Development of Re-tightening Method for Bolted Joints Using Gradient of Tightening Torque

Abstract

A re-tightening method is generally used to check whether the bolted joint has been tightened with the prescribed tightening torque. The advantage of the re-tightening method is that it can be easy to check the tightening torque. On the other hand, the tightening torque in re-tightening process becomes always larger than the initial tightening torque because static coefficient of friction at mating thread surfaces and mating bearing surfaces is higher than dynamic coefficient of friction. Consequently, the tightening torque in re-tightening process causes excessive clamping forces on the joints. Therefore, it is not easy to estimate the initial tightening torque accurately without the excessive tightening torque and clamping force. In this study, we have proposed a new re-tightening method to estimate accurately the initial tightening torque without increasing the clamping force. In the re-tightening process, the gradient of the tightening torque T to the rotation angle θ in the re-tightening process. We have conducted the experiments using M8 steel bolts. As a result, the initial tightening torque estimated with an error of less than 10% in all conditions.