The maximum screw-out torque of the locking nut is affected by many factors.
For the study of the low-cycle fatigue performance of the locking nut, the thread diameter, helix angle and tooth bevel angle remain unchanged, and only the maximum elastic recovery force FNmax and equivalent friction angle ρe of the thread piece will change to a certain extent after repeated use. Therefore, it is only necessary to analyze the change law of the maximum screw-out torque of the locking nut when it is subjected to cyclic load from these two aspects.
1.Material strain hardening When the material is cyclically loaded, “cyclic strain hardening” or “cyclic strain softening” will occur, 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 stable state. The low-cycle fatigue of the locking nut is carried out under the condition of constant strain. The strain hardening or softening of the thread piece will affect the size of its maximum screw-out torque. The alloy steel used to manufacture the locking nut belongs to the cyclic strain hardening material. The hardening of the material will increase the elastic recovery force FN of the thread piece and increase the screw-out torque.
2.Low-cycle fatigue Low-cycle fatigue refers to fatigue stress approaching or exceeding the yield limit of the material. The material has a certain amount of plastic deformation in each strain cycle. The service 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 locking nut, the stress at the root of the thread piece is large, and some areas of the surface are in a yield state, while the strain in the central area of the root of the thread piece is very small, and the strain situation is relatively complex. The area with high strain at the root of the thread piece undergoes reciprocating loading, which is prone to low-cycle fatigue, reducing the pressure of the thread piece and the screw-out torque.
3.Friction coefficient The friction angle is an important factor affecting the screw-out torque, and the existence of friction is the basis for the normal operation of the locking nut.
When the locking nut is working, the contact surface has pressure and friction under the elastic restoring force of the thread piece. During repeated use, the rough positions and edges of the contact surface are smoothed and smoothed under the action of reciprocating friction, and 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 precision, there are sharp corners on the edge of the thread or the size matching between parts is not coordinated. During the initial assembly, the screw-in and screw-out torque may fluctuate to a certain extent, and a certain number of running-in times are required to obtain a more accurate lock nut reuse characteristic.
5.After the closing value material and the geometric parameters of the nut are determined, the change of the closing value has an important influence on the reuse characteristics of the lock nut.
As the closing value increases, the deformation of the thread sheet increases when it is opened, the strain of the thread sheet increases, the strain cycle hardening phenomenon intensifies, and the thread sheet pressure FN increases, which has a tendency to increase the screw-out torque; on the other hand, the width of the thread sheet decreases, the total area of the thread sheet decreases, the friction with the bolt decreases, the strain of the thread sheet increases, and the low-cycle fatigue performance decreases, which has a tendency to reduce the maximum screw-out torque.
Under the combined effect of multiple factors, the change of the maximum screw-out torque with the number of repeated uses is difficult to predict, and it can only be observed through experiments.