Self-loosening of fastened systems is known as a severe damage mechanism besides the loss of preload due to relaxation and as a result of this the failure of the joint. Main problem is, that self-loosening mostly leads to a rapid preload loss when occurs.
In the 1970s first systematic investigations due to this are reported from Junker et al. using a transverse-vibrational test stand . Nowadays numeric calculation approaches are available (e.g. ). All approaches show that very small displacements from loading before complete sliding are sufficient to induce self-loosening — which is a screw rotation against its tightening direction without material rupture of the screw.
Today the mechanism of self-loosening under uni-axial transverse load of single-screwed joints with plain bearing surface is understood and predictable. Also combined loads of vibration and rotation became the focus of university-based research. The challenge is to transfer the knowledge about the mechanism to component fastened systems with multi-screw joints. The fundamental mechanism is not sufficient for component design. This is the reason why up to now no prediction in advance is established in guidelines.
First, this paper shows the time-sequence of self-loosening in general with its different stages. The second step is to work out important influences on self-loosening which will be shown by existing calculations. Then a stress-based calculation and a better criterion for self-loosening will be developed.
Next step, if analytics come to their limits, is the numeric simulation of system behavior. With this the critical preload for self-loosening must be determined to ensure safety of the connection. Following from this screw joints can be dimensioned without risk of self-loosening. The simulation procedure includes right modelling with boundary conditions as well as defining evaluation procedure for a ‘self-loosening-safety-margin’. With simulation of a single-screw joint it gets clear that either analytical or numerical approach can be used. But already for two screws in a fastened system the limits of known equations become obvious.
Finally, results of a vibrational test are shown before conclusions and outlook.