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A substance used to reduce friction between parts or objects in relative motion.



any of the substances that exhibit a lubricating action. These substances are used in the lubrication of machine parts in relative motion and in the pressure shaping of metals. Substances used as lubricants include oils derived for the most part from petroleum and synthetic oils, greases, solids (graphite, molybdenum disulfide, polymers with fillers), gases (air, vapors of hydrocarbons, halogen derivatives of methane), and surfactants (soaps, glycerol). Lubricants are placed in groups and grades depending on their state of aggegation, properties, and purpose.


Rozenberg, Iu. A. Vliianie smazochnykh masel na dolgovechnost’ i nadezhnost’ detalei mashin. Moscow, 1970.
Tovarnye nefteprodukty, ikh svoistva i primenenie: Spravochnik. Edited by N. G. Puchkov. Moscow, 1971.
Maiorova, L. A. Tverdye neorganicheskie veshchestva v kachestve vysokotemperaturnykh smazok. Moscow, 1971.


A gas, liquid, or solid used to prevent contact of parts in relative motion, and thereby reduce friction and wear. In many machines, cooling by the lubricant is equally important. The lubricant may also be called upon to prevent rusting and the deposition of solids on close-fitting parts.

Crude petroleum is an excellent source of lubricants because a very wide range of suitable liquids, varying in molecular weight from 150 to over 1000 and in viscosity from light machine oils to heavy gear oils, can be produced by various refining processes (see table). In order to standardize on nomenclature for oils of differing viscosity, the Society of Automotive Engineers (SAE) has established viscosity ranges for the various SAE designations (see table).

It is often desirable to add various chemicals to lubricating oils to improve their physical properties or to obtain some needed improvement in performance. These include viscosity-index improvers, pour-point depressants, antioxidants, anti-wear and friction-reducing additives, and dispersants.

Synthetic lubricants may be superior to mineral lubricants in some applications. The main advantage of synthetics is that they have a greater operating range than a mineral oil. Included in this class are esters, containing oxidation inhibitors and sometimes mild extreme pressure additives, silicones, and the polyglycols, such as polypropylene and ethylene oxides.

Viscosity of oils for various applications
Viscosity in centistokes
Application at 25°C (77°F) Primary function
Engine oils
SAE 10W 60–90 Lubricate piston rings,
SAE 20 90–180 cylinders, valve gear,
SAE 30 180–280 bearings; cool piston;
SAE 40 280–450 prevent deposition on
SAE 50 450–800 metal surfaces
Gear oils Prevent metal contact
SAE 80 100–400 and wear of spur
SAE 90 400–1000 gears, hypoid gears,
SAE 140 1000–2200 worm gears; cool
gear cases
Aviation engine oils 220–700 Same as engine oils
Torque converter fluid 80–140 Lubricate, transmit
Hydraulic brake fluid 35 Transmit power
Refrigerator oils 30–260 Lubricate compressor
Steam-turbine oil 55–300 Lubricate reduction
gearing, cool
Steam cylinder oil 1500–3300 Lubricate in presence of
steam at high temperatures

The most useful solid lubricants are those with a layer structure in which the molecular platelets will readily slide over each other. Graphite, molybdenum disulfide, talc, and boron nitride possess this property. A unique type of solid lubricant is provided by the plastic polytetrafluoroethylene (PTFE). The principal difficulty encountered with the use of solid lubricants is that of maintaining an adequate lubricant layer between the sliding metal surfaces.

A lubricating grease is a solid or semifluid lubricant comprising a thickening (or gelling) agent in a liquid lubricant. Other ingredients imparting special properties may be included. An important property of a grease is its solid nature; it has a yield value. This enables grease to retain itself in a bearing assembly without the aid of expensive seals, to provide its own seal against the ingress of moisture and dirt, and to remain on vertical surfaces and protect against moisture corrosion, especially during shut-down periods.

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