Friction coefficient testing: From very small to M80

Friction is a force between objects in contact with each other. Friction suppresses the movement of the two objects against each other and can be seen as a counterforce to the movement. The friction coefficient (µ) indicates the ratio of the frictional force to the contact pressure between two objects. Friction pairs also occur in screw connections on the contact surfaces between the screw head and the bearing surface and between the screw thread and the nut thread.

Depending on the friction coefficient of the screw connection, up to 90% of the applied torque turns into friction and therefore mainly into heat. Only the remaining 10% is available for elongating the screw and thus generating the clamp force. As the prevailing clamp force within the screw connection cannot be measured directly in most applications, the screw connection is tightened to a specified torque. The clamp force resulting from the applied torque depends on the friction coefficient and decreases as the friction coefficient increases. Too little clamp force can lead to loosening or even complete release of the screw connection. Excessive clamp force, on the other hand, can lead to elongation of the screw and thus to permanent plastic deformation. The tensioned components can also be damaged if the clamp force is too high.

Screw connections are the backbone of many constructions and applications. It must always be ensured that the desired clamp force is achieved in the connection. As a result, systems with a set (i.e. known and specified) friction coefficient are required to ensure a stable assembly process. Friction coefficient testing enables the friction coefficient to be determined experimentally and thus guarantees the desired friction coefficient.

If the friction coefficient is unknown, the screw connections often have to be overdesigned to achieve the necessary safety in the design. On the other hand, if the friction coefficient is known, it may be possible to reduce the dimensions of the screw connection, which can fulfil, for example, the requirements of lightweight construction.