The coefficient of friction is a unit-less dimension that describes the relationship between pretension and torque.
It is determined by two factors: thread friction and head friction. Thread friction describes the characteristics that arise when bolt and nut threads meet. In contrast, head friction occurs where the lower side of the head of the bolt meets surface of the joint. The load pressure that affects the surfaces is the tension, which is also responsible for the elongation of the bolt and the corresponding spring effect. Torque, on the other hand, affects the surface of the thread and the contact area, it is divided into three partial torques: thread torque, head friction and the part translated directly into tension. This means: the tension ultimately achieved with prescribed torque is dependent on the coefficient of friction.