Doerken MKS, specialists in coating and anti-corrosion protection, are experts in the fastener field. You can visit the Doerken MKS US team at Booth 401 during the 2020 Fastener Fair USA show at the Charlotte Convention Center,  July 13-15. Also invaluable is the Doerken Knowledge Base from which this article is drawn:

Every day, thousands upon thousands of screws and bolts are used to fasten components together. That may sound routine initially, however, specialists also must take a closer look. Because if bolts and screws are too loose, components may move. If they are fastened too tightly, components and the fasteners themselves may be damaged.

The objective is to fix components to each other in a way that enables them to fulfil their function. When fastening, the torque must be correct in order to achieve the required clamp force (i.e. the force that a fastener uses to hold two components together). This is not always easy. In practice, tightening instructions often require assembly of fasteners to a specified torque value. However, the resultant clamp force is dependent on several factors. These include the geometry of the screw and the friction between the fastener and components.  

Surfaces often have a major influence on this. The friction between rougher surfaces is greater than between smooth ones. If fasteners are lubricated, friction is reduced.  This applies to both external lubrication via grease or wax and integrated lubrication via the addition of a lubricant to the coating formulation. Friction effects such as those between fastener and component are detailed with the total coefficient of friction (µtot).

This comprises the friction in the thread (µth) as well as that between the head and head contact surface (µb). A very low total coefficient of friction runs the risk of the fastener working itself loose. If the factor is very high, fasteners need to be tightened with a high torque and the efficiency of the fasteners decreases.

In order to achieve the desired clamp force with a specified torque, the tightening instructions must assume a total coefficient of friction of the fasteners. Industrial companies and associations often specify a coefficient of friction window for this. Read more at https://www.doerkenusa.com/knowledge-base/glossary/screw-connections/