Fastener dynamics: Optimum placement, effect of thread dimensional conformance, and threadlocker life.

摘要

Each year more than 200 billion fasteners are manufactured and are used in almost every industry in U.S. However, there exists a fastener-loosening problem when fasteners are under dynamic environments, such as vibration, shock, and thermal cycling. This dissertation reports on a study of three issues related to vibration-induced fastener loosening.;First, this dissertation examines the concept of optimum placement of fasteners in structural assemblies based on dynamic considerations. Calculations for an inertial loaded, cantilevered, Euler-Bernoulli beam show that the dynamic shear stress is maximum near the dynamic nodal lines, and essentially vanishes near the anti-nodes. Experiments with a compound cantilever beam with one fastener reveal that fastener loosening occurs more readily when the bolt and nut are placed near a nodal line. This result indicates an innovative way to reduce fastener loosening by optimizing placement of fasteners based on the dynamic shear force in the structural assemblies.;Whether or not the thread dimensional conformance affects fastener mechanical performance has been a controversy in the fastener industry for over 25 years. In this dissertation, the effect of thread dimensional conformance on vibration and shock-induced loosening is investigated by performing both inertial loaded compound beam tests and military standard tests (MIL-STD-1312-7A, 1984). The test specimens include combinations of bolts and nuts within dimensional conformance as specified by ASME Standard B1.1-1989, as well as bolts with undersized pitch and major diameters and nuts with oversized pitch and minor diameters. Data from the compound cantilever beam tests show significantly degraded resistance to vibration for fastener combinations with undersized pitch and major bolt diameters or oversized pitch and minor nut diameters, compared to fastener combinations within conformance. Data from the more severe MIL-STD-1312-7A tests also show degraded resistance to vibration for the nonconforming fastener combinations, however, the difference is not as significant as in the beam tests.;Finally, this dissertation presents a test method to evaluate the life of threadlocking adhesives under dynamic loading. Threadlocking adhesive is a product to reduce or even to eliminate the vibration-induced loosening. However, there does not exist a test method for assessment of the threadlocking adhesives under dynamic loading. The work includes design of a test apparatus, development of a test plan and test procedure. Five threadlocking adhesives were tested. The test data were fitted to Weibull-power models, which can be used to compare and predict the dynamic life of threadlocking adhesives at normal operating conditions.