Steel screws on aluminium nuts: Different engagement ratio tapped threads compared to threaded inserts with a proper tolerance choice

Higher engagement length over screw diameter ratios (ER), are recommended to avoid thread shearing off in applications, where high strength steel screws are tightened into nuts of materials with lower strength. The aim of this study was to determine the frictional coefficients of bolts for three different engagement ratios, to be compared to those of a commercial threaded insert, whose use is also suggested for the aforementioned application. The tests were run over five repeated tightenings, on M6 class 12.9, black oxidized screws in lubricated conditions. Retightening does not affect friction, whereas ER does and the insert exhibits the highest friction coefficient. The ER threshold to avoid thread stripping was also estimated by experimental, numerical and analytical methods.