Influencing factors for locking the safety nut
The maximum unscrewing torque is affected by many factors. For the research on the low-cycle fatigue performance of the locking safety nut, the pitch diameter of the thread, the helix angle and the bevel angle of the tooth shape remain unchanged. Only the maximum elastic restoring force FNmax and the equivalent friction angle ρe of the thread piece will appear to a certain extent after repeated use. change. Therefore, it is only necessary to analyze the variation law of the maximum unscrewing torque when the locking safety nut is subjected to cyclic load from these two aspects.
1. Material strain hardening
When the material is cyclically loaded, the phenomenon of “cyclic strain hardening” or “cyclic strain softening” occurs, that is, under the condition of constant 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 lock lock safety nut is carried out under the condition that the strain is constant, and the strain hardening or softening of the threaded piece will affect the size of its maximum unscrewing torque. The alloy steel used to make the lock nut is a cyclic strain hardening material, and the hardening of the material will increase the elastic restoring force FN of the threaded sheet and increase the unscrewing torque.
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, and the fatigue curve is generally expressed by the ε-N curve . The finite element calculation results show that after the bolt is screwed into the lock nut, the stress at the root of the threaded piece is large, and part of the surface area is in a yield state, while the strain in the central area of the root of the threaded piece is small, and the strain situation is more complicated. The area with high strain at the root of the thread piece experiences reciprocating loading, which is prone to low cycle fatigue, which reduces the pressure of the thread piece and reduces the screw-out torque.
3. Coefficient of friction
The friction angle is an important factor affecting the torque, and the existence of friction is the basis for the normal operation of the locking safety nut. When the lock nut is working, the contact surface has pressure and friction under the action of the elastic restoring force of the thread piece. During the repeated use, the rough position and edges and corners of the contact surface are ground and smoothed under the action of cyclic friction. The coefficient of friction becomes smaller, which in turn reduces the maximum unscrewing torque of the nut.
4. Manufacturing and assembly
Due to the limitations of manufacturing technology and accuracy, the edge of the thread has sharp corners or the dimensional coordination between parts is not coordinated. During the initial assembly, the torque of screwing in and screwing out may fluctuate or fluctuate to a certain extent, which requires a certain number of running-in times. In order to obtain a more accurate reusability of the locking safety nut.
5. Closing value
After the material and the geometric parameters of the nut are determined, the change of the closing value has an important influence on the repeated use characteristics of the lock safety nut. When the closing value becomes larger, the deformation of the threaded piece increases, the strain of the threaded piece increases, the strain cycle hardening phenomenon intensifies, and the pressure FN of the threaded piece increases, which has a tendency to increase the unscrewing 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 combined action of various factors, the change of the maximum unscrewing torque with the number of repeated uses is difficult to predict, and it can only be observed through experiments.