Fasteners, Powers Screws, Connections

Helical thread screw was an important invention.

Power Screw, transmit angular motion to liner motion

Transmit large or produce large axial force

It is always desired to reduce number of screws

Typical cup screws

Other types of screw heads

Screw Designations

• United National Standard UNS

• International Standard Organization

Roots and crest can be either flat or round

Pitch diameter produce same width in the thread and space,

Single and Double threaded screws

Double threaded screws are stronger and moves faster

Multiple-threaded screws

Coarse thread Designated by UNC

• Fine Thread UNF, is more resistance to loosening, because of its small helix angle.

• They are used when Vibration is present• Class of screw, defines its fit, Class 1 fits have

widest tolerances, Class 2 is the most commonly used

• Class three for very precision application• Example:1in-12 UNRF-2A-LH, A for Ext. Thread

and B for Internal, R root radius• Metric M10x1.5 10 diameter mm major

diameter,1.5 pitch

Square and Acme Threads are used for the power screw

Preferred pitch for Acme Thread

1/51/51/61/61/81/101/121/141/16p,in

1 1/417/83/45/81/23/85/161/4d, in

Mechanics of Power Screws

Used in design to change the angular motion to linear motion, Could you recall recent failure of power screw leading to significant causalities

What is the relationship between the applied torque on power screw

and lifting force F

Torque for single flat thread

)sec

sec(

2

fld

fdlFdT

m

mmR

)(2

)(2

fld

lfdFdT

fld

fdlFdT

m

mmL

m

mmR

If the thread as an angle α, the torque will be

Wedging action, it increases friction

Stresses in the power Screw

pnd

F

A

V

pnd

F

pnd

Fd

T

tr

trb

tmB

3

2

3

6

2/

163

Shear stress in the base of the screw

Bearing stress

Bending stress at the root of the screw

Shear stress in the thread

nt number of engaged thread

Definition of important Terminologies

Major diameter d, Minor diameter dr Mean dia or pitch diameter dp

Lead l, distance the nut moves for one turn rotation

Bolt terminology

Main characteristics of Bolts

Some important properties

Load that a bolt can sustain

bolt.ppt - 10 of 12

product dissection 3/4/2009

Bolt Manufacturing Processes

Forging (upsetting)

Rolling

Thread-rolling processes: a) reciprocating flat dies; and b) two-roller dies.Threaded fasteners, such as bolts, are made economically by these processes at high rates of production

a) b)

bolt.ppt - 11 of 12

product dissection 3/4/2009

Manufacturing Processes -continued

Turning on screw machines

(a) Differences in the diameters of machined and rolled threads. (b) Grain flow in machined and rolled threads. Unlike machining, which cuts through the grains of the metal, rolled threads have improved strength because of cold working and favorable grain flow.

bolt.ppt - 9 of 12

product dissection 3/4/2009

Tightening Torque

It is typical on engines for bolts to have a specified tightening torque. Why?

It results in a quantified preload on the boltsInsures that parts never separate Maintains friction (no sliding to shear forces)Insures even distribution of loading

• prevent warpage of mating parts• uniform pressure distribution over seal or gasket

Prevents bolt from looseningReduces fatique effects

bolt.ppt - 8 of 12

product dissection 3/4/2009

Materials

Let’s look at the example of engine head bolts

The head isalumimum, so why are the head bolts made of steel ?

In what cases might you want to use an aluminum bolt?

bolt.ppt - 7 of 12

product dissection 3/4/2009

Bolt Grades

Grade indicates the tensile strength of the bolt

Determined by bolt material and heat treating

Loading to the fasteners and their Failure considerations

Bolts are used to clamp two or more parts

It causes pre tension in the bolt Grip length is the total thickness of parts and washers

l

ld

l t

t2

ldh

L’ effective grip= h+t2 if t2<d

=h=d/2 for t2 d

lt=L’- ld

Failure of bolted or riveted joints

Type of Joints

• Lap Joint (single Joint) But Joint

Example 1

Example 2

Example 2

Example 3

Weld

Weld under Bending

Brazing

• Joining process

• A filler metal is placed between two workpieces and heated until melted

• Two main types of Brazing– Ordinary– Braze welding

• Use of flux is very important

Filler Metals• Available in a wide range of brazing temperatures

• They come in a wide range of shapes

• Choice of the filler metal and its composition are important

• Diffusion of the filler metal in to the workpeice is an important consideration

Fig: a) Brazing b) Braze welding operation

Fluxes

• The use of flux in brazing is very important

• Generally made of:– Borax– Boric acid– Borates– Fluorides– Chlorides

• Wetting agents may also be added

Brazing Methods

• Torch Brazing

– Performed by heating the joint with a torch

– Depositing the filler metal in the joint

– Suitable part thickness (0.25 – 6.0)mm

– Not a automated process

– More than one torch can be used in this process

• Furnace Brazing

– Precleaned & Preloaded with brazing metal

– Heated in a furnace

Fig:An example of furnace brazing a)before b) after

Brazing Methods

Other Types Of Brazing

• Induction Brazing

• Resistance Brazing

• Dip Brazing

• Infrared Brazing

• Diffusion Brazing

Braze Welding

• Prepared like fusion welding

• Filler metal is deposited at the joint with the use of an oxyacetylene torch

• Considerably more filler is used

• Temperature is minimal compared to that of fusion welding; part distortion is minimal

Brazing Process Capabilities

• Dissimilar metals can be assembled with good joint strength

• Shear strength of brazed joints can reach 800Mpa

Fig:Joint Designs commonly used in brazing operations.The clearance between the two parts being brazed is an important factor in joint strength.If the clearance is to small, the molten braze metal will not fully penetrate the interface.

Good/Poor Designs

Soldering

• Different types of soldering

– Torch– Furnace– Iron– Induction– Resistance– Dip– Infrared– Ultrasonic– reflow (paste)– Wave

Reflow Soldering

• Solvents present in the paste are evaporated

• The Flux in the paste is activated and the fluxing action occurs

• The components are carefully preheated

• The solder particles are melted and wet the joint

• The assembly is cooled

Wave Soldering

• Popular approach to attaching circuits to circuit boards

Fig: a)Screening or stenciling paste onto a printed circuit board: 1) Stenciling process 2) a section of a typical stencil pattern b) wave soldering process

Types Of Fluxes

• Inorganic acids or salts – clean the surface rapidly

• Noncorrosive resin-based – used in electrical applications

• Soldering is used extensively in electronics industry

Adhesive Bonding

• Products are joined and assembled by the use of Adhesives

• Adhesives properties to be considered– Strength– Toughness– Resistance to various fluids– Ability to wet the surface to be bonded

Types of adhesives

Surface must be clean for joining parts

• Should avoid joints that might be subjected to peeling forces• Design for adhesive bonding

Adhesive Peeling Test

Fig : Characteristic behavior of (a) brittle (b) tough adhesive in a peeling test

Joint Design in Adhesive Bonding

Fig:Various joint design in adhesive bonding.

Configurations for adhesive bonds

Fig : (a) single lap (b) double lap (c) scarf (d) strap

Mechanical Fastening

• Threaded Fasters– Bolts– Screws– Nuts

• Other Fastening Methods– Stapling– Crimping– Snap-in Fasteners– Shrink and press fits

Rivets

Fig:a)solid b)tubular c) split (bifurcated) d) compression

Fig : Design guidelines for riveting (a)Exposed shank is too long; the result is buckling instead of upsetting (b)Rivets should be placed sufficiently far from edges to avoid stress concentrations (c)Joined sections should allow ample clearance for riveting tools (d) section curvature should not interfere with the riveting process

Joining Plastics

• Heat softens the plastic to a molten state

• Then pressure added & fusion takes place

• External Heat Sources– Hot air– Heated tools & dies– Electrical-Resistance– Lasers

• Internal Heat Sources– Ultrasonic welding– Friction welding