Threaded Fastener Locking With Safety Wire and Cotter Pins

Journal of Failure Analysis and Prevention - This paper presents test results and analyses of safety lock wire and cotter pins with castellated nuts used as locking features in threaded fasteners....     

Development of an anti-loosening fastener and comparing its performance with different other threaded fasteners

Threaded fasteners are widely used for temporary joining of different components that require occasional disassembling. However, threaded fasteners may have the problem of loosening under vibrating conditions, which may lead to decreasing clamping force and, thereby, a system failure. In the present experimental investigation, anti-loosening ability of various fastening elements, such as conventional nut, nylock nut, flat washer, spring washer, inside and outside serrated washer, is tested with a conventional M16 high-tension steel bolt. A hybrid double nut is used to clamp the plates in which a conventional nut is used to tighten first, and one nylock nut is then placed at the outer side for further tightening. An adhesive-bonded nut is also tested, where an adhesive is placed between bolt and nut threads. All these fasteners are tested in terms of their loosening characteristics. Accelerated vibrating conditions are used for the test on an indigenously made test rig. A split bolt with taper pin is introduced for the first time to reduce loosening of fasteners. The split bolt with taper pin shows considerably good anti-loosening property compared with other threaded fasteners tested in this work.     

Three-dimensional finite element analysis of threaded fastener loosening due to dynamic shear load

The two most widespread causes of failure of threaded fasteners subjected to dynamic loads are fatigue and vibration induced loosening. This paper presents results of a study on failure of threaded fasteners by vibration induced loosening caused due to dynamic shear loads. Previous experimental work has revealed that fastener loosening occurs as a result of complete or localized slip at the thread and head contact surfaces. A three-dimensional finite element (FE) model is used to study details of four different loosening processes that are characterized by either complete or localized slip at the head and thread contacts. The FE model is found to be capable of adequately modeling factors that influence slip and predicting the different loosening processes. Primary factors that influence slip at fastener contacts are discussed. The results show that loosening can occur at relatively low shear loads due to the process of localized slip.     

Failure of cadmium plated maraging steel tension bolt

Cadmium electroplated (EP) high strength steel fasteners are widely used for assembling threaded joints as cadmium gives lubrication and provides excellent corrosion resistance. However, in-house-failure case histories revealed that in some cases delayed failure of cadmium EP high strength steel fasteners occurred. Recently, one such premature failure of a cadmium EP 18 Ni 1800 MPa maraging steel tension bolt occurred at a sustained stress of about 40% of the ultimate tensile stress (UTS), after 11,500 h. For their successful functional application, cadmium EP fasteners require proper and adequate baking treatment after electroplating. This paper highlights details of analysis carried out on the failed bolt to find out the reason for the failure, using standard metallographic and NDT techniques. Remedial measures to minimise, if not totally prevent, such failure are also discussed.     

Tests on Loosening of Aviation Threaded Fasteners with Different Washer Configurations

This paper presents results from an experimental investigation on loosening of aviation threaded fasteners with internal teeth lock washers and plain washers. The tests are performed on a transverse test machine which provides dynamic shear. Five different configurations are tested: (1) only lock washer, (2) lock washer on plain washer, (3) no washers, (4) only plain washer, and (5) lock washer on hardened plain washer. The resistance to loosening of these configurations is assessed by comparing loss of preload versus cycles of dynamic shear loading. Internal teeth washers used alone under a plain nut without additional plain washer are found to provide the worst case with respect to loosening resistance. Use of a plain washer under the lock washer provides significant improvement in resistance to loosening. In addition to providing protection to clamped part surfaces and better load distribution in a joint, the data presented reveals the use of plain washers under a plain nut can provide improved resistance to loosening. Relative hardness of the components, e.g., plain washer compared to lock washer, influence the performance.     

室温下C/C复合材料紧固件拧紧特性

本文研究了C/C复合材料紧固件在不涂防松胶（简称不涂胶）和涂防松胶（简称涂胶）两种状态下的拧紧特性。试验测试了两种状态下C/C复合材料紧固件拧紧力矩与预紧力的关系,并对两种状态下紧固件摩擦系数和拧紧力矩分配关系进行了深入分析。结果表明:涂胶与不涂胶相比,相同拧紧力矩作用下,螺栓预紧力增大20%~82%,且不涂胶状态下预紧力与拧紧力矩线形关系较好;不涂胶和涂胶两种状态下,螺纹副摩擦系数分别为0.41、0.35,端面摩擦系数分别为0.59、0.41,涂胶后两对摩擦副摩擦系数均降低;另外,涂胶和不涂胶两种状态下施加拧紧力矩在端面摩擦副、螺纹摩擦副和附加力矩中比例分配规则略有不同。      

Direct Verification of Threaded Fastener Locking Compounds and Adhesives

This article explores the feasibility of a method to directly verify that thread lock compound or adhesive has cured sufficiently to provide secondary locking. The application of a torque after assembly and cure time was found to provide a viable verification test. Fastener material and coatings were found to significantly affect cure. For example, medium-strength anaerobic compound used with fasteners made of inactive material such as stainless steel had curing issues which prevented determination of a useful verification test torque. However, modifications such as higher-strength compound or primer can be introduced making this method viable even for fasteners made with inactive materials and coatings. This article outlines a process to implement this method in practice. This requires sample tests with representative product to determine a test verification torque. This process is particularly useful in identifying curing and locking performance issues, and provides guidance for modification so that the method can be successfully implemented in practice. Test results show that application of verification test torque with or without standard vibration test exposure does not degrade the locking performance.     

Hydrogen-Induced Cracking (HIC) of Hardened and Tempered Steel Fastener Used in Space Application

Cadmium-plated 0.35C–3.5Ni–1.5Cr–0.5Mo steel threaded fasteners of 1230 MPa properties class are used for aerospace applications. These fasteners were torqued to 13 N·m. Few fasteners parted into two pieces while in use under sustained assembly load for a period of 50 days. The fracture surface of the failed fasteners had two distinct regions when viewed under microscope at higher magnification. Fractography revealed that the larger region consisted of predominantly intergranular features, whereas the smaller region had features of microvoid coalescence. From the metallography evidences it was concluded that the fastener failure was due to hydrogen embrittlement. Electro-deposit of cadmium was identified to be the main source for hydrogen entrapment, which could not be compelled completely by post-plating baking treatment.     

Caution is advised when specifying stainless steel fasteners

Stainless steel fasteners are often chosen to replace carbon steel fasteners in many engineering applications. These fasteners offer some advantages over their carbon steel counterparts—most notably, increased corrosion resistance. However, stainless steel fasteners have a tendency to gall, or cold weld, when in contact with a mating stainless steel fastener of similar composition when the proper precautions are not taken. Galling can vary in severity, and may be only a nuisance when minor, but lead to fastener failure in the extreme. This paper describes the galling of a threaded stainless steel ejector rod from a laser guided training round (LGTR) used by the U.S. Navy. Also presented are the precautions that should have been taken to avoid galling.     

Parametric study of threaded fastener loosening due to cyclic transverse loads

This paper presents results from an experimental investigation of mechanical loosening in bolted joints due to cyclic transverse loads. The influence on the resistance to loosening of basic parameters such as preload, fastener material elastic modulus, nominal diameter, thread pitch, hole fit and lubrication is quantified. Sixty-four tests have been performed as part of a nested-factorial design in which the nominal diameter is the nesting factor of preload, thread pitch and hole fit. A statistical analysis identifies the factors and interactions that significantly affect the resistance to loosening and it is found that the preload and the fastener elasticity are the most influencing parameters. A statistical model is developed that predicts the level of loosening reached by a threaded fastener under defined conditions. The analysis shows that optimum conditions to avoid fastener loosening are high preload, low modulus of elasticity, large diameter, lubrication, tight fit and fine threads.     

Correlation-based phase space beam characterization

A generalized correlation-based definition for moments of arbitrary order is introduced that can also accommodate mixed spatial and angular moment. Moreover, a transformation law forthese moments for propagation through linear optical systems is derived. This law has the same form as the corresponding propagation law of the moments defined in terms of the Wigner distribution function. The correlation-based moments can be used to fully characterize beams of arbitrary states of coherence.     

Analysis of dowelled timber to timber moment-resisting joints

Dowelled joints, widely used in timber structures, are designed to transfer shear forces and bending moments between timber members. The anisotropic non-linear behaviour of the timber beneath the fasteners controls the stiffness of these joints. At the ultimate load-carrying capacity, the failure modes result from the shear stresses induced by the load distribution among the fasteners. The paper presents the experimental results obtained for beam to column joint with or without reinforcement using glued plywood plates. Based on these results, a two-dimensional finite element model was developed in two stages to describe the three-dimensional behaviour of the joint. At the single fastener scale, the model considers the non-linearity induced by the timber embedding and the fastener bending. At the structural scale, the modelling approach considers the timber as an elastic orthotropic material whereas each fastener is modelled by two non-linear springs. The elastic-plastic behaviour of each spring element is defined by the local scale model defined in two perpendicular directions. The load distribution among the fasteners is compared to the analytical results according to design rules. Considering the global load displacement curves, the results show that the modelling approach provides a good estimation of the structural response.     

室温下C/SiC复合材料螺纹紧固件的拧紧特性

探讨了C/SiC陶瓷基复合材料螺纹紧固件在室温下的拧紧特性。试验测试了紧固件在拧紧和拧松过程中, 力矩与预紧力两者的对应关系, 记录了预紧力在短时、 长时内的减小比率, 并用显微镜观察了螺纹面的磨损情况。另外, 分析了材料非线性拉伸行为对拧紧状态所造成的影响。结果表明: 拧紧、 拧松力矩与预紧力之间近似呈线性关系, 螺纹面和支承面的平均摩擦系数分别为0.52和0.46; 随拧紧力矩增大, 螺纹面产生一定程度磨损, 螺纹之间的相互嵌入作用减弱, 因此预紧力在拧紧后的降幅减小, 稳定性提高; 对螺栓进行适当的预拉伸处理, 提高材料的弹性极限后, 可提高紧固件的抗松弛能力。     

Effect of Reuse of Locknuts with Prevailing Torque Locking Feature

This paper presents results from tests on the reuse of locknuts with a prevailing torque locking feature. Aerospace and non-aerospace products are tested. Many qualification specifications provide reuse requirements for prevailing torque locking features in terms of unseated reuses. This paper provides data for comparison of locknut reuse for unseated and seated to preload conditions. All locknuts tested, except one, maintain required prevailing torque levels over 15 unseated reuses as typically required. The unseated reuse tests exhibit a moderate decrease in prevailing torque with reuse with the largest decrease usually occurring during the first reuse. The effect of seating to preload is a higher loss in prevailing torque compared with the unseated results. Reuse in seated conditions does not generally meet unseated requirements. Between 10 and 25% of the assembly prevailing torque is lost during initial seating to preload. This loss continues with reuse but at a lower rate. However, inconsistent fluctuations in prevailing torque are observed for tests with high preload of 85% yield strength of the bolt. For all the seated to preload reuse tests, the tightening torque to achieve a set preload increased with reuse, despite the corresponding reduction in prevailing torque. This phenomenon results from the breakdown of the solid film or as-received lubricant provided with the locknuts.     

External CFRP tendon members: Secondary reactions and moment redistribution

The response of prestress secondary reactions in the post-elastic range has been a topic of much controversy. Due to the brittleness of FRP (fiber reinforced polymer) composites, external FRP tendon members may have different moment redistribution characteristics compared to conventional concrete members. This paper presents a numerical investigation into the secondary reactions and moment redistribution in prestressed concrete continuous members with external CFRP tendons. The investigation parameters include the initial prestress level and the pattern of loading. The secondary reactions are computed using a newly developed method based on the linear transformation concept combined with a nonlinear finite element analysis. The results indicate that the secondary reactions increase quicker after concrete cracking and nonprestressed steel yielding. As a consequence, the secondary moment should be included in the design moment. The moment redistribution behavior for symmetrical loading is shown to be quite different from that for unsymmetrical loading. The study also shows that the effect of initial prestress on the moment redistribution is rather important.     

A mixed finite element formulation for timoshenko beam on winkler foundation

 A mixed formulation for Timoshenko beam element on Winkler foundation has been derived by defining the total curvature in terms of the bending moment and its second order derivation. Displacement and moment have been chosen as primary variables, while slope and first derivation of moment have been chosen as secondary variables. The behaviour matrix for Timoshenko beam element has been obtained in mixed form by using weak formulation with equilibrium and compatibility equations. The presented formulation makes the analysis of beams free of shear locking. Received: 10 July 2002 / Accepted: 14 January 2003     

Erweiterte Prüfmethoden zur sicheren Rad- und Fahrwerksverschraubung der E-Mobilität

Changes towards electrified vehicles lead to an overall increase of loads in chassis components, particularly in wheel attachments. Especially wheel attachments are facing increased dynamic loads, caused by increased axle loads, wheel dimensions, cornering loads due to tire grip and center of gravity improvements…to name some examples. To prevent fastener failures in future, new measurement and analysis procedures are being presented, enabling a complete and accurate verification of superimposed dynamic stresses caused by dynamic axial and bending loads in wheel & chassis fasteners at its critical failure area. Furthermore load-specific S−N-curves are required to accomplish an appropriate damage accumulation. This detailed understanding of loads occurring at fasteners itselfs enables improved safety in all chassis fasteners, provided these loads are known and taken into account at an early stage of a development process.     

Loosening-resistance evaluation of double-nut tightening method and spring washer by three-dimensional finite element analysis

The mechanisms of loosening-resistance components are investigated within the framework of the three-dimensional finite element method (FEM). In this paper, we have evaluated the ability of double-nut tightening method (DN) and spring washer (SW) to resist self-loosening due to transverse loading. We have found that if locking state is properly achieved in the tightening process, DN shows significant loosening resistance regardless of the magnitude of locking force. It was observed in this case that thread surface on the upper nut retains stuck state even if bearing-surface undergoes complete slip. However, if the locking process is not performed properly, the ability to resist loosening completely disappears. On the other hand, it is shown that SW accelerates loosening rotation of nut. The stuck area on the contact surfaces is reduced to two corner edges of the SW and the rotational force around these edges thus drastically leads to loosening before complete bearing-surface slip.     

Pull-out steel fasteners from a laminated composite subjected to external pressure

Single steel fasteners of circular cross-section have been pulled out of predrilled laminated glass/epoxy composite cylinders located in a pressure cell in which external pressures of up to 56 MPa were applied. An analysis is presented which relates the external pressure, p_o, to the interfacial pressure, p_i, in terms of the elastic properties of the fastener and the composite and the radial clearance between them in the absence of an external pressure. The value of the coefficient of static friction, μ = 0·1, calculated from these experiments suggests that the observed axial pull-out forces of 2·5 kN for similar fasteners fired directly into the composite material, cannot be accounted for solely in terms of interfacial friction. Some evidence is presented which suggests that plastic deformation of the fastener surface into interstices within the surrounding material during impact penetration contributes to the initial resistance to pull-out.     

自攻螺钉在胶合木中的拉拔破坏模型

随着木结构领域内力学紧固件的发展，具有足够长度和优化螺纹的自攻螺钉或螺纹杆为实现具有高强力学性能的木连接提供了一种新的技术方案。自攻螺钉在木材中的拉拔性能是影响螺钉连接力学性能的重要因素，为了充分利用自攻螺钉的轴向受力优势，应当避免埋置在木材中的自攻螺钉发生拔出破坏。不同于现有基于经典Volkersen理论的计算方法，该文提出一种适用于轴向受力螺纹紧固件的力学模型，首次在模型中引入一种包含螺纹的“装配单元”，用于研究自攻螺钉在胶合木中的拔出破坏机制，解决自攻螺钉的锚固长度计算问题。模型根据拔出破坏的典型现象重新定义了拔出破坏面，考虑了螺纹对木材破坏区域的影响，并在模型简化、公式推导和算法设计上体现出木材的局部受力状态和拔出破坏面上的剪应力、剪力不连续传递现象。理论计算与试验结果有较好的一致性，计算结果还表明，在发生拉断破坏以前，自攻螺钉和螺纹杆的拔出破坏荷载与埋置长度基本呈线性关系。