The maximum tightening torque of the lock nut is affected by many factors. For the research on the low-cycle fatigue performance of the lock nut, the pitch diameter of the thread, the helix angle and the bevel angle of the thread remain unchanged. Only the maximum elastic restoring force FNmax of the thread piece and the equivalent friction angle ρe will appear to a certain extent after repeated use. change. Therefore, it is only necessary to analyze the change law of the maximum screw-out torque when the lock 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" will appear, that is, in the case of constant 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 cyclically stable state. The low-cycle fatigue of the lock nut is carried out under the condition of constant strain, and the strain hardening or softening of the threaded piece will affect the maximum screw-out torque. The alloy steel used to manufacture the lock nut is a cyclic strain hardening material. The hardening of the material will increase the elastic restoring force FN of the threaded piece, and the screwing 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 span 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 lock nut, the stress at the root of the thread is larger, and the surface area is in a yielding state, while the strain in the center of the root of the thread is very small and the strain situation is more complicated. The area with higher strain at the root of the threaded piece undergoes reciprocating loading, which is prone to low-cycle fatigue, which reduces the pressure of the threaded piece and the screw-out torque.
3. Friction coefficient
The friction angle is an important factor that affects the tightening torque, and the existence of friction is the basis for the normal operation of the lock nut. When the lock nut is working, there is pressure and friction on the contact surface under the action of the elastic restoring force of the threaded piece. In the process of repeated use, the rough position and edges and corners of the contact surface are ground and become smooth under the action of reciprocating friction. The friction coefficient becomes smaller, thereby reducing the maximum screw-out torque of the nut.
4. Manufacturing and assembly
Due to manufacturing technology limitations and accuracy, there are sharp corners on the thread edges or the dimensional coordination between parts is not coordinated. During the initial assembly, the screw-in and screw-out torque may have certain fluctuations or fluctuations, which require a certain number of running-in In order to obtain more accurate lock nut repeated use 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 repeated use characteristics of the lock nut. The larger the closing value, the greater the deformation of the thread when opening, the increased strain of the thread, the intensified strain cyclic hardening, and the larger the pressure FN of the thread, which has a tendency to increase the unscrewing torque; on the other hand, the width of the thread is reduced , The total area of the threaded piece decreases, the friction with the bolt decreases, the strain of the threaded piece increases, and the low-cycle fatigue performance decreases, which has a tendency to reduce the maximum screw-out torque. Under the combined action of many factors, the variation of the maximum screw-out torque with the number of repeated use is difficult to predict, and it can only be observed through experiments.
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