Abrasion
General wearing of a surface by constant scratching, due to the presence of foreign matter such as metallic particles grit, or dirt in the lubricant. It may also cause a break down of material (such as gears tooth’s surfaces). Lack of lubrication may result in abrasion.

Abrasive wear
(Cutting wear) comes when hard surface asperities or hard particles that have embedded themselves into a soft surface and plough grooves into the opposing harder surface, e.g., journal.

Absolute filtration rating
Largest diameter of hard spherical particles that will pass through a filter under specified test conditions. Indicator of largest opening in the filter elements.

Absolute Viscosity
Term used interchangeably with viscosity to distinguish it from either commercial or kinematics viscosity. Is the ratio of shear stress to shear rate. It is property of fluid, a fluid’s internal resistance to flow. Its common unit is the poise. Absolute viscosity divided by the fluid density equals kinematics viscosity. Occasionally referred to as dynamic viscosity. Absolute viscosity and kinematics viscosity are expressed in fundamental units. Commercial viscosity such as Saybolt viscosity is expressed in arbitrary units of time, usually seconds.

Absorber Filter
A filter medium that holds contaminant by mechanical means.

Absorption
The assimilation of one material into another; in petroleum refining, the use of an absorptive liquid to selectively remove components from a process stream.

Accumulator
A container in which fluid is stored under pressure as a source of fluid power.

Acid
In a restricted sense, any substance containing hydrogen in combination with a nonmetal or nonmetallic radical and capable of producing hydrogen ions in solution.

Acid Sludge
The residue left after treating petroleum oil with sulfuric acid for the removal of impurities. It is a black, viscous substance containing the spent acid and impurities.

Acid Treating
A refining process in which unfinished petroleum products, such as gasoline, kerosene and lubricating oil stocks, are contacted with sulfuric acid to improve their color, odor, and other properties.

Acidity
In lubricants, acidity denotes the presence of acid-type constituents whose concentration is usually defined in terms of total acid number. The constituents vary in nature and may or may not markedly influence the behavior of the lubricant.

Additive
A compound that enhances some property of, or imparts some new property to, the base fluid. In some hydraulic fluid formulations, the additive volume may constitute as much as 20 percent of the final composition. The more important types of additives include anti-oxidants, anti-wear additives, corrosion inhibitors, viscosity index improvers, and foam suppressants.

Additive Level
The total percentage of all additives in an oil.

Additive stability
The ability of additives in the fluid to resist changes in their performance during storage or use.

Adhesion
The property of a lubricant that causes it to cling or adhere to a solid surface.

Adhesive wear
Is often referred to as galling, scuffing, scoring, or seizing. It happens when sliding surfaces contact one another, causing fragments to be pulled from one surface and to adhere to the other.

Adsorbent filter
A filter medium primarily intended to hold soluble and insoluble contaminants on its surface by molecular adhesion.

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 anti-wear additive on metal surfaces.

Adsorptive filtration
The attraction to, and retention of particles in, a filter medium by electrostatic forces, or by molecular attraction between the particles and the medium.

Aeration
The state of air being suspended in a liquid such as a lubricant or hydraulic fluid.

Agglomeration
The potential of the system for particle attraction and adhesion.

Air Breather
A device permitting air movement between atmosphere and the component in/on which it is installed.

Air, Compressed
Air at any pressure greater than atmospheric pressure.

Air Entrainment
The incorporation of air in the form of bubbles as a dispersed phase in the bulk liquid. Air may be entrained in a liquid through mechanical means and/or by release of dissolved.

Alkali
Any substance having basic (as opposed to acidic) properties. In a restricted sense it is applied to the hydroxides of ammonium, lithium, potassium and sodium. Alkaline materials in lubricating oils neutralize acids to prevent acidic and corrosive wear in internal combustion engines.

Almen EP Lubricant Tester
A journal bearing machine used for determining the load-carrying capacity or extreme pressure properties of gear lubricants.

Ambient Temperature
Temperature of the area or atmosphere around a process (not the operating temperature of the process itself).

Analytical ferrography
The magnetic precipitation and subsequent analysis of wear debris from a fluid sample This approach involves passing a volume of fluid over a chemically treated microscope slide which is supported over a magnetic field. Permanent magnets are arranged in such a way as to create a varying field strength over the length of the substrate. This varying strength causes wear debris to precipitate in a distribution with respect to size and mass over the Ferrogram. Once rinsed and fixed to the substrate, this debris deposit serves as an excellent media for optical analysis of the composite wear particulates.

Anhydrous
Free of water, especially of crystallization.

Aniline Point
The minimum temperature for complete miscibility of equal volumes of aniline and the sample under test ASTM Method D 611. A product of high aniline point will be low in aromatics and naphthenes and, therefore, high in paraffins. Aniline point is often specified for spray oils, cleaning solvents, and thinners, where effectiveness depends upon aromatic content. In conjunction with API gravity, the aniline point may be used calculate the net heat of combustion for aviation fuels.

Anti-foam 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.

Anti-freeze Solution
A fluid, such as ethylene or propylene glycol, which is added to or used to replace the water in the cooling system of engines in order to prevent freezing.

Anti-friction bearing
A rolling contact type bearing in which the rotating or moving member is supported or guided by means of ball or roller elements. Does not mean without friction.

Antiknock
Resistance to detonation or pinging in spark-ignition engines.

Anti-oxidants
Prolong the induction period of a base oil in the presence of oxidizing conditions and catalyst metals at elevated temperatures. The additive is consumed and degradation products increase not only with increasing and sustained temperature, but also with increases in mechanical agitation or turbulence and contamination - air, water, metallic particles, and dust.

Antistatic (additive)
An additive that increases the conductivity of a hydrocarbon fuel to hasten the dissipation of electrostatic charges during high-speed dispensing, thereby reducing the fire/explosion hazard.

Antiwear (additives)
Improve the service life of tribological elements operating in the boundary lubrication regime. Antiwear compounds (for example, ZDDP and TCP) start decomposing at 90° to 100°C and even at a lower temperature if water (25 to 50 ppm) is present.

API Gravity
A gravity scale established by the American Petroleum Institute and in general use in the petroleum industry, the unit being called "the A.P.I. degree." This unit is defined in terms of specific gravity as follows:

Apparent Viscosity
The ratio of shear stress to rate of shear of a non-Newtonian fluid such as lubricating grease, calculated from Poiseuille’s equation and measured in poises. The apparent viscosity changes with changing rates of shear and temperature and must therefore, be reported as the value at a given shear rate and temperature (ASTM Method D 1092). Derived from or characterized by, the presence of the benzene ring.

Ash
A measure of the amount of inorganic material in lubricating oil. Determined by burning the oil and weighing the residue. Results expressed as percent by weight.
Ash Content
The percent by weight of residue left after combustion of an oil sample (ASTM Method D 482).

Asperities
Microscopic projections on metal surfaces resulting from normal surface-finishing processes. Interference between opposing asperities in sliding or rolling applications is a source of friction, and can lead to metal welding and scoring. Ideally, the lubricating film between two moving surfaces should be thicker than the combined height of the opposing asperities.

Asphalt
Black to dark-brown solid or semisolid cemetitious material which gradually liquifies when heated and in which the predominating constituents are bitumen’s. These occur in the solid or semisolid form in nature; are obtained by refining petroleum; or are combinations with one another or with petroleum or derivatives thereof.

Asphaltic
Essentially composed of, or similar to, asphalt; frequently used to describe lubricating oils derived from crude oils which contain asphalt.

ASTM Colorimete
Apparatus widely used for determining the color of lubricating oils/(ASTM Method D 1500). The color so determined in known as ASTM color.

ASTM Distillation
A distillation test made on such products as gasoline and kerosene to determine the initial and final boiling points (ASTM Method D 86).

ASTM Gum Test
An analytical method for determining the amount of existing gum in a gasoline; by evaporating a sample from a glass dish on an elevated-temperature bath (ASTM Method D 381 and ASTM Method D 525).

ASTM Melting Point
The temperature at which was first shows a minimum rate of temperature change; also known as the English melting point.

ASTM Viscosity Classification
A method of specifying viscosity levels for industrial lubricants; does not denote quality.

Auto Ignition
The spontaneous ignition, and the resulting very rapid reaction, of a portion or all of the fuel-air mixture in an engine. The flame speed is many times greater than that which follows normal spark ignition. The noise associated with it is called knock.

Aviation Method
A method for determining the knock-limited power, under lean-mixture condition, of fuels for use in spark-ignition aircraft engines (ASTM Method D 614).

Atomic absorption spectroscopy
Measures the radiation absorbed by chemically unbound atoms by analyzing the transmitted energy relative to the incident energy at each frequency. The procedure consists of diluting the fluid sample with methyl isobutyl ketone (MIBK) and directly aspirating the solution. The actual process of atomization involves reducing the solution to a fine spray, dissolving it, and finally vaporizing it with a flame. The vaporization of the metal particles depends upon their time in the flame, the flame temperature, and the composition of the flame gas. The spectrum occurs because atoms in the vapor state can absorb radiation at certain well-defined characteristic wave lengths. The wave length bands absorbed are very narrow and differ for each element. In addition, the absorption of radiant energy by electronic transitions from ground to excited state is essentially and absolute measure of the number of atoms in the flame and is, therefore, the concentration of the element in a sample.

Axial-load bearing
A bearing in which the load acts in the direction of the axis of rotation.

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