Tightening and Self-loosening Behavior of Double-nut Tightening System by Three-dimensional Finite Element Method

Double-nut tightening fastening is widely known as one of the methods for preventing loosening. However, it is also well known that unless a self-locking state is properly realized the effect of loosening prevention is lost. There are not many precedents where double-nut performance was verified on the basis of these features. For this reason it is believed that it is very important to consider the loosening-prevention mechanism of these fasteners. In this paper, we analyze and explain the locking mechanism of a double-nut tightening system by three-dimensional finite element method. In addition, we also analyzed the loosening under shear loading and investigated the relation between locking state and loosening-prevention performance. Qualitative accordance with experimental results of tightening analysis presented by Yamamoto et al. was obtained. What was also made clear was that if the locking is properly realized, double-nut fastening is a very effective loosening-perevention method. On the other hand, if locking is not achieved properly, this method cannot be relied upon for loosening-prevention.