Guide to choosing locknuts

What is a lock nut

Locknuts, also often referred to as self-locking nuts, are an agreed term for nuts with a locking function. In fact, in the relevant standards, the locking nut has a more "high class" scientific name - the effective torque type nut（Prevailing torque nuts）。

What is an effective torque locking nut?

A nut which, by virtue of its effective torque characteristics, cannot be turned freely ona the matching threads and which provides a degree of protection against turning back in addition to the clamping or compression forces. The effective torque generated by the nut is the torque required to rotate the nut on the matching threads without axial load.

When to use locknuts

Short clamping lengths

When the clamping length of the joint pair is short (Lk/d < 3), the elastic elongation of the bolt cannot compensate for the deformation of the clamped material. In this case, the clamping load of the coupling is reduced or even completely lost when using a normal nut. The following diagram shows an example of a nut with specification M8：

Nuts and combination materials assembly

Combined materials refer to products that are made of different materials with different properties combined together, such as the epdm washers shown in the picture below.

When ordinary nuts are used with combined materials, the actual assembly effect of the nut is affected by the hardness of the material, the surface treatment and the quantity of combined materials due to the different deformation variables of the different materials under force. If, in this case, there is also a high degree of vibration or a long-term load working environment, then the locking capacity achieved by the common nut is very limited and the locking nut assembly must be selected.

Assembly of nuts and soft materials

When soft materials such as rubber, wood, plastic, aluminium and copper are used as load-bearing parts in the assembly of a joint, locking nuts must be used. The surface of the soft material is subjected to large deformations, which do not provide an effective axial load on the joint and the joint cannot achieve and maintain a high level of clamping load. When mounting torques are applied to the joint, the soft material continues to deform plastically, which eventually leads to the failure of the normal nut.

Vibrating environment

The joint consisting of a common nut assembly is subjected to vibration for a long period of time, resulting in reduced clamping load and loosening of the joint. Locking nuts in the same working environment can effectively prevent loosening of the joint.

The significance of using locking nuts

It is unrealistic to require a common nut to achieve the required locking force without forcing the mounting torque to be set. Different installation tools and different installers can affect the assembly of a normal nut. Most of the connection pairs assembled in this case will loosen during use. According to analysis, 80% of the connection pairs will loosen due to a gradual weakening of the clamping load. This loosening, in turn, means a great safety risk, a production risk. The use of locknuts significantly reduces the risk of failure of the connection pair.