Investigation of the thread coefficient of friction when impact tightening bolted joints

The friction conditions in bolted joints are decisive for the quality of the tightening process and therefore for the quality of the joint. One of the many influences on the tribological systems in bolted joints is the tightening process, which in case of the impact tightening process has not yet been researched fully yet. This paper aims to investigate the differences in thread friction between manual and impact tightening during the tightening process and in relation to the thread friction during the loosening process. A full factorial study with the factors preload and tightening tool, with two factor levels each, was conducted. The measurements were analyzed qualitatively and a linear regression model was derived. The results show for the tests with the torque wrench as expected the approximatively same coefficient of friction (COF) for tightening and loosening of the bolted joint. Whereas for the impact wrench it is shown that the COF while tightening is in average 70% higher than the corresponding coefficient of friction while loosening. To depict the correlation of the tightening thread COF with the loosening thread COF a factor λ is introduced and is going to be investigated in further research.