A Study on the Variation of Bolt Tightening Torque in the calibrated wrench method

Abstract

In the tightening of a large number of bolts, the coefficient of friction has a large influence on the variation of initial axial force. The tightening work coefficient a, which indicates the variation of tightening torque, also affects the variation of initial axial force, but has not been studied in detail. Based on the results of tightening tests conducted by the Japan Screw Research Association, a was estimated to be 0.080 for oil-lubricated fastening, 0.048 for rust-preventive oil-lubricated fastening, and 0.140 for fastening without lubrication at the 90% confidence limit, when initial axial tension is 70% of yield point. According to the experimental results using the design of experiment method (L₈(2⁷)) orthogonal table), a was 0.059 for fastening with anaerobic adhesive and 0.131 for fastening without lubrication at the 90% confidence limit with human error.