Effect of Component Twist on Friction in a Bolted Joint

Abstract

This paper presents an analysis of the assembly and disassembly processes for components in a bolted joint. The analysis shows a small amount of torsional elastic twist in joint components from applied torque during assembly and disassembly significantly affects thread and nut bearing friction. It reveals that after the assembly process, the nut bearing interface friction torque alone acts to counter not only the torque associated with bolt stretch, also known as the inherent self-loosening torque, but thread interface friction torque as well. Previous models and analyses of bolted joints have missed this point. Further, analysis of the disassembly process with applied removal torque reveals thread friction does not act to counter self-loosening and removal torque until sometime after nut bearing interface slip occurs. Only then do both thread friction and nut bearing friction counter self-loosening torque and removal torque. This is the state commonly assumed after the assembly process and for the duration of the disassembly process. The analysis presented reveals the effects of elastic twist of components in a bolted joint on thread friction and nut bearing friction and provides improved understanding of the state and behavior of bolted joints.