Criterion for Preventing Self-Loosening of Preloaded Countersunk Head Threaded Fasteners

This study develops an analytical formula for determining the minimum initial preload required to prevent the self-loosening of preloaded countersunk fasteners that are subjected to cyclic transverse loading. The formula is based on mathematical modeling of the self-loosening behavior of the fastener. The accurate prediction of the minimum bolt preload required for preventing loosening would reliably enable the use of that minimum threshold preload as a primary locking feature in critical bolted joint applications. An experimental setup and test procedure is established to compare the model prediction with the experimental data. The focus of this paper is to investigate the effect of thread pitch, excitation amplitude, as well as the bearing friction coefficient on the threshold value of the bolt preload that would prevent loosening.