Influence of Seating Conditions
Basically, all described tightening processes treat only the torque or pre-load force that occurs at the time of the screw-tightening. A settling process in the screw-connection actually leads, as shown in the tension picture D (Basics Screwdriving Technology)in basically all cases to a reduction of the remaining torque and/or pre-load forces.
Among other things, these settling conditions occur during the assembly of soft materials, such as during the assembly of plastic or if a seal is present between two parts that need to be assembled.
An Example: a silicone flat-gasket is tightened to a pump with four screws. The Result: the Pump will leak.
This situation often leads to problems for the Quality control of screwdriving equipment. Only in rare cases is it possible to predict occurrences. In most all cases, those occurrences have to be determined with extensive test done on the original parts. Here, DEPRAG can support your efforts with a comprehensive screw-joint analysis. The result of such a screw-joint analysis indicates the most-suitable torque and ideal tightening-process that should be applied. Settling conditions are naturally taken into consideration.
A multitude of screw-connections has shown that the majority of the settling occurs during the first millisecond following the tightening.
This means, that an improvement can be achieved in some cases by lowering the driver speed, since the tightening process is lengthened and the bulk of the settling has already occurred during tightening.
If this measure is not sufficient, there is also the possibility of a multi-stage tightening process. In such a case, it is possible to tighten the fastener to about 80% of the final torque, then apply a short waiting period and consequently tighten the fastener to the required final torque. It is practically possible to use any combination of pre-torque, stand-by, loosening and renewed tightening of any connection.