A Study Of Loosening Process Of Stainless Steel Bolt And Method For Resisting Loosening Under Transverse Loading

Bolted joints are commonly used in the assemblies of medicine,transportation,energy and aeronautics due to their advantages,such as the ability to develop a clamping force,and the ease of disassembly for maintenance.However,it has been widely observed that bolted joints would loosen when subjected to external loading,such as vibration,impact and temperature variation.It will lead to the reduction of the preload,then result in the loosening of bolted joints,which can hit the normal service of mechanical structure.Stainless steel bolts possess relatively high strength,high ability of corrosion resistance and high quality of appearance.Therefore,they are widely used in mechanical parts.In this study,a device was designed to simulate the loosening process under transverse loading and measure the relative rotation between the bolt and the nut.Loosening experiments were carried out under several loading conditions with different preloads and transverse displacements.Besides,finite element modeling was conducted to obtain the slip amplitude at threads.Then the loosening process of stainless steel bolts was analyzed.It was found that the loosening process of bolt can be divided into two stages.During the first stage,the clamping force reduces rapidly.Local cyclic plasticity occurring near the roots of the engaged threads results in cyclic strain ratcheting and stress relaxation,and thus causes the rapid loss of clamping force.During the second stage,the clamping force reduces relatively slowly.Loosening is attributed to the slippage at the thread surface and the fretting wear occurring on the threads.Additionally,with the increasing preload and the decreasing transverse displacement amplitude,the slip amplitude at threads increases and the degree of loosening becomes greater.Moreover,some loosening tests were conducted to evaluate the anti-loosening ability of several types of fasteners which are widely used in assembles(thread locker,nylon insert nut,double nuts,spring washer and plain washer).The results show that the anti-loosening ability of the thread locker is the best,the nylon insert nut less,then the double nuts and the spring washer,plain washer is the worst.This result can contribute to selecting the right anti-loosening measure in practical mechanical structure.