Patent ID: 7398700

Claim:
A bolt tightening apparatus applicable to a bolt having bolt tightening torque properties, in which a torque gradient expressed in terms of the ratio of an additional tightening torque exerted on a bolt to its additional turning angle has a first torque gradient in an initial bolt tightening stage and a second torque gradient in a late bolt tightening stage subsequent to the initial bolt tightening stage, a magnitude of the second torque gradient being greater than that of the first torque gradient, the apparatus comprising: a bolt turning device operative to turn the bolt around a bolt axis; a torque detecting device operative to detect a torque applied to the bolt; a bolt turning angle detecting device operative to detect a bolt turning angle; a control device operative to control an operation of said bolt turning device; and a corrective additional-turning angle calculating device operative to calculate a conective additional-turning angle that is equivalent to an amount of additional tightening of the bolt to be added to a predetermined amount of initial tightening of the bolt, wherein said control device is configured so to operate said bolt turning device until a bolt tightening torque detected by said torque detecting device becomes a predetermined snug torque and subsequently operate said bolt turning device until a bolt turning angle detected by said bolt turning angle detecting device becomes a total turning angle that is equivalent to the sum of a predetermined initial-set turning angle and the corrective additional-turning angle calculated by said corrective additional-turning angle calculating device, and said conective additional-turning angle calculating device is configured so as to calculate a first corrective additional-turning angle and a second corrective additional-turning angle, the first corrective additional-turning angle being to compensate for a situation in which a bolt turning angle corresponding to said snug torque on an extension line of said bolt tightening torque properties with the second torque gradient is greater than a bolt turning angle corresponding to said snug torque on a line of said bolt tightening torque properties with the first torque gradient, and said second corrective additional-turning angle being to compensate for a decrease of an effective bolt axial force that is caused by an existence of said bolt tightening torque properties with the first torque gradient, the sum of the first corrective additional-turning angle and the second corrective additional-turning angle being said corrective additional-turning angle.