The paper aims to contribute to the understanding of scuffing occurrence. The evolution of surface topography during four balls tests using two types of grease was investigated. For testing, a four balls experimental setup was used. The test rig allows loading the contact between balls while maintaining relative motion between surfaces. Constant velocity is maintained for 10 seconds as per the EN ISO 20623:2003 procedure. The wear scar produced during testing was investigated by means of an optical 3D profilometer and the roughness parameters were analysed. Experimental results reveal that the values of arithmetical mean height Sa and root mean square height Sq, have a significant growth and they are correlated with the occurrence of the scuffing phenomenon.

The paper presents some typical results regarding the evolution of friction torque during the four balls scuffing tests. The tests were conducted in 10 seconds running periods having 100 N loading steps, according to ISO 20623:2017. For testing, 12.7 mm bearing balls manufactured by SKF were used and as lubricant, U90Ca3 grease was used. A special device was conceived and built to evaluate the friction torque between balls using a force transducer mounted on the testing machine and a data acquisition module. A correlation between the loading, wear scar dimensions and friction torque was obtained. The results can be used to determine the precursory moment of scuffing initiation in four ball test using different lubricants. Some remarks concerning the working parameters inter-correlation during tests were also made.