Contact Mechanics Approach to Determine Effective Radius in Bolted Joints

The effective radius is a very important factor in determining frictional torque under bolt head/nut is bolted joints. Since the effective radius is hard to be precisely determined in practice, the mean radius, which is the mean value of the inner and outer radii of the contact area under bolt head/nut, is used. In this paper, contact mechanics analysis is used to determine the real contact pressure distribution between bolt head/nut and the joint surface. Based on the pressure distribution, the effective radius can be precisely calculated. Effect of the surface roughness, underhead load distribution, underhead load value, and the ratio of the maximum to minimum contact radii are investigated. The results are compared with the result from theoretical formulas and the value of mean radius. These data will be very useful in enhancing the reliability and safety of bolted joints.