Abstract
Bolts and nuts are widely used in various fields as an important joining machine element. In this mechanism, some problems always exist that loosening is induced by vibrations and external loads; screws are broken due to fatigue and so on. 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 the first pitch decreases more than 12% with increasing initial clearance of outer cap nut. Stress concentration appearing at the first 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 hands, it is found that anti-loosening performance of SPR can be realized when the outer cap has high yield stress.
