Self-Loosening Analysis of Bolt-Nut Tightening System Subjected to Axial Loading by Three-Dimensional Finite Element Method

Abstract

Loosening of bolt-nut tightening system is still unsolved problem even in recent days. Recently, we have investigated the loosening mechanism of bolt-nut tightening system subjected to shear loading and have succeeded the reproduction of experimental result in the framework of three-dimensional finite element method. In this paper, we have applied the scheme to the loosening problems induced by axial loading. As a result, the loosening mechanism has been clarified. The resulting loosening rate and its dependence on friction coefficients show qualitative agreement with the Kumakura's experiments. Critical loading range of loosening also shows quantitative agreement with the experimental result. It is found that the evolution of bolt-nut relative rotations in the loading Process and unloading Process are different each other, that is, the relative rotation is enhanced in the loading process and is inhibited in the unloading process. The analysis involving different Young modulus of bolt, nut and moving plate also indicates that loosening is originated by the slippage at bearing surface induced by nut deformation. As an application to actual tightening system, the loosening of the piston nut in hydraulic cylinder has been investigated. It is found that the accumulation of rod torsion in the loading process and the simultaneous rotation of rod and nut induce the loosening.