1. Material strain hardening: When materials are subjected to cyclic loading, the phenomenon of “cyclic strain hardening” or “cyclic strain softening” occurs, which means that under constant amplitude cyclic strain, the stress amplitude increases or decreases with the increase of the number of cycles. After several cycles, the stress amplitude enters a cyclic stable state. The Low-cycle fatigue of the lock nut is carried out under the condition that the strain is constant, and the strain hardening or softening of the thread piece will affect the maximum screw out torque. The alloy steel used for manufacturing lock nuts belongs to the cyclic strain hardening material. Material hardening will increase the elastic recovery force FN of the threaded piece and increase the tightening torque.
2.The friction angle is an important factor affecting the tightening torque, and the existence of friction is the basis for the normal operation of the locking nut. When the locking nut is working, there is pressure and seat friction on the contact surface under the elastic restoring force of the threaded piece. During repeated use, the contact surface is subjected to cyclic friction, and the coarse and fine positions and edges are smoothed, resulting in a smaller friction coefficient and a decrease in the maximum tightening torque of the nut.
3.Due to manufacturing technology limitations and precision reasons, there may be sharp corners at the thread edges or mismatched dimensional fit between parts. During the initial assembly, there may be some fluctuations or fluctuations in the screw-in and screw-out torque, which requires a certain number of runs in order to obtain more accurate locking nut reuse characteristics.
4.After determining the geometric parameters of the material and nut, the change in the closing value has a significant impact on the reuse characteristics of the locking nut. The larger the closing value, the larger the deformation of the thread piece when it opens, the higher the strain of the thread piece, the more strain cyclic hardening phenomenon, and the larger the pressure FN of the thread piece, which has the trend of increasing the screw out torque. On the other hand, the width of the thread piece decreases, the total area of the thread piece decreases, the friction with the bolt decreases, the strain of the thread piece increases, and the Low-cycle fatigue performance decreases, which has the trend of reducing the maximum screw out torque. Under the combined action of multiple factors, it is difficult to predict the variation of the maximum torque with the number of repeated uses, and it can only be observed through experiments.
Post time: Jul-10-2023