Effect of Pitch Difference between the Bolt-Nut Connections upon the Anti-Loosening Performance and Fatigue Life

In this paper, the effect of a slight pitch difference between a bolt and nut is studied. Firstly, by varying the pitch difference, the prevailing torque required for the nut rotation, before the nut touches the clamped body, is measured experimentally. Secondly, the tightening torque is determined as a function of the axial force of the bolt after the nut touches the clamped body. The results show that a large value of pitch difference may provide large prevailing torque that causes an anti-loosening effect although a very large pitch difference may deteriorate the bolt axial force under a certain tightening torque. Thirdly, a suitable pitch difference is determined taking into account the anti-loosening and clamping abilities. Fourthly, fatigue experiments are conducted using three different values of pitch difference for various stress amplitudes. It is found that the fatigue life could be extended when a suitable pitch difference is considered Furthermore, the chamfered corners at nut ends are considered, and it is found that the finite element analysis with considering the chamfered nut threads has a good agreement with the experimental observation. Finally, the most desirable pitch difference required for improving both anti-loosening and fatigue life is proposed