Variation of Coefficient of Friction between Bearing Surfaces in Bolt Tightening Process

In the conventional torque control method, clamping force scatters widely in each bolt tightening due to the scatter of coefficients of friction between screw surfaces and between bearing surfaces. It has been a common knowledge that the coefficients of friction are generally influenced by roughness of the mating surfaces and lubricated conditions. However, it must be noted that the configuration error such as burr on edge of bearing surfaces is also an important influencing factor. In this study, variations of coefficient of friction between bearing surfaces against bolt head types with different conic angles of bearing surfaces and with configuration error such as burr on the edge of bearing surfaces have been investigated. The conic angles of bearing surfaces in the each bolt head types are 0°, +1° and —1° to bolt head and to outside. As the result, in case of using bolt head type with +1° to bolt head and to outside, the variations of the coefficient of friction is relatively small. In cases of using other bolt head types, the coefficients of friction vary remarkably. Accordingly, the bolt head type with +1° to bolt head and to outside is considered the optimum type for bolt tightening. The optimum bolt head type is also effective for increasing reliability of the new torque control method which simultaneously loads with torque and compressive force to a bolt.