Comparative analysis of fatigue failures in rolling contacts lubricated with a grease and a base oil

Grease lubrication is applied to almost every situation, especially for rolling bearing lubrication. In practical situations, a large proportion (more then 80%) of all rolling bearings are lubricated by some sort of grease. Usually, a rolling bearing fails by fatigue, when a spall is formed and it comes out of the contact surface. As it is well known, spalling is prone to appear in heavily loaded EHL contacts. Its presence depends essentially on the Hertzian pressure, thus the maximum shear stress. Also, the spalling phenomena are accelerated by the presence of an indent in the contact surfaces that can be caused by the passage of hard particles in the contact. The main objective of this work is the experimental evaluation and the surface analysis of fatigue failures, mainly spalling and micro pitting, when lubricating greases were used in a pure rolling contact and to compare them with the base oils in the same conditions. The experimental work is performed with a twin disc machine, adapted for grease lubrication. The procedures are described and the results obtained with one base oil and one grease are shown. The operating conditions in the test apparatus resembled those present in a ball bearing.