Review on anti-loosening methods for threaded fasteners

Hao GONG,Jianhua LIU,Huihua FENG

School of Mechanical Engineering,Beijing Institute of Technology,Beijing 100081,China

KEYWORDS Anti-loosening designs;Anti-loosening structures;Bolted joints;Loosening failure;Preload test

Abstract Threaded fasteners naturally tend to loosen under vibration,impact,or alternating thermal load.Various anti-loosening methods or strategies are widely applied for preventing loosening,including a few anti-loosening designs and anti-loosening structures.In this review,a variety of influencing factors that improved the anti-loosening ability and helped guide the anti-loosening designs of threaded fasteners are summarized.Second,the anti-loosening structures are classified into two categories,that is,conventional and new-style anti-loosening structures.The former refers to widely used structures in engineering applications,while the latter refers to novel structures not available in the markets.The research on the evaluation and comparison of various conventional anti-loosening structures in terms of preventing loosening is summarized in detail and the newstyle anti-loosening structures from five different aspects are also surveyed based on different structural characteristics.In the future,the effects of combined factors on loosening should be considered to achieve optimal anti-loosening designs.More quantitative research on the mathematical relationship between the anti-loosening performance of conventional structures and structural parameters must be conducted.Furthermore,novel structures with excellent abilities to prevent loosening are needed,and issues of difficulties in installation and disassembly,low strength at thread,large deformation,and temperature dependence also need to be addressed.

1.Introduction

Threaded fasteners with the reputation of‘‘industrial rice”are standard fundamental components in the industry,providing the functions of connecting,fastening,and sealing.They are applied widely in mechanical engineering,aerospace engineering,civil engineering,and marine engineering,among others,due to their low cost,good interchangeability,and ease of installation and disassembly.Although threaded fasteners are commonly small and even inconspicuous,they play a significant role in guaranteeing product reliability.

However,threaded fasteners are susceptible to loosening after assembly due to severe service environment exposure.A great deal of research has showed that loosening behavior often occurs when threaded fasteners are subjected to vibration,impact,or alternating thermal load.Loosening results in the loss of preload and harms the integrity and reliability of a product.A survey on automobile dealer service managers in the United States indicated that 23% of the service problems were attributed to the loosening of threaded fasteners.Bolt fatigue fracture or the nut falling off the bolt induced by loosening can even cause fateful consequences.Fig.1 (a) shows a railway accident due to a high-speed train derailment in the United Kingdom in 2007.This accident resulted from nuts detaching from the bolts due to loosening and allowing the switch rail to be struck by the inner faces of passing train wheels.Fig.1(b) shows the collapse accident at Space Lost,an amusement park in eastern Shenzhen,China,causing six deaths and 10 injuries.The main cause was loosening that triggered the fatigue fracture of the bolt.

Fig.1 Two severe accidents caused by loosening of threaded fasteners.

In engineering application,various anti-loosening methods or strategies have been applied or proposed for improving the anti-loosening ability and to avoid loosening failure.These anti-loosening methods or strategies can be categorized into two groups.One group enhances the anti-loosening performance by the structural design of threaded fasteners,for example,the designs of preload,friction coefficient,fit clearance,and shape.The other group applies anti-loosening structures to improve loosening resistance.The former primarily focuses on the effects of various factors on the preload retention of threaded fasteners subjected to external loads and finds the optimal structural design of threaded fasteners for resisting loosening.The latter can be traditionally divided into three basic categories based on different anti-loosening principles.The first category is non-detachable anti-loosening,which employs locking adhesive and welding to bond internal and external threads and make them non-removable.The second category is the mechanical locking installation,which employs components including the split pin and tab washer to prevent the relative rotational loosening between internal and external threads.The third category is the frictional locking installation,which achieves anti-loosening performance by increasing the frictional forces between internal and external threads,or between bearing surface and nut(or bolt head),such as double nut and spring washer.

This review focuses on anti-loosening designs and detachable anti-loosening structures.For the convenience of review,the detachable anti-loosening structures are mainly classified into two categories,that is,conventional structures and newstyle structures.Conventional structures refer to the antiloosening structures that have been widely applied in engineering applications,including double nut,spring washer,wedge washer,and eccentric double nut.New-style structures refer to the anti-loosening structures that were recently proposed,appearing only in the literature or patents and are not available in the market.Although a few scholars have reviewed the research on various anti-loosening methods,these studies were limited to different helical spring-locked washers.Therefore,a comprehensive review on anti-loosening methods is still lacking,which is the motivation for this study.

In this study,various anti-loosening methods and strategies are systematically outlined.The review is organized as shown in Fig.2.In Section 2,the research on anti-loosening designs of threaded fasteners is reviewed.A variety of influencing factors improving anti-loosening ability that guides the structural design of threaded fasteners are summarized.In Section 3,the research on the evaluation of conventional anti-loosening structures in the form of washer and nut is reviewed.The anti-loosening capabilities,mechanisms,or optimizations of various conventional structures are summarized.In Section 4,the research on the comparison of conventional anti-loosening structures in terms of loosening resistance is reviewed.In Section 5,the research on new-style anti-loosening structures is reviewed.Some novel anti-loosening structures and their anti-loosening principles proposed in recent literature or patents are introduced.In Section 6,the paper is concluded with the perspectives and future research directions on antiloosening methods.This review will further knowledge of the research status of anti-loosening methods,and help guide the development of anti-loosening designs and the selection of proper anti-loosening structures in engineering applications.

Fig.2 Framework of review on anti-loosening method.

2.Anti-loosening designs for threaded fasteners

Anti-loosening designs refer to the optimal designs of various structural parameters of threaded fasteners with the aim of improving anti-loosening performance.These structural parameters include fastener position,preload,friction coefficient,clamped length,thread pitch,fit clearance,hole clearance,and nominal diameter.The effects of these structural parameters on the anti-loosening capability of threaded fasteners have been extensively investigated,and are used as the basis for anti-loosening designs.Following is a summary of related research work.

Early in 1969,Junker’s experimental resultsshowed that transverse vibration (the direction perpendicular to the bolt axis) can cause large-scale loosening behavior,compared with axial vibration.Transverse vibration was considered as the primary vibrational form inducing loosening.In 2007,Zhang et al.demonstrated that for a given bolted joint,there was an increase in resistance to loosening when the angle between the external vibration load and the pure transverse direction gradually rose from 0 to 90 degrees.Therefore,for a known external vibration load,the axial direction of the threaded fasteners should be designed parallel to the vibration direction as far as possible for enhancing anti-loosening performance.

Preload is an essential factor,which is considered in the anti-loosening design of threaded fasteners.Existing research resultshave demonstrated that the larger the preload,the better the anti-loosening ability.However,excessive preload may cause surface collapse,which adds the risk of failure.Therefore,the preload should be designed to be in a reasonable range.The lower limit of the range depends on the amplitude of the transverse vibration.In general,for a threaded fastener with a given preload,there exists a critical amplitude of transverse vibration,below which loosening will not occur.That is,for a given vibration amplitude,there is a critical preload above which loosening will be prevented.The critical preload corresponds to the aforementioned lower limit.Some scholars have systematically investigated the critical amplitudes of transverse vibration under different preloads.

Junkerproposed that loosening originated from complete slippage on the bearing and thread surfaces.The critical transversal force (i.e.,the vibration amplitude) corresponding to complete bearing slippage was used as a criterion indicating loosening.Nishimura et al.proposed the equation for critical relative slippage () under complete bearing slippage,below which the threaded fasteners will stay fastened.Pai and Hessfound that loosening could still occur when the transversal displacement was less than.It was experimentally observed that loosening initiated when the transversal displacement reached approximately 46%-66% of.Izumi et al.found that loosening initiated when the thread surface reached a complete slippage state regardless of the slippage state at the bearing surface.In another study,it was calculated by finite element analysis(FEA)that a small degree of loosening initiated when the vibration force reached approximately 50%-60% of the critical loading necessary for complete bearing slippage.Dinger and Friedrichfound that,for a specific threaded fastener,the transversal displacement causing loosening can be as low as 0.55 mm,whereas the value ofcorresponding to complete bearing slip was 0.86 mm.Gong et al.proposed a novel method of finite element simulation to accurately calculate the critical transversal force.

Eq.(1) is widely used to calculate critical transversal force,whereis the preload,μ is the frictional coefficient of bearing surface,andis the loosening factor.As can be seen from the above research,the value ofis approximately 0.4-0.6 and is affected by many factors.Onceis precisely determined,the critical preload corresponding to the lower limit of preload range for a threaded fastener with a given vibration load can be accurately calculated using Eq.(1):

Some studieshave demonstrated that increasing friction coefficients of the surfaces of thread and bearing can reduce the preload loss of threaded fasteners under cyclic transverse vibration.Gong et al.also validated that large friction coefficients will improve anti-loosening performance from the perspective of critical transverse force.However,the increase in friction coefficients may enlarge the torque factor,indicating that the generated preload will decrease for the same torque compared with normal threaded fasteners.Overall,the friction coefficients at the surfaces of thread and bearing can be designed a little larger for better antiloosening performance.However,the tightening torque must be added to obtain the expected preload.

The large clamped length may result in extensive bending and elastic deformation,which may affect the anti-loosening ability.Some scholarshave demonstrated from different views that for a given transversal force,increasing the clamped length will harm the anti-loosening ability.For example,Pai and Hessobtained that the loosening factor for a 63.5 mm bolted joint was seen to be about 30% higher than that for a 76.2-mm bolted joint.Gong et al.also found that shorter clamped length had the larger critical transversal force,which represented anti-loosening performance better.However,for a given transversal displacement,the conclusion is opposite.Zhang et al.revealed that increasing the clamped length could enhance loosening endurance limits in terms of the controlled relative displacement between the two clamped plates.Therefore,it is essential to determine the form of transverse vibration load(transverse force or transverse displacement) in the antiloosening design for threaded fasteners.To improve the anti-loosening performance,the clamped length should be shorter for the vibration load in the form of transversal force while it should be larger for the vibration load in the form of transversal displacement.

The pitch difference between a bolt and nut may induce the prevailing torque required for the nut rotation and influence the anti-loosening capability.Noda et al.experimentally found that the large pitch difference induced an extra prevailing torque,which made the disassembly more difficult,although a very large pitch difference might deteriorate the bolt axial force.A suitable pitch difference should be determined taking into account the anti-loosening and clamping abilities.The results of the experiment and mathematical model from Nassar and Housarialso showed that a tighter thread fit between the bolt and the nut threads reduced the loosening rate under transverse vibration.The loosening rate increased almost exponentially when the thread clearance was increased.Therefore,tighter thread fits are also recommended to reduce loosening behavior.

A bolted joint with a fine thread pitch is seen to be tighter than that with a coarse thread pitch during assembly.Much research has demonstrated by experiment,finite element analysis,and theoretical modelthat fine thread pitch can effectively decrease the preload decay under cyclic transverse vibration.Hole clearance is also a factor affecting antiloosening performance.Similarly,it has been validated by experiment,finite element analysis,and theoretical modelthat large hole clearance will exacerbate loosening.Therefore,fine thread pitch or smaller hole clearance can be applied for better anti-loosening performance of threaded fasteners.

In addition,Gong et al.investigated the influence of nominal diameter and material type on the anti-loosening ability from the view of critical transversal force.The simulation results indicated that the bolted joints with different nominal diameters and materials had almost the same critical transversal force,which meant the same anti-loosening ability.However,the maximum preload that bolted joints with different materials and nominal diameters can bear is not the same.Therefore,as far as possible,in order to enhance the antiloosening performance,the threaded fasteners with high strength and large nominal diameter should be selected.

3.Evaluation of conventional Anti-loosening structures

Although conventional anti-loosening structures have been applied widely in engineering applications,including plain washer,spring washer,and double nut,the knowledge of their anti-loosening abilities and mechanisms is generally empirical.Many scholars evaluated the anti-loosening performance of conventional structures by experiments and FEA.The experimental methods include the standard Junker transverse vibration experiment (see Appendix A),non-standard Junker transverse vibration experiment,impact experiment,random vibration experiment and long-term service experiment.The non-standard Junker transverse vibration experiment and long-term service experiment are explained herein.The former means that the test apparatus of generating cyclic transverse vibration is manufactured simply and may not accord with national or international standards.The latter refers that the preload declines in different threaded fasteners are evaluated after the threaded fasteners are in long-term service.Furthermore,the anti-loosening mechanisms of different structures by experiments and FEA were revealed,and the structures might be optimized.

Conventional anti-loosening structures are mainly classified into two forms according to different structural characteristic,that is,washer form and nut form.Anti-loosening structures in the form of a washer comprise various anti-loosening washers,including plain washer,spring washer,Belleville washer,wedge washer,and serrated washer,as shown in Fig.3(a).Antiloosening structures in the form of a nut include adding a regular nut or changing the shape of a regular nut,for example,double nuts,eccentric double nuts,wedge locking nut,prevailing torque nut,and ratcheted nut,as shown in Fig.3(b).Subsequently,the research methods on the evaluation of above conventional anti-loosening structures are summarized in Table 1.

Table 1 Summary of research methods on evaluation of conventional anti-loosening structures.

Fig.3 Images of various conventional anti-loosening structures.

3.1.Anti-loosening structures in the form of a washer

A plain washer generally has a larger outer diameter and is applied under a nut or bolt head.The plain washer was initially designed to provide protection to clamped part surfaces and better loading distribution on the joint surface.It was not specialized for resisting loosening.Some scholars tested the anti-loosening ability of the plain washer,but the results were controversial.

Izumi et al.established the three-dimensional finite element model of a bolted joint with a plain washer and simulated cyclic transverse vibration.They found that the plain washer showed lower critical loosening load and higher loosening rate,as compared with the conventional nut,indicating that the plain washer had no loosening resistance.This conclusion was supported by Pinelli et al..They employed a new measuring method to estimate the tension on the bolt installed with a plain washer that had been in service for more than a year.The measured data showed that the bolt with the plain washer had a poor record of maintaining the tension in the specified range.In addition,the experimental results from Panja and Dasalso demonstrated that the bolted joint with a plain washer had the same preload loss with the regular bolted joint under transverse vibration.

However,some researchers applied Junker’s testto experimentally assess the performance of a plain washer in terms of resisting loosening under transverse vibration and obtained contradicting results.Dravid et al.found that the loosening angle was smaller and the critical vibration cycle initiating loosening was larger when a plain washer was used with the nut.This demonstrated that the plain washer played an important role in preventing loosening in the bolted joint.The experimental data of Hess et al.revealed that the use of plain washers under a nut can provide improved loosening resistance.The performance was influenced by the washer hardness.The mechanism for this improvement appeared to be from the changes in bearing area distribution and friction.Moreover,additional experimental evidencesuggested that a plain washer can reduce the preload decay,showing better anti-loosening performance.

The above contradictory conclusion might be due to the effects of the material of clamped plates,preload,and vibration condition.Overall,the plain washer is not an effective anti-loosening structure and should be cautiously applied in engineering applications for improving anti-loosening performance.

The spring washer is formed by splitting and twisting the circular plain ring.It works as an anti-loosening component that enhances the holding capability of general industrial assemblies.Under preload,a spring washer is compressed until it becomes flat,and two sharp edges cut into the contact surfaces of the clamped plate and nut (or bolt head).The frictional forces on the bearing surfaces are increased to improve the loosening resistance.

The Junker test results from Dravid et al.indicated that the loosening rate was almost halved when a spring washer was used at a bolted joint,compared with the case where no washer was used.Pinelli et al.measured the tension on bolts with spring washers that had been in service for a long time.It was found that no instances of consistent bolt loosening below the allowable range of tensions were observed.Additionally,many comparative experimentsdemonstrated that bolted joints with spring washers had less preload loss than regular bolted joints when subjected to cyclic transverse vibration.Therefore,the anti-loosening performance of the spring washer can be observed.

However,when the actual external vibration is violent,material wear likely occurs on the bearing surface.The embedding state of the two edges may not be kept,especially for the hard bearing surface.In such a situation,the spring washer may lose its original anti-loosening ability and loosening may be accelerated.Izumi et al.performed threedimensional finite element analysis on a bolted joint with a spring washer subjected to cyclic transverse vibration.The embedding of edges into the bearing surfaces was not considered.The results showed that the spring washer accelerated the loosening rotation of nut since the stuck area on the contact surfaces was reduced to two corner edges of the spring washer,and the rotational force around these edges led to loosening before complete bearing surface slippage.Mo et al.applied transverse vibration in experiments with large amplitude to the bolted joints with and without a spring washer,and they also found that the spring washer did not provide an obvious anti-loosening effect.

Furthermore,the embedding of the edges possibly generates scratches on the bearing surface under vibration,inducing the risk of potential failure.Therefore,many scholars and engineers suggest removing spring washers in bolted joints.Currently,the German Institute for Standardization has revised the standard for the spring washer,while the fastener design manual of NASA also no longer recommends spring washers.

Belleville washers,also known as conical spring washers,are specifically designed for heavy-duty bolted sections such as bus bars,transformers,rectifiers,heat exchangers,and transmissions.The results of experimentand FEAconducted by Sawa et al.showed that the Belleville washers had less preload decrease under cyclic transverse vibration compared withregular threaded fasteners,representing excellent antiloosening performance.The general explanation of Belleville washers on loosening resistance is that the stored strain energy can counteract the relaxation in the bolt.Based on this explanation,Carcaterra and Ngaileevaluated the strain energy storage in Belleville washers and optimized the geometrical parameters for preventing loosening.

Yokoyama et al.employed three-dimensional FEA to further evaluate the loosening resistance performance of Belleville washers.The results revealed that when a transverse load sufficient to induce complete bearing-surface slip was applied and the preload was low enough,Belleville washers were not fully compressed.Belleville washers showed superior antiloosening performance.Otherwise,Belleville washers could not prevent loosening.The analysis of contact states indicated that the smaller stuck region as well as the smaller equivalent diameter of friction torque contributed to the larger loosening rotation.However,the above findings must be validated by experiment.

A wedge washer consists of a pair of locking washers with a cam on one side and radial teeth on the other side.A typical product is the Nord-lock washer in Sweden.The results obtained by experimentand FEAshowed that a bolted joint with a wedge washer had a stronger locking ability compared with a conventional bolted joint.The general mechanism is that the wedge-shaped cams lead to an increase in preload during loosening and in turn the loosening is inhibited by the increased preload.

Recently,Liu et al.further revealed the anti-loosening mechanism by FEA.Their results implied that the wedge washer caused an increase in preload and a decrease in the slipping area on the thread surface,thus weakening the loosening.This was the process of negative feedback adjustment to stop loosening and maintain the preload.They also systematically investigated the effects of various factors on the antiloosening performance of the wedge washer.In addition,Misiekconducted a detailed study on the tightening process for conventional bolted joints with and without wedge washers.

As shown in Fig.3(a5),a serrated washer has a circle of serrations,and can be classified into inside and outside serrated washers.After tightening,these serrations will cut into contact surfaces,resulting in larger frictional force.Thus,loosening becomes difficult.Few scholars have specifically investigated the anti-loosening performance and mechanism of the serrated washer.Only some comparison experiments were conducted in previous literature.The serrated washers provided some antiloosening tendency compared with pure bolted joints.In addition,the experimental results also indicated that the outside serrated washer had better anti-loosening ability than the inside serrated washer.

3.2.Anti-loosening structures in the form of a nut

Double nuts,as the name suggests comprises two nuts,that is,a power nut(bottom nut)and a locking nut(upper nut).When double nuts are used in engineering applications,the power nut is first tightened followed by the locking nut.The results from experimentand simulationhave demonstrated that double nuts can effectively prevent loosening under transverse vibration compared with a single nut,due to the increased contact area.This implies that the frictional forces required to overcome during loosening are much larger.

However,the anti-loosening performance of double nuts is significantly affected by the tightening process.Li et al.varied different axial force distributions on the power and locking nuts and found that the locking ability was enhanced with the increase in the torque applied to the locking nut.Izumi et al.further evaluated the loosening resistance of double nuts and clarified the anti-loosening mechanism by threedimensional FEA.The simulation results indicated that if the locking state was properly achieved in the tightening process,double nuts showed significant loosening resistance regardless of the magnitude of the locking force,since the thread surface on the upper nut retained the stuck state even if the bearing surface underwent complete slippage.However,if the locking process was not performed properly,the ability to resist loosening completely disappeared.

Therefore,double nuts must be properly tightened in order to achieve excellent anti-loosening performance.In other words,if double nuts are not properly tightened,they may lose anti-loosening ability.This explains results of some vibrational experiments wherein double nuts did not show good loosening resistance.

Eccentric double nuts are composed of two nuts;one is convex and the other is concave.The convex nut with an eccentric boss and a concave nut shaped in a perfect circle are brought together to mechanically generate a powerful locking effect transverse to the bolt shaft with the wedge principle.A typical product is a Hardlock nut in Japan.

Sawa et al.from Hiroshima University repeatedly evaluated the anti-loosening performance of eccentric double nuts using Junker’s vibration experimentand FEA.In the transverse vibration experiment,they found that the preload loss with eccentric double nuts was the least compared with nine other anti-loosening structures,while the single nut almost completely loosened after several hundred cycles.According to the results of FEA,the eccentric double nut can even prevent the loosening of the bolted joint with the incline at the bearing surfaces.

The wedge locking nut has a modified thread profile,compared with the internal thread of a regular nut.The key characteristic of a wedge locking nut is a special wedge ramp at the root of the thread.NASA’s Goddard Space Flight Center found that the wedge locking nuts could stay tight until the bolts failed due to fatigue,while the regular nuts began to loosen within the first few minutes of vibration load application.Boettcher et al.also demonstrated the excellent antiloosening capability of a wedge locking nut by impact experiment,compared with traditional anti-loosening structures.A widely acknowledged opinion holds that the large frictional force and uniform stress distributionon the wedge thread results in the superior anti-loosening ability of a wedge locking nut.

However,Li et al.experimentally found that when the initial preload was small enough,the wedge locking nut easily loosened under transverse vibration.When the initial preload was large enough,the engaged wedge internal thread and regular external threads would degenerate to regular engaged threads,thus also diminishing the loosening resistance.This result implied that the proper initial preload of wedge locking nut should be controlled to 35%-55% of the yield limit.In addition,Gong et al.conducted a systematic numerical and experimental study to explain quantitative anti-loosening performance evaluation of a wedge locking nut and to determine the optimal ramp angle.The results demonstrated that a 30-degree wedge ramp could generate the best anti-loosening ability.

A prevailing torque nut usually refers to the kind of antiloosening nut with a prevailing torque,widely employed to prevent loosening of threaded fasteners in engineering applications.A torque is needed to rotate the nut down the thread of an untightened bolt,including non-metallic nylon insert nut,all-metal prevailing torque nut with slots,all-metal prevailing torque nut with an elliptical shaped top of the thread,and all-metal prevailing torque nut with spring steel inserts.Larger friction torque is overcome during loosening;thus the superior anti-loosening performance is achieved.

Richeswas the first to investigate the loosening characteristics of commercially available prevailing torque nuts under transverse vibration.His work showed that prevailing torque nuts did not fully loosen under transverse vibration.It was also found that the higher the prevailing torque,the larger the retained preload.The experimental results from Finkelstonindicated that a prevailing torque stopped the loosening process when the prevailing torque counteracted the loosening torque.Eccles et al.further studied the conditions of loosening and the complete detachment of prevailing torque nuts.It was found that if the magnitude of the axial loading exceeded the residual preload in the bolt retained from sustaining transverse movement alone,all-metal prevailing torque nuts could completely detach from bolts.Additionally,Jung et al.systematically optimized the shape and material of the spring in an allmetal prevailing torque nut with spring steel inserts.The performance of the newly developed locking nut with a spring was compared with that of the general nut and bolt connection without a spring and the effectiveness of the locking nut was verified.

A ratcheted nut has ratchets on the bearing surface to enlarge friction force and anti-loosening ability.Similar to the serrated washer,ratchets may cut into the contact surface of the clamped plate under preload to resist the loosening between internal and external threads.Jeong et al.demonstrated that the threaded fasteners with ratcheted surface require about 1.7 times the anti-releasing torque compared with the regular ones.However,when severe impact load is applied,the material of the ratcheted surface wears down easily,causing the antiloosening ability to degenerate.For example,Sase et al.found that the ratcheted nut almost had the same preload loss compared with a conventional nut under impact load.

Currently,the quantitative relationship between the antiloosening performance and structural and loading parameters(for example,friction coefficient,pitch,helix angle,vibrational cycle,transverse force,among others)for regular threaded fasteners under cyclic transverse vibration has been established mathematically.For example,Yang et al.developed a novel criterion for preventing the self-loosening of preloaded threaded cap screws under cyclic transverse loading.Sun et al.proposed a preliminary theoretical foundation for the evaluation of bolt anti-loosening performance and design parameters.However,for conventional anti-loosening structures,it can be seen from above research that the quantitative research on the mathematical relationship between the antiloosening performance and structural parameters is still lacking,which requires to be researched further in the future.

4.Comparison of conventional Anti-loosening structures

The aforementioned conventional anti-loosening structures are widely used in engineering applications.However,there are differences in anti-loosening performance between these structures.It is essential to understand their differences in antiloosening ability to select the appropriate anti-loosing structure for a given application.Scholars have quantitatively compared the anti-loosening abilities of different structures.It has been demonstratedthat the periodic transverse vibration is the main form of loading inducing the loosening of threaded fasteners.Therefore,cyclic transverse vibration is extensively applied to different anti-loosening structures,forcing them to loosen as much as possible.The data of preload versus vibration cycle are obtained,based on which the loosening curves are plotted.The retention rate of preload at the set vibration cycle can be used as an indication to quantitatively evaluate the anti-loosening performance of different structures.

The general research methods include experimental and FEA methods.The Junker test machineis a standard experimental setting,introduced in detail in Appendix A.Other researchers have manufactured non-standard Junker machines to generate cyclic transverse vibration in experimental research work.The FEA method constructs the finite element models of different anti-loosening structures and then simulates the periodic transverse vibration.Table 2 summarizes the comparative results on the anti-loosening performance of different structures through experiment and FEA.

Table 2 Comparative results on the anti-loosening performance of different structures.

First,thread adhesive locking has the best resistance to loosening,superior to the anti-loosening structures in the forms of washer and nut.For example,Cheatham et al.demonstrated that the preloads in the bolted joints with thread adhesive decreased more slowly than those in the prevailing torque nuts using Junker test apparatus.The experimental results using non-standard Junker machine from Bhattacharya et al.and Panja and Dasalso indicated that chemical lock(also called adhesive bounded nut)had the most retention rate of preload,compared with other anti-loosening nuts andwashers,regardless of different materials,sizes,and initial preloads.However,it is difficult to disassemble the threaded fasteners with thread adhesive locking despite the good antiloosening performance,which limits the application scope of thread adhesive locking,especially when requiring repeated assembly and removal.

The structures in the form of a nut show better antiloosening performance in terms of resisting loosening than the washer.Saha et al.confirmed that the nylock nut has better anti-loosening performance than the serrated washer and spring washer.Sawa et al.,and Mahato and Dasfound that the eccentric double nuts and prevailing torque nuts (nylon nut and slotted nut) have smaller preload loss compared with various anti-loosening washers under cyclic transversal vibration.However,there are two exceptional anti-loosening structures,that is,double nuts and plate spring washer.The former’s anti-loosening ability depends on the tightening method.When tightened properly,excellent anti-loosening performance is achieved.Otherwise,its anti-loosening performance is even worse than anti-loosening washers.The latter may provide excellent anti-loosening ability,even better than prevailing torque nuts when the initial preload is proper.

Second,among the anti-loosening structures in the form of a nut,eccentric double nuts generally show outstanding antiloosening ability compared with other anti-loosening nuts.A type of hybrid double nut,that is,the bottom one is a regular nut and the upper one is a nylon insert nut,was also demonstrated to show excellent anti-loosening capability compared with the prevailing torque nut (nylock insert nut) and regular double nuts.Therefore,in engineering applications,eccentric double nuts and hybrid double nuts can be recommended for better anti-loosening performance.

Among the anti-loosening structures in the form of a washer,the wedge washer and plate spring washer have the best anti-loosening abilities while the spring washer and outside and inside serrated washers show considerable antiloosening abilities.The differences in anti-loosening abilities of the spring washer and outside and inside serrated washers cannot be clearly differentiated,and their anti-loosening abilities might be affected due to violent vibration.Many researchers have demonstrated that the plain washer showed little anti-loosening performancecompared with the normal bolted joints.However,the combination of soft washer and serrated washer may effectively prevent loosening,even beyond the single serrated washer.

Third,the performance and the difference of various antiloosening structures in the form of nut and washer may significantly be affected by the initial preload,joint material,and size.For example,Sawa et al.found that when the initial preload corresponded to 30%of the bolt yield stress,the eccentric nut,nylon insert nut,and slotted nut ranked in the top three,in terms of preventing loosening,while the eccentric nut,norlock washer,and plate spring washer ranked in the top three when the initial preload corresponded to 75%of the bolt yield stress.Cheatham et al.experimentally demonstrated that high-tension steel bolts installed with different nuts had lower preload decrease than low carbon steel bolts under transverse vibration.

5.New-Style Anti-loosening structures

To improve the loosening resistance of threaded fasteners subjected to vibration and impact,scholars have explored and invented a few new-style anti-loosening structures,which may not be available in the market.Related research work was surveyed on these new-style anti-loosening structures with respect to the following five aspects:(1) changing the internal thread of the nut;(2) changing the external thread of the bolt;(3) changing the bearing surface of the nut or bolt head;(4)changing the external and internal thread of the bolt and nut simultaneously;(5) changing the material type of the bolt or nut.Table 3 summarizes the key references and the antiloosening principles of various new-style structures,which will be subsequently introduced in detail.

Table 3 Key references and anti-loosening principles of various new-style structures.

(1) Changing the internal thread of the nut

This kind of new-style anti-loosening structure is similar to conventional prevailing torque nuts.Interference is generated between the internal and external threads by changing the shapeor pitchof the internal thread of the nut,as shown in Fig.4.An additional torque is required to overcome the interference before the bearing surface contact.Accordingly,larger loosening torque should be satisfied once loosening initiates to improve the anti-loosening ability.

Fig.4 Schematic of interference between internal and external threads.

Typically,the super lock nutdivides the internal thread into upper and lower threads,between which there is a thin walled tube.Thus,a phase difference between lower and upper threads is produced due to the deformation of the thin walled tube along the axial direction,which induces the interference and contrary forces on the surfaces of the upper and lower threads.The nut with variable pitchchanges the previous constant pitch to variable,which results in interference between the variable internal thread and standard external thread,and more uniform stress distribution on the thread contact surface.It has been validated that they both show better anti-loosening ability than regular threaded fasteners.

(2) Changing the external thread of the bolt

Considering that the external thread of the bolt is easier to manufacture in practice,some scholars change the shape of the external thread of the bolt engaged with the regular nut to enhance loosening resistance.Novel anti-loosening bolts are mainly classified into three categories based on different antiloosening mechanisms.

The first category generates interference between the modified external thread and a regular internal thread.Similar to the aforementioned super lock nut and nut with variable pitch,an additional torque due to interference is required during the assembly.Thus,the anti-loosening performance is improved.For example,Ranjan et al.modified the geometry of the thread in a bolt to be a cubic function of rotation,instead of linear rotation.Based on this modification,a mismatch between internal and external threads is produced resulting in interference during the tightening process,and additional torque is required to overcome this interference,giving rise to the enhanced anti-loosening property.

Second,it is known that reducing the lead angle can inhibit the transfer of torque and effectively prevent loosening.According to this anti-loosening idea,a new screw named step-lock bolt (SLB)was developed,as shown in Fig.5(a),that has eight steps with zero degree of lead angle at the circumference of the thread.In another study,SLB was optimized to increase the scope of the application.

Third,the shape of the external thread in the bolt is changed and the modified bolt is matched with double nuts for preventing loosening using the axial force locking method.The typical bolt with right-and left-hand threads was invented by Tangas shown in Fig.5(b),named Tang’s bolt in China.The instruction for using Tang’s bolt is to tighten a power nut with right-hand thread first,followed by a lock nut with left-hand thread.The anti-loosening mechanism is that the power nut rotates in the loosening direction,causing the lock nut to rotate in the tightening direction.Accordingly,loosening is prevented because the axial force is increased by the tightening of the lock nut.Another example,double thread bolts (DTBs) are machined with both coarse and fine pitch threadsas shown in Fig.5(c).A power nut with coarse pitch thread is first added to the DTB and then a lock nut with fine pitch thread is tightened during assembly.DTBs have excellent anti-loosening performance since the loosening velocity of the lock nut is slower than that of the power nut,and the movement of the power nut is completely suppressed by the lock nut.However,the thread intensity is weakened due to the changed shape at thread,which may bring about an unexpected damage to threaded fasteners during service.

Fig.5 Typical anti-loosening structures by changing external thread of bolt.

(3) Changing the external thread of the bolt

The friction behavior of bearing surface on the nut or bolt head also plays an important role in resisting the loosening of threaded fasteners.Some scholars have made efforts to modify the shape of the bearing surface to achieve better antiloosening performance.According to different anti-loosening mechanisms,this type of anti-loosening structures can be divided into three categories.

The first category is to increase the interference between internal and external threads by making the bearing surface with an angle of inclination.Sawa et al.have demonstrated that the retention rate of preload is higher with the increase of the inclination angle under transverse vibration.Yang et al.proposed a novel anti-loosening nut with an inclination of 1 degree and a small slit around the side,as shown in Fig.6(a).Its superb loosening resistance has been validated by finite element analysis.The second category is to use the axial force locking method.For example,Sun et al.designed a spherical bolt with a matched spherical gasket as shown in Fig.6(a),which generates additional bolt tension when the transverse load increases.This new anti-loosening structure has an increasing critical transverse load and thereby improved its anti-loosening ability.The third category is to increase the friction force on the bearing surface.For example,Ren proposed a novel nut with ratchet on the bearing surfacematched with a ratcheted washer,as shown in Fig.6(c).Its superior anti-loosening ability was also validated by FEA.

Fig.6 Novel anti-loosening structures by changing bearing surface of nut or bolt head.

(4) Changing the external and internal thread of the bolt and nut simultaneously

Recently,Gong et al.proposed a modified Iwan model to represent the local slippage behavior on the thread surface.It was found that the cyclic component force along the radial direction dominates the local slippage accumulation on the thread surface.Accordingly,they simultaneously changed the shape of the external and internal threads and designed several novel thread structures,as shown in Fig.7.The anti-loosening mechanism inhibits the slippage or relative motion occurring on the thread surface along the radial direction.The FEA method has validated their excellent anti-loosening abilities compared with regular threaded fasteners.However,the large plastic deformation between internal and external threads may occur under the excessive preload,which will decrease the antiloosening performance of this type of novel anti-loosening structures.

Fig.7 Novel anti-loosening structures by simultaneously changing external and internal thread of bolt and nut.101

(5) Changing the material type of bolt or nut

Shape memory alloys(SMAs)are a new kind of metals with the capability to ‘‘remember” their original shape when activated by temperature change.Some scholars have applied SMAs to threaded fasteners with the aim of resisting loosening.These anti-loosening structures are mainly of two categories:one is a nut with SMA and the other is bolt with SMA.

Zhang et al.proposed a new self-locking SMA nut and found that the transverse clamping force of an SMA nut on regular bolt due to the shape memory effect always existed,preventing vibrational loosening.Li et al.further reported the anti-loosening mechanism of an SMA nut.It indicated that the self-locking frictional moment in the thread connection of the new nut made of the alloy was considerably increased,and the uniformity of the axial load distributed among every screw tooth was also improved.

Guoproposed an anti-loosening bolt with SMA and experimentally found that the axial restoring force and radial restoring force were produced on the thread surface,which could be transformed into frictional torque to prevent rotation between internal and external threads.Jiang et al.proposed a macroscopic constitutive model to reproduce the uniaxial transition ratcheting behaviors of the SMA undergoing cyclic loading.It was demonstrated by FEA that the superelastic SMA bolts based on the proposed constitutive model had lower preload loss under cyclic transverse vibration.From above research,we can see that the anti-loosening structures by changing the material type of bolt or nut strongly depend on the temperature change,which may limit their applications.

6.Conclusions and prospects

The anti-loosening designs of threaded fasteners play a significant role in guaranteeing the reliability of a product.This paper summarized the research work on the effects of various factors on loosening,which provided the basis for antiloosening designs.It can be concluded that increasing factors such as the preload,friction coefficients of bearing,and thread surfaces,and reducing factors such as the hole clearance,thread fitting clearance,and thread pitch contribute to loosening resistance.Meanwhile,issues such as clamping abilities and surface damage during the actual tightening process should also be considered.For example,large preload might lead to the risk of surface damage.Large friction coefficients of bearing and thread surfaces and small thread fitting clearance enhance the loosening resistance but might deteriorate the bolt axial force under a given tightening torque.In the existing research,there are few investigations on the effects of the interactions of various factors on loosening,which may suggest that the actual anti-loosening designs of threaded fasteners are not optimal.Therefore,in the future,the effects of coupled factors on loosening should be further studied to determine the optimal design parameters for achieving the best antiloosening performance.

Next,this paper summarized the research work on the evaluation and comparison of conventional anti-loosening structures,which helped researchers strengthen understanding of anti-loosening performance and the mechanisms of different conventional structures.It can be concluded that,in terms of preventing loosening,the structures in the form of a nut that increase the frictional force at the thread surface are much better than those in the form of washer that increase the frictional force at the bearing surface.Especially,eccentric double nuts and prevailing torque nuts showed excellent anti-loosening abilities in all cases.Regarding the anti-loosening washers,the plain washer and spring washer are likely to lose antiloosening performance due to the wear on the bearing surface by acute vibrations.They should be limited in engineering applications for resisting loosening.Instead,the wedge washer and Belleville washer are recommended to effectively prevent loosening.In addition,the anti-loosening capabilities of different structures are affected by the magnitude of preload,material,and nominal diameter.Although some qualitative conclusions can be drawn from previous research,the antiloosening performance of different structures must be further quantified.In other words,the mathematical relationship between the anti-loosening performance and structural parameters should be explored in the future.In this way,appropriate anti-loosening structures can be selected for a given application.

Finally,as found in the recent literature and patents,researchers designed various anti-loosening structures by changing the shapes of the thread surface or bearing surface or by using novel material.Results of experiment and simulation illustrated their superior anti-loosening abilities compared with regular threaded fasteners.The related research work is summarized in detail.It can be concluded that the antiloosening mechanisms comprise prevailing torque,mechanical locking,inhibiting local slippage accumulation,and structural deformation of SMA.However,some other issues might appear despite good anti-loosening performance,including difficulties in installation and disassembly,low strength at thread,large deformation,and temperature dependence.Therefore,in the future,it is necessary to explore more novel structures with excellent loosening-prevention abilities,as well as address the aforementioned issues.The advanced surface treatment process may provide a solution.

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

This study was supported by the National Natural Science Foundation of China (No.51675050 and 51935003).

Junker test apparatus is widely used to evaluate the antiloosening performance of various threaded fasteners,and also adopted by national standards.Fig.A1(a)and(b)show the picture and the schematic of Junker test apparatus,respectively.It can be seen that Junker test apparatus consists of fixed base,moving plate and several sensors monitoring the preload,shear load,and transversal displacement.Using this test apparatus,the nut and bolt are tightened together with the required preload.Then,a shear load is applied to the joint through the transverse movement of the plate between the nut and bolt.The transverse movement is generated by an eccentric structure,by which the transversal amplitude and vibration frequency are adjustable.When the apparatus begins to work,connected computer screen will show the changes of preload and loosening angle with the increasing vibration cycles.According to the loosening curve,we can obtain the rates of preload decay and loosening rotation;so that the antiloosening capabilities of different threaded fasteners can be evaluated quantitatively.

Fig.A1 Junker test apparatus.