Abstract

A method for yield tightening of screws by use of a wrench including a device for detecting a tightening torque in the actual tightening process, a device for detecting a tightening angle and an electric motor for applying a torque to each screw, a device for driving the wrench, and a controller including a device for communication with an external device. After the actual tightening torque reaches a certain value, an average torque rate is obtained by an integration using four or more pieces of torque data and is compared with a preset target torque rate, judging a yield point. The integration is performed for each minimum angle for which new torque data can be obtained. The tightening process is stopped when the judgement of the yield point has been given in succession a larger number of times than 1/2 of the number of torque data used for the integration. Further, a value obtained by subtracting a certain value from the actual tightening angle is integrated and the area of a right triangle inscribed in the area of the integrated value is obtained. The tightening process is stopped also when a difference between the integrated value and the area of the right triangle is greater than an area calculated using a preset target angle.