Towards an understanding of the loosening characteristics of prevailing torque nuts

Abstract

Prevailing torque nuts are an extremely popular method of providing resistance to vibration-induced self-loosening of fasteners. Such nuts have a self-contained prevailing torque feature that provides a degree of resistance to rotation. Although such nuts are frequently used, it is not widely realized that they can occasionally come completely detached from bolts. The mechanism by which this can occur has hitherto been unidentified since it has not been possible to replicate detachment under laboratory testing. This article identifies a general condition that can result in the complete loosening and detachment of prevailing torque type nuts. This mechanism involves the application of an axial load when transverse joint slip is occurring. This article describes a modified Junker test machine that allows the application of axial loading to a joint while experiencing transverse displacement. Tests have been completed using an intermittent as well as a constant axial load. Loading in both modes has been demonstrated to result in the complete detachment of this nut type. Based on this investigation, if the magnitude of the axial loading exceeds the residual preload in the bolt retained from sustaining transverse movement alone, the all-metal type of prevailing torque nut can completely detach. Applications that involve shear and axial loading being simultaneously applied to a joint are numerous in engineering.