There are fastened structures composed of rotary disc and shaft, which are fastened with bolts and nuts with tapered bearing surfaces. Those are loaded with rotating-bending force. It has been difficult to explain clearly the rotating loosening mechanism of such joint's nuts by the previously proposed mechanism. In this study, the rotating mechanism of this nut was investigated from theoretical and experimental view points. Finally two types of mechanisms were derived. One is as follows: High pressure contact area is formed by external load oriented in the radial direction of a disc. It gives very small difference of radii between the bearing surface of disc and that of nut. Then, with the revolution of the disc, it makes occurrence of two friction torques in opposite direction, which are Tw and Ts torques on bearing and on threads respectively. When revolving disc, relative rotating direction of nut is dominated by bigger torque of Tw and Ts. If Tw is bigger than Ts, the rotating direction of nut is the same as the disc revolution. If Ts is bigger than Tw, the rotating direction is opposite to disc revolving direction. Taking into consideration this mechanism, some tests were carried out by changing magnitude of the friction coefficients both of bearing surface and of threads surface with greasing and degreasing intentionally. Loosening and tightening rotations were confirmed to occur just as predicted from the above mentioned mechanism. The other is due to the eccentricities caused by dimensional errors around the bolt, the nut and the tapered bearing surface of rotary disc. In this case, the direction of the force transmitted through the high pressure contact area changes from the center of bolt axis. So, the transmitted force can cause the torque which loosens and tightens the nut during one revolution of rotary disc, therefore relative rotation of nut results in small fluctuating of loosening and tightening. Combining two types of mechanisms, repeating small angle of loosening and tightening, nuts are rotated inclined to loosening or tightening direction after many revolutions of disc.
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