Tightening procedures in screwdriving technology
The reliable assembly of fasteners
Screwdriving technology is the key to numerous assembly tasks. Screw fastenings are popular due to their resilience, part reusability and because of the option of undoing the connection at any time without damaging parts. Therefore, screws are the most frequently utilised standardised machine elements and are available in a wide range of designs.
The most important task for assembly fastening involves the application of a defined pre-load force. The pre-load force must be determined so that on one hand, the intended function can still be executed despite any applied operating force, and on the other hand, that the permissible load on the screw connection is not exceeded. It is the frequently unknown seating conditions of the screw connection which can be particularly problematic, as well as any assembly-related fluctuations of the pre-load force.
Series screw assembly does not allow effective determination of the required preload force. Therefore, as an alternative, indirect measurements must be used to control the assembly process. Generally, this means the tightening torque of the screw assembly. Additionally, other experimentally determined parameters such as angle, screw-in time, friction etc., can act as reference variables for the assembly process. Furthermore, innovative procedures have been developed for the detection of the head seating point in order to improve the constancy of the pre-load force.
Torque-controlled tightening procedure
Within screwdriving technology, torque is still used most frequently as the control parameter. The pre-load force resulting from the tightening torque is significantly influenced by the fluctuating coefficient of friction as well as the screwdriving device’s torque spread. In particular one must differentiate between head friction and thread friction. The sum of these fluctuating frictional conditions results in fluctuations of the pre-load force of up to 50% or more, despite high torque repeatability. This means that the screw connection always needs to be so over-dimensioned that it is not overtightened if a deviation in the upper range occurs and that the required pre-load force is still applied in the case of a lower deviation. “Despite these drawbacks, torque-controlled tightening has emerged as the most popular tightening procedure. This is because technical realisation is relatively simple.”, explains Daniel Guttenberger, Deprag screwdriving Technology Product Manager.
Angle-controlled tightening procedure
During a so-called angle procedure, both the torque and angle of the screw connection are used as control parameters. In this case, it is the angle and not the torque which is used as the control parameter for the final tightening stage. This means that the screw is tightened to a specific threshold torque and tightening continues from there at a predetermined post-tension angle. The torque can be monitored as an additional control parameter.
More information: Deprag schulz gmbh u. CO.
