The most important application for bolting in assembly technology is the application of a defined preload force. The preload force must be determined in such a way that, on the one hand, the intended function is still given at every possible operating force and, on the other hand, the permissible load on the screw connection is not exceeded.

In series production, it is difficult to determine the preload force achieved. Indirect measured variables must therefore be used to control the assembly process. As a rule, this is the tightening torque of the screw connection. In addition, other specifications such as angle of rotation, screw-in times or friction values can also be determined experimentally, which serve as reference variables for the assembly process. Furthermore, methods for detecting the screw head contact have been developed in order to improve the consistency of the preload force.

In addition to the torque-controlled and angle-controlled tightening process, the friction value-dependent torque bolting and the yield point-controlled tightening process, Deprag has developed the adaptive tightening process CFC Clamp Force Control. This tightening method ensures that the preload force remains constant even with fluctuating insertion torques. A complete screw connection consists of the head contact detection screw pattern and a screw connection based on differential torque or a screw connection based on angle of rotation. The main component is head contact detection. For this purpose, a mathematical evaluation function is continuously formed on the basis of the torque curve. If the trend line calculated in this way exceeds a defined limit value, the head support is considered to have been detected - the torque and angle of rotation are saved at the time of the head support.

The calculation method combines two advantages: It is robust against random fluctuations and increases in the torque curve, which are not caused by the actual head support, and the algorithm is generally valid, so that the user does not have to set any parameters relevant for the calculation.

The screwdriving process is used when there are variable boundary conditions that lead to widely varying insertion torques. "Fluctuations result, for example, from changes in the screw and/or hole geometry, the structure of the component material, changing surface conditions of the screw thread, springy elements or settling phenomena," says Daniel Guttenberger, Product Manager Deprag Schraubtechnik. By detecting the head contact, Clamp Force Control ensures a uniform initial state for the subsequent final tightening, which ultimately results in a significantly better consistency of the preload force. Typical application examples are direct screw connections in plastic or metal. "Depending on the application and changing boundary conditions, a suitable tightening method can be determined for each fastening case, thereby improving the consistency of the preload force," adds Guttenberger. as