Coefficient of Friction (Friction Coefficient)

Friction in bolted joints occurs at the threaded and head contact surfaces, and its coefficient of friction has a significant impact on the achievable preload force. Targeted friction control with defined coefficient of friction ranges, as well as suitable bolt locking elements such as NSK® washers or wedge washers, is essential for ensuring assembly quality and operational safety. Coefficients of friction below 0.05 are critical, as they can negate self-locking and promote spontaneous loosening of the joint; therefore, elements with undefined friction behavior should be avoided.

Friction is a mechanical resistance force that occurs at the contact surfaces of bolted joints and is influenced by adhesion, surface roughness and surface deformation. In bolted joints, a distinction is made between thread friction and head friction, both of which significantly influence the resulting preload force. The coefficient of friction (μ), a dimensionless parameter, describes the ratio of the frictional force to the normal force and is crucial for the design of assembly parameters. Excessively high friction coefficients lead to under-preload with an increased tendency to loosening, whilst excessively low friction coefficients can lead to over-preload, thread damage or fracture. Friction coefficients below 0.05 are critical, as these can negate the self-locking effect and allow the joint to loosen spontaneously.

Targeted control of the coefficient of friction requires defined friction coefficient ranges (e.g. 0.12–0.16) for the screw, nut and securing element, as well as appropriately selected lubricants where necessary.  Screw locking elements such as tension washers, wedge locks and NSK® washers can help to stabilise assembly conditions by specifically influencing head friction. Elements with undefined friction behaviour, such as spring rings or toothed washers, are not recommended for safety reasons; in addition, practical coefficient of friction tests taking all connection components into account are required.

Back to the glossary overview

Ask our AI assistant about screw locking devices