Methodology for evaluating the tightening torque-clamping force relationship and friction coefficients in bolted joints

In bolted joints, clamping force is determined by the material, geometrical dimensions, and surface conditions of the assembly. There is a considerable risk of various industrial accidents if the clamping force is not evaluated accurately. Therefore, a method of determining tightening torque-clamping force relationship and friction coefficients in bolted joints was proposed by analyzing subdivided theoretical formulas of the input tightening torque. These formulas calculate the friction coefficients between the threads of the bolt and nut, and between the bolt underhead and contact area. A test device was also developed to measure the input tightening torque, clamping force, and thread friction torque. The torque-clamping force relationship and those friction coefficients for an assembly of the bolt, nut, and body parts in the vehicle chassis were obtained using the device. The accuracy of the derived results using the test device was verified by measuring the torque-clamping force relationship of the same assembly in the actual part. A highly reliable structure can be produced owing to the precise control of the clamping force according to the environmental conditions of the bolted joints.