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Bolt-nut tightening systems are widely used in mechanical structures since the disassembly for maintenance is easy without much cost. However, vibration induced loosening has been unsolved subject. In this paper, we have paid attention to the mechanisms of small loosening process due to micro bearing-surface slip in the framework of the three-dimensional finite element method (FEM). Results show good agreement with Kasei’s experimental results. It was found that early-stage nut rotation obtained by the experiments originates from bolt-nut simultaneous rotation induced by tightening torsion of bolt and does not correspond to the loosening rotation. Therefore, loosening rotation must be defined by the relative rotational angle of nut with respect to bolt. It was also found that the small loosening initiates when the vibrational force reaches about 50 to 60 % of that bearing-surface slip occurs. It is necessary to pay attention to the contact state of both bearing surface and thread surface for the consideration of loosening of bolt-nut tightening systems. Contact states are classified into three kinds of states, that is, complete slip involving no sticking region, micro slip involving no ever-sticking region over a vibration cycle, and localized slip involving ever-sticking region over a vibration cycle. A total of nine kinds of contact states are defined by the combination of the bearing surface and thread surface. It is also found that loosening rotation can proceed when micro slip or complete slip occurs at both thread and bearing contact surfaces.

Small Loosening of Bolt-Nut Tightening System due to Micro Bearing-Surface Slip: Finite Element Method Study