Tests and analysis of secondary locking features in threaded inserts

This paper presents results from an experimental investigation of loosening of threaded inserts. The tests are performed on a transverse test machine which provides transverse shear. Three different configurations are tested: insert with no secondary locking feature, insert with prevailing torque locking feature, and insert with thread adhesive locking feature. Both secondary locking features are found to provide improved resistance to loosening. The improvement with inserts with prevailing torque locking feature is found to be minimal at first, then increase with decreasing preload and eventually level out with a fraction of initial preload retained. Inserts with thread adhesive are found to provide significant improvement throughout the duration of the test. Loosening characteristics are quantified with the following five parameters extracted from measured preload versus cycles data: percent loss of initial preload, initial rate of preload loss, secondary rate of preload loss, steady-state value, and final preload value. Inserts with a prevailing torque feature are found to have a statistically significant effect on the secondary rate of preload loss, the steady-state preload, and the final preload, but no significant effect on the percent loss of initial preload and the initial rate of preload loss. Inserts with thread adhesive are found to have a statistically significant effect on the percent loss of initial preload, initial rate of preload loss, and final preload.