Tribological Aspects of the Self-Loosening of Threaded Fasteners

Abstract

Practically every engineering product with any degree of complexity uses threaded fasteners. Although threaded fasteners are generally considered a mature technology, significant problems exist with their use. This study has investigated a number of issues with the tightening and self-loosening of threaded fasteners.

- It was found that upon repeated tightening of electro-zinc plated fastners significant wear of the contact surfaces of the bolt/nut thread and nut face occured. This wear was accompanied by an increase in the friction coefficient causing a reduction in the clamp force provided for an assembly when tightened to a specific torque value.

- The self-loosening characteristics of prevailing torque nuts were also investigated. It was found that there was a significant loss of prevailing torque when a fastner self-loosened when compared to the prevailing torque when being deliberately disassembled. The current standard test for prevailing torque nuts on re-use does not reflect this surprising result and leads to a significant over-estimate of the capability of this class of nut to resist self-loosening. This is a contribution to knowledge on this topic.

- A further major original finding of this study has been that if an axial load is also acting on a joint which is experiencing transverse slip, prevailing torque nuts can continue to self-loosen leading to their possible detachment from bolts. A number of accidents have been caused by such detachments, but the cause was not understood partly because this detachment could not be reproduced on the standard loosening test. Work reported in this thesis has been found that if an external axial load is acting whilst the joint is experiencing transverse slip, under the approprioate conditions, the loosening process will continue until nut detachment occurs.

- A series of tests has been completed in which the forces needed to tighten and loosen threaded fastners were measured whilst the joint was being subjected to transverse slip/vibration. Measurements were made of the frictional resistance forces in the circumfrential direction and the loosening torque acting on a fastner under transverse slip conditions. It was found that the loosening torque range varied between two positive limits rather than between zero and an upper limit as anticipated by the theory. It was also found that the friction coefficient in the circumfrential direction in the threads is greated than that on the nut face bearing surface during conditions of transverse slip.