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An investigation on the anti-loosening characteristics of threaded fasteners under vibratory conditions Anirban Bhattacharya a, , Avijit Sen b , Santanu Das b a Department of Mechanical Engineering, Thapar University, Patiala 147004, Punjab, India b Department of Mechanical Engineering, Kalyani Government Engineering College, Kalyani 741235, West Bengal, India article info abstract Article history: Received 8 October 2007 Received in revised form 13 August 2008 Accepted 13 August 2008 Available online 1 October 2008 A signicant advantage of the screw fastener is its capability of being mantled and dismantled using simple tools. However, threaded fasteners have inherent and inevitable limitations that they loosen eventually under vibrating environment leading to higher frequency of routine maintenance of the components, the absence of which may result in fatal accidents. In the present work, an attempt has been made to test the anti-loosening ability of various locking screw fasteners, such as nylock nut, aerotight nut, chemical lock, cleveloc nut, at washer, nylon washer, serrated washer and spring washer with bolts of different materials, sizes and types with different initial clamping forces under the accelerated vibrating conditions obtained in an indigenously made testing rig. The loss of clamping force gives an indication of the extent of loosening. Their anti-loosening characteristics are compared with respect to initial clamping forces. On the basis of the test results, chemical locking has been found to show best anti- loosening characteristics followed by nylock and aerotight nut. Loosening is found to be considerably less, when the initial clamping force is more than 1.1 ton for metric threaded bolts, and the same for BSW bolts is found at a force above 0.8 ton, when the bolts are fastened with conventional nuts. © 2008 Elsevier Ltd. All rights reserved. Keywords: Threaded fasteners Vibrating condition Anti-loosening Tightening torque 1. Introduction The history [1] of screw fasteners is believed to have begun in TigrisEuphrates (Shatt-al-Arab) region almost 3000 years ago. At the initial stages, the cross section of screw threads was plate-like and they were used for irrigation purposes. According to an Encyclopedia, in ancient Greece people started applying screw to press olives. If this is true, then they are the rst people to realize great potential of screw. The next application that followed was the usage as feeders [1]. Leonardo da Vinci [2] is credited with the mooting and implementation of this important usage of screw threads. He was the man who started the era of development of this fastener by making sketches of them showing his ideas concerning application of screws. Within half a century after Leonardo died, drastic and revolutionary changes were made. The most drastic change came in its shapefrom square to triangle. This invention was made by gunsmiths somewhere in Europe. Although this assumption has not been conrmed yet, it is certain that a bolt at the end of a gun barrel in the early 1940s had a triangular shape. The potential of screw fasteners was slowly realized by people who nally accepted it by 1779 with the rst implementation made exclusively during the construction of Iron Bridgein Telford, England. After this, there evolved wide ranges of screw fasteners and they began to be used in various applications. The numerous advantages realized by the use of threaded fasteners are ability to generate high clamping forces and retention of the same for very long duration, easy assembly and disassembly without damage of the components, wide range of applications of Mechanism and Machine Theory 45 (2010) 12151225 Corresponding author. Department of Mechanical Engineering, Thapar University, Patiala 147004, Punjab, India. Tel.: +91 175 2393365, 2393086 (ofce). E-mail address: [email protected] (A. Bhattacharya). 0094-114X/$ see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mechmachtheory.2008.08.004 Contents lists available at ScienceDirect Mechanism and Machine Theory journal homepage: www.elsevier.com/locate/mechmt

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Transcript of 1-s2.0-S0094114X08001651-main

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Mechanism and Machine Theory 45 (2010) 1215–1225

Contents lists available at ScienceDirect

Mechanism and Machine Theory

j ourna l homepage: www.e lsev ie r.com/ locate /mechmt

An investigation on the anti-loosening characteristics of threaded fastenersunder vibratory conditions

Anirban Bhattacharya a,⁎, Avijit Sen b, Santanu Das b

a Department of Mechanical Engineering, Thapar University, Patiala – 147004, Punjab, Indiab Department of Mechanical Engineering, Kalyani Government Engineering College, Kalyani – 741235, West Bengal, India

a r t i c l e i n f o

⁎ Corresponding author. Department of MechanicalE-mail address: [email protected] (A. Bhat

0094-114X/$ – see front matter © 2008 Elsevier Ltd.doi:10.1016/j.mechmachtheory.2008.08.004

a b s t r a c t

Article history:Received 8 October 2007Received in revised form 13 August 2008Accepted 13 August 2008Available online 1 October 2008

A significant advantage of the screw fastener is its capability of being mantled and dismantledusing simple tools. However, threaded fasteners have inherent and inevitable limitations thatthey loosen eventually under vibrating environment leading to higher frequency of routinemaintenance of the components, the absence of which may result in fatal accidents. In thepresent work, an attempt has been made to test the anti-loosening ability of various lockingscrew fasteners, such as nylock nut, aerotight nut, chemical lock, cleveloc nut, flat washer,nylon washer, serrated washer and spring washer with bolts of different materials, sizes andtypes with different initial clamping forces under the accelerated vibrating conditions obtainedin an indigenously made testing rig. The loss of clamping force gives an indication of the extentof loosening. Their anti-loosening characteristics are compared with respect to initial clampingforces. On the basis of the test results, chemical locking has been found to show best anti-loosening characteristics followed by nylock and aerotight nut. Loosening is found to beconsiderably less, when the initial clamping force is more than 1.1 ton for metric threadedbolts, and the same for BSW bolts is found at a force above 0.8 ton, when the bolts are fastenedwith conventional nuts.

© 2008 Elsevier Ltd. All rights reserved.

Keywords:Threaded fastenersVibrating conditionAnti-looseningTightening torque

1. Introduction

The history [1] of screw fasteners is believed to have begun in Tigris–Euphrates (Shatt-al-Arab) region almost 3000 years ago.At the initial stages, the cross section of screw threads was plate-like and they were used for irrigation purposes. According to anEncyclopedia, in ancient Greece people started applying screw to press olives. If this is true, then they are the first people to realizegreat potential of screw. The next application that followed was the usage as feeders [1]. Leonardo da Vinci [2] is credited with themooting and implementation of this important usage of screw threads. He was themanwho started the era of development of thisfastener by making sketches of them showing his ideas concerning application of screws.

Within half a century after Leonardo died, drastic and revolutionary changes were made. The most drastic change came in itsshape—from square to triangle. This invention was made by gunsmiths somewhere in Europe. Although this assumption has notbeen confirmed yet, it is certain that a bolt at the end of a gun barrel in the early 1940s had a triangular shape. The potential ofscrew fasteners was slowly realized by people who finally accepted it by 1779 with the first implementation made exclusivelyduring the construction of “Iron Bridge” in Telford, England. After this, there evolved wide ranges of screw fasteners and theybegan to be used in various applications.

The numerous advantages realized by the use of threaded fasteners are— ability to generate high clamping forces and retention ofthe same for very long duration, easy assembly and disassembly without damage of the components, wide range of applications of

Engineering, Thapar University, Patiala – 147004, Punjab, India. Tel.: +91 175 2393365, 2393086 (office)tacharya).

All rights reserved.

.

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threaded coupling, standardization leading to cheap production and easy availability for replacement etc. Such unique advantagemakes possible usage of screw threads in numerous engineering applications like holding the components together—parts of largemachines that must be made in small units for ease in manufacturing, assembling or shipping, for production and transmission ofpowers e.g. in lead screws of lathes; screw or press, for adjusting and obtaining accurate movements as in micrometers, pumping,pulling, sealing, etc.

However, prolonged and sustained vibration in some typical mode can result in loosening of screw fasteners very easily inseveral cases—heralding the major drawback of screw fasteners.

2. Review of literature

In the year 1945, Goodier and Sweeny [3] tested only a dynamically loaded bolted joint. In spite of their failure to obtain acomplete self-loosening of threaded fasteners, they offered an explanation of partial loosening of threaded fasteners. They pointedout that for axially loaded joint, pulsating tension of a clamped bolted connection creates radial sliding motions between thethread flanks of the bolt and nut or the interface of the clamped bearing surfaces. The reasons for this are the contraction of the boltaccording to Poisson's ratio and dilation of the nut walls caused by axial tension.

In the year 1964, Hongo [4] conducted some experiments on axial loading of nut and bolt assemblies. He varied the axial tensileforce of a bolt and nut fastening having JIS M20 coarse screw threads 100 times in the range of 250 to 3000 kg in a reciprocalfashion. He examined whether there was any relative rotation between the bolt and nut by observing the oscillation of a beam oflight projected on to a mirror pasted on the bolt. The results showed that the bolt did not continue to rotate in a direction thatwould loosen the nut. Goodier and Sweeny [3] in a similar way had reported detecting a relative rotation of 6.28×10−3 radian byvarying the axial tensile force of the bolt 100 times. Hongo [4] could not accept this conclusion of Goodier and Sweeny [3] that “…the bolt and nut undergo relative rotation in the direction that would loosen the fastening as long as there is variation in the axialtensile force of the bolt” [3].

In the year 1966, Paland [5] tested various types of threaded fasteners for axial loading and gave the rule of looseningarithmetically and bymeasuring the tangential strain on the surface of the nut. He came to the conclusion that a loaded nut widenselastically in a radial direction at the area near the bearing surfaces and contracts in upper part.

This very small amount of radial displacement by expansion of the nut would explain why Paland, in spite of heavy impactloading in an axial direction of the bolt, still needed a small external off-torque to turn the nut so as to loosen completely.

In the year 1969, Junker [6] described themechanism of loosening on the basis of friction between the flank surfaces. Accordingto him, the theory of mechanism of self-loosening of nut and bolt is based on the well-known law of physics that defines the effectof friction on two interacting solid bodies. As soon as the friction force between two solid bodies is overcome by an external forceworking in one direction, an additional movement in any other direction can be caused by the action of forces that can beessentially smaller than the friction force.

He tested cap screws, spring washers and free spinning locking screws with respect to its anti-loosening characteristics. Sincetransversely loaded joints tend more to self-loosening, the test procedure suggested imitates these actual conditions. The firstattemptwasmadewith a device consisting of two parts clamped together by the specimen, with load cell and displacement pickupto record transverse load, preload and displacement. He also reported thatmaximumvalues of vibration energy (transverse force xdisplacement) were significantly different for various locking elements.

Sase et al. [7,8] tested the effectiveness of screw threads, spring washers, nylon inserted nuts, double nuts and eccentric nuts offew sizes to resist loosening. Test results showed that the popularly known anti-loosening fasteners did not possess muchresistance to loosening.

In the year 1998, Sase et al. [8] introduced and evaluated the Step Lock Bolt (SLB) with regard to its anti-loosening performanceusing a displacement based loosening device. They found the presence of desirable anti-loosening characteristics of SLB. Thedisplacement and turning angle of the bolts and the nuts were examined in loosening tests.

Following the experimental procedure and conclusions drawn by Sase et al. [7], a testing rig was designed and fabricated by agroup lead by Das and co-workers [9,10], where a constant vibrating force of constant frequency and amplitude is applied at theright angle of the bolt axis. In this set-up, several tests were carried out with BSW and metric bolts of different materials, sizeswith various types of nut and washer arrangements to conclude that nyloc nuts give substantial resistance to loosening comparedto other fasteners.

2.1. Definition of screw loosening

Materials fastened using screws are held together by the tensile force generated by the elongation of the bolt shaft (the bolt axisforce) and by the force of compression generated in the objects being tightened (the tightening force) [1]. These two forces remainin balance as long as no external forces are applied to the objects being fastened by the screws. The general term for the forcesinvolved in pulling or fastening the two materials together is the pretension force.

In some situations, such as in the course of usingmachinery, the pretension force applied at the time that thematerials formingthe machinery were originally fastened may decrease for a variety of reasons. This spontaneous decrease in the pretension force iswhat is described in general terms as screw loosening.

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2.2. Loosening without relative rotation between bolt and nut

In this case, some residual plastic deformation, such as stretching of bolt, shrinking of an intermediate piece, or smoothening ofthe contact surface like the sides of the thread ridges and the bearing surface of the nut, invariably exists. Those problems can besolved by selection of proper material for bolt and nut [1,3,4].

2.3. Loosening caused by relative motion of the bolt and nut

There are two types of relative motion which occur in threaded fasteners. One is the relative motion between the nut and thebolt. And the other is the relative motion between the nut/bolt and clamping surfaces.

There are three common causes of the relative motion occurring in the threads [1,6]:

1. Bending of parts, which result in forces being induced at the friction surface. If slip occurs, the head and threads will slip whichcan lead to loosening.

2. Differential thermal effects caused as a result of either differences in temperature or differences in clamped materials.3. Applied forces on the joint can lead to shifting of the joint surfaces leading to bolt loosening.

2.4. Vibration loosening of nut and bolt

When nut is subjected to vibrating force, a cycle of alternative tensile and compressive forces starts acting on mating surfacebetween the nut and bolt. But since the mating surface has two angles namely, the lead angle and flank angle, the force gets splitinto three mutually perpendicular components. One of these components acts along the axis of the bolt, the other act in a radialdirection and the remaining one act tangentially to mating surface.

So the force that acts along the axis tries to stretch the bolt and deforms it. The radial force acts to deform the thread profile andtangential force generates a moment in the reverse direction that favours loosening.

If relative motion occurs between the threaded surfaces and/or other contact surfaces of the clamped and clamping partsbecause of an external force, the direction of which is either tangential or radial, the bolted connection will become free of frictionin the circumferential direction. This means that the preload acting on the thread creates a force in a circumferential direction andresults in the rotational loosening of the bolt or nut.

For axially loaded joint, pulsating tension of a clamped bolted connection creates radial sliding motions between the threadflanks of the bolt and nut or the interface of the clamped bearing surfaces. The reasons for this are the contraction of the boltaccording to Poisson ratio and dilation of the nut walls caused by axial tension. Thus axially loaded nut widens elastically in radialdirection at the area near the bearing surfaces and contracts in upper part. These create a relative motion between the nut and boltwhich favours loosening of the fastener.

For dynamically loaded joints, the relative motion between thread flanks and other contact surfaces of the bearing areas canoccur inmagnitudes up to themaximumallowance of the thread. These large effects appear when transverse loading, which has tobe transmitted by grip friction, exceed the friction force between the clamped parts. The resultant transverse slippage between theclamped parts forces the bolt to assume a pendulummovement, which leads to relative motion in the thread hole and thus in thethread flanks.

If the amplitude of such transverse slippage of the bolt is large enough, slippage of nut or bolt head bearing surface will finallyoccur and make the join totally free of friction in a circumferential direction. It can be easily realized too that, contrary to theconditions to the axial loading, relative motion between the flanks will occur in all parts of the nut threads when the joint slipsunder transverse force. Thus, the internal off-torque force becomes sufficient to turn the bolt or nut completely loose as soon as thefriction is eliminated from the bearing area as well as from the thread area. Such transverse slippages aremore common in practicethan usually accepted. Experience shows that these joints most frequently fail by self-loosening [1,4,6–8].

2.5. Prevention of loosening by means of design

From the above discussion it can be declared that loosening caused by relative rotation can be minimized if [1],

➢ The lead angle is reduced:Relative slip depends on lead angle. With the increase of lead angle relative slip also increases and vice versa. If helix anglebecomes greater than friction angle then thread becomes overhaul. If lead angle reduces to zero then thread will not be able totransfer torque.

➢ The flank angle is made as small as possible—almost zero:A very small flank angle results in increased fastening torque as well as due to vibration when it tries to rotate in the looseningdirection it needs more torque. So a very small flank angle increases the anti-loosening property of thread.

➢ Reducing relative slip between bearing surface nut and the fastened material by introducing a taper between these twosurfaces:In this case contact area as well as friction force increases. But assembly becomes difficult. Intermediate part must have propercounter profile.

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2.6. Locking fasteners

A variety of locking fasteners are used nowadays offered by major companies. For example, conventional spring lock washersare no longer specified, because it has been shown that they actually aid self-loosening rather than prevent it. There are amultitude of thread locking devices available. Through the efforts of the American National Standards Subcommittee B18:20 onlocking fasteners, three basic locking fastener categories have been established. They are [1] free spinning, friction locking, andchemical locking.a. Free Spinning Type: The free spinning type is plain bolts with a circumferential row of teeth under the washer head. These are

ramped, allowing the bolt to rotate in the clamping direction, but lock into the bearing surface when rotated in the looseningdirection. The ‘Whizlock’ is in this category.

b. Friction Locking: Friction locking categories can be sub-divided into two groupings, metallic and non-metallic. The metallicfriction locking fastener usually has a distorted thread which provides a prevailing torque; an example of this category is the‘Philidas’ nut. Non-metallic friction locking devices have plastic inserts which provide a thread locking effect; an example beingthe ‘Nyloc’ nut.

c. Chemical Locking: The chemical locking category is adhesives which fill the gaps between the male and female threads andbond them together; ‘Loctite’ is an example. Such adhesives are now available in micro-encapsulated form and can be pre-applied to the thread.

2.7. Some anti-loosening fasteners

Many new threaded fasteners have been introduced to claim to have considerable anti-loosening ability. One of such attemptshas provided a wedge ramp which can draw the crest of the bolt tightly. ‘Step lock bolt’ has been reported [1,7,8] to show anti-loosening property by putting ‘step parts’ having no lead angle at certain positions on to the bolt. Combinations of two nuts withsmall eccentricity in the sliding part of the convex top of the lower nut have also been tried for self-locking of the fasteners undervibratory conditions. In Aero-tight nut, a torque prevailing nut of all metal construction, the nut is slotted in two places which,after the nut has been tapped, are bent slightly inwards and downwards. When the nut is screwed onto the bolt thread the twoslotted parts are forced back to their original position [1]. In Cleveloc nut, the collar of the nut is elliptical in cross section and it isthis that provides the flexible locking element. The nut is pre-lubricated to reduce the tightening torque [1].

3. Experimental investigation

3.1. The set up

From previous works it is seen that the loosening mechanism becomes most pronounced when the direction of vibration isperpendicular to the longitudinal axis of the bolt.

The testing rig has thus been designed to obtain the following:

1. A repeated oscillatory motion to achieve vibration of fixed frequency and amplitude.2. Measurement of clamping force at any instant.3. Any desired number of oscillations.

To obtain the above requirements the following machines and/or mechanisms have been made use if:

1. A motor provides the basic rotary motion and the motion is transferred to another shaft by a belt–pulley arrangement.2. A cam mounted on the rotating shaft incorporates rotating motion to the oscillatory motion onto a rocking plate at a fixed

frequency.3. A load cell measures the clamping force between the two plates clamped by the bolt to be tested, at any instant.4. A proximity switch measures the number of oscillation by measuring the rotation of the shaft.

The rocking plate transmits this vibration to another plate which is clamped to a fixed structure with the help of the nut andbolt assembly under test. By this way, the nut and bolt assembles the two plates out of which one is fixed and the vibration istransferred to the other plate via the rocking plate. The vibration provided is of repeated hammering nature. Due to this vibration,the fasteners will begin to loosen and the corresponding clamping force will decrease. The schematic diagram of the testing deviceis shown in Fig. 1, as also referred in Ref. [10]. Fig. 2 shows the pictorial view of the testing rig.

This continuous decrement in clamping force with the number of revolutions will be a measure of loosening. The aboveproposition is the key behind the designing and fabrication of the test rig.

The eccentricity of the cam is so designed that at the end of the rocking plate, where the bolts is clamped, the amplitudebecomes 0.175 mm and the plate rocks at a fixed frequency of 3 Hz. The frequency of the vibration can be changed by changing thepulley step.

The indigenously made testing machine consists of a motor, a belt–pulley drive, a rotating cam, a rocking plate, a testing areafor fastening and a load cell (compression type, Make–Sushma, Bengaluru, India, Model SLC-302) along with the indicator.

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Fig. 1. Schematic diagram of the testing rig.

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3.2. Experimental conditions

For carrying out the experiments and to judge the anti-loosening ability of the screwed fasteners the bolts used were Metrictype (M16 and M10, both of High Tension Steel (HTS), having 2 mm and 1.5 mm pitch respectively and 5/8 in. BSW HTS threadedbolts and Low Carbon Steel (LCS) bolt, having 11 TPI; 3/8 in. HTS, LCS and Stainless Steel (SS) bolt having 16 TPI. The initialtightening torque was 1.5 ton for M16 HTS bolt, 0.8 ton for M10 HTS bolt and 3/8 in BSWHTS, SS, LCS bolts, 1.3 ton for 5/8 in. BSWHTS, SS bolts. The number of oscillations up to which the constant decrease in clamping force measured was 10,200. The differenttypes of nuts and washers used for the clamping of the fasteners were Conventional Nuts, Flat Washer, Spring Washer, InsideSerrated, Outside Serrated washers, Double nuts, Nylock nut, Nylon washer, Cleveloc Nut, Chemical lock (Locktite) and Aerotightnut. All the experiments were repeated for three times.

Fig. 2. Pictorial view of the testing rig.

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Fig. 3. Comparison of loosening for Metric High Tension Steel Bolt (M16) with different nuts.

Fig. 4. Comparison of loosening for Metric High Tension Steel Bolt (M10) with different nuts.

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4. Results and discussions

Fig. 3 shows the comparison of loosening for metric (M 16) High Tension Steel Bolt with different nuts. The plot shows that theuse of flat washer can reduce the loss of clamping force to a little extent. The double nut can also effectively reduce loosening.Chemical locking is found to be a comparatively good anti-loosening fastener under vibration, following nylock nut. On the otherhand, the inside serrated washer shows a better anti-loosening ability than outside serrated washer.

Loosening tendency of metric (M 10) High Tension Steel Bolts with different nuts is compared in Fig. 4. Here the use of flatwasher and nylon washer can reduce the loosening of conventional nut to some extent. Nylon washer has better anti-loosening

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Fig. 5. Comparison of loosening for 5/8 in. BSW High Tension Steel Bolt with different nuts.

Fig. 6. Comparison of loosening for 5/8 in. BSW Stainless Steel Bolt with different nuts.

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ability than flat washer if properly used between two flat washers. Chemical lock has the better anti-loosening property thanothers. Nylock nut also has considerable ability to resist loosening under vibration. Double nut, if properly tightened, can providegood resistance to loosening. Outside serratedwasher and spring washer lies in themiddle on the basis of anti-loosening property.The loosening with conventional nut and flat washer is always faster than all other nuts. If the threaded fasteners are categorizedaccording to their anti-loosening property, then conventional nut, flat washer shows very little effect on anti-loosening; springwasher, nylonwasher and serratedwashers exhibit moderate anti-loosening ability, whereas double nut, nylock nut, and chemicallock provide the best anti-loosening ability under vibratory conditions.

Fig. 5 and Fig. 6 show the variation of loosening torque with number of oscillations for a 5/8 in. BSW bolt of high tension steeland stainless steel bolts respectively. In Fig. 5, it is seen that the nylock nut and chemical locking nut have comparatively good anti-loosening property. Cleveloc nut cannot prevent loosening effectively. Spring washer and serrated washer (both inside and

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Fig. 7. Comparison of loosening for 3/8 in. BSW High Tension Steel Bolt with different nuts.

Fig. 8. Comparison of loosening for 3/8 in. BSW Stainless Steel Bolt with different nuts.

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outside), can reduce loosening to some extent, whereas Fig. 6 shows that Chemical lock nut has comparatively high resistance toloosening followed by nylock nut. Inside and outside serrated washer have also enough ability to resist loosening under vibration.Spring washer has considerable anti-loosening effect compare to flat washer.

Figs. 7, 8 and 9 provide the variation of loosening torque with number of oscillations for a 3/8 in. BSW bolt of high tension steel,stainless steel and low carbon steel bolts, respectively. Among the three types of fastening materials, low carbon steel being aductile material, can deform easily under high tightening torque resulting in high contact friction between the fastening elementsand hence, tends to resist loosening. However, high tension steel and stainless steel are commonly harder materials than lowcarbon steels, and do not deform easily making it suitable for repetitive use. Due to high surface finish of stainless steels, actualcontact area between the fastening elements becomes large leading to high contact friction reducing the tendency for possibleloosening under vibration. According to Fig. 7, very little improvement has been found using Flat washer over conventional nut,nylon washer has better resistance to loosening than a flat washer, if properly used between two flat washers. On the other hand,cleveloc nut does not have enough anti-loosening property like aerotight nut. Double nut has good anti-loosening ability if

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Fig. 9. Comparison of loosening for 3/8in. BSW Low Carbon Steel Bolt with different nuts.

Fig. 10. Comparison of loosening for 3/8 in. BSW Low Carbon Steel Bolt with conventional nut for different initial clamping force.

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properly tightened. Chemical lock shows comparatively better method of reducing loosening under vibration followed by nylocknut. Fig. 8 shows that chemical locking has better anti-loosening property compared to the others. The nylock nut showsconsiderable resistance to loosening followed by outside serrated washer. Spring washer and double nut can also reduce thetendency to loosening under vibration to some extent. However, double nut does not show (Fig. 9) good results consistently. Flatwasher can reduce the loosening to some extent. Spring washer and outside serrated washer are found to reduce looseningconsiderably. Chemical locking is the best method of preventing loosening followed by nylock nut.

Fig. 10 shows the comparison of loosening for 3/8″ BSW low carbon steel bolt with conventional nut when tested with differentinitial clamping forces. The plot shows that for every initial clamping force, the nature of loosening is almost the same, i.e. for firstfew oscillations, the loosening is higher and after certain number of oscillations the rate of loosening becomes slower. Thepercentages of loosening after 5000 oscillations corresponding to the different initial clamping forces are shown in Table 1.

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Table 1Loosening at different initial clamping forces for 3/8 in. BSW low carbon steel bolt.

Initial clamping force (ton) Percentage of loosening

0.4 8.510.5 14.910.6 13.480.7 9.140.8 8.070.9 3.321.0 3.191.1 31.2 1.2

Fig. 11. Comparison of loosening for M16 High Tension Steel Bolt with conventional nut for different initial clamping force.

Table 2Loosening at different initial clamping forces for M16 high tension steel bolt.

Initial clamping force (ton) Percentage of loosening

0.8 15.810.9 6.41.0 6.01.1 5.91.2 3.41.3 31.4 2.51.5 2.51.6 2.71.7 31.8 3.6

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From the Table 1, it can be said that for higher initial clamping force, total loosening is lower for BSW low carbon steel bolt. Butfor extensively higher clamping force, the thread as well as the bolt may be deformed and the loosening is lower when the initialclamping force is higher than 0.8 ton.

Fig. 11 shows the comparison of loosening for M16 High Tension Steel Bolt with conventional nut when tested with differentinitial clamping force. Here, it is seen that for all initial clamping force, the loosening trends are almost the same. The initialloosening rate is quite high. The total loss of clamping force for different initial clamping forces is shown in Table 2.

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In the case of M16 high tension steel bolt, the percentage of loosening was calculated after 10,200 oscillations and it was found(Table 2) that the loosening is minimumwhen the initial clamping force is more than 1.1 ton. The reason behind this may be that,for higher initial clamping force, the fastener may undergo higher deformation at the contact points leading to more frictionalresistance to loosening. However, very high force may damage the fastener. Hence, appropriate initial tightening torque is to beapplied on a threaded fastener that can restrict loosening.

5. Conclusions

From the results obtained of repetitive experiments with different combinations of fastening elements, the followingconclusions may be drawn.

(i) Not only the bolt material, but other fastening elements such as nuts or washers also play a key role behind the anti-loosening property of the fasteners; the initial tightening torque also plays a significant role behind the self-lockingproperty of fasteners.

(ii) Possibly because of high surface finish, the stainless steel bolts show better resistance to loosening than that of high tensionsteel and low carbon steel bolts.

(iii) Out of several anti-loosening fasteners tested, chemical locking provides the best anti-loosening ability followed by nylocknut and aerotight nut. Aerotight and nylock nuts have less possibility to loose under hostile vibrating conditions, but theeffect decreases considerably with repetitive use.

(iv) Spring washer, inside serrated and outside serrated washers also provide considerable anti-loosening property, but getsdamaged after single use.

(v) It is found that flat washers and nylon washers do not prevent loosening, but spring washers and double nuts showconsiderable resistance against loosening.

(vi) For metric as well as BSW threaded bolts, with the increase in clamping force, the percentage of loosening shows an overalldecrease because of possible deformation and/or larger contact area between fastening elements.

Acknowledgement

The authors sincerely acknowledge the All India Council for Technical Education, New Delhi for the financial support toundertake the project vide sanction letter Ref. 8018/RDII/BOR/R&D(244)/99-2000 dated March 24, 2000.

References

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