Vibration-induced loosening of threaded fasteners

A new mathematical model is developed to evaluate the self-loosening of threaded fasteners when subjected to cyclic transverse load. The model explains the loosening mechanism during the third stage of self-loosening. The effect of some key variables is predicted by the model and validated experimentally.;The mathematical model is developed from the force and moment components analysis during the cyclic transverse loading of a tightened joint. The equations of motion are then used to study the bolt sliding and rotation. The bolt angular motion is converted to tension loss in the bolt using an experimentally established tension-rotation signature. The torque components acting on the bolt are divided into thread and frictional bearing torque components. The sum of the two torque components gives the resultant loosening torque, which determines if the bolt will rotate in the loosening direction. The magnitude of the resultant torque and the duration in which it remains greater than zero determine the loosening rate of the fastener.;The model provides an insight into the self-loosening mechanism of tightened threaded fasteners. The outlined mechanism shows that when the transverse load is applied some sectors at the bearing surface and between the threads have more frictional resistance than other sectors, which make them a pivot point about which the bolt rotates loose. Therefore, the bolt rolling on one edge takes place instead of linear sliding.;An experimental procedure and test set-up are developed in order to validate the mathematical model results, and to investigate the effect of several key variables that are related to the bolt, joint, and the transverse cyclic load on the self-loosening of threaded fasteners. Variables investigated in this work include the variation of the thread dimensions within the tolerance allowed for conforming fasteners, thread fit, hole clearance, thread pitch, initial clamp load, amplitude and frequency of the cyclic transverse displacement, thread friction, bearing friction, and joint stiffness.;Theoretical and experimental data are presented and discussed in light of the effect of the investigated key-variables on the loosening of tightened threaded fasteners. The results identify the variables that have more significant effect.