is the mixture of a fluid lubricant (usually a
petroleum oil) and a thickener (usually a soap)
dispersed in the oil. Because greases do not flow readily, they are used
where extended lubrication is required and where oil would not be retained.
The thickener may play as important a role as the oil in lubrication.
Soap thickeners are formed by reacting (saponifying) a metallic
hydroxide, or alkali, with a fat,
fatty acid, or ester.
The type of soap used depends on the grease properties desired. Calcium
(lime) soap greases are highly resistant to water, but unstable at
high temperatures. Sodium soap greases are stable at high
temperatures, but wash out in moist conditions. Lithium soap greases
resist both heat and moisture. A mixed-base soap is a combination of soaps,
offering some of the advantages of each type. A complex soap is
formed by the reaction of an alkali with a high-molecular-weight
fat or fatty acid to form a soap, and the simultaneous reaction of the
alkali with a short-chain organic or inorganic acid
to form a metallic salt (the complexing agent). Complexing agents
usually increase the dropping point
of grease. Lithium, calcium, and aluminum greases are common. alkalis in
complex-soap greases. Non-soap thickeners, such as clays, silica
gels, carbon black, and various synthetic organic materials are also used in
grease manufacture. A multi-purpose
grease is designed to provide resistance to heat, as well as water, and
may contain additives to increase load-carrying ability and inhibit rust.
Also see block grease,
bulk appearance,
colloid, consistency (grease),
fire-resistant grease,
penetration (grease).