Effect of Initial Assembly Gaps on Thread Strip and Fatigue Assessment of Bolted Joint

Abstract

<div class="section abstract"><div class="htmlview paragraph">Bolted joint is a popular method for assembly of mechanical systems which are typically designed by considering members to be in full contact without initial gap. However, manufacturing imperfections or part tolerances can introduce gaps between members. This initial gap is proven to have an adverse effect on the performance of bolted connection. The gap introduces additional bending moments (B.M.) during tightening operation and affects the loads shared by the threads thereby aggravating thread strip and fatigue performance. The aim of this paper is to provide a robust approach for predicting this premature failure of bolted joint due to initial gaps in assembly. VDI 2230 industry guideline for fastener assessment does not account for bending effect due to initial gap. To address this limitation, a “Coupled Analytical and FEA based” approach is developed to accurately capture initial bending moment and its effect on distribution of loads between the engaged threads. Results with initial gap show that there is a significant non-uniform distribution of shear forces on engaged threads, which reduces safety margins by 50% when compared with no-gap condition. The proposed approach was validated with experimental results on a sub-assembly of power electronic device. The authors expect this body of work will enable the readers to identify and mitigate the risk of bolted joint failure in compact assemblies like those in powder-dense EV and allied applications where initial gap in assembly is possible due to presence of multiple part tolerances (e.g., Choke, PCB, housing standoff etc.) and threads are present in soft material (e.g., copper busbar). Also, gap closure with sufficient clamping is desired for low contact resistance and high electrical performance.</div></div>