Experimental analysis of fatigue failures (spalling) in grease lubricated contacts

Abstract

Abstract In practical situations, a large proportion (about 80%) of all rolling bearing are lubricated by some sort of grease. Usually, a rolling bearing fails by fatigue, when a spall is formed on the contact surface. Spalling is prone to appear in heavily loaded EHL contacts and its presence depends essentially on the Hertzian pressure, thus the maximum shear stress. The depth of the cracks is similar to the depth of maximum shear stress. Crack initiation is also dependent on the lubrication conditions, namely on the specific film thickness Λ . Values of Λ greater than 3 virtually eliminate spalling, whereas values closer to 1 (or even smaller) tend to promote it. The concept of specific film thickness or lubricant parameter Λ , however, has a limited application if one is dealing with grease lubrication. Prediction of fatigue, in general, and spalling in particular, should be handled with cautions. The main purpose of this paper is to present an experimental analysis of fatigue failures, mainly spalling, when lubricating greases were used in a pure rolling contact and to compare them with the base oils in the same conditions. The tests were performed in a four-ball machine and in a twin disc machine. A discussion of the morphological analysis of the fatigue failures is done, comparing the results obtained for grease and base oil lubricated contacts.