Influences of squareness error at bolt bearing surface on a frictional coefficient between bearing surfaces and on tightening accuracy

It is well known that a main cause of tightening error on the torque control method is changes of the frictional coefficients in thread surfaces and bearing surfaces in bolt tightening. It has been shown in the previous study that the frictional coefficients depend on surface roughness of contact surface, lubrication conditions and bolt configuration errors. The frictional coefficients can be changed by the bolt configuration errors such as a bend of bolt shank and inclination of bolt bearing surface, if large wear and seizure do not occur on the lubricated surfaces.In this study, an influence of the squareness error of bolt bearing surface on the tightening accuracy has been investigated. Results clearly showed that the squareness error of bearing surface causes variations of torque coefficients and the clamping force error. It was shown by FEM analysis that the variation of torque coefficient between bearing surfaces is caused by change of an apparent frictional coefficient. Tightening accuracy also depended on the squareness error of bearing surface.