TIGHTENING STATE DETECTING METHOD FOR BOLT

PURPOSE:To detect a tightening state of a bolt with accuracy by calculating a torque rate adequately reflecting fastening torque actually acting on the bolt. CONSTITUTION:In tightening a bolt using an angle tightening method through an intermediary such as a washer up to a plastic region, a line segment 23 is acquired according to a torque rate of a final curved part 20c with a modified seat Pm as a starting point and a final torque value Te as an end point, and the line segment 23 is moved in parallel so that the final torque value Te becomes the end point when a tightening angle theta reaches a target tightening angle theta1, the modified seat point Pm is moved to a position where a line segment 23' after parallel movement crosses a lateral axis 22. Thus, an estimated rotation angle DELTAthetap that the tightening angle theta reaches the target tightening angle theta1 with the modified seat point P where the line segment 23' crosses the lateral axis 22 reflects an actual estimated rotation angle with accuracy.