Threaded Fastener Secondary Locking Requirements

Secondary locking features in threaded fasteners are in widespread use in machinery, structures, and systems. In this paper, the mechanism for loosening of threaded fasteners is explained qualitatively and defined quantitatively in terms of the self-loosening moment inherent to threaded fasteners and external load-induced loosening moments. Equations for loosening moments are defined. This paper provides analysis which quantifies the locking action required to prevent loosening due to the inherent self-loosening moment and external load loosening moments in threaded fastener joints. This paper provides the basis and method for engineers to properly design or specify secondary locking features in threaded fasteners to provide sufficient locking and prevent loosening. Requirements for secondary locking feature moments are developed in terms of loosening moments and factor of safety. Test data and calculations are provided to substantiate the requirements. Examples are provided for common secondary locking features including prevailing torque and adhesive.