へたりがボルト締結体の軸力低下に与える影響評価

Abstract

Loose bolts and the loss of clamp force can lead to serious accidents such as broken or missing parts. Loosening of bolts is classified into two types: rotational loosening and non-rotational loosening. The former is caused by external load that rotates the bolt and nut. The latter is caused by material deformation due to temperature change or load. The basic principle of non-rotational loosening has not yet been clarified in many cases. That reason is that the changes are minute and long-lasting, and the factors that affect them are diverse. In this study, the phenomenon known as "initial loosening" among non-rotational loosening was examined. This is due to the flattening of small irregularities on the material surface by the surface pressure of tightening, which decreases the thickness of the fastener, thereby reducing the axial force of bolt. Therefore, experiments were conducted in this paper to elucidate the basic principles of initial loosening. Specifically, the axial force, surface roughness, number of plates, and thickness of the fastener were changed without applying external force, and the changes in axial force were measured by strain gauges to evaluate their effects.