Optimal Fastener and Preload Selection

Selecting fastener pre-load and strength for tension fastened joints has traditionally been an iterative tradeoff between separation, strength and slip requirements. In this paper, equations are presented for the maximum external load that a fastened joint can support and the optimal pre-load to achieve this load while simultaneously satisfying safety factors for separation, strength and slip. The equations are based on linear joint theory and joint stiffness. The optimal equations account for variations in joint stiffness due to variations in joint geometry and materials. Furthermore the equations account for variations in joint preload, load-plane factor, friction coefficients and thermal loading. The external load may be axial, shear or a combination of both. The equations can be plotted to create a 3D “Fastener Design Space” that represents the maximum acceptable joint external tension and shear loads as a function of the joint preload. The design space is normalized by fastener strength to make it applicable to any fastener. Any combination of preload, tension and shear that falls within the fastener design space represents a safe joint design. Plotting multiple load combinations provides a quick method for evaluating the safety of multiple joints and load cases. Alternatively the Fastener Design Space can be used to determine the maximum load capacity and optimal preload for a set of joints with varying properties.