Experimental Determination of the Coefficient of Friction on a Screw Joint

Abstract

This paper deals with the coefficient of determination of screw connection friction between the thread and the matrix. The coefficient of friction was measured using a laboratory device with an M20 screw connection without any grease and, subsequently, plastic grease was added (CX80 silicone, lithium, and copper grease). When grease is added, the friction in the threads and screw heads is limited and consistently retained. When tightening by torque, which represents the prevailing assembly method in standard screwing practice, only part of the torque is effectively used to create axial force and pre-stress. The rest of the torque is employed in friction suppression between threads and converted into heat. In general, the coefficient of friction depends on diverse factors such as the roughness of the thread surface, the gradient angle of the helix, and the grease properties. The tightening torque represents a primary parameter in the experimental measurements, monitored using a digital torque spanner, and generates an axial force in the screw. Based on the aforementioned parameters, the objective of this paper was to monitor changes in the coefficient of friction between the thread of the screw and the matrix in the case of different grease types. The actual coefficient of friction was calculated through the exponential equation of the torque balance. First measured was the load of the bolted joint without the use of grease, where the average value of the coefficient of friction was 0.44732; this value served as a benchmark for comparison to the measurements with the use of grease. The measurements showed that the value of the friction coefficient was reduced by 30.57% when using lithium grease, by 40.56% when using silicone grease, and by 47.64% when using copper-based grease, making the latter the most suitable for the application. Without appropriate greasing, friction suppression was accompanied by extremely high torques, which resulted in insufficient screw prolongation.