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Although it is very convenient to use grease for lubrication of rolling contact bearings, where some bearings come pre-packed with grease ready for use, grease lubrication becomes unsuitable if operating temperature becomes high.
The high temperature may be because of high ambient temperature environment, or because heat evolved in bearing due to friction from high speed or heavy loading. Sometimes use of oil becomes more logical if lubricating intervals for grease lubrication becomes too short, perhaps due to leakage from seals.
In oil lubrication, heat generated from bearings are able to be transfered to a larger volume of oil which in turn can be pumped through heat exchangers for cooling. In this way, oil functions both as a lubricating agent as well as a cooling agent.
Oils can also have additives to improve their properties. Some examples are anti-oxidants, corrosion protection additives, anti-foaming additives, surface tension additives, wetting agents, and extreme pressure additives. These additives are put in according to application of oil.
Compared to greases, oils can enable bearings to be operated at a wider range of temperatures. However, there are limits to this, especially at higher temperatures. At high temperatures of 90 degree C and above, mineral oils oxidize rapidly and they lose their properties to lubricate. Synthetic oils are increasingly being used for higher temperature applications.
Which to use? Oil or grease? The choice is yours!
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