Grease
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- For the musical play, see Grease (musical). For the 1978 movie, see Grease (movie).
Grease is a lubricant of higher initial viscosity than oil, consisting originally of a calcium, sodium or lithium soap jelly emulsified with mineral oil. Greases are a type of shear thinning or pseudo-plastic fluid, which means that the viscosity of the fluid is reduced under shear. After sufficient force to shear the grease has been applied, the viscocity drops and approaches that of the base mineral oil (or that of the EP additive for EP greases under heavy load). This sudden drop in shear force means that grease is considered a plastic fluid, and the reduction of shear force with time makes it thixotropic.
Greases are employed where heavy pressures exist, where oil drip from the bearings is undesirable, and/or where the motions of the contacting surfaces is discontinuous so that it is difficult to maintain a separating lubricant 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 greases are labeled "EP", which indicates "extreme pressure". Under high pressure or shock loading, normal grease can be compressed to the extent that the greased parts come into physical contact, causing friction and wear. EP grease contains solid lubricants, usually graphite and/or moly, to provide protection under heavy loadings. The solid lubricants bond to the surface of the metal, and prevent metal to metal contact and the resulting friction and wear when the lubricant film gets too thin.
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.