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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.
REFERENCESRozenberg, 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 in centistokes|
|Application||at 25°C (77°F)||Primary function|
|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|
|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|
|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.