The Influence of Ball Surface Properties on the Friction and Wear Results of the Four-Ball Tribo-Test

Lubricity is critical for maintaining the reliability of mechanical systems. Consistent monitoring of in-service lubricants and the development of new formulations require reliable test equipment and standardised procedures. The four-ball tribo-test is among the most widely used and acknowledged methods. Like other methods, its effectiveness depends on the procedures and specimens used. Therefore, in this study, we aim to emphasise the influence of ball-specimen surface parameters on lubricity results, a factor that is often insufficiently acknowledged in reported experiments. Accordingly, lubricity tests of the same motor oil were performed using balls obtained from different suppliers. The experimental procedure was identical for all ball specimens. It was found that balls from different series exhibit distinct surface properties, leading to differences in lubricity even when the same lubricating oil was used. Based on the results obtained, it was concluded that the outcome is a multiparameter solution and not the result of a single surface parameter. On the other hand, if balls from the same series are used, the results will meet the standard requirements for repeatability regardless of their surface conditions.