A lubricating grease is usually a mixture of 85 to 90 percent mineral oil or synthetic oil together with a thickener. In a majority of all greases, thickener is a metallic soap. One example is lithium stearate for lithium soap.
The function of thickener, metallic soap, is to hold lubricating oil in a semi-liquid state for easier handling.
When there is rise in temperature, oil bleeds out from thickener and functions as a lubricating agent. When temperature drops again, thickener soaks up oil again to become semi-solid once more.
The type of grease chosen for a particular bearing lubrication application must therefore be chosen very carefully. High temperature grease used in low temperature applications may cause bearings to seize due to lack of lubrication because oil does not bleed out. The common types of grease in use for rolling contact bearings are calcium, sodium and lithium greases.
Calcium Soap Greases
These do not dissolve in water. They are recommended for installations exposed to water at temperatures below 60 degree C. They offer good protection against salt water in marine environments.
Sodium Soap Greases
Also called soda greases, they may be utilized over a wide range of temperatures up to 120 degree C. However, if too much water penetrates into bearings, there is a risk that grease will be washed out and lubricating properties become deteriorated.
Lithium Soap Greases
These have excellent resistance to high temperatures. They can also be used over a wider range of temperatures from -50 to 150 degree C. They are not water soluble.
Additives are also added to some greases to improve their properties. Some examples of these additives are anti-rust, anti-oxidants, extreme pressure additives, and stabilizers.