Tests on Loosening of Aviation Threaded Fasteners with Different Washer Configurations

This paper presents results from an experimental investigation on loosening of aviation threaded fasteners with internal teeth lock washers and plain washers. The tests are performed on a transverse test machine which provides dynamic shear. Five different configurations are tested: (1) only lock washer, (2) lock washer on plain washer, (3) no washers, (4) only plain washer, and (5) lock washer on hardened plain washer. The resistance to loosening of these configurations is assessed by comparing loss of preload versus cycles of dynamic shear loading. Internal teeth washers used alone under a plain nut without additional plain washer are found to provide the worst case with respect to loosening resistance. Use of a plain washer under the lock washer provides significant improvement in resistance to loosening. In addition to providing protection to clamped part surfaces and better load distribution in a joint, the data presented reveals the use of plain washers under a plain nut can provide improved resistance to loosening. Relative hardness of the components, e.g., plain washer compared to lock washer, influence the performance.