Coefficients of friction
The coefficient of friction is a parameter with no unit of measurement that describes the relationship of preload force and torque. It is determined by two factors: the characteristics of the pairing of screw thread and nut thread (thread friction) and of the pairing of head bottom and head contact area on the nut (head friction). The loading pressure exerted on the surfaces is the preload force, which is also responsible for the screw’s elongation and the corresponding spring effect. The torque in turn impacts on the thread surface and contact area and splits into three sub-torques: thread friction, head friction and the element that gets implemented directly in preload force. That means: given a specified level of torque, the preload force ultimately achieved depends on the coefficient of friction.
Ensuring consistent coefficients of friction
For automatic screw connections, i.e. for industrial manufacturing, in particular, the setting of consistent coefficients of friction is especially important. That’s because in such processes the screw or bolt gets turned by machine until the pre-set torque is reached. Any fluctuations in the coefficients would result in levels of preload force higher or lower than desired, which can impair the operational safety of the manufactured pieces/components.
Numerous factors to be taken into account
In order to be able to create a consistent and reproducible range for a threaded connection’s coefficient of friction, various factors need to be taken into account. First, it is important to know that the nature of the materials / of the material pairing to be worked on has a major influence on the coefficients of friction – for example, steel/steel or steel/aluminium. The surface finish is also a very key factor. Is the material soft, hard, roughened, bare metal or coated? Do the paired surfaces fit precisely into each other or is there any unevenness due to the manufacturing process? Are any lubricants such as grease or oil being used? What size are the screws? What shape is the thread? How deep are they to be screwed in?
Using high-performance coating systems it is possible to adjust the coefficient of friction accordingly to a given specification and to set it individually within a consistent range.
Detailed information on determining and on methods of calculating the coefficients of friction of fasteners is contained in DIN EN ISO 16047, 2005-10 and VDA 235-203.