Contaminant and moisture analysis of bearing grease

When selecting a grease for high temperature applications, thermal stability, oxidation resistance and temperature limits must be considered. In non-relubricable applications, when operating temperatures are above 121°C, it is important to select a refined mineral oil or a stable synthetic oil as the base oil. Table 28. Grease Temperature Range Contaminants Abrasive Particles When rolling bearing models are operated in a clean environment, the main cause of bearing damage is fatigue of the rolling contact surfaces. However, when particulate contaminants enter the bearing system, it can cause damage such as galling, which can shorten the life of the bearing. When contaminants in the environment or metal burrs from certain components in an application contaminate the lubricant, wear becomes the primary cause of bearing damage. If, due to particulate contamination of the lubricant, bearing wear becomes significant, critical bearing dimensions can change, which can affect machine operation.
Bearings operating in contaminated lubricants will experience higher initial wear rates than those operating in non-contaminated lubricants. However, this rate of wear decreases quickly when the lubricant is no longer encroaching further because the contaminant shrinks in size as it passes through the bearing contact surface during normal operation. Moisture and moisture are important factors in bearing damage. Grease can provide a protective measure against such damage. Certain greases, such as calcium complex and aluminum complex greases, are extremely resistant to water. Sodium-based grease is soluble in water and therefore cannot be used in applications containing water. Whether it is dissolved water or suspended water in lubricating oil, it can have a fatal impact on the fatigue life of bearings. Water can corrode bearings, and corrosion can reduce bearing fatigue life. The exact mechanism by which water reduces fatigue life is not fully understood. But it has been suggested that water can get into the microcracks in the bearing raceways, which are caused by repeated cyclic stresses. This results in corrosion and hydrogen embrittlement of microcracks, greatly reducing the time required for these cracks to propagate to unacceptable cracking sizes. Water-based fluids such as water glycol and converted emulsions have also shown a reduction in bearing fatigue life. While the water from which it comes is not the same as contaminated water, the results support previous arguments about water contaminating lubricants.