Mechanisms of Loosening and Secondary Locking of Jack Bolt Nuts

This paper presents test results from a study on the locking performance of jack bolt nuts with and without added secondary locking. A Junker-type test apparatus is used to apply dynamic transverse force loading to induce loosening. The test data reveal there are two mechanisms of loosening for jack bolt nuts, namely slip of the nut body and slip of the jack bolts. Furthermore, the data show the dominant mechanism of loosening in jack bolt nuts is nut body slip. Since the only currently available option for secondary locking for these types of fasteners is holes in the jack bolts heads for lockwire, additional methods for secondary locking for both mechanisms of loosening are developed and tested. For the test parameters used in this study, it is found that even without any secondary locking, loosening occurs only at relatively low preload levels. The effect of introducing secondary locking to only the jack bolts is a modest decrease in the rate of loosening. The effect of adding secondary locking to only the main nut body is a significant decrease in the rate of loosening. Adding mechanical or adhesive secondary locking to the main nut body and mechanical locking to the jack bolts eliminates loosening even at low preload levels.