Grease

Greases are used where a mechanism can only be lubricated infrequently and where a lubricating oil would not stay in position. They also act as valuable sealants to prevent ingress of water and dust. Grease-lubricated bearings have greater frictional characteristics due to their high viscosity. Under shear, the viscosity drops to give the effect of an oil-lubricated bearing of approximately the same viscosity as the base oil used in the grease. 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 °C to 220 °C (350 °F to 400 °F). However the maximum usable temperature for Lithium-based grease is 120 °C.
Grease used for axles are composed of a compound of fatty oils to which tar,graphite or mica is added to increase the durability of the grease and give it a better surface.
A true grease consists of an oil and/or other fluid lubricant that is mixed with another thickener substance, a soap, to form a solid. The term soap is used in the chemical sense sense, meaning a metallic salt of a fatty acid which forms an emulsion with the 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 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.