Structures composed of a rotary disk and a shaft, which are fastened with bolts and nuts having tapered bearing surfaces, are loaded with a rotating-bending force. Upon investigation, two rotating mechanisms of the nut were derived. In one mechanism a high-pressure contact area is formed at the nearest loading point on threads and bearing surfaces. This leads to a difference in the curvature radii between the bearing surface of the disk and that of the nut. During the revolution of the disk, two friction torques occur in opposite directions on the bearing surface and the threads, respectively. The relative rotating direction of the nut is dominated by the greater torque. The other mechanism is due to the eccentricities caused by dimensional errors of the bolt, nut, and disk. By combining the two mechanisms, the rotations of the nuts either cause a loosening or tightening after many revolutions of the disk.

Self-loosening mechanisms of a bolt were investigated by Finite Element Method, under the assumption of a twist at the center of a circular joined structure in which the bolt was set along a certain pitch circle. In this structure, the bolt is loosened by combining the translational and rotational external loads. In the case of a large pitch circle structures in which self-loosening occurs, the directions of friction shear forces on the threads were along concentric circles; however, the instantaneous center of rotation was located one-side near the thread surface, and the center was eccentric with the axis of the bolt. If the radius of the pitch circle is set smaller, the instantaneous center of rotation moves closer to the center of the bolt, and finally reaches to the same position at the center of the bolt. On the other hand, the directions of friction shear forces on pitch diameter of one thread were calculated theoretically using the inclination and friction on a pressure flank. The results were in good agreement with FE analysis. By considering these mechanisms, it was estimated that the number of occurrence of self-loosening in one vibration cycle changes at the border when the diameter value of the pitch circle equals that of the screw threads. If the diameter of the pitch circle becomes smaller than that of the screw threads, the number changes from two to one. With the exception of torsional center-fastened structures, since the pitch circle is very small, self-loosening of general joined structures will occur twice in one vibration cycle.

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