The Effect of Thread Dimensional Conformance on Vibration-Induced Loosening

This paper presents the results from dynamic tests that investigate the effect of thread dimensional conformance of fasteners on vibration-induced loosening. Test specimens include combinations of bolts and nuts within dimensional conformance as specified by ASME Standard B1. 1-1992, as well as bolts with undersized pitch and major diameters and nuts with oversized pitch and minor diameters. Tests were performed using an inertial loaded compound cantilever beam apparatus that subjects the test fasteners to a vibration environment. Data from the tests show a significantly degraded resistance to vibration for the fastener combinations with undersized pitch and major bolt diameters or oversized pitch and minor nut diameters, compared to fastener combinations within conformance. Specifically, the lower 90 percent confidence limit for the time to failure for conforming specimens was 782 seconds compared to 8 to 29 seconds for nonconforming specimens.