Research on the relationship between thermal aging of lubricating grease outside the contact area of bearing rolling elements and bearing lubrication failure

Abstract

This study examines the lubrication and tribological performance of lithium-based grease subjected to varying degrees of thermal aging. The paper elucidates the degradation in anti-oxidant, rheological, and tribological performance of the grease, shedding light on the lubrication failure mechanism of lithium-based grease under thermal influence. At the molecular level, lithium-based greases undergo chemical degradation due to the thermal oxidation of their base oil. This process produces macromolecular polymers and oxygenated organic substances that accumulate as the degree of aging increases, progressively reducing the original lubrication efficiency and chemical stability of the grease. From a microstructural viewpoint, persistent thermal effects lead to a gradual reduction in the helical structure of the ribbon fibers within the thickener. This leads to the increased fiber diameter and its breakage, reduced binding capacity of the base oil, heightened oil separation, and a continuous decrease in structural strength and recovery capability. The rapid evaporation of the base oil reduces the grease oil storage capacity. As the aging process advances, aged grease soap fibers fragment and re-aggregate into bundles. This aggregated thickener structure hardens the remaining grease, and reduces its oil supply capacity. The friction pair surface is not adequately replenished with oil, which leads to direct contact at the interface and causes the surface contact fatigue damage. The friction process generates more wear products, which in turn cause the abnormal vibrations and noise. The combined degradation of grease performance and the deterioration of the friction pair surface conditions exacerbate bearing vibration failure.