An Asymmetric Wrenching System with High Torque Transfer Capability for Aerospace Pins

The installation of common threaded aerospace fasteners by the application of a torque to a nut or collar is made possible by an internal wrenching element or recess feature adapted to the threaded end of a pin, which accepts a mating anti-rotation key designed to partially balance the applied torque. In applications such as the mechanical joining of composite structures accomplished by wet clearance fit installations of permanent fasteners, high nut or collar seating torques not adequately opposed by frictional resistance at the contact surfaces of the fastener and joint members effectively shift a greater proportion of the torque reaction requirement onto the recess and mating anti-rotation key which in turn can experience high torsional stresses exceeding their design capability and result in frequent service failures. In particular, the industry standard hexagonal recess and key have been shown to be highly susceptible to such severe conditions with failure rates as high as 20% in field applications. A superior asymmetric recess design was developed for use in aerospace pins to enhance the reaction torque capability of the fastener and mating key over the current baseline, without compromising established functional requirements, enabling fastener use in more demanding service applications. In this paper, the installation performance capability of threaded pins with the asymmetric recess is compared to equivalent substitutes with alternative recess designs through a series of standard industry tests conducted across a range of part sizes. Numerical simulations and experimental test results demonstrate that the asymmetric recess and key exceed the torsional strength and fatigue life of the current industry standard and other commercially available designs, offering the potential to increase productivity and reduce the cost of structural assembly and maintenance operations.