Abrasion: a wearing, grinding or rubbing away by friction, usually (but not always) involving the action of particles against or between surfaces.

Absolute (Dynamic) Viscosity: the ratio of shear stress to shear rate, representing a fluid's internal resistance to flow. Although the fundamental unit of absolute or dynamic viscosity is the poise, results are often expressed in centipoise (cP). ( See VISCOSITY in Technical Library)

Absorption: the process by which one substance draws itself into another substance. An example is a sponge picking up water, or on oil recovering gasoline from wet natural gas

Accumulator (hydraulics): an in-line vessel in which hydraulic fluid is stored under pressure in a system to be used as a source of fluid power.

Acid: any one of various hydrogen-containing molecules or ions capable of giving up a proton to a base, of accepting an unshared pair of electrons from a base, or of reacting with a base to form a salt. A more limited definition restricts the acid to a hydrogen-containing substance that contains a non-metallic radical and produces hydrogen ions when placed in solution.

Acid digestion: process of dissolving a sample in an acid matrix, usually accompanied by heating.

Acidity (in lubricants): denotes the presence of weakly and strongly acidic materials whose total concentration is usually defined in terms of the AN (Acid Number).

Acid Number(AN): the number of mg of potassium hydroxide required to neutralize one gram of an oil sample.

Actuator: a mechanical device, like a cylinder or hydraulic motor, used to convert hydraulic energy into mechanical energy.

Addendum (gears): distance between the pitch circle and the tooth crest.

Additive: a chemical substance added to oils, fuels, and coolants to impart specific beneficial properties to the finished products. Additives create new fluid properties, enhance inherent properties and reduce the rate at which undesirable changes take place in a fluid during service. (See ADDITIVES in KNOWLEDGE CENTER)

Adhesion: the force of molecular attraction exerted between two surfaces in contact. (In lubricants the property of a lubricant that causes it to cling or adhere to a solid surface).

Adsorption: the adhesion of an extremely thin layer of molecules (gases, dissolved solids, or liquids) to the surfaces of solids or liquids with which the molecules are in contact.This process is utilized in a broad range of fluid and additive applications to concentrate one material on the surface of another

Aeration: combining or charging a fluid with a gas, as in oil being aerated with air.

Aerosol: a highly dispersed suspension of fine solid or liquid particles in a gas.

Aftercooling: the process of cooling compressed gases under constant pressure after the final stage of compression.

Age hardening: an undesirable process during which a solid (a grease, an elastomeric seal or rubber hose) hardens with prolonged storage.

Agglomeration: the action of combining a number of small particles into a larger single mass, often the operating principle for additives which promote particle attraction and clumping.

AGMA: American Gear Manufacturers Association. One of its activities is to establish & promote standards for gears & gear lubricants.(see AGMA Lubrication numbers in VISCOSITY CLASSIFICATION SYSTEMS in KNOWLEDGE CENTER)

Air entrainment: the presence of air bubbles throughout an oil as a result of agitation and/or the release of dissolved air because of a sudden change in environment. Entrained air is usually obvious from the hazy, opaque, or bubbly physical appearance of the liquid, while dissolved air can only be determined by specific testing.

Air line lubricator: an oil reservoir attached to an air line that provides automatic air-borne lubrication to air operated power consuming equipment by means of venturi action.

Air-oil lubrication: a system of lubrication in which small quantities of oil are injected into an air line that terminates at a bearing or other lubrication point. The velocity of the air moves the oil, which remains in droplet form, along the periphery of the fluid conductor to the point of need; the clean, dry air, being unheated, helps cool the lubrication point. Since the lubricant does not return to a reservoir, these systems are classified as all-loss systems.

Air oil separator: a mechanical device that defoams oil, using a centrifugal oil trap (a defoamer), or any oil condensing device in an air line.

Aliphatic: one of three types of hydrocarbons found in fuels or lubricants. Typically, aliphatics are visualized as linear molecules with no reactive chemical sites (see MINERAL BASE OILS in Technical Library).

Aniline point: lowest temperature at which equal volumes of aniline ( a benzene derivative) is soluble in a specified quantity of a petroleum product; serves as a measure of the solvent or “grease-cutting” power of a hydrocarbon; generally, the lower the aniline point, the more effective the solvent. Paraffinic hydrocarbons have higher aniline points than aromatic types.

ANSI: American National Standards Institute (a member of the ISO). Among other things, ANSI standards are used to evaluate load ratings for ball and roller bearings.

Antifoam agent : an additive used to reduce foaming in petroleum products . Typically either a Silicone oil to break up large surface bubbles or various kinds of polymers that decrease the amount of small bubbles entrained in the oils.

Anti-friction bearings: (see rolling element bearings).

Antioxidant: an additive to retard oxygen-related deterioration, especially oxidation of lubricants.

antiwear additive – additive in a lubricant that reduces friction and excessive wear Anti-seize compounds (pipe dope): grease-like substances with graphite, molybdenum disulfide and metallic particles dispersed throughout, primarily to prevent seizure on threaded joints.

API: American Petroleum Institute, the trade association of the petroleum producers, refiners, marketers & transporters.

API Gravity: measures the density of petroleum fluids. It is derived from specific gravity and was developed to express density in whole numbers with one or two decimal places. For example, an API gravity of 25.5 is the same as a specific gravity of 0.9013. This does not compare to an H₂O constant. This unit is defined in terms of specific gravity at 60°F (SPG 60°F/15.6°C) as follows:
Degrees API Gravity =
SPG (60°F/15.6°C)
- 131.5

Apparent viscosity: Terms characterizing the resistance to flow of liquids whose viscosities vary with the rate of shear. It can be evaluated in a capillary-type instrument where it is defined as the shear stress at the capillary wall divided by the mean rate of shear as computed from the Poiseuille equation: it is expressed in fundamental viscosity units at a given rate of shear.

Aromatics: cyclic unsaturated hydrocarbons identified by one or more benzene rings (ring-structured hydrocarbons that contain unsaturated double bonds). Benzene is the simplest aromatic .Aromatics are usually more reactive & have higher solvency than parrafins and napthenes. Aromatics readily react with other active molecular groups, such as nitrates, sulfonates, etc. Aromatics are extensively used as petrochemical building blocks

Ash content: percentage of non-combustible residue of a lubricating oil or fuel. Ash reveals the presence of metals, including the calcium, magnesium and zinc introduced by additives.

Asperities: microscopic projections on metal surfaces, invisible to the naked eye, that creates peaks and valleys, even after grinding or machining. When two surfaces are in sliding contact, these imperfections cause interference that results in friction; without proper lubrication, wear, scoring or welding will follow. Ideally the lubricating film should be thicker than the combined height of the opposing asperities.

Asphalt/bitumen: a soft black or dark brown tacky residual material containing asphaltenes, compounds of sulfur, nitrogen and tar. Asphalt, derived from petroleum, is solid at normal temperatures; as an adjective, “asphalt” is often used to describe viscous open gear compounds or black, tacky greases.

Asphaltenes: asphaltic materials soluble in aromatic solvents but insoluble in naphtha.

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 large number of properties used to describe, identify, or specify petroleum products is determined in accordance with ASTM test methods.

Auto ignition temperature: the minimum combustion temperature for a vapor- air mixture without an open flame. It permits evaluation of the fire hazards of vapors.

automatic transmission fluid (ATF) – a fluid used in automatic transmissions in motor vehicles. ATFs must have a suitable coefficient of friction, good low-temperature viscosity, and antiwear properties. Other necessary properties are: high oxidation stability, anti-corrosion, anti-foaming, and compatibility with synthetic rubber seals.

Axial load bearing: a bearing that supports an axial thrust (a load exerted in line with the length or the axis of a shaft).