When selecting grease for high temperature applications, thermal stability, oxidation resistance and temperature limits must be considered. In applications that cannot be relubricated, when the operating temperature is above 121°C, it is very important to choose refined mineral oil or synthetic oil with stable performance as the base oil. Table 28. Lubricating Grease Temperature Range Pollutant Abrasive Particles When the rolling bearing model is operated in a clean environment, the main cause of bearing damage is the fatigue of the rolling contact surface. However, when particulate contamination enters the bearing system, it will cause damage such as scratches, which will shorten the life of the bearing. When contaminants in the environment or metal burrs of certain parts in the application contaminate the lubricant, wear will become the main cause of bearing damage. If, due to particulate contamination of the lubricant, bearing wear becomes significant, critical bearing dimensions will change, which will affect the operation of the machine. When a bearing is operated in contaminated lubricant, its initial wear rate is higher than that of a non-contaminated lubricant. However, when the lubricating substance does not enter further, this wear rate will quickly decrease, because the size of the contaminant will shrink when the contaminant passes through the bearing to contact the surface during normal operation. Moisture and humidity are important factors that cause bearing damage. Grease can provide a protective measure against such damage. Certain greases, such as complex calcium-based and complex aluminum-based greases, have extremely high water resistance. Sodium-based grease is soluble in water, so it cannot be used in water-containing applications. Both the dissolved water and the suspended water in the lubricating oil can have a fatal effect on the fatigue life of the bearing. Water can corrode the bearing, and corrosion will reduce the fatigue life of the bearing. The exact mechanism by which water can reduce fatigue life is not yet fully understood. However, some people have proposed that water will enter the micro-cracks on the bearing raceway, and the micro-cracks are caused by repeated cyclic stress. This can lead to corrosion and hydrogen embrittlement of micro-cracks, and the time required for these cracks to propagate to unacceptable crack sizes is greatly reduced. Water-based fluids such as water glycol and converted emulsions have also shown a reduction in bearing fatigue life. Although the water from it is not the same as the polluted water, the results support the previous argument about water-contaminated lubricants.