Grease is a lubricant of higher viscosity than oil, consisting originally of a calcium, sodium or lithium soap jelly emulsified with mineral oil. Greases are employed where heavy pressures exist, where oil drip from the bearingss is undesirable, and where the motions of the contacting surfaces is discontinuous so that it is difficult to maintain a separating film in the bearing. Grease-lubricated bearings have greater frictional characteristics at the beginning of operation, causing a temperature rise which tends to melt the grease and give the effect of an oil-lubricated bearing. Calcium and sodium base greases are the most commonly used; sodium base greases have higher melting point than calcium base greases but are not resistant to the action of water. Lithium based grease has a drip temperature at 350° to 400°F and it resists moisture hence it is commonly used as lubricant in household products such as garage door openers. Graphite, either by itself or mixed with grease, is also employed as a lubricant. Teflon is added to some greases to improve on the lubricating property. Gear greases consist of rosin oil, thickened with lime and mixed with mineral oil, with some percentage of water. Special purpose greases contain glycerol and sorbitan esters. They are used, for example, in low-temperature conditions.
Some silicone-based lubricants are also marketed as grease. Whether they can be classified as grease in the regular sense is uncertain. Silicone grease is an amorphous fumed, silica thickened, polysiloxane-based compound.
Some rendered animal fats are known as greases. Rendered chicken fat becomes the commodity known as yellow grease. Animal greases may have been used as lubricants in the past, but this is not now common in developed nations.
For the musical play Grease, see Grease (musical).
