ねじ部品の形状誤差を考慮したトルク法の締め付け精度の改善

抄録

  Torque control method is most commonly used when tightening bolted joints because of its easy operation. However, the method involves the essential problem of fairly large scatter in bolt force. It has been reported that even if the same torque is applied, bolt forces show a considerable scatter, e.g., ranging from 25% to 35%. A scatter in the coefficients of friction on nut bearing surface and thread pressure flank is a primary source of bolt force scatter. Meanwhile, the effect of equivalent friction diameters at nut bearing surface and bolt head surface cannot be ignored. The scatter in equivalent friction diameter is caused by imperfect geometry, i.e., the flatness deviation at the bearing surfaces. In this paper, the magnitudes of equivalent friction diameters are evaluated in detail by FEA, using the measured data of flatness deviation for a number of commercial nuts and bolts. It is shown that the bolt force is likely to be scattered by as much as 10% of the target value, owing to flatness deviation. Based on comprehensive calculations by considering imperfect geometry, a strategy to effectively suppress bolt force scatter is proposed.