Yasuo Fujioka

Rotating loosening mechanism of a nut connecting a rotary disk under rotating-bending force

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 highpressure 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.

Calculated Behavior and Effective Factors for Bolt Self-Loosening Under a Transverse Cyclic Load Generated by a Linearly Vibrating Washer

It is common knowledge that a bolt is apt to loosen due to slippage between the contact surfaces of joined parts. Loosening tests using real parts enable precise scrutiny of real phenomena under the influence of multiple factors such as slip distance, surface roughness, and coefficient friction. However, estimating the influence of the individual factors is very difficult because the friction forces of real contact surfaces are compiled based on variations in friction coefficients, meaning friction is not stable. Therefore, the effects of factors were investigated using Finite Element Analysis (FEA) to control friction coefficients. The procedures were as follows. Assuming a joined structure consisting of a bolt, nut, and washer, bolt axial tension was generated through constant movement of a washer in the bolt’s axial direction, following which the washer was constantly vibrated in one direction transverse to the bolt axis. This vibration generated displacements equivalent to the degree of slippage between the two clamped parts. During vibration, the rotating angles of the bolt and the contact pressure of the threads and bearing surfaces were calculated. The results were as follows. The vibrating displacements of a washer have considerable influence on the rotational loosening of a bolt. In cases where there was only minor displacement of the washer vibrations, the rotational loosening angle rapidly decreased, although the loosening did not cease completely. Therefore, the magnitude of what is called “critical slip” was not confirmed under the conditions of this study. In addition, the friction coefficient has a significant influence on the rotational loosening of a bolt. When the respective friction coefficient values of the threads and bearing surfaces are not balanced, rotational loosening cannot continue. Surface roughness readily affects contact pressure, so it tends to make the contact pressure localized. In particular, high-pressure areas were affected by several projections set on the threads. However, under those conditions the rotational loosening did not differ greatly from the results of the fine surface models subject to the same vibrating amplitude and friction coefficient. Consequently, the localized contact pressure had little evident effect on loosening. Above all, FEA reproduced the loosening of the bolt, and the reference made in this analysis is useful.Copyright

Behavior and Mechanisms of Bolt Self-Loosening Under Transverse Load Due to Vibrations of a Washer Along an Arc

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.Copyright

Non-Rotational-Loosening Behavior of Bolted Joint Constructed From Steel Tube and Plates

The reduction in axial tension was investigated in bolted joints, which comprise two hot-rolled steel plates and a carbon steel tube sandwiched between the plates. After tightening the bolts, vibration tests were carried in the following two cases. In the first case, flat-faced steel tubes without having any serrations on both ends of the tube were used, and the effect of wave profiles of the external load was investigated. These wave profiles are those of random waves, quasi-random waves that are obtained by only eliminating random waves with small amplitudes, and 7-step programmed waves generated from random waves. In each test, the maximum and the minimum amplitudes are the same. In the other case, the effect of the convex height of the serrations, which are cut on the end face of a tube, was investigated. In addition to the flat faces, the end faces of the tubes were serrated with three different heights for the purpose of comparison. In this case, the vibrations were applied only by means of sinusoidal waves. The effect of the wave profile on the external load is as follows. During the period of low cyclic loading, the bolt axial tension was reduced in the following order: the reduction was the minimum for the 7-step programmed waves, followed by the quasi-random waves, and was maximum for the random waves. As the number of cycles increases, the scattering range of bolt axial tensions corresponding to three types of waves becomes smaller. Furthermore, a comparison of this result with a fatigue test on mild steels indicates that the vibrating wave affects the reduction in axial tension, this is evident from the wave profiles and frequency. With regard to the convex height of the serration, after 106 cycles, the data shows that the lower the convex height, smaller is the reduction in the axial tension. The value of relative micro-slippage of the contact surfaces between the carbon steel tube and the steel plates is small at the sharp serrations. Therefore, the reduction in the axial tension is assumed to be mainly due to the cumulative effect of the plastic deformations.Copyright

Rotating Loosening Mechanism of Nut Connecting Rotary Disc Under Rotating-Bending Force

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.Copyright