Research On Factors Affecting Bolt Loosening Under Impact Load

Bolt connection is one of the most commonly used fastening methods,which plays an important role in transferring force(or moment)and ensuring the tightness and reliability of sealing structure.Loosening is one of the main failure modes of bolt connection,and the decrease of clamping force is the main sign of loosening.Bolt loosening under impact load is very easy to occur,which may cause huge losses.Therefore,it is of great significance to study the influence of various factors on bolt loosening under impact load for understanding bolt loosening mechanism and optimizing anti-loosening design.This paper mainly includes the following aspects:1)A bolt connection model without screw elevation angle was established,and the validity of the model is verified.Finally,finite element simulation analysis is carried out to study the influence of impact load amplitude,initial preload and friction coefficient between threaded meshing surfaces on bolt loosening during material loosening period under impact load.The results show that bolt loosening is more likely to occur with the increase of impact load amplitude.For the whole life of bolt loosening,there exists an initial preload range and a friction coefficient range between meshing surfaces,which makes bolt loosening the most difficult to occur.2)By analyzing the mathematical expression of threaded surface,an accurate model of bolt connection with screw elevation angle was established,and the validity of the model is verified.Finally,the finite element simulation analysis is carried out to study the influence of impact load amplitude,initial preload,friction coefficient between threaded meshing surfaces,friction coefficient between joints and pitch on bolt loosening.The results show that bolt loosening occurs more easily with the increase of impact load amplitude and pitch,and the friction coefficient of joint surface between connectors has no effect on bolt loosening.The bigger the pulse width of impact load is,the larger the reduction of clamping force of bolt is,but the slower the decrease is.There is an initial pre-tightening force range and a friction coefficient range of the thread meshing surface,which makes bolt loosening the most difficult to occur.This verifies the previous analysis.3)The impact test device of bolt loosening was built,and the impact test analysis of bolt loosening was carried out.The effects of impact load amplitude,initial preload and impact loadpulse width on bolt loosening were studied.The results show that bolt loosening is more likely to occur with the increase of impact load amplitude.The bigger the pulse width of impact load is,the larger the reduction of clamping force of bolt is,but the slower the decrease is.There is an initial preload range,which makes bolt loosening the least likely to occur.It is consistent with the influence of impact load amplitude,initial preload,impact load pulse width and other factors on bolt loosening in the previous simulation analysis.