Grease Lubrication Review
Grease is a semifluid to solid mixture of a
fluid lubricant, a thickener, and additives. The fluid
lubricant that performs the actual lubrication can be
petroleum (mineral) oil, synthetic oil, or vegetable oil.
The thickener gives grease its characteristic consistency
and is sometimes thought of as a three-dimensional
fibrous network or sponge that holds the oil in
place. Common thickeners are soaps and organic or inorganic
A true grease consists of an oil and/or other fluid lubricant that is mixed with a thickener, typically a soap, to form a solid or semisolid. 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 viscosity drops and approaches that of the base lubricant, such as the mineral oil. 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. It is often applied using a grease gun, which applies the grease to the part being lubricated under pressure, forcing the solid grease into the spaces in the part.
The majority of greases on the market
are composed of mineral oil blended with a soap thickener.
Additives enhance performance and protect the grease and
lubricated surfaces. Grease has been described as a
temperature-regulated feeding device: when the lubricant
film between wearing surfaces thins, the resulting heat
softens the adjacent grease, which expands and releases oil
to restore film thickness.
Powdered solid greases
Powdered solids may also be used as thickeners, especially as clays, which are used in some inexpensive, low performance greases. Fatty oil-based greases have also been prepared with other thickeners, such as tar, graphite, or mica, which also increase the durability of the grease.
Engineering assessment and analysis of greases
Lithium-based greases are the most commonly used; sodium and lithium-based greases have higher melting point (dropping point) than calcium-based greases but are not resistant to the action of water. Lithium-based grease has a dropping point at 190 to 220 °C (350 to 400 °F). However the maximum usable temperature for lithium-based grease is 120 °C.
The amount of grease in a sample can be determined in a laboratory by extraction with a solvent followed by e.g. gravimetric determination.
Solid lubricating additives including PTFE, graphite, and molybdenum disulphide are added to some greases to improve their lubricating properties. 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 molybdenum disulfide, 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.
Copper is added to some greases for high pressure applications, or where corrosion could prevent dis-assembly of components later in their service life. Copaslip is the registered trademark of one such grease produced by Molyslip Atlantic Ltd, and has become a generic term (often misspelled as "copperslip" or "coppaslip") for anti-seize lubricants which contain copper.