Tightening Reliability of Bolted Joints in Calibrated Wrench Method
(Improvement of Initial Clamping Force Distribution and Optimum Tightening Torque by Elliptical Confidence Limit) 2020-01-0218
On tightening bolted joints, the calibrated wrench method is used in manufacturing industries for a large amount of tightening work. It is important to give high initial clamping force in respect of tightening reliability, prevention of self-loosening, the prevention from fatigue breakage, and so on. In this method the clamping force of bolted joints is controlled by grasping the wrench torque. However, since the clamping force is indirectly applied by wrench, it varies greatly in lot of tightening in factory, etc. Therefore, the calibrated wrench method is not so accurate from the viewpoint of clamping force control. It is thought that distribution of this kind is conventionally varied in a rhombus. When tightening torque and clamping force are considered to be two independent random variables, the clamping force is distributed within an elliptical confidence limit. We also show that the distribution of equivalent stress is obtained in an elliptical confidence limit. Considering the permitted limit for working load stress on a bolted joint, elliptical distribution has big margin to yield point than the shape of a conventional rhombus. Using this feature, a higher target tightening torque can be set than before. This study indicates that higher tightening torque and initial clamping force can be obtained with smaller variation than before. Finally, we establish a method for maintaining the tightening reliability that involves applying high clamping force by increasing the target design tightening torque using the elliptical confidence limit.