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Abrasion -- a general wearing away of a surface by constant scratching, usually due to the presence of foreign matter such as dirt, grit, or metallic particles in the lubricant. It may also cause a break down of the material (such as the tooth surfaces of gears). Lack of proper lubrication may result in abrasion. Abrasive wear -- (or cutting wear) comes about 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., a journal. Absolute filtration rating -- the diameter of the largest hard spherical particle that will pass through a filter under specified test conditions. This is an indication of the largest opening in the filter elements. Absolute Viscosity -- a term used interchangeably with viscosity to distinguish it from either kinematic viscosity or commercial viscosity. Absolute viscosity is 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 fluid density equals kinematic viscosity. It is occasionally referred to as dynamic viscosity. Absolute viscosity and kinematic viscosity are expressed in fundamental units. Commercial viscosity such as Saybolt viscosity is expressed in arbitrary units of time, usually seconds. Absorbent 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. AC Fine Test Dust (ACFTD)-- A test contaminant used to assess both filters and the calibration of particle counting instruments. 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. 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. Acid Number (AN) -- A measure of the acidity of a petroleum product. Usually the weight in milligrams of potassium hydroxide (KOH) required to neutralize all the acidic constituents in one gram of lubricating oil. Also known as Neutralization Number. (Previously known as TAN or Total Acid Number) 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 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.
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