A study on lubrication mechanism and wear scar in sliding circular contacts

Abstract

The effects of test method, normal load and sliding speed on the lubricating characteristics of a paraffinic base oil were experimentally investigated using a four-ball tester with a contact resistance instrument. Results show that at light load, elastohydrodynamic lubrication can only be found in the specific speed method, but a little wear scar still existed in the standard test method because of the contact initially. The specific speed method has a higher contact resistance than those of the standard test method at light load, but this is not always true at high load. According to the measured data of friction coefficient, contact resistance, and wear scar, and the observation of the wear surface, the lubrication mode can be clearly classified into four regimes under the various normal loads and sliding speeds. The transition curve between boundary lubrication and initial seizure makes no significant difference for both specific speed and standard test methods. This result shows that the initial seizure is significantly influenced by the load and sliding speed rather than by the test method. In the partial elastohydrodynamic lubrication (P-EHL) regime, the relationship between the wear scar diameter D and the contact resistance Rc has been quantitatively and qualitatively obtained. The diameter of the wear scar can be predicted by measuring the contact resistance Rc using this empirical equation. The transition curve between the P-EHL and elastohydrodynamic lubrication is in very good agreement with the prediction using the minimum film thickness obtained by Hsu and Lcc with the assumption of Λ = 3.