1. Material strain hardening
When the material is cyclically loaded, the phenomenon of "cyclic strain hardening" or "cyclic strain softening" will appear, that is, under the condition of equal amplitude cyclic strain, the stress amplitude will increase or decrease with the increase of the number of cycles. After several cycles, the stress amplitude enters a cyclic steady state. The low cycle fatigue of the hexagonal compression opposite side lock nut is carried out under the condition that the strain is constant, and the strain hardening or softening of the threaded sheet will affect the maximum twisting torque. The alloy steel used to manufacture the hexagonal compression lock nut belongs to the cyclic strain hardening material, and the hardening of the material will increase the elastic recovery force FN of the threaded piece, and the twisting torque will increase.
2. Low cycle fatigue
Low-cycle fatigue means that the fatigue stress is close to or exceeds the yield limit of the material. The material has a certain amount of plastic deformation in each strain cycle. The life is generally in the range of 102 to several times 104. The fatigue curve is generally represented by the ε-N curve. . The finite element calculation results show that after the bolt is screwed into the hexagonal compression lock nut, the stress at the root of the threaded piece is relatively large, and some areas of the surface layer are in a yield state, while the strain in the central area of the root of the threaded piece is very small, and the strain situation is more complicated. The area with high strain at the root of the threaded piece experiences reciprocating loading, which is prone to low-cycle fatigue, which reduces the pressure of the threaded piece and the unscrewing torque.
3. Friction coefficient
The friction angle is an important factor affecting the unscrewing torque, and the existence of friction is the basis for the normal operation of the hexagonal compression lock nut. When the hexagonal lock nut is working, there is pressure and friction on the contact surface under the action of the elastic restoring force of the thread piece. In the process of repeated use, the rough position and corners of the contact surface are ground and smoothed under the reciprocating friction. It becomes smoother and the coefficient of friction becomes smaller, thereby reducing the maximum unscrewing torque of the nut.
4. Manufacture and assembly
Due to the limitations of manufacturing technology and precision, there are sharp corners on the edge of the thread or the size of the parts is not coordinated. During the initial assembly, there may be certain fluctuations or fluctuations in the screw-in and screw-out torque, which requires a certain number of running-in. In order to get a more accurate hexagonal pressure on the side locknut reusable characteristics.
5. Closing value
After the geometric parameters of the material and the nut are determined, the change of the closing value has an important influence on the reusable characteristics of the hexagonal compression lock nut. As the closing value becomes larger, the deformation of the threaded piece increases, the strain of the threaded piece increases, and the strain cyclic hardening phenomenon intensifies, and the pressure FN of the threaded piece increases, which tends to increase the twisting torque; on the other hand, the width of the threaded piece decreases , the total area of the threaded piece decreases, the friction with the bolt decreases, the strain of the threaded piece increases, the low cycle fatigue performance decreases, and the maximum unscrewing torque tends to decrease. Under the joint action of many factors, it is difficult to predict the change of the maximum unscrewing torque with the number of repeated uses, and it can only be observed through experiments.
