Stress Reduction Effect and Anti-Loosening Performance of Outer Cap Nut by Finite Element Method

Previously several kinds of anti-loosening bolts and nuts were invented; however, they usually need a certain amount of prevailing torque even before the nut touches a fastened plate. A new outer cap nut named “Super loose proof (SPR)” has been developed to overcome such inconvenience. At first this outer cap nut can be rotated smoothly by hand until the nut touching the fastened plate; however, after fastening the outer cap nut, anti-loosening performance can be realized by deforming the outer cap and producing thread contact force at the outer cap region. In this study, stress concentration and tightening-loosening behavior are analyzed by axi-symmetric and three-dimensional finite element methods. Under a certain bolt-axial force, the load distribution of 1st pitch decreases more than 12% with increasing initial clearance of outer cap nut. Stress concentration appearing at the 1st pitch of the bolt can be about 10% smaller than that of conventional nut, reflecting the increase of the thread contact force at the outer cap region. On the other hand, it is found that anti-loosing performance of SPR can be realized when the outer cap has high yield stress.