly andinstallation of threaded fasteners.

The new 'Torque-Hed' detail (Fig. 10) is a six wingself-centring arrangement for screw heads. This

design is particularly suitable where high torque

driving is required. The head does not easily 'cam-out' thus preventing undue damage to screwdriver

bits. If painted over, the heads are easier to cleanthan many other recess headed screws. It shouldalso be noted that in emergency the head can be

turned by using a conventional screw driver blade.

Manufacturers: The Torrington Co. Ltd.

Touch and close fasteners

Fig. 11 shows a greatly enlarged view of a touchand close fastener which is sold under the name of

'Velcro'. The fastener consists of two nylon strips,

one with thousands of liny hooks and the other withmany tiny loops. When pressed together the hooksgrip the loops to give a tight, secure closure. Toseparate the fastener, the two strips are simplypeeled apart.

The fastenings are flexible and can be washed, dry-cleaned and ironed. Being plastics they will not cor-rode or jam. Most applications for Velcro are to

be found where flexible materials have to be fixedin position.

Manufacturers: Seleetus Ltd.

Ultrasonic plastics assembly

In the last two or three years the assembly of plas-tics components by means of ultrasonic energy hasemerged from the laboratory to become a recog-nised - and increasingly accepted - industrial tech-nique. Already it is estimated * that 200-300 ultra-sonic assembly equipments are being used indus-

164

Fig. 10. (Right).

Fig. 11 . (Below).

Fig . 1 2 . (Below right)

Special multiple ultra-

sonic welding headdesigned for a large

assembly task. Eachhorn is powered bya separate generator.

LARGE LARGEDRIVING .DRIVINGAREA / RADIUS

DRIVER WING

SECTION XX

DRIVING FEATURES

RECESS APPROACH PACE

DRIVER ENGAGEMENT

'' '':

.

trially in Britain and it is safe to predict that the

number will rise rapidly as realisation of the ad-

vantages of ultrasonics spreads.

Ultrasonics cannot handle all plastics assemblyjobs but, where it can be used, joints are producedrapidly and automatically and are reliable, incon-

spicuous and indeed attractive. Basically there are

three techniques for ultrasonic assembly , welding,

staking and metal-to-plastics insertion.

Whatever technique is used in a particular applica-

tion, the equipment and the method of applying ul-

trasonics are substantially the same. The ultra-

sonic vibrations are initiated by an electronic gen-

erator which converts the 50 Hz mains electricity

supply into electrical energy of the required ultra-

sonic frequency - generally 20, 000 Hz. This elec-

trical energy drives a transducer (converter) built

into the head of a column-mounted probe. Thetransducer converts the electrical energy into mech-anical vibration, which is imparted to the work-piece by the probe tip, known as a 'horn'. Thevibration is transmitted to the extremities of the

workpiece, where it is either internally reflected

or transmitted to the adjacent medium.

Typical standard equipment for ultrasonic assembly- the Dawe Sonic Welder Type 1133 - uses a genera-

tor rated a 1700 inch-pounds per second. A similar

unit (Type 1134) has a generator- rated at 3200 inch-

pounds per second.>

Several transducers and probes may be mountedtogether in a combined unit, as shown in Fig. 12, to

enable comparatively large workpieces to be ultra-

sonically assembled; the associated generators maybe rack-mounted if required.

Welding

Mechanical vibration from the transducer, imparted

to the workpiece by way of the horn, spreads through

the workpiece. At a joint line, the adjacent med-ium is solid and unable to respond to the high-fre-

quency vibration of the workpiece in contact with

the horn. This results in high-frequency rubbing of

the workpiece against its companion, causing the

ultrasonic energy to be dissipated as frictional

heat. The result is local heating and melting of

the plastics in the immediate vicinity of the joint

line, giving a strong thermal weld.

The principle is similar to spin welding except that

the relative motion is reciprocating instead of ro-

tary and joints of many shapes may be made. Toobtain the best joints in practice the joint profile

should be specially designed, with a raised ridge

on one joint surface to act as an energy 'director'

(Fig. 13), concentrating and localising the heating

165

THREADED BORE-

ENERGY DIRECTOR

effect. On the application of ultrasonics this ridgerapidly melts and the molten material spreadsevenly across the joint profile. Handling of work-pieces may be mechanised and horns and joint pro-files may be specially designed. Joints which arevirtually homogeneous can be produced very rapidlyand automatically with almost zero rejection rate.

Ultrasonic welding of rigid thermoplastics is al-ready widely used for automatic assembly of pla-stics bowls, cosmetic jars, flash cubes and manyother components, where adhesives, solvents anddirectly applied heat are at a disadvantage.

In the case of flash cubes - a particularly good ex-ample - the use of ultrasonics is virtually essential.It is necessary to seal the cover to the base of theflash cube with a joint of high strength, since aforce of about 3 lb. is typically required to extracteven a correctly fitted flash cube from its socketand some allowance must be made for faulty fitt-

ing. An adhesive would be too messy - it would haveto be kept away from the reflectors and non- join-ing surfaces, to preserve both the properties of thereflectors and the pleasing, even sparkling, appear-ance of the cube, which accounts for at least partof its attraction to the buying public. In any casean adhesive would hardly be suitable for high-vol-ume production and heat could not be directly ap-plied because of the proximity of the flash bulbs.

Fig. 14. Plastics tubes are easy to seal ultrason-ically since the vibrations decontaminate the

joint zone .

Fig. 13. (Far left) Suitableprofile for ultrasonic butt

wold showing recommendedrelative dimensions of

energy director.

Fig. 16;. (Left) Suitable

proportions for plastics

head and metal insert

designed for ultrasonic

assembly.

The four-bulb flash cube was, in fact, designed forultrasonic assembly right from the start. Ultra-sonic energy is applied around the circumferenceof the base, immediately above the welding line.

In other applications, where this can be arranged,it minimises energy requirements and gives themost economical joint. In the case of rigid thermo-plastics it is also possible to weld remotely, sincethe ultrasonic energy is transmitted by the work-piece to the joint line. The range at which it ispossible to carry out remote welding depends on anumber of factors, such as the power imparted tothe plastics by the horn and the sound-transmittingproperties of the plastics. With good horn-plasticscoupling and suitable thermoplastic materials arange of six inches or more is practicable.

With non-rigid plastics, generally in the form offilm, sheeting or tubing, the horn must be applieddirectly above the joint line. Since the ultrasonicshas the secondary effect of cleaning the joint zoneof contaminants and extraneous matter, ultra-sonics is particularly suitable for sealing plasticstubes (Fig. 14), sachets and similar non-rigid con-tainers which are filled via the joint prior to sealing.

Most commonly used injection-moulded plasticscan be ultrasonically sealed or welded without theuse of solvents, heat or adhesives. Weldabilitydepends on their melting temperature, modulus ofelasticity, impact resistance, coefficient of frictionand thermal conductivity. General-purpose styreneis the best material for ultrasonic assembly be-cause of its high modulus and low melting tempera-ture. Conversely, fluorocarbon resins, which can-not be welded, have a low modulus, high meltingtemperature and low coefficient of friction. Gen-erally, the softer the plastics, the more difficult

it is to weld the part remotely (where the horn ismore than i in. from the joint). Low-modulus mat-erials such as polyethylene, polypropylene and buty-rate can be welded, provided the horn can be posi-tioned close to the joint area.

Both similar and dissimilar thermoplastic mat-erials may be welded if their melting temperaturesare of the same order. Higher power and longerweld times are needed for materials with a highmelting point and, if one material melts beforethe other, it becomes extremely difficult to obtaina satisfactory joint.

166

Manufacturers of PHILLIPS

"POZIDRIV" SCREWSADVANTAGES:the driving faces of the recess are vertical, which

"fc" Eliminates cam-out, or driver disengagement

"At Reduces operator fatigue

"A" Reduces wear on driver

"A" Reduces damage during driving

"^T All resulting in overall reduction in costs

With the recess being shallower there is an increased head to

shank strength.

BISSEL STREETBIRMINGHAM 5

Telephone: 021-692 1135 (10 linesl

Telex: 33474

London Office: 212, CHEAM COMMON ROAD, WORCESTER PARK, SURREY

Telephone: 01-337 0017. London Telex: 'Fraimfil' London 25514.

Metric

!

We specialise exclusively in metric fasteners ex stock:

HI-TENSILE, MILD STEEL, STAINLESS STEEL, BRASS ETC.

BOLTS, SCREWS, NUTS, STUDDING, WING NUTS, SELF-LOCKING NUTS,

DOWEL, TENSION & TAPERED PINS, ETC., ETC.

THREADS TO I.S.O., D.I.N, and SYSTEME-FRANCE

METRIC ALLSCREWS LTD.

PEASE POTTAGESUSSEX

Telephone CRAWLEY (OCY3) 25811/2

167

0.5D RADIUS0.5D RADIUS

H°h

Jjvjb:£ Jf1^

STANDARO LCW PROFILE

Fig.16. Relative dimensions oF horn and stud

profiles For standard and low-proFile staked headForms .

Insertion

In the case of insertion, a hole (not necessarilycircular) of slightly smaller dimensions than that

of the insert to be received is first pre-mouldedin the plastics, to provide an interference fit and to

guide the insert into place. For a fully interlock-ing assembly, the metal insert is generally knurled,undercut, or otherwise shaped to resist the loadsimposed on the finished assembly.

Ultrasonic energy may be applied to the metal orthe plastics, but is generally applied in practice tothe metal if it is an insert since it has a smallervolume, better sound-transmitting properties andconsequently wastes less energy. The ultrasonicvibration gives rise to frictional heat at the joint

or interface, causing momentary melting and flow-ing of the plastics and allowing the insert to be driv-

en home. The ultrasonic energy is generally ap-plied for less than one second but during this timethe plastics flows around the knurls, flutes, under-cuts or threads to encapsulate the insert.

A typical example is the assembly of a steel insertinto a knob of impact styrene (Fig. 15) for use as alocking device. The insert should be threaded orknurled because the finished assembly has to with-stand torque and axial shear forces when pressureis brought to bear both on the plastics and insertsurfaces.

Insert/hole design varies with each application butin all cases a sufficient volume of plastics must bedisplaced to fill the voids created by knurled orundercut areas of the insert. A slight excess ofmolten material is generally preferable to insuffi-cient interference, which may result in a joint ofinadequate strength.

Staking

Ultrasonic staking of metal to plastics employs thesame principles as welding and insertion but jointdesign is very different. In staking, a hole in themetal receives a plastics stud which is then formedinto a head by ultrasonic energy to hold the metalin place. The process is very similar to riveting.Staking requires ultrasonic energy only at the sur-face of the plastics stud so that the initial contactarea between horn and plastics must be kept small.The horn is specially designed, and usually under-cut to the shape of stud head required. One of twohead forms, having a high or low profile (Fig. 16),will suit the majority of applications.

Unlike welding or insertion, staking requires thatout-of-phase vibration should take place betweenhorn and stud surfaces. Light initial contact pres-sure is therefore applied over a very small initialarea. The progessive melting of the plastics underthis light but continuous pressure forms the re-quired stud head. As with welding and insertion,some trial and error may be necessary to obtainthe optimum settings of pressure, hold time andweld time but the result, when set up, is an opera-tion suitable for rapid production with very lowrejection rate.

REFERENCES

1. Stafford, R. D. 'Ultrasonic assembly techniquesfor plastic components'. Paper 2, session 5, 'Plas-tics and the production engineer', conference pre-print. The Plastics Institute and the Institution of

Production Engineers, June 1967.

2. Kolb D. J. 'Designing plastic parts for ultra-sonic assembly'. Machine Design. The PentonPublishing Co. , Cleveland, Ohio. March 16, 1967.

168

Suppliers of Fasteners

CIRCLIPS

Acme Spring Co. Ltd.

Aircraft Materials Ltd.

Alder Hardware Ltd.

Automotive Engineering Ltd.

Baileys of Aldridge.

British Lock Washers Ltd.

George Cotton & Sons.

Cross Manufacturing Co. (1938)Ltd.

Everbright Fasteners Ltd.

Firth Cleveland Fastenings Ltd.

Charles E. Greehill Ltd.

Helical Springs Ltd.

Lamp Manufacturing & Railway Supplies Lid.

C. Lindley & Co. Ltd.

Metric AUscrews Ltd.

Morlock Industries Ltd.

Spafax (1965) Ltd.

Spring Washers Ltd.

Wellworthy Ltd.

EYELETS

Aircraft Materials Ltd.

Alder Hardware Ltd.

Copper & Asbestos Washer Co. Ltd.

E. J. Francois Ltd.

Ross Courtney & Co. Ltd.

Ceo. Tucker Eyelet Co. Ltd.

Clifford Whatmoufih Ltd.

THREADED INSERTS

Peter Abbott & Co. Ltd.

Alder Hardware Ltd.

Anglo-Swiss Screw Co. Ltd.

Armstrong Patents Co. Ltd.

The Automatic Standard Screw Co. (Halifax) Ltd.

Avdel Ltd.

Bar Production (Bromsgrove)Ltd.Cranes Screw & Colgryp Castor Co. Ltd.

Cross Manufacturing Co. (1938) Ltd.

Datim Screw Co. Ltd.

Everbright Fasteners I-td.

Expandite Ltd.

Firth Cleveland Fastenings Ltd.

C. J. Fox & Sons Ltd.

E. J. Francois Ltd.

GKN Screws & Fasteners Ltd.

Harris & Edgar Ltd.

Industrial Fasteners Ltd.

Instrument Screw Co. Ltd.

Irlam Engineering Co. (1942) Ltd.

Jesse Haywood & Co. Ltd.

Jukes Coulson. Stokes & Co. Ltd.

Isaac Jackson & Sons (Fasteners) Ltd.

Lamp Manufacturing & Railway Supplies Ltd.

Long-Lok Ltd.

Metric AUscrews Ltd.

Midland Screw Co. Ltd.

Precision Screw Manufacturing Co. Ltd.

Prestincert Ltd.

Screw Machine Products Ltd.

Segmatic Ltd.

Tappex Thread Inserts Ltd.

Geo. Tustin Ltd.

Thos. W. Ward Ltd.

Woodberry Chillcott & Co. Ltd.

Crompton Parkinson Ltd.

Nyloy Screws Ltd.

NUTS - BLACKPeter Abbott & Co. Ltd.

Alder Hardware Ltd..

Annfield Metal Fasteners Ltd.

Arcon Engineering Co.

Avon Manufacturing (Warwick) Ltd.

B.A.R. Fasteners Ltd.

Baxters (Bolts Screws & Rivets) Ltd.

G. F. Bridges (Glynwed Distribution Ltd.

)

John Bullough Ltd.

Carr & Nichols Ltd.

George Cooper (Sheffield) Ltd.

David Etchells (Forgings & Fasteners) Ltd.

Firth Cleveland Fastenings Ltd.

GKN Bolts & Nuts Ltd.

Industrial Fasteners Ltd.

Isaac Jackson & Sons (Fasteners) Ltd.

James & Tatten Ltd.

C. Lindley & Co. Ltd.

P. & W. MacLellan Ltd.

Macnays Ltd.

Samuel Marden & Son Ltd.

Metric AUscrews Ltd.

Wm. Motherwell & Co. Ltd.

Nettlefold & Moser Ltd.

Nuts & Bolts (Darlaston) Ltd.

Prestwich Parker Ltd.

Benjamin Priest & Sons Ltd.

Charles Richards & Sons Ltd.

G. H. Smith & Co (Bankhall) Ltd.

Spafax (1965) Ltd.

Swinnerton & Co (Stourbridge) Ltd.

Thos. W. Ward Ltd.

Williams Bros (Sheffield) Ltd.

NUTS - LOCKING

Peter Abbott & Co. Ltd.

Aircraft Materials Ltd.

Alder Hardware Ltd.

Arcon Engineering Co.

Armstrong Patents Co. Ltd.

Avdel Ltd.

Bar Production (Bromsgrove) Ltd.

H.J. Barlow & Co. Ltd.

Baxters (Bolts Screws & Rivets) Ltd.

Benton Engineering Co. Ltd.

G. F. Bridges ( Glynwed Distribution Ltd).

Brown Bros (Aircraft) Ltd.

Carr Fastener Co. Ltd.

Carr & Nichols Ltd.

George Cooper (Sheffield) Ltd.

Crane's Screw & Colgryp Castor Co. Ltd.

Crew & Sons Ltd.

Datim Screw Co. Ltd.

Deltlght Industries Ltd.

:)avid Etchells (Forgings & Fasteners) Ltd.

Everbright Fasteners Ltd.

Tirth Cleveland Fastenings Ltd.

C. J. Fox & Sons Ltd.

GKN Screws & Fasteners Ltd.

Arthur Gise Ltd.

industrial Fasteners Ltd.

Irlam Engineering Co. (1942) Ltd.

Jukes Coulson, Stokes & Co. Ltd.

Isaac Jackson & Sons (Fasteners) Ltd.

James & Tatten Ltd.

C.W. Juby Ltd.

Lamp Manufacturing & Railway Supplies Ltd.

C. Lindley & Co. Ltd.

London Metal Warehouses Ltd.

P. & W. MacLellan Ltd.

Macnays Ltd.

Samuel Marden & Son Ltd.

Metric AUscrews Ltd.

Midland Screw Co. Ltd.

James Mills Ltd.

Fredk. Mountford (Birmingham) Ltd.

Nettlefold & Moser Ltd.

Stephen Newall & Co. Ltd.

Nuts & Bolts (Darlaston) Ltd.

Palnut Co. Ltd. , TheR.A. Poole & Co. (Sutton) Ltd.

Preswich Parker Ltd.

Benjamin Priest & Sons Ltd.

Charles Richards & Sons Ltd.

G.H. Smith & Co. (Bankhall) Ltd.

Spafax (1965) Ltd.

Spensall Eng. Co. Ltd.

Swinnerton & Co (Stourbridge) Ltd.

Telco Ltd.

Geo. Tustin Ltd.

Thos. W. Ward Ltd.

Whitehouse Industries Ltd.

Williams Bros (Sheffield) Ltd.

Woodberry Chillcott & Co. Ltd.

Nyloy Screws Ltd.

NUTS - CLINCH ft ANCHOR

Peter Abbott & Co. Ltd.

Aircraft Materials Ltd.

Alder Hardware Ltd.

Avdel Ltd.

Bar Production (Bromsgrove) Ltd.

Barton Rivet Co. Ltd.

Baxters (Bolts Screws & Rivets) Ltd.

Benton Engineering Co. Ltd.

G. F. Bridges (Glynwed Distribution Ltd.

)

Brown Bros (Aircraft) Ltd.

Carr & Nichols Ltd.

Crane's Screw & Colgryp Castor Co. Ltd.

Deltight Industries Ltd. •

Everbright Fasteners Ltd.

Firth Cleveland Fastenings Ltd.

C.J. Fox & Sons Ltd.

GKN Screws & Fasteners Ltd.

Instrument Screw Co. Ltd.

Jukes Coulson, Stokes & Co. Ltd.

Douglas Kane Group Ltd.

C. Undley & Co. Ltd.

P. & W. MacLellan Ltd.

Metric AUscrews Ltd.

Midland Screw Co. Ltd.

James Mills Ltd.

Nettlefold & Moser Ltd.

Spirol Pins Ltd.

Tappex Thread Inserts Ltd.

Geo. Tustin Ltd.

Thos. W. Ward Ltd.

Whitehouse Industries Ltd.

Williams Bros (Sheffield) Ltd.

NUTS - CAGED

Peter Abbott & Co. Ltd.

Aircraft Materials Ltd.

Alder Hardware Ltd.

Brown Bros (Aircraft) Ltd.

Carr & Nichols Ltd.

Cranes Screw & Colgryp Castor Co. Ltd.

Everbright Fasteners Ltd.

Firth Cleveland Fastenings Ltd.

C. J. Fox & Sons Ltd.

James & Tatten Ltd.

P. & W. MacLellan Ltd.

Metric AUscrews Ltd.

Tappex Thread Inserts Ltd.

Thos. W. Ward Ltd.

Whitehouse Industries Ltd.

NUTS - SINGLE THREADED

Alder Hardware Ltd.

Arcon Engineering Co.Bar Production (Bromsgrove) Ltd.

John Bradley & Co. Ltd.

G. F. Bridges (Glynwed Distribution Ltd. ).

Brown Bros (Aircraft) Ltd.

Carr Fastener Co. Ltd.

Carr & Nichols Ltd.

Crane's Screw & Colgryp Castor Co. Ltd.

Datim Screw Co. Ltd.

Deltight Industries Ltd.

David Etchells (Forgings & Fasteners) Ltd.

Everbright Fasteners Ltd.

Firth Cleveland Fastenings Ltd.

C.J. Fox & Sons Ltd.

GKN Bolts & Nuts Ltd.

Industrial Fasteners Ltd.

Irlam Engineering Co. (1942) Ltd.

C.W. Juby Ltd.

Arnold Kinnings & Son Ltd.

London Metal Warehouses Ltd.

P. & W. MacLellan Ltd.

Metric AUscrews Ltd.

Midland Screw Co. Ltd.

James Mills Ltd.

Fredk. Mountford (Birmingham) Ltd.

R.A. Poole & Co. (Sutton) Ltd.

Prestwich Parker Ltd.

Spafax (1965) Ltd.

Spirol Pins Ltd.

Ucan Products Ltd.

Thos W. Ward Ltd.

Nyloy Screws Ltd.

NUTS -PLAIN

Peter Abbott & Co. Ltd.

Aircraft Materials Ltd.

Alder Hardware Ltd.

Annfield Metal Fasteners Ltd.

Arcon Engineering Co.

Automatic Standard Screw Co. (Halifax) Ltd.

Avon Manufacturing (Warwick) Ltd.

B. A. R. Fasteners Ltd.

Bar Production (Bromsgrove) Ltd.

N. J. Barlow & Co. Ltd.

John Bradley & Co. Ltd.

G. F. Bridges (Glynwed Distribution Ltd.

)

Brown Bros (Aircraft) Ltd.

John Bullough Ltd.

Carr & Nichols Ltd.

George Cooper (Sheffield) Ltd.

Crane's Screw & Colgryp Castor Co. Ltd.

Crew ft Sons Ltd.

Datim Screw Co. Ltd.

Deltight Industries Ltd.

Thos. Eaves Ltd.

David Etchells (Forgings & Fasteners) Ltd.

Everbright Fasteners Ltd.

Firth Cleveland Fastenings Ltd.

E. J. Francois Ltd.

GKN Bolts & Nuts Ltd.

GKN Screws & Fasteners Ltd.

Arthur Gise Ltd.

169

Thomas Haddon & Stokes Ltd.Industrial Fasteners Lid.

Irlam Engineering Co. (1942) Ltd.Jukes Coulson. Stokes & Co. Ltd.

Isaac Jackson & Sons (Fasteners) Ltd.James & Tatten Ltd.

C. W. Juby Ltd.Arnold Kinnings & Son Ltd.

C. Lindley & Co. Ltd.London Metal Warehouses Ltd.

Is

. 6, W. MacLellan Ltd.

Samuel Harden & Son Ltd.Metric Allsc-ews Lid.Midland Screw Co. Ltd.

James Mills Ltd.

Motherwell & Co. Ltd.Fredk. Mountford (Birmingham) Ltd.

Stephen Newall & Co. Lid.

R. A. Poole & Co. (Sutton) Ltd.

Prestwich Parker Ltd.

Charles Richards & Sons Ltd.

Screw & Rivet Co. Ltd.Simpson-Turner Ltd.G. H. Smith & Co. (Bankhall) Ltd.

Spafax (1965) Lid.

Spensall Eng. Co. Ltd.

Telco Ltd.

E. H. Thompson & Son (London) Ltd.

Ucan Products Ltd.

Thos. W. Ward Ltd.Williams Bros (Sheffield) Ltd.Woodberry Chillcott & Co. Ltd.

Nyloy Screws Lid.

NUTS - WELD

Alder Hardware Ltd.

B.A.R. Fasteners Ltd.

Baxters (Bolts Screws & Rivets) Ltd.

G. I'. Bridges (Glynwed Distribution Ltd).

Carr & Nichols Ltd.

Crane's Screw & Colgryp Castor Co. Ltd.

Deltight Industries Ltd.

Firth Cleveland Fastenings Ltd.

C. J, Fox & Sons Ltd.

GKN Bolts & Nuts Ltd.Arthur Gise Ltd.Industrial Fasteners Ltd.

James & Tatten Ltd.P. &. W. MacLellan Ltd.

Metric Allscrews Lid.

Midland Screw Co. Ltd.

James Mills Ltd.

Stephen Newall Ac Co. Ltd.

Screw & Rivet Co. Ltd.

Thos. W. Ward Ltd.

Williams Bros (Sheffield) Ltd.

KSM Siud Welding Ltd.

PLASTICS FASTENERS

Alder Hardware Ltd.Avon Manufacturing (Warwick) Ltd.Black & Luff Ltd.

G. F. Bridges (Glynwed Distribution Ltd).British Screw Co. Ltd.

Carr Fastener Co. Ltd.

Crane's Screw & Colgryp Castor Co. Ltd.

Deltight Industries Ltd.

Dzus Fastener Europe Ltd.Expandite Ltd.

Firth Cleveland Fastenings Ltd.

C.J. Fox & Sons Ltd.

E..7. Francois Ltd.

CKN Screws & Fasteners Ltd.ITW Ltd., Fastox Division.

P. & W. MacLellan Ltd.Metric Allscrews Ltd.

Midland Screw Co. Ltd.

Ross, Courtney & Co. Ltd.

Simpson-Turner Ltd.Tower Manufacturing Co. Ltd.Geo. Tucker Eyelet Co. Ltd.t. can Products Ltd.

Moulded Fasteners Ltd.

Nyloy Screws Lid.

BOLTSPeter Abbott & Co. Ltd.Aircraft Materials Ltd.Alder Hardware Ltd.

Annfield Metal Fasteners Ltd.

Arcon Engineering Co.The Auto Machinery Co. Ltd.Avon Manufacturing (Warwick) Ltd.B.A.R. Fastener* Ltd.

Berber & Colman Ltd.

H.J. Barlow & Co. Ltd.Baxters (Bolts Screws & Rivets) Ltd.John Bradley & Co. Ltd.G. F. Bridges (Glynwed Distribution Ltd).Brown Bros (Aircraft) Ltd.John Bullough Ltd.

Carr & Nichols Ltd.Chalfont Aluminium Roofing Supplies Ltd.George Cooper (Sheffield) Ltd.Cooper & Turner Ltd.Crane's Screw & Colgryp Castor Co. Ltd.Crew & Sons Ltd.

Deltight Industries Ltd.Thomas Eaves Ltd.David Etchells (Forgings & Fasteners) Ltd.Everbright Fasteners Ltd.

Firth Cleveland Fastenings Ltd.

H. Fordsmith Ltd.

E. J. Francois Ltd.GKN Bolts & Nuts Ltd.

Arthur Gise Ltd.

Harris & Edgar Ltd.

Harrison (Birmingham) Brassfoundry Ltd.Industrial Fasteners Ltd.Irlam Engineering Co. (1942) Ltd.Jesse Haywood & Co. Ltd.

Jukes Coulson, Stokes & Co. Ltd.Isaac Jackson & Sons (Fasteners) Ltd.James & Tatten Ltd.

C.W. Juby Ltd.

Douglas Kane Group Ltd.Arnold Kinnings & Son Ltd.Lamp Manufacturing & Railway Supplies Ltd.C. Lindley & Co Ltd.

London Metal Warehouses Ltd.Long-Lok Ltd.

Macnays 'Ltd.

Samuel Marden & Son Ltd.Metric Allscrews Ltd.Midland Screw Co. Ltd.James Mills Ltd.Fredk. Mountford (Birmingham) Ltd.

Nettlefold & Moser Ltd.

Stephen Newall & Co. Ltd.

Nuts & Bolts (Darlaston) Ltd.R.A. Poole & Co. (Sutton) Ltd.Prestwich Parker Ltd.

Price & Orphin Ltd.

Charles Richards & Sons Ltd.Screw & Rivet Co. Ltd.

Simpson-Turner Ltd.

G.H. Smith & Co. (Bankhail) Ltd.Spafax (1965) Ltd.

Spensall Eng. Co. Ltd.

Swinnerton & Co. (Stourbridge) Ltd.Telco Ltd.

E. H. Thompson & Sons (London) Ltd.The Torrington Co. Ltd.

Geo. Tustin Ltd.Unbrako Ltd.

Thos. W. Ward Ltd.

Warne Wright Engineering Ltd.Whitehouse Industries Ltd.Williams Bros (Sheffield) Ltd.Woodberry Chillcott & Co. Ltd.

Nyloy Screws Ltd.

PINS - SOLID & TUBULAR

Peter Abbott & Co. Ltd.Aircraft Materials Ltd.Alder Hardware Ltd.

Anglo-Swiss Screw Co. Ltd.Bar Production (Bromsgrove) Ltd.Barton Rivet Co. Ltd.

G. E. Bissell & Co. Ltd.

Brown Bros (Aircraft) Ltd.Datim Screw Co. Ltd.Deltight Industries Ltd.

Everbright Fasteners Ltd.

Exors. of James Mills Ltd.

Firth Cleveland Fastenings Ltd.H. Fordsmith Ltd.

C. J. Fox & Sons Ltd.

Arthur Gise Ltd.Grover & Co. Ltd.

Harris & Edgar Ltd.Industrial Fasteners Ltd.

Jesse Haywood & Co. Ltd.

Jukes Coulson, Stokes & Co. Ltd.Isaac Jackson & Sons (Fasteners) Ltd.

C. Lindley & Co. Ltd.Ltandaff Engineering Co. Ltd.P. & W. MacLellan Ltd.Macnays Ltd.

Marples & Beasley Ltd.Metric Allscrews Ltd.

Stephen Newall & Co. Ltd.Nuts &. Bolts (Darlaston) Ltd.Precision Screw Manufacturing Co. Ltd.Spirol Pins Ltd.

The Torrington Co. Ltd.Trinity Engineering Co.Geo. Tustin Ltd.

Ucan Products Ltd.Unbrako Ltd.Williams Bros (Sheffield) Ltd.Woodberry fhiUcott & Co. Ltd.Crompton Parkinson Ltd.

QUICK OPERATING FASTENERS

Alder Hardware Ltd.Avdel Ltd.

Howard S. Cooke & Co. Ltd.Deltight Industries Ltd.Dzus Fastener Europe Ltd.Firth Cleveland Fastenings Ltd.

C. J. Fox & Sons Ltd.

GKN Screws & Fasteners Ltd.ITW Ltd. Fastex Div.

170

Isaac, .lackson & Sons (Fasteners) Ltd.Douglas Kane Group Ltd.Metric Allscrews Ltd.Ross, Courtney & Co. Ltd.Silenlblock Ltd.Nyloy Screws Ltd.

RIVC I S - BUNPeter Abbott & Co. Ltd.

Aircraft Materials Ltd.Alder Hardware Ltd.Avdel Ltd.

Brown Bros (Aircraft) Ltd.Carr Kas:ener Co. Ltd.

Chalfon: Aluminium Roofing Supplies Ltd.Datim Screw Co. Ltd.Industrial Fasteners Ltd.

James & Tatten Ltd.Douglas Kane Group Ltd.Llanda:T Engineering Co. Ltd.Metric A:iscrews Ltd,Tappcx Thread Inserts Ltd.Geo. Tucker Eyelet Co. Ltd.

Thos W. Ward Ltd.Clevoilmi Rivets & Tools Ltd.

RIVETS - SOLID & TUBULAR

Aircraft Materials Ltd.

Alder Hardware Ltd.Avdel lad.

Bar Production (Bromsgrove) Ltd.Barton Kivet Co. Ltd.

Baxters ; Holla, Screws & Rivets) Ltd.Bifurcaa.:) & Tubular Rivet Co. Ltd.Black & Luff Ltd.

John Bradley & Co. Ltd.

Brown Bros (Aircraft) Ltd.Cooper & Turner Ltd.

Crane's Screw & Colgryp Castor Co. Ltd.Datim Screw Co. Ltd.

Deltight Industries Ltd.

Everbnghi Fasteners Ltd.Hall fc Mice Ltd.

Industrial Fasteners Ltd.

Jesse Hc,> wood & Co. Ltd.James *. l'a:ten Ltd.

C. Lindley & Co. Ltd.

Llandaff IJr.gmeering Co. Ltd.London Metal Warehouses Ltd.P. & \\ . MacLellan Ltd.Metric A.lscrews Ltd.Midland Screw Co. Ltd.

Motherwe.l & Co. Ltd.Fredk. Mountford (Birmingham) Ltd,

S. & 0. Rivet Co. Ltd.

Screw tv J^vet Co. Ltd.Tower Manufacturing Co. Ltd.

Trinity Kr.gineering Co.Geo. Tustin Ltd.

Thos. \\\ Ward Ltd.

Williams Hros (Sheffield) Ltd.Cleve-lun Rivets & Tools Ltd.

Nyloy Screws Ltd.

SCRFWS - MACHINEPeter Abhott & Co. Ltd.Aircraf* Materials Ltd.Alder Hardware Ltd.Anglo-Suiss Screw Co. Ltd.Annfieit Metal Fasteners Ltd.Arcon Lugi-ieering Co.Automa-ic Standard Screw Co. (Halifax) Ltd.Avon Manufacturing (Warwick) Ltd.B.A. R. /.isteners Ltd.Bar Production (Bromsgrove) Ltd.Barber & Colman Ltd.

H.J. Barlow & Co. Ltd.

Baxters Mioks. Screws & Rivets ) Ltd.John Bradley & Co. Ltd.

G. F. Krioges (Glynwed Distribution Ltd).Brown Hros (Aircraft) Ltd.

Carr c* \i<:tiols Ltd.

George : coper (Sheffield) Ltd.Crane's Screw & Colgryp Castor Co. Ltd.Datim Screw Co. Ltd.

Deltigh; Industries Ltd.Thos. Lr.ves Ltd.

David Etchells (Forgings & Fasteners) Ltd.Everbright Fasteners Ltd.Firth Cleveland Fastenings Ltd.E.J. Francois Ltd.

GKN Screws & Fasteners Ltd.Arthur Gise Ltd.

Thomas Haddon & Stokes Ltd.John Hlt-fctou & Co. Ltd.lndustr.al Fasteners Ltd.

Irlam Kngir.eering Co. (1942) Ltd.Jesse Hitywood & Co. Ltd.Jukes Couison, Stokes & Co. Ltd.James ^ fallen Ltd.C.W. Juby Ltd.Lamp Manufacturing & Railway Supplies Ltd.C. Lindley & Co. Ltd.

Linread Lid.

London Metal Warehouses Ltd.Long-Lok Ltd.

P. & W. MacLellan Ltd.Macnays Ltd.

Metric Allscrews Ltd.

Midland Screw Co. Ltd.

Motherwell fit Co. Ltd.

Fredk. Mountford {Birmingham) Ltd.

Nettlefold & Moser Ltd.

Stephen Newall & Co. Ltd.

R.A. Poole & Co (Sutton) Ltd.

Screw Machine Products Ltd.

Screw & Rivet Co. Ltd.

Segmatac Ltd.

Simpson-Turner Ltd.

G. H. Smith fit Co (Bankhall) Ltd.

Spensall Eng. Co. Ltd.

Swinnerton fit Co (Stourbridge) Ltd.

Telco Ltd.

E.H. Thompson & Son (London) Ltd.

The Torrington Co. Ltd.

Geo. Tustin Ltd.

Williams Bros (Sheffield) Ltd.

Woodberry Chilicott & Co. Ltd.

Ephraim Phillips Ltd.

Holo-Krome Ltd.

Nyloy Screws Ltd.

SCREWS - SELF TAPPING & SIMILARPeter Abbott & Co. Ltd.

Alder Hardware Ltd.

Annfield Metal Fasteners Ltd.

Avon Manufacturing (Warwick) Ltd.

Barber fit Colman Ltd.

Baxters (Bolts, Screws fit Rivets) Ltd.

G. F. Bridges (Glynwed Distribution Ltd).

Crane's Screw fit Colgryp Castor Co. Ltd.

Datim Screw Co. Ltd.

Deltight Industries Ltd.

Everbright Fasteners Ltd.

GKN Screws fit Fasteners Ltd.

Industrial Fasteners Ltd.

ITW Ltd. Fastex Div.

James fit Tatten Ltd.

C. Lindley fit Co. Ltd.

Linread Ltd.

London Metal Warehouses Ltd.

P. fit W. MacLellan Ltd.

Macnays Ltd.

Metric Allscrews Ltd.

Midland Screw Co. Ltd.

Fredk. Mountford (Birmingham) Ltd.

Nettlefold & Moser Ltd.

R.A. Poole & Co (Sutton) Ltd.

Screw Machine Products Ltd.

G.H. Smith fit Co (Bankhall) Ltd.

Spafax (1965) Ltd.

Tappex Thread Inserts Ltd.

Telco Ltd.

The Torrington Co. Ltd.

Williams Bros (Sheffield) Ltd.

Woodberry Chilicott fit Co. Ltd.Ephraim Phillips Ltd.

Nyloy Screws Lid.

SCREWS -SEXPeter Abbott fit Co. Ltd.

Alder Hardware Ltd.

Arcon Engineering Co.

Automatic Standard Screw Co (Halifax) Ltd.

Avon Manufacturing (Warwick) Ltd.

B.A.R. Fasteners Ltd.

Bar Production (Bromsgrove) Ltd.

H.J. Barlow & Co. Ltd.

Baxters (Bolts, Screws fit Rivets) Ltd.

John Bradley & Co. Ltd.

G. F. Bridges (Glynwed Distribution Ltd).

Brown Bros (Aircraft) Ltd.

John Bullough Ltd.

Carr & Nichols Ltd.

George Cooper (Sheffield) Ltd.

Crane's Screw fit Colgryp Castor Co. Ltd.

Crew & Sons Ltd.

Datim Screw Co. Ltd.

Deltight Industries Ltd.

Thos. Eaves Ltd.

David Etchells (Forgings & Fasteners) Ltd.

Everbright Fasteners Ltd.

Firth Cleveland Fastenings Ltd.

H. Fordsmith Ltd.

GKN Bolts & Nuts Ltd.

GKN Screws & Fasteners Ltd.

Arthur Gise Ltd.

John Hickton fit Co. Ltd.

Industrial Fasteners Ltd.

Irlam Engineering Co (1942) Ltd.

Jukes Coulson, Stokes fit Co. Ltd.

Isaac Jackson & Sons (Fasteners) Ltd.

James fit Tatten Ltd.

C.W. Juby Ltd.

Arnold Kinnings fit Son Ltd.

C. Lindley fit Co. Ltd.

London Metal Warehouses Ltd.

P. & W. MacLellan Ltd.

Macnays Ltd.

Metric Allscrews Ltd.

Midland Screw Co. Ltd.

James Mills Ltd.

Motherwell fit Co. Ltd.

Fredk. Mountford (Birmingham) Ltd.

Nettlefold fit Moser Ltd.

Stephen Newall fit Co. Ltd.

Nuts fit Bolts (Darlaston) Ltd.

R.A. Poole fit Co (Sutton) Ltd.

Prestwich Parker Ltd.

Price fit Orptain Ltd.

Benjamin Priest & Sons Ltd.

Charles Richards fit Sons Ltd.

Screw Machine Products Ltd.

Screw fit Rivet Co. Ltd.

Simpson-Turner Ltd.

G.H. Smith & Co (Bankhall) Ltd.

Spafax (1965) Ltd.

Spensall Eng. Co. Ltd.

Swinnerton & Co (Stourbridge) Ltd.

Telco Ltd.

The Torrington Co. Ltd.

Unbrako Ltd.

Whitehouse Industries Ltd.

Williams Bros (Sheffield) Ltd.

Woodberry Chilicott fit Co. Ltd.

Holo-Krome Ltd.

Nyloy Screws Ltd.

SPRING STEEL CUPSAcme Spring Co. Ltd.

Aircraft Materials Ltd.

Alder Hardware Ltd.

British Lock Washers Ltd.

Carr Fastener Co. Ltd.

Howard S. Cooke St Co. Ltd.

George Cotton fit Sons.

Crane's Screw & Colgryp Castor Co. Ltd.

Cross Manufacturing Co (1938) Ltd.

Everbright Fasteners Ltd.

Firth Cleveland Fastenings Ltd.

Charles E. Greenhill Ltd.

Hall & Rice Ltd.

Helical Springs Ltd.

Industrial Fasteners Ltd.

ITW Ltd. . Fastex Division,

James & Tatten Ltd.

Lamp Manufacturing fit Railway Supplies Ltd.

Morlock Industries Ltd.

Spafax (1965) Ltd.

Spring Washers Ltd.

Swinnerton &. Co (Stourbridge) Ltd.

WASHERS

Peter Abbott & Co. Ltd.

Acme Spring Co. Ltd.

Adams fit Benson Ltd.

Aircraft Materials Ltd.

Alder Hardware Ltd.

Anderton (Spring Pressings) Ltd.

Anglo-Swiss Screw Co. Ltd.

Arcon Engineering Co.

Avon Manufacturing (Warwick) Ltd.

B.A.R. Fasteners Ltd.

Bailey's of Aldridge.

Bar Production (Bromsgrove) Ltd.

Barber & Colman Ltd.

H.J. Barlow fit Co. Ltd.

Baxters (Bolts, Screws & Rivets) Ltd.

John Bradley & Co. Ltd.

G. F. Bridges (Glynwed Distribution Ltd).

British Lock Washers Ltd.

G. fit S. Brough Ltd.

Carr fit Nicholls Ltd.

Chalfont Aluminium Roofing Supplies Ltd.

Charles (Wednesbury) Ltd.

George Cooper (Sheffield) Ltd.

Copper fit Asbestos Washer Co. Ltd.

George Cotton fit Sons.Crane's Screw fit Colgryp Castor Co. Ltd.

Crew fit Sons Ltd.

Cross Manufacturing Co. (1938) Ltd.

Deltight Industries Ltd.

David Etchells (Forgings fit Fasteners) Ltd.

Everbright Fasteners Ltd.

E. J. Francois Ltd.

GKN Bolts fit Nuts Ltd.

GKN Screws fii Fasteners Ltd.

Arthur Gise Ltd.

John fit Joseph Goodare Ltd.

Charles E. Greenhill Ltd.

Grover fit Co. Ltd.

Thomas Haddon fit Stokes Ltd.

Hampton fit Beebee Ltd.

John Hickton fit Co. Ltd.

Industrial Fasteners Ltd.

International Engineering Concessionaires Ltd.

Jukee Coulson. Stokes & Co. Ltd.

Isaac Jackson fit Sons ( Fasteners) Ltd.

James fit Tatten Ltd.

C.W. Juby Ltd.

Richard Klinger Ltd.

Lamp Manufacturing & Railway Supplies Ltd.

C. Lindley fit Co. Ltd.

London Metal Warehouses Ltd.

P. fit W. MacLellan Ltd.

Macnays Ltd.

Samuel Marden fit Son Ltd.

Marples fit Beasley Ltd.

Metric Allscrews Ltd.

Midland Screw Co. Ltd.

Moorside Machining Co. Ltd.

Morlock Industries Ltd.

Wm. Motherwell fit Co. Ltd.

Fredk. Mountford (Birmingham) Ltd.

Nettlefold fit Moser Ltd.

Stephen Newall fit Co. Ltd.

Nuts fit Bolts (Darlaston) Ltd.

R.A. Poole & Co. (Sutton) Ltd.

The Positive Lock Washer Co. Ltd.

Prestwich Parker Ltd.

Price fit Orphin Ltd.

Benjamin Priest fit Sons Ltd.

Charles Richards fit Sons Ltd.

Ross. Courtney & Co. Ltd.

Screw fit Rivet Co. Ltd.

G.H. Smith fit Co (Bankhall) Ltd.

Spafax (1965) Ltd.

Spensall Eng. Co. Ltd,

Spring Washers Ltd.

Swinnerton fit Co (Stourbridge) Ltd.

E.H. Thompson fit Son (London) Ltd.

Toledo Woodhead Springs Ltd.

Tower Manufacturing Co. Ltd.

Geo. Tustin Ltd.

Ucan Products Ltd.

Thos. W. Ward Ltd.

Williams Bros (Sheffield) Ltd.

John Williams (Wishaw) Ltd.

Woodberry Chilicott & Co. Ltd.

Nyloy Screws Ltd.

STRUCTURAL WASHERSAdamB fit Benson Ltd.

Alder Hardware Ltd.

Arcon Engineering Co.B.A.R. Fasteners Ltd.

Bailey's of Aldridge.

G. F. Bridges (Glynwed Distribution Ltd).

George Cooper (Sheffield) Ltd.

Cooper fit Turner Ltd.

Everbright Fasteners Ltd.

GKN Bolts fit Nuts Ltd.

Arthur Gise Ltd.

John fit Joseph Goodare Ltd.

Hampton fit Beebee Ltd.

John Hickton fit Co. Ltd.

Industrial Fasteners Ltd.

James fit Tatten Ltd.

Richard Klinger Ltd.

London Metal Warehouses Ltd.

P. fit W. MacLeUan Ltd.

Macnays Ltd.

Samuel Marden fit Son Ltd.

Metric Allscrews Ltd.

The Midland Screw Co. Ltd.

Nettlefold & Moser Ltd.

Nuts fit Bolts (Darlaston) Ltd.

Prestwich Parker Ltd.

Benjamin Priest & Sons Ltd.

G.H. Smith fit Co (Bankhall ) Ltd.Swinnerton fit Co (Stourbridge) Ltd.

Thos. W. Ward Ltd.Williams Bros (Sheffield) Ltd.

John Williams (Wishaw) Ltd.

Nyloy Screws Ltd.

SCREWS - WOOD

Peter Abbot fit Co. Ltd.

Aircraft Materials Ltd.

Alder Hardware Ltd.

Annfield Metal Fasteners Ltd.

Avon Manufacturing (Warwick) Ltd.

U. A. R. Fasteners Ltd.

C. P. Bridges (Glywed Distribution Ltd.

Deltight Industries Ltd.

Kphraim Phillips Ltd.

Kverbright Fasteners Ltd.

GKN Screw & Fasteners Ltd.

Industrial Fasteners Ltd.

James & Tatten Ltd.

C. Lindley & Co. Ltd.

London Metal Warehouses Ltd.

P & W MacLellan Ltd.

Macnays Ltd.

Fredk. Mountford (Birmingham) Ltd.

Nettleford & Moser Ltd.

Nyloy Screws Ltd.

R. F. Overton Ltd.

R.A. Poole & Co. (Sutton) Ltd.

G.H. Smith & Co. (Bankhall) Ltd.

Swinnerton fit Co. (Stourbridge) Ltd.

Telco Ltd.

Ucan Products Ltd.

William Hros. (Sheffield) Ltd.

Woodberry Chilicott fit Co. Ltd.

PROJECTION WELDED FASTENERS

Alder Hardware Ltd.

B.A.R. Fasteners Ltd.

Barton Rivet Co. Ltd.

Baxters (Bolts Screws fit Rivets) Ltd.

Black fit Luff Ltd.

John Bradley fit Co. Ltd.

Carr Fastener Co. Ltd.

Crane's Screw fit Colgryp Castor Co. Ltd.

Crompton Parkinson Ltd.

David Etchells (Forgings fit Fasteners) Ltd.

Firth Cleveland Fastenings Ltd.

C.J. Fox fit Sons Ltd.

Thomas Haddon fit Stokes Ltd.

John Hickton fit Co. Ltd.

Jesse Haywood fit Co. Ltd.

KSM Stud Welding Ltd.

Linread Ltd.

Midland Screw Co. Ltd.Screw fit Rivet Co. Ltd.

The Torrington Co. Ltd.

Trinity Engineering Co.

171

Mtmetouts'i

FAST AND RELIABLE DELIVERIES

of Socket Screws contribute in no small

measure to the efficiency of British

Industry. This type of service is a Holo-

Krome speciality, because Holo-Krome

Distributors and Stockists all over the

country carry large and comprehensive

stocks . . . and are backed by the

efficiency of Holo-Krome's customer-

orientated production methods.

Indeed, Industry can always depend on

the Quality and Fast Delivery of Holo-

Krome Thermo- Forged Socket Screws.

ANDORDER NUWiSSl%^2S2

If you would like a free Holo-KromeSocket Screw Selector, or Samples, or

Price Lists of Standard Sizes of SocketScrews having British, American andMetric Thread Forms, please write on

yourcompany letterhead to Holo-Krome.

HOLO-KROMEHead Office £r Factory — Holo-Krome Limited, Kingsway West, Dundee. Tel. No. 69261, Telex 76241.

Sales Office &• Stock Depot—Holo-Krome Limited, Park Lane, Birmingham 21. Tel. No. 021 553 1037, Telex 338140

172

Suppliers Addresses

Peter Abbott & Co. Ltd. .

191 Francis Road,Leyton. E. 10.

01-539 0631

Acme Spring Co. Ltd.,

Bull Lane Works.West Bromwich,Staffs.

021-553 0756

Adams & Benson Ltd.

,

Union Lodge.Albion,

West Bromwich,Staffs.

021-553 0561

Aircraft Materials Ltd.

,

Midland Road.N.W. 1.

01-387 6151

Alder Hardware Ltd.

,

Beaconsfield Road,

Hayes,Middx.01-573 7766

Anderton (Spring Pressings) Ltd.

.

Hithercroft Road,Wallingford.

Berks.Wallingford 2081

Anglo-Swiss Screw Co. Ltd.,

Trout Rood.

West Drayton.Middx.West Drayton 3644

Annfield Metal Fasteners Ltd.

.

Overton Mill.

Overton,Basingstoke,

Hants.

Overton 303

Arcon Engineering Co.

,

Wallsuches,Horwich.

Bolton,

Lanes.Horwich 68215

Armstrong Patents Co. Ltd.

,

Eastgate,Beverley,Yorks.Beverley 882212

Auto Machinery Co. Ltd.,

Aldermoor Lane.Coventry,

Warks.Coventry 52261

Automatic Standard Screw Co. (Halifax) Ltd.

Charles Street,

Halifax,

Yorca.Halifax 65967

Automotive Engineering Ltd.

.

The Green.Twickenham,Middx.01-894 1161

Avdel Ltd.

,

Welwyn Garden City,

Herts".

Welwyn Garden 28161

Avon Manufacturing (Warwick) Ltd.

,

Montague Road.

Warwick.Warwick 41737

B.A.R. Fasteners Ltd..

Brinton Division.

Wednesbury.Staffs.

021-556 0951

Bailey's of Aldridge,

Redhouse Industrial Estate,

Aldridge.Nr. Walsall.

Staffs.

Aldridge 52288

Bar Production (Bromsgrove) Ltd.

.

Sherwood Road,Bromsgrove.Wores.Bromsgrove 3241

Barber & Colman Ltd.

.

Marsland Road.

Sale,

Ches.Sale 2277

H. J. Barlow & Co. Ltd.,

Mounts Works,Wednesbury,Staffs.

Wednesbury 0906

Barton Rivet Co. Ltd.

.

Hampton Road,

Droitwich.

Wores.

Droltwich 2021

Baxters (Bolts, Screws & Rivets) Ltd.,

Sheepcote Street.

Birmingham, IS.

021-643 0105

Benton Engineering Co. Ltd.

.

Tonbridge Road.

Harold Hill.

Romford.Essex.Ingrebourne 43864

Bifurcated & Tubular Rivet Co. Ltd.

,

MandeviUe Road.

Aylesbury.

Bucks.Aylesbury 5911

G. E. Bissell & Co. Ltd.,

Crown Works,

Malt Mill Lane,

Halesowen.Worcs.021-599 2241

Black & Luff Ltd.

,

Pershore Road South.

Birmingham, 30.

021-458 4371

John Bradley & Co. Ltd.

,

101-111 Holloway Head,

Birmingham, 1.

021-643 4781

G. F. Bridges (Glynwed Distribution) Ltd.

Bordesley Green,Birmingham. 9.

021-772 5511

British Lock Washers Ltd.

,

Bridgnorth Road.

Wombourn,Wolverhampton.Staffs.

Wombourn 2431

The British Screw Co. Ltd.

.

153 Kirkstall Road,

Leeds, 4.

Leeds 30541

G. & S. Brough Ltd,

,

25/29 Commercial St.,

Birmingham, 1.

021-643 3574

Brown Bros (Aircraft) Ltd...

Bedford Road,Northampton,Northants.Northampton 35181

John Butlough Ltd.

.

Bag Lane,Atherton,Manchester,Lanes.Atherton 4151

173

Camloc Industrial Fixings (UK) Ltd.,

12 Hampton Court Parade,

East Molesey.

Surrey.01-979 7363

Carr Fastener Co. Ltd.

,

Stapleford.

Nottingham.

Sandiacre 2661

Carr & Nichols Ltd.

,

Bolton Road,Atherton,Manchester.Atherton 2431

Chalfont Aluminium Roofing Supplies Ltd.

,

Newcastle upon Tyne, 4,

Northumberland.Newcastle 35226

Charles (Wednesbury) Ltd.,

Bridge Works,Wednesbury,Staffs.

021-556 2261

Howard S. Cooke & Co. Ltd.

,

Arrow Road,

Redditch,

Worcs.Redditch 3231

George Cooper (Sheffield) Ltd.

,

Sheffield Road,

Sheffield, 9,

Yores.Sheffield 41026

Cooper & Turner Ltd.

,

Vulcan Works,Vulcan Road,

Sheffield, 9,

Yores.Sheffield 42091

Copper *t Asbestos Washer Co. Ltd.

,

Northgate,

Aldridge,Walsall.Staffs.

Aldridge 52951

George Cotton & Sons.

Lockfield Avenue.Brimsdown,Enfield.

Middx.01-804 3033

Crane's Screw & Colgyrp Castor Co. Ltd.

72 Floodgate Street,

Birmingham. 5.

021-772 3274

Crew & Sons Ltd.

.

Newey Street.

Dudley,

Worcs.Dudley 57231

Cross Manufacturing Co. (1938) Ltd.,

Combe Down.Bath BA2 5RR.Somerset.Combe Down 2355

Datim Screw Co. Ltd.

,

Brooker Road,

Waltham Abbey,Essex.01-97 24738

Deltlght Industries Ltd.

,

Fairfield Street.

Wandsworth,S.W. 18.

01-870 3262

Dzus Fastener Europe Ltd.

Farnham Trading Estate,

Farnham,Surrey.

Farnham 4422

Thos. Eaves Ltd.

,

58 Holloway Head,Birmingham, 1,

021-692 1481

David Etchells (Forgings & Fasteners) Ltd.Blaenau Ffestiniog.

Merioneth.N. Wales.Blaenau Ffestiniog 493

Everbright Fasteners Ltd,

,

162 Colne Road,Twickenham.Middx.01-894 7553

Exors. of James Mills Ltd..

Bredbury Works,Woodley,Stockport,

Cheshire.061-430 2231

Expandite Ltd.

,

Plilplug Div.

.

Western Road,Bracknell,Berks.RG12 1-RH.

Bracknell 3200

Firth Cleveland Fastenings Ltd.

,

Treforest,Glam.Treforest 2633

H. Fordsmith Ltd.,

Hadfield Street Works,

Combrook,

Manchester, 16.

061-872 1615

C. J. Fox & Sons Ltd.

,

117 Victoria St.,

London.S.W. 1.

01-834 0204

E. J. Francois Ltd.

.

62/68 Rosebery Avenue,London,E.C.I.01-837 9157

GKN Bolts & Nuts Ltd.

,

Atlas Works.P.O. Box -No. 12.

Darlaston.S. Staffs.

021-526 3100

GKN Screws & Fasteners Ltd.

,

Heath St. Div.

,

P.O. Box No. 61.

Heath St.

Smethwick,Warley.Worcs.021-558 1441

Arthur Gise Ltd.,

Cooksey Road.Small llcalh,

Birmingham, 10.

021-772 4961

•John & Joseph Goodare Ltd.

,

Noose Lane,Willenhall,

Staffs.

Willenhall 66553

Charles E. Greenhill Ltd.

,

Enterprise Works,Queen Street,

Redditch,

Worcs.Redditch 2657

Grover & Co. Ltd.

,

Britannia Works.Carpenters Road,Stratford,

E. 15.

01-534 4342

Thomas Haddon & Stokes Ltd.

,

Globe Works,Deritend,

Birmingham, 12.

021-772 2312

Hall & Rice Ltd.,Old Meeting Street,

West Bromwlch,Staffs.

West Bromwich 1287

Hampton & Beebee Ltd.

,

Franchise St.

,

Kings Hill,

Wednesbury.Staffs.

021-526 2801

Harris & Edgar Ltd.

,

Progress Works,222 Purley Way,Croydon,CR9 4JH,Surrey.-01-686 4891

Harrison (Birmingham) Brassfoundry Ltd.Bradford Street Works,Birmingham. 12.

021-772 3421

Helical Springs Ltd.

.

Dock Road,Lytham St. Annes,Lanes.

Lytham 7971

John Hickton & Co. Ltd.

,

Stourbridge Road,Halesowen,Birmingham.021-550 1169

Industrial Fasteners Ltd.

,

Hempsted Lane,Gloucester.

Gloucester 25171

Instrument Screw Co. Ltd. ,

206 Northolt Road,South Harrow,Middx.01-422 1141

International Engng. Concessionaires Ltd.

,

Walton -on-Tnames

.

Surrey.Walton-on-Thames 22211

Irlam Engineering Co. (1942) Ltd.

,

Grosvenor Street,

Ashton-under-Lyne,Lanes.061-330 5291

Jesse Haywood & Co. Ltd. .

Foundry Lane,Smethwick,Birmingham, 40.

021-558 3027

Jukes Coulson, Stokes & Co. Ltd.

,

Howards Works,

Second Avenue,E. 13.

01-472 2283

ITW Ltd. , Fastex Div.,470-474 Bath Road.Cippenham.Slough.

Bucks.Burnham 4333

Isaac Jackson & Sons (Fasteners) Ltd.

,

Glossop,Derbys.Glossop 2091

James & Tatten Ltd.

,

P.O. Box No. 5,

Berryhill,Stoke-on-Trent.Stoke-on-Trent 24724

C. W. Juby Ltd.,

Alpha Works,White House Road,Ipswich.Suffolk.

Ipswich 41222

Douglas Kane Group Ltd.

,

Swallowfields.

Welwyn Garden City,

Herts.Welwyn Garden 21261

Arnold Kinnings & Sons Ltd.

,

Norwood Road,Southport.

Lanes.Southport 3182

Richard Klinger Ltd.,Klingerit Works,Sidcup.

Kent.01-300 7777

174

Lamp Manufacturing & Railway Supplies Ltd.Vinceni I.ane,

Dorking.

Surrey.Dorking 4411

C. Lindley & Co. Ltd.,34 K.ngleiield Road,London.N. 1.

01-254 6431

Linread Ltd.

,

P.O. Box No. 21,

Cox Street,

Birmingham, 3.

021-236 9822

Llanda.T Engineering Co. Ltd.

,

Paper Mill Road.Canton.Cardiff.

Wales.Cardiff 563242

London Metal Warehouses Ltd.

,

Summer Road,Thames Ditton,

Surrey.01-398 4121

Long-l,ok Ltd.,

Buckingham Ave.

,

Trading Estate.

Slough.

Bucks.Slough 26741

P. fcW. MacLeUan Ltd.

,

120 Cornwall St.

.

Glasgow SI,

Scotland.041-427 4061

Mscnays-Ltd.,48-50 West Street.

Mlddlesborough.Yoris.Middlcsborough 48144

Sanvje: Marden & Son Ltd.,Wellington Road.Ashton-under-Lyne,Lanes,Ashlon 5136

Marples ^ Beasley Ltd.

,

Marhee Works,South Road.Birmingham, 19.

021-55-i 8471

Metric Ailscrews Ltd.,Pease i'ottage,

Sussex

.

OCY 3 25811

Midland Screw Co. Ltd.

.

46 Floodgate St.

,

Birmingham. 5.

021-772 3513

James Mills Ltd.

,

Knights Road,Tyse;ey.Birmingham, 11.

Acocks Green 1175

Moorside Machining Co. Ltd.

,

Ebor .Mills,

Dubb Lane,Bingley.

Yorks."

Bing.ey 2211

Morloc'x Industries Ltd.,

P.O. Box Mo. 2,

Wombourn.Nr. Wolverhampton,Staffs.

Wombourn 2431

Wm. Motherwell & Co. Ltd.

.

32-42 Partisan Street,

Glasgow, s. 1,

Scotland.

041-429 1047

Fredk. Mountford (Birmingham) Ltd.

,

Abberley Street,

Smethwick,Warley.Worcs.021-658 3101

Nettleiok: & Moser Ltd.

,

170-194 Borough High St.

.

London,S.E.I.01-407 7111

Stephen Newall & Co. Ltd..

James Street.

,

Helensburgh,Scotland.

Helensburgh 2121

Nuts & Bolts (Darlaston) Ltd.

,

Foster Street,

Darlaston,Staffs.

021-526 2201

R. P. Overton Ltd.

,

Ashley House,Spokes Road,Wigmore,Gillingham,

Kent.Medway 32191

Palnut Co. Ltd.,

Arthur Street.

Hove,Sussex.

Brighton 70427

R. A. Poole & Co. (Sutton) Ltd.

,

Mantis House,Willow Walk,Sutton,

Surrey.01-644 1251

Positive Lock Washer Co. Ltd. ,

34 Dalmarnock Road,

Glasgow,S.E.041-556 1873

Precision Screw Manufacturing Co. Ltd.

Longacres.Willenhall.

Staffs.

Willenhall 65621

Prestincert Ltd.,

540 Great Cambridge Road.Enfield,

Middx.01-363 5393

Prestwick Parker Ltd.

,

Bag Lane,Atherton,Manchester,Lanes.

Atherton 2561

Price & Orphin Ltd..

Canal Road.Newtown.Mont.

.

Wales.Newtown 6644

Benjamin Priest & Sons Ltd.,Old Hill Works.P.O. Box No. 38.

Cradley Heath,Warley,Worcs.Cradley Heath 6G501

Charles Richards & Sons Ltd.

.

Darlaston.Wednesbury,Staffs.

James Bridge 3188

S. & D. Rivet Co. Ltd.

.

Temple Road,Leicester.Leicester 36541

Ross. Courtney & Co. Ltd.

,

Ashbrook Road,Upper Holloway,

N. 19.

01-272 0551

Screw Machine Products Ltd.

.

Wooburn Green.Nr. High Wycombe.Bucks.

Bourne End 22741

Screw & Rivet Co. Ltd.

.

Penn Street Works,Wolverhampton

,

Staffs.

Wolverhampton 29041

Segmatac Ltd.

,

Priory Road,

Kenilworth.Warks.

Kenilworth 52358

Silentblock Ltd.

,

Manor Royal.

Crawley,Sussex.

Crawley 27733

Simpson- Turner Ltd.,

Irvine Industrial Estate,

Irvine,

Scotland.

Irvine 2922

G. H. Smith & Co. (Bankhall) Lid.

,

Bankhall Bolt Works.Stanley Road,Liverpool, 5.

051-922 2128

Spafax (1965) Ltd.

.

Box, Chippenham,Wilts.

Box 721

Spensall Eng. Co. Ltd.

.

Great Wilson Street,

Leeds, 11.

Yorks.Leeds 34803

Spirol Pins Ltd.,Windmill Road,Sunbury-on-Thames,Middx.

Sunbury-on-Thames 86165

Spring Washers Ltd.,

Smestow,Wornbourn,Wolverhampton,Staffs.

Wombourn 2431

Swinnerton & Co. (Stourbridge) Ltd.

,

Hall Street,

Stourbridge,

Worcs.Stourbridge 4255

Tappex Thread Inserts Ltd.,Masons Road.Stratford-on-Avon.

Warks.Stratford-on-Avon 4081

Telco Ltd.

,

Alma Road.Enfield.

Middx.01-804 1282

E. H. Thompson & Son (London) Ltd.Skelton Works,Chaucer Road,Forest Gate,E.7.01-472 7094

Toledo Woodhead Springs Ltd.,

Aycliffe Ind. Estate,Darlington,

Co. Durham.Aycliffe 2371

The Torrington Co. I td.

,

Torrington Avenue,Coventry,Warks.Coventry 74241

Tower Manufacturing Co. Ltd.,

Central Works,Shrub Hill,

Worcester.Worcester 27272

Trinity Engineering Co.

,

Hampton Road,Droitwich,Worcs.Droitwich 2426

Geo. Tucker Eyelet Co. Ltd.,Walsall Road.

Birmingham, 22b.

021-356 4811

Geo. Tustin Ltd.

,

New Street,

West Bromwich,Staffs.

021-553 1784

Ucan Products Ltd.

,

27 Lyon Road,Hersham,Walton on Thames.Surrey.Walton on Thames 40111

Unbrako Ltd.

.

P. O. Box No. 38,

Burnaby Road,Coventry,Warks.Coventry 88722

Thos. W. Ward Ltd.

.

Albion Works.Savile Street.

Sheffield, 4.

Sheffield 26311

Warne Wright Eng. Ltd.

,

Warne Wright House.Keeley Street,

Birmingham, 9.

021-772 2921

Wellworthy Ltd.

,

Stanford Road,

Lymington,Hants.

Lymington 2231

Clifford Whatmough Ltd.

,

Vesta Street,

Manchester, 4.

061-273 2624

Whitehouse Industries Ltd.

,

Monkhill,Pontefract,

Yorks.Pontefract 4141

Williams Bros. (Sheffield) Ltd,

,

Green Lane,Sheffield, 3.

Sheffield 27868

John Williams (Wishaw) Ltd.,Excelsior Iron Works,Wishaw.Scotland.Wishaw 2466

Woodberry Chillcott & Co. Ltd.

Atlas Street.

Feeder Road,Bristol, 2.

Bristol 70407

ADDENDUM

Clevedon Rivets & Tools Ltd.

Reddicap Trading Estate.

Sutton Coldfield,

Warks.021-354 5238

Ephraim Phillips Ltd.

,

212, Cheam Common Road,Worcester Park,Surrey.01-337 0017

KSM Stud Welding Ltd.

,

1, Farnham Trading Estate,Farnham,Surrey.Farnham 2 1 101

Moulded Fasteners Ltd.,

Vestry Estate,

Otford Road,Sevenoaks.Kent.

Sevenoaks 56176

Holo-Krome Ltd..

Kingsway West,Dundee.Dundee 69261

Crompton Parkinson Lid.

Crompton House,Aldwych,W.C.2.01-242 3333

Nyloy Screws Ltd.

274, King Street,

Hammersmith,W.6.01-748 9973

175

INDEX TO ADVERTISERS

Avdel Ltd. i07

Bifurcated & Tubular Rivet Co. Ltd. 109

British Screw Co. Ltd. 4

Carr Fastener Co. Ltd. 55

Clevedon Rivets & Tools Ltd. i60

Crompton Parkinson Ltd. 85

Dawe Instruments Ltd. 162

Dzus Fastener (Europe) Ltd 96

Ephraim Phillips Ltd. 167

Fox & Sons Ltd. , C. J. 53

Firth Cleveland Fastenings Ltd (insert)

Holo-Krome Ltd. 172

Instrument Screw Ltd. 27

I.T.W. Ltd. (ii)

K. S. M. Stud Welding Ltd. 92

Long-lok Ltd. 39

McNays Ltd. 133

Metric Allscrews Ltd. 167

Morlock Industries Ltd. 133

Nyloy Screws Ltd. 34

Salter & Co. Ltd., George. 17

Telco Ltd. 59

Tucker Eyelet Co. Ltd., George. 99

Unbrako Ltd. 21

176

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