A lubricant is a material between two moving surfaces that reduces the friction and wear
between them. In the early days, lubricants were made from rendered animal fats
and vegetable oils. Now, there mostly synthetic materials with much greater
effectiveness. Lubricants are extremely important to keep wheels, gears, bearings,
and other moving parts running at high speeds at high temperatures.
Lubricants can be grouped in 3 classes: oils with lower viscosity
(resistance to flow) used in low pressure applications, greases with higher viscosity used in
high pressure applications, and solids, e.g., graphite, teflon,
(MoS2), polytetraflouroethylene (PTFE).
Typically, oil lubricants contain about 90% base oil, which is
most often a derivative of petroleum, called mineral
oils, but also vegetable and synthetic oils, and about 10%
additives. The additives reduce friction and wear, increased viscosity, resistance to
corrosion and aging. Additives also improve oil's ability to take contaminants into
solution, so they can be filtered.
Grease is a lubricant of higher viscosity than oil, consisting originally of a
calcium, sodium and
lithium soap jelly emulsified with mineral oil.
Greases are used where heavy pressures
exist, where oil drip from the bearings is undesirable, where the motions of the contacting
surfaces is discontinuous so that it is difficult to maintain a separating film in the bearing, and
where it is necessary stabilize the greases against the high temperature deterioration.
Grease-lubricated bearings have greater frictional characteristics at the beginning of operation
compared to oil-lubricated bearings, but the rise in temperature melts the grease and give the
effect of an oil-lubricated bearing.
Graphite, a solid form of carbon either by itself or
mixed with grease, is also employed as a lubricant.
Teflon is added to some greases to improve the lubricating property. Gear greases
consist of rosin oil, thickened with lime and mixed with mineral oil, with some water.
Wiki n.p. Talc
(soapstone) is another important lubricant, e.g., "talcum powder".