Experimental study of friction in aluminium bolted joints

This study aims at developing an experimental tool useful to define accurately the friction coefficients in bolted joints and, therefore, at relating precisely the tightening torque to the bolt preloading force in some structural components of front motorbike suspensions. The components under investigation are clamped joints made of aluminium alloy. The preloading force is achieved by applying a specific torque to the fastener, by means of a click-type torque wrench. Appropriate specimens have been designed by the authors in order to study the tribological aspects of the tightening phase. Then, experimental tests have been performed by applying the Design of Experiment (DOE) method in order to obtain a mathematical model for the friction coefficients. Three replicas of a full factorial DOE at two levels for each variable have been carried out. Levels include cast versus forged aluminium alloy, anodized versus spray-painted surface, lubricated versus unlubricated screw, and first tightening (fresh unspoiled surfaces) versus sixth tightening (spoiled surfaces). The study considers M8x1.25 8.8 galvanized screws.