Patent ID: 6954682

Claim:
A bolt tightening method applied when tightening a bolt by using a bolt tightening device in such a case that a first torque gradient which is a ratio of a bolt tightening torque to a bolt turning angle in an initial bolt tightening stage differs from a second torque gradient which is the ratio of the bolt tightening torque to the bolt turning angle in a succeeding final bolt tightening stage, said method comprising the steps of: calculating the first torque gradient in the initial bolt tightening stage; calculating an intermediate torque gradient which is the ratio of the bolt tightening torque to the bolt turning angle in a stage of transfer from a region of the first torque gradient to a region of the second torque gradient; determining an inflection point joining the first and second torque gradients based on the calculated values of the first torque gradient and the intermediate torque gradient; determining an amount of additional tightening of the bolt based on the inflection point and the intermediate torque gradient; calculating the second torque gradient upon transferring to the region of the second torque gradient; determining a theoretical seating point of the bolt based on the calculated value of the second torque gradient; and controlling an amount of tightening of the bolt based on an amount of initial tightening set by regarding the theoretical seating point as a starting point of tightening and the amount of additional tightening.