Lubricants

Lubricants, substances applied to the bearing, guiding, or contact surfaces of machinery to reduce friction between moving parts. The process of applying lubricants is called lubrication. Natural lubricants may be fluid, or semifluid, such as organic and mineral oils; semisolid, such as grease; or solid, such as graphite. Synthetic lubricants include the silicones and special products that can operate at very high temperatures, as in advanced automobile diesel engines. Such products may take the form of coatings that enable moving parts to lubricate themselves, and of oils that decompose without leaving behind friction-producing deposits.

Historically, vegetable oils and animal fats and oils were the chief sources of lubricants. Since the late 19th century, more than 90 percent of all lubricants have been obtained from petroleum or shale oil, which are abundant and can be distilled and condensed without decomposing.

The requisites of a good lubricant are body or density, resistance to corrosive acids, fluidity, minimal frictional or tensional resistance, high burning and flash points, and freedom from oxidation or gumming. Chemical tests exist for determining all of these properties in lubricants (see Isotopic Tracer).

Lubricants enable machinery to function continuously by preventing abrasion, or so-called seizing, of metal parts caused by heat expansion. Some lubricants are also coolants and thus prevent material deformities caused by heat. Thin-film lubricants are used for heavy loads, where internal friction in the lubricant and consequent loss of power have to be avoided. Thick-film, or fluid, lubricants are used in machinery in which moving or oscillating conditions are moderate. Lubricants today are often applied mechanically for better control, usually by valves, rotating rings or chains, immersion devices, splash devices, or centralized reservoirs and pumps. Grease-type lubricants are applied by packing, pressing, or pumping. The method of application, as well as the selection of lubricant, is important for efficiency.