Glossary of Lubrication Terms

This glossary is designed to help the understanding of some of the terms used in Tribology and Lubrication Engineering. Quickly look up a definition or explanation for a topic.
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Absolute Viscosity – The ratio of shear stress to shear rate. It is a fluid's internal resistance to flow.  The common unit of absolute viscosity is the poise. Absolute viscosity divided by the fluid’s density equals kinematic viscosity.

Absorption – The assimilation of one material into another.

Acid Number – The number of milligrams of potassium hydroxide required to neutralize one gram of an oil sample. ASTM D664 uses a potentiometric titration; D974 uses a color-indicator titration. Also known as neutralization number.

Additive – A chemical substance added to a petroleum product to impart or improve certain properties. Common petroleum product additives include antifoam agent, antiwear additive, corrosion inhibitor, demulsifier, detergent, dispersant, emulsifier, EP additive, oiliness agent, oxidation inhibitor, pour point depressant, rust inhibitor, tackiness agent and viscosity index (VI) improver.

Adsorption – Adhesion of the molecules of gases, liquids, or dissolved substances to a solid surface, resulting in relatively high concentration of the molecules at the place of contact (e.g., the plating out of an antiwear additive on metal surfaces). 

AGMA – American Gear Manufacturers Association, which, as one of its activities, establishes and promotes standards for gears and gear lubricants.

AGMA Lubricant Numbers
– AGMA specification covering gear lubricants. The viscosity ranges of the AGMA numbers (or grades) conform to the International Standards Organization (ISO) viscosity classification system.

Aniline Point – Lowest temperature at which equal volumes of aniline (a benzene derivative) is soluble in a specified quantity of a petroleum product, as determined by test method ASTM D611; hence, an empirical measure of the solvent power of a hydrocarbonthe lower the aniline point, the greater the solvency. Paraffinic hydrocarbons have higher aniline points than aromatic types.

ANSI (American National Standards Institute) – An organization of industrial firms, trade associations, technical societies, consumer organizations and government agencies, intended to establish definitions, terminologies, and symbols; improve methods of rating, testing, and analysis; coordinate national safety, engineering and industrial standards; and represent U.S. interests in international standards work.

Antifoam Agent
– One of two types of additives used to reduce foaming in petroleum products:  silicone oil to break up large surface bubbles, and various kinds of polymers that decrease the amount of small bubbles entrained in the oils.

Antioxidant – A chemical additive, which increases a lubricant’s oxidation resistance, which lengthens its service and storage life.

Antiwear Additive – Additive in a lubricant that reduces friction and excessive wear.

API (American Petroleum Institute) – A trade association of petroleum producers, refiners, marketers and transporters, organized for the advancement of the petroleum industry by conducting research, gathering and disseminating information, and maintaining cooperation between government and the industry on all matters of mutual interest.

Asperities – Microscopic projections on metal surfaces resulting from normal surface finishing processes. Ideally the lubricating film should be thicker than the combined height of the opposing asperities.

Aromatic – Cyclic unsaturated hydrocarbons identified by one or more benzene rings or by chemical behavior similar to benzene. Aromatics are usually more reactive and have higher solvency than paraffins and naphthenes. Aromatics readily undergo electrophilic substitution; that is, they react to add other active molecular groups such as nitrates, sulfonates, etc.  Aromatics are used extensively as petrochemical building blocks.

ASTM (American Society for Testing and Materials)
– An organization devoted to the promotion of knowledge of the materials of engineering, and the standardization of specifications and methods of testing. A preponderance of the data used to describe, identify, or specify petroleum products is determined in accordance with ASTM test methods.

Automatic Transmission Fluid (ATF) – A functional fluid for automatic transmissions in motor vehicles. Automatic transmission fluids must have a suitable coefficient of friction, good low-temperature viscosity, and antiwear properties. Other necessary properties are high oxidation stability, anticorrosion, antifoaming, and compatibility with synthetic rubber seals.