The task of screws and bolts is to join two or more components, actuated by adherence. In contrast to other fastening techniques such as riveting or welding, a connection established by screws or bolts can be loosened again at a later time point.
The decisive factor for the fastening outcome is the force with which the components are held together – so-called pretension. This should not be too great, as otherwise the components will be damaged and the bolt subjected to excessive stress. It should also not be too little, as the components will otherwise not be held together with sufficient force and will be able to move. The pretension of a screwed fastening cannot be measured directly, or only with comparatively great effort. A further key element of screwed fastenings is thetightening torque, the force acting on the bolt from the tightening movement. In comparison to pretension, tightening torque can be measured relatively easily using a torque wrench.
In addition, knowledge of coefficients of friction is elementary for the precise determination of pretension and tightening torque and therefore ultimately for establishing a secure connection. Similarly, trueness of gauge and setting behaviour also play an important role in this respect.