Volume 5 (2011) Issue 11 Pages 627-639
The mechanism for a bolt and nut self loosening under repeated bolt axial tensile load has yet to be clarified, despite much investigation into this phenomena. In this paper, the self loosening mechanism is derived from basic strength of materials equations, which results in the following conclusions. (1) When load is applied, a slip occurs on the screw thread surface, and the bolt shank twists clockwise, that is it descends on the lead angle of the screw thread. At the end of this process, counterclockwise restitution torque TS1- is generated by the twisted angle of the bolt shank. However, there is no rotation of the nut. (2) When the load is released, if TS1- exceeds the friction torque TW0 on the nut bearing surface generated by the decreased bolt axial tension, a slip occurs on the nut bearing surface, and the bolt and the mating nut rotate counterclockwise as one unit. To satisfy the relationship TS1->TW0, the maximum bolt axial tension F1 must be larger than cF0. Here F0 is the minimum bolt axial tension and the coefficient c depends on the bolt shape and the friction coefficient. The rotational behavior of the bolt and the nut derived from this analysis concurs with experimental and FEM calculation results of other researchers. For steel joints, it is believed that rotational loosening rarely occurs when there is no separation between joined parts.