Loosening-resistance evaluation of double-nut tightening method and spring washer by three-dimensional finite element analysis

The mechanisms of loosening-resistance components are investigated within the framework of the three-dimensional finite element method (FEM). In this paper, we have evaluated the ability of double-nut tightening method (DN) and spring washer (SW) to resist self-loosening due to transverse loading. We have found that if locking state is properly achieved in the tightening process, DN shows significant loosening resistance regardless of the magnitude of locking force. It was observed in this case that thread surface on the upper nut retains stuck state even if bearing-surface undergoes complete slip. However, if the locking process is not performed properly, the ability to resist loosening completely disappears. On the other hand, it is shown that SW accelerates loosening rotation of nut. The stuck area on the contact surfaces is reduced to two corner edges of the SW and the rotational force around these edges thus drastically leads to loosening before complete bearing-surface slip.