Lubricant

Lubricant

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
		power
Hydraulic brake fluid	35	Transmit power
Refrigerator oils	30–260	Lubricate compressor
		pump
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.