Influencing Factors On Maximum Unscrewing Torque Of Locking Nuts

The maximum unscrewing torque of locking nuts is affected by a variety of factors. In the research on the low-cycle fatigue performance of locking nuts, the thread pitch diameter, helix angle and thread flank angle remain constant. Only the maximum elastic restoring force of the thread segment and the equivalent friction angle will change to a certain extent after repeated use. Therefore, the variation rule of the maximum unscrewing torque of locking nuts under cyclic loads can be analyzed from these two aspects.

1. Material Strain Hardening

Materials exhibit cyclic strain hardening or cyclic strain softening under cyclic loading. Under constant-amplitude cyclic strain, the stress amplitude increases or decreases with the growth of cycle times and finally reaches a stable cyclic state. The low-cycle fatigue test of locking nuts is carried out under constant strain conditions. The strain hardening or softening of thread segments directly affects the maximum unscrewing torque. Alloy steel used for manufacturing locking nuts is a typical cyclic strain hardening material. Material hardening increases the elastic restoring force of thread segments and further raises the unscrewing torque.

2. Low-Cycle Fatigue

Low-cycle fatigue refers to the fatigue state where the cyclic stress is close to or exceeds the yield limit of the material. The material produces a certain amount of plastic deformation in each strain cycle, with a fatigue life ranging from 10² to tens of thousands of cycles, which is generally described by the ε-N fatigue curve.

Finite element calculation results show that after the bolt is screwed into the locking nut, the root of the thread segment bears high stress, and partial surface areas are in a yield state, while the central area of the thread root presents low and complex strain characteristics. The high-strain area at the thread root undergoes repeated cyclic loading, which is prone to low-cycle fatigue damage. This reduces the bearing pressure of thread segments and leads to the decrease of unscrewing torque.

3. Friction Coefficient

Friction angle is a critical factor affecting unscrewing torque, and friction force is the foundation for the normal operation of locking nuts. During operation, the contact surface maintains continuous pressure and friction force under the elastic restoring force of thread segments. After repeated use, the cyclic friction continuously polishes the microscopic sharp edges and rough protrusions on the contact surface, making the surface smoother. The reduction of friction coefficient lowers the friction resistance, thereby decreasing the maximum unscrewing torque of the nut.

4. Manufacturing and Assembly Factors

Restricted by manufacturing process and precision, thread edges may have sharp corners, burrs and mismatched dimensional fit between parts. The initial screwing and unscrewing torque will fluctuate abnormally. Stable and accurate reusable performance of locking nuts can only be obtained after a certain number of running-in cycles.

5. Closing Deformation Value

After the material and geometric parameters of the nut are determined, the closing deformation value exerts a significant influence on the reusability of locking nuts. A larger closing value increases the opening deformation and strain of thread segments, intensifies cyclic strain hardening, improves the elastic pressure of thread segments, and tends to increase the unscrewing torque.

On the other hand, excessive closing deformation reduces the width and effective contact area of thread segments, decreases friction force, increases thread strain, deteriorates low-cycle fatigue performance, and tends to reduce the maximum unscrewing torque. Affected by the superposition of multiple contradictory factors, the variation of maximum unscrewing torque with repeated service cycles cannot be predicted simply by theoretical analysis and can only be verified through experimental observation.