Contamination and moisture analysis of bearing grease

When selecting grease for high temperature applications, thermal stability, oxidation resistance and temperature limit must be considered. In non re lubricating applications, 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 Grease temperature range pollutants abrasive particles when the rolling bearing model operates in a clean environment, the main reason for bearing damage is the fatigue of the rolling contact surface. However, when particulate pollutants enter the bearing system, they will cause damage such as scratch, which will shorten the service life of the bearing. When the lubricant is polluted by pollutants in the environment or metal burrs of some parts in the application, wear will become the main cause of bearing damage. If the bearing wear becomes significant due to the particulate contamination of the lubricant, the key bearing dimensions will change, which will affect the operation of the machine.
The initial wear rate of bearing running in contaminated lubricant is higher than that running in non contaminated lubricant. However, this wear rate will decrease rapidly when the lubricant does not invade further, because the size of the pollutant will be reduced when the pollutant passes through the bearing contact surface during normal operation. Moisture and moisture are important factors leading to bearing damage. Grease can provide a protective measure against such damage. Some greases, such as compound calcium base and compound aluminum base greases, have extremely high water resistance. Sodium base grease is soluble in water, so it cannot be used in water containing applications. Both dissolved water and suspended water in lubricating oil can have a fatal impact on the fatigue life of bearings. Water can corrode the bearing, and corrosion will reduce the fatigue life of the bearing. The exact mechanism of water reducing fatigue life has not been fully understood. However, it has been suggested that water will enter the microcracks on the bearing raceway, which are caused by repeated cyclic stress. This leads to corrosion and hydrogen embrittlement of microcracks, greatly reducing the time required for these cracks to expand to unacceptable crack sizes. 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 different from the polluted water, the results support the previous argument that water pollutes lubricants.