三次元有限要素法による軸方向外力作用下でのボルト・ナット締結体のゆるみ挙動解析

摘要

ボルト・ナットを用いた締結は他の締結法と比べ取り付け，取り外しが容易であり低価格のため様々な機械・構造物に用いられている．しかし，ボルト締結体に外力が作用することでゆるみが生じるという問題があり，現代においてなおゆるみに起因する事故が生じている.筆者らの研究ではねじ山の螺旋形状の接触を考慮した三次元有限要素法を用いて軸直角方向外力を受けるボルト・ナット締結体のゆるみの解析を行い，山本・賀勢らの実験と非常に良い一致を示すことを示した．%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.