Tightening Torque for Locking Nuts: Should Locking Torque Be Included?

Locking nuts play a critical role in various industrial applications, particularly in fields such as clean energy, diesel engines, rail locomotives, and steam turbines. Among the considerations surrounding locking nuts, the tightening torque is of particular importance. When designing the tightening torque for a locking nut, the question arises: Should the locking torque be included in the calculation?

Classification of Locking Nuts

Locking nuts come in a variety of types, including all-metal locking nuts, non-metallic insert locking nuts, and metal clip locking nuts. These types differ in their design, with all-metal locking nuts using methods such as end-face compression at three points or opposite-side flattening to achieve various locking characteristics.

Considerations for Tightening Torque of Locking Nuts

When designing the tightening torque for a locking nut, several factors must be considered. In addition to the normal thread tightening torque (MG) and end-face tightening torque (MK), the thread rotation moment (MU) and the resistance torque from any end-face attachments (MKzu) must also be taken into account. Notably, for high pre-tightening force connections, the VDI 2230 standard suggests that the thread rotation moment (MU) can be ignored, provided that the bolt is tightened to high pre-tightening force.

Measured Friction Coefficient of Locking Nuts

The friction coefficient is a critical parameter in the design of locking nuts, directly affecting the final pre-tightening force. For example, in nylon insert locking nuts, experimental results show that the friction coefficient changes as the tightening angle increases. When the locking nut is tightened to a certain level, both the thread friction coefficient and the total friction coefficient decrease as the axial force increases.

Considerations for Insertion Torque of Locking Nuts

When designing the tightening torque for a locking nut, it is also important to consider the impact of insertion torque. From actual test results, it is evident that if the designed pre-tightening force reaches at least 40% of the guaranteed load, there is typically no need to further account for the insertion torque. This aligns with the recommendations of the VDI 2230 standard for high pre-tightening forces.

Special Considerations for Insertion Torque in Certain Areas

In specific applications, such as the piston rod of a shock absorber and the mounting nut, care must be taken to limit the tightening torque. This is due to the small outer diameter of the piston rod, which provides limited contact area. To avoid causing crushing or permanent plastic deformation of the mount, the tightening torque must be restricted while still meeting the requirements.

In conclusion, the design of a locking nut requires a comprehensive consideration of factors such as thread friction coefficient, tightening torque, and insertion torque. For high pre-tightening forces, the pre-tightening force should be designed to meet at least 40% of the guaranteed load, as suggested by the VDI 2230 standard. However, for locking nuts with teeth, additional torque should be considered in all cases. Therefore, experience and accumulated knowledge are valuable references when designing locking nuts, ensuring that the desired locking effect is achieved reliably and consistently under various working conditions.