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Glossary

Abrasive Wear
Wear due to hard particles or hard protuberances forced against and moving along a solid surface.

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. A Beta Raio of 75 or higher is normally considered to be equivalent to the absolute filtration rating.

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 (see viscosity). Absolute viscosity divided by the fluid's density equals kinematic viscosity.

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. (a.k.a. neutralization number)

Additive
A compound that enhances some property of, or imparts some new property to, the base fluid. he more important types of additives include anti-oxidants, anti-wear additives, corrosion inhibitors, viscosity index improvers, and foam suppressants.

Adhesive Wear
Wear due to localized bonding between contacting solid surfaces leading to material transfer between the two surfaces or loss from either surface.

AGMA
American Gear Manufacturers Associations, establishes and promotes standards for gears and gear lubricants.

Analytical Ferrography
Technique whereby particles from an oil sample deposited by a ferrograph are identified to aid in establishing wear mode inside an oil-wetted path of a machine.

Aniline Point
The 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.

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-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.

Anti-oxidants
A chemical additive which increases a lubricant’s oxidation resistance, which lengthens its service and storage life.

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
An additive in a lubricant that reduces friction and excessive wear.

API
American Petroleum Institute, a trade organization for the oil and natural gas industry.

API gravity
A measure of how heavy or light a petroleum liquid is compared to water: if its API gravity is greater than 10, it is lighter and floats on water; if less than 10, it is heavier and sinks

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.

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.

ASTM International
Formerly American Society for Testing Materials, a society for developing standards for materials and test methods.

Atomic Absorption Spectroscopy
AAS is a spectroanalytical procedure for the quantitative determination of chemical elements using the absorption of optical radiation (light) by free atoms in the gaseous state.

Babbitt
Any of several alloys, predominantly lead, tin, copper, antimony and arsenic, used for the bearing surface in a plain bearing.

Bactericide
A chemical agent that helps to prevent the formation of bacteria, often used as additives in coatings and corrosion inhibitors.

Base Number
The number of milligrams of acid required to neutralize one gram of an oil sample. ASTM D974 uses hydrochloric acid and a color-indicator titration; D2896 uses perchloric acid in a potentiometric titration.

Base Stock
The base fluid, usually a refined petroleum fraction or a selected synthetic material, into which additives are blended to produce finished lubricants.

Beta Ratio (ß)
The ratio of the number of particles of a minimum given size upstream of the filter to the number of particles of the same size and larger found downstream, under specified test conditions (see "Multi-Pass Test").

Biodegradability
Defined as the ability of a substance to degrade over time to carbon dioxide and water in the presence of water, nutrients and microorganisms.
Readily biodegradable - at least 60-70% (depending on the test type) of the sample oil is degraded.
Inherently biodegradable - 20-60% of the sample oil is degraded.
Persistent - less than 20% of the sample oil is degraded.

Black Oils
Any of various dark-colored, low-grade, oils used to lubricate slow-moving or rough-surfaced machinery where high-grade lubricants are impractical or too expensive.

Blotter Test
Technique used to identify excessive engine soot, evaluate a lubricant’s dispersancy and detect the presence of glycol, diesel fuel and other contaminants in diesel engine crankcase lubricants. (See Paper Chromatography)

Boundary Lubrication
A form of lubrication between two rubbing surfaces without development of a full-fluid lubricating film. Boundary lubrication can be made more effective by including additives in the lubricating oil that provide a stronger oil film, thus preventing excessive friction and possible scoring. There are varying degrees of boundary lubrication, depending on the severity of service. For mild conditions, oiliness agents may be used; by plating out on metal surfaces in a thin but durable film, oiliness agents prevent scoring under some conditions that are too severe for a straight mineral oil. Compounded oils, which are formulated with polar fatty oils, are sometimes used for this purpose. Anti-wear additives are commonly used in more severe boundary lubrication applications. The more severe cases of boundary lubrication are defined as extreme pressure conditions; they are met with lubricants containing EP additives that prevent sliding surfaces from fusing together at high local temperatures and pressures.

Brookfield Viscosity
A measure of the viscosity characteristics of a lubricant under low temperature and low shear conditions. The apparent viscosity of oil, usually determined by test method ASTM D2983. The apparent viscosity of a non-Newtonian fluid is valid only for the shear rates and temperature at which it is determined. The Brookfield viscometer provides a known rate of shear by means of a spindle of specified configuration that rotates at a known constant speed in the fluid. The torque imposed by fluid friction can be converted to absolute viscosity units (centipoise) by a conversion factor or equation.

BS&W
An acronym for the material that settles to the bottom of a storage tank, namely bottoms, sediment and water. Laboratories sometimes quantify and report this information when examining oil in service.

Carbon Residue
The measure of the coke forming tendency of oils at high temperatures.

Catalyst
a substance which speeds a chemical action without undergoing a chemical change itself during the process.

Cavitation
The formation of an air or vapor pocket (or bubble) due to lowering of pressure in a liquid, often as a result of a solid body, such as a propeller or piston, moving through the liquid; also, the pitting or wearing away of a solid surface as a result of the collapse of a vapor bubble. Cavitation can occur in a hydraulic system as a result of low fluid levels that draw air into the system, producing tiny bubbles that expand explosively at the pump outlet, causing metal erosion and eventual pump destruction. Cavitation can also result when reduced pressure in lubricating grease dispensing systems forms a void, or cavity, which impedes suction and prevents the flow of greases.

Centipoise (cP)
A dynamic viscosity measurement unit, used to report absolute viscosity.

Centistoke (cSt)
A kinematic viscosity measurement unit.

Channeling (grease)
A term describing the usually desirable tendency of grease to form a channel by working down in a bearing, leaving shoulders of unworked grease that serve as both reservoirs and seals.

Channeling (liquids)
The undesired formation of troughs or channels in flow-type lubricants due to thickening during cold weather. Since such behavior occurs near the pour point of the lubricant, lubrication may be marginal until the lubricant warms up from being worked (see channeling point).

Channeling Point (gears)
A federal test that measures the tendency of lubricants
at low temperatures to form plastic structures of sufficient strength to resist flow
under gravitational forces only. This test is specified and required for MIL 2105-
type gear oils.

Chromatography
A powerful method for analyzing fluids and determining their components by selective adsorption or size exclusion, using liquid or gas as the eluent. In the adsorptive procedure, the substance flows slowly through a column of adsorbent; as different substances pass at different speeds, they separate from each other and can sometimes be isolated and identified. In other cases, the chromatogram (a trace of the signal from the detector) is utilized to fingerprint a lubricant. Liquid chromatography is used for lubricants because of their low volatility. Paper chromatography, an adsorptive method, is often used to examine or establish the sludge or dispersive characteristics of a lubricant. Gel permeation chromatography, a size exclusion method, separates polymeric (oxidized oil/sludge) material from a lubricant base stock by molecular weight.

Cloud Point
The temperature at which waxy crystals in an oil or fuel form a cloudy appearance when cooled under standard conditions.

Compounded Oil
A mixture of petroleum oil with animal or vegetable fat or oil. Compounded oils have a strong affinity for metal surfaces; they are particularly suitable for wet-steam conditions and for applications where lubricity and extra load-carrying ability are needed. They are not generally recommended where long-term oxidation stability is required.

Condition Monitoring
Determining the condition of a machine by interpretation of measurements taken either periodically or continuously while the machine is running.

Consistency
A basic property describing the softness or hardness of a grease, i.e., the degree to which a grease resists deformation under the application of force. Consistency is usually measured by means of a cone penetration test such as ASTM D217. The consistency of a grease depends on the viscosity of the base oil and the type and proportion of the thickener. It can also be affected by recent agitation; to take this phenomenon into consideration, grease is usually subjected to working (a standard churning process) prior to measuring its penetration value.

Contaminant
Any foreign or unwanted substance that can have a negative effect on system operation, life or reliability.

Contamination Control
A broad subject that applies to all types of material systems (including both biological and engineering). It is concerned with planning, organizing, managing, and implementing all activities required to determine, achieve and maintain a specified contamination level.

Coolant
A fluid used to remove heat. See Cutting fluid.

Corrosion
A chemical or electrochemical reaction between a material, usually a metal surface, and its environment that can produce a deterioration of the material and its properties.

Corrosion Inhibitor
An additive for protecting lubricated metal surfaces against chemical attack by water or other contaminants. There are several types of corrosion inhibitors. Polar compounds wet the metal surface preferentially, protecting it with a film of oil. Other compounds may absorb water by incorporating it in a water-in-oil emulsion so that only the oil touches the metal surface. Another type of corrosion inhibitor combines chemically with the metal to present a non-reactive surface.

Cutting Fluid
Any fluid applied to a cutting tool to assist in the cutting operation by cooling, lubricating or other means.

Cylinder Oil
A lubricant for independently lubricated cylinders, such as those of steam engines and air compressors; also for lubrication of valves and other elements in the cylinder area. Steam cylinder oils are available in a range of grades with high viscosities to compensate for the thinning effect of high temperatures; of these, the heavier grades are formulated for super-heated and high-pressure steam, and the lighter grades for wet, saturated, or low-pressure steam. Some grades are compounded for service in excessive moisture; see compounded oil. Cylinder oils lubricate on a once-through basis.

Dark Metallo-Oxide Particles
Partially oxidized ferrous wear particles indicating high heat during generation most likely due to lubricant starvation.

Demulsibility
The ability of a fluid that is insoluble in water to separate from water with which it may be mixed in the form of an emulsion.

Demulsifier
An additive that promotes oil water separation in lubricants that are exposed to water or steam.

Deposits
Oil-insoluble materials that result from oxidation and decomposition of lube oil and contamination from external sources and engine blow-by. These can settle out on machine or engine parts. Examples are sludge, varnish, lacquer and carbon.

Detergent
An important component of engine oils and some industrial lubricants, such as paper machine oils and hydraulic fluids; helps control deposits by preventing contaminants of combustion from directly contacting metal surfaces and, in some cases, by neutralizing acids. A detergent is usually a metallic (commonly barium, calcium or magnesium) compound, such as a sulfonate, phosphonate, thiophosphonate, phenate, or salicylate. Because of its metallic composition, a detergent leaves a slight ash when the oil is burned. A detergent is normally used in conjunction with a dispersant.

Dielectric Strength
A measure of the ability of an insulating material to withstand electric stress (voltage) without failure. Fluids with high dielectric strength (usually expressed in volts or kilovolts) are good electrical insulators. (ASTM D877.)

Dispersant
An additive that helps prevent deposits by holding the insoluble products of oil oxidation and fuel combustion in suspension in the oil.

Distillation Method for Water
A method involving distilling the fluid sample in the presence of a solvent that is miscible in the sample but immiscible in water. The water distilled from the fluid is condensed and segregated in a specially-designed receiving tube or tray graduated to directly indicate the volume of water distilled.

DN Factor
Used as a guide to lubricant selection for rolling contact bearings, it is also called a speed factor, the product of the bore of a rolling contact bearing, expressed in mm (D), and the speed in rpm. Values up to 300,000 permit use of normal NLGI 2 grease; higher values indicate fluid oil or specially formulated greases, and values in the 1,000,000 range require oil-mist or air-oil lubrication or specially formulated greases.

Dropping Point
The temperature at which grease passes from a semi-solid to a liquid state as specified under ASTM D566. Considered to be temperature where thickener system fails.

Elastohydrodynamic (EHL or EHD) Lubrication
A lubrication phenomenon occurring during elastic deformation of two non-conforming surfaces under high load. A high load carried by a small area (as between the ball and race of a rolling contact bearing) causes a temporary increase in lubricant viscosity as the lubricant is momentarily trapped between slightly deformed opposing surfaces.

Emission Spectroscopy
A method of chemical analysis that uses the intensity of light emitted from a flame, plasma, arc, or spark at a particular wavelength to determine the quantity of an element in a sample.

Emulsibility
The ability of a non-water-soluble fluid to form an emulsion with water.

Emulsifier
An additive that promotes the formation of a stable mixture, or emulsion, of oil and water. Common emulsifiers are: metallic soaps, certain animal and vegetable oils, and various polar compounds.

Emulsion
A two-phase liquid system in which small droplets of one liquid are immiscible in, but uniformly dispersed throughout, a second, continuous phase. Generally of a milky or cloudy appearance, emulsions may be of two types; oil-in-water (where water is the continuous phase) and water-in-oil (where water is the discontinuous phase). Oil-in-water emulsions are used as cutting fluids because of the need for the cooling effect of the water. Water-in-oil, or invert, emulsions are used where the oil, not the water, must contact a surface - as in rust preventives, non-flammable hydraulic fluids, and compounded steam cylinder oils (see Compounded oil); such emulsions are sometimes referred to as invert emulsions. Emulsions are produced by adding an emulsifier.

Extreme Pressure (EP) Additive
A lubricant additive that prevents sliding metal surfaces from seizing under conditions of extreme pressure. At the high local temperatures associated with metal-to-metal contact, an EP additive combines chemically with the metal to form a surface film that prevents the welding of opposing asperities, and the consequent scoring that is destructive to sliding surfaces under high loads. Reactive compounds of sulfur, chlorine, or phosphorus are used to form these inorganic films.

False Brinelling
A form of wear categorized under frictional corrosion, occurs with mild adhesion and small amplitude oscillations.

Fatigue Wear
Wear of a solid surface caused by fracture arising from material fatigue.

Ferrography
A method of particle analysis using precision magnets to strip iron-laden and other susceptible particles from a used lubricating oil for study; results indicate extent of equipment wear and likelihood of imminent failure. Direct-reading ferrography uses optical sensors to measure the density of particles collected and the ratio of large particles to small (fatigue-related catastrophic failure generally is characterized by generation of particles larger than 10-15 microns). Analytical ferrography employs microscopic and photographic evaluation of wear particles. The test provides in-depth analysis of particle composition (e.g., steel, copper, bronze) and type of wear (e.g., corrosion, metal-to-metal contact).

Fire Point
The temperature to which a combustible liquid must be heated so that the released vapor will burn continuously when ignited under specified conditions.

Fire-resistant Fluid
A lubricant used especially in high-temperature or hazardous hydraulic applications. Three common types of fire-resistant fluids are: (1) water-petroleum oil emulsions, in which the water prevents burning of the petroleum constituent; (2) water-glycol fluids; and (3) non-aqueous fluids of low volatility, such as phosphate esters, silicones, and halogenated hydrocarbon-type fluids.

Flash Point
The temperature to which a combustible liquid must be heated to give off sufficient vapor to form momentarily a flammable mixture with air when a small flame is applied under specified conditions. (ASTM D92.)

Fretting
Small amplitude oscillatory motion, usually tangential, between two solid surfaces in contact.

Friction
Resistance to sliding exhibited by two surfaces in contact with each other. Basically there are two frictional properties exhibited by any surface; static friction and kinetic friction.

FTIR (Fourier Transform Infrared) Spectroscopy
A molecular analysis of lubricant where infrared light absorption is used for assessing additive depletion, contaminant buildup (soot and incorrect oils) and base stock degradation (oxidation, nitration and sulfation.

Full-fluid-film Lubrication
The presence of a continuous lubricating film sufficient to completely separate two surfaces, as distinct from boundary lubrication. Full-fluid-film lubrication is normally hydrodynamic lubrication, whereby the oil adheres to the moving part and is drawn into the area between the sliding surfaces, where it forms a pressure or hydrodynamic wedge.

Fungicide
A substance that kills, prevents or retards the growth of fungi. Fungicides and biocides are most often used with fluids like soluble oils that contain water.

Gas Chromatography
A form of chromatography using an inert gas as the carrier medium. See Chromatography.

Grease
A lubricant that is a solid to semi-fluid dispersion of a thickening agent (thickener) in a liquid. A lubricating grease may be formulated with additives that impart special properties such as resistance to oxidation or wear.

Heat transfer oils
Oils utilized as heat transfer media; typical applications include asphalt plants, grease plants, etc. These oils have low volatility and contain additives to inhibit cracking and sludging.

Hydraulic Fluid
A fluid serving as the power transmission medium in a hydraulic system. The most commonly used fluids are petroleum oils, synthetic lubricants, oil-water emulsions, and water-glycol mixtures. The principal requirements of a premium hydraulic fluid are proper viscosity, high viscosity index, anti-wear protection (if needed), good oxidation stability, adequate pour point, good demulsibility, rust inhibition, resistance to foaming, and compatibility with seal materials. Anti-wear oils are frequently used in compact, high-pressure, and capacity pumps that require extra lubrication protection.

Hydrodynamic Lubrication
A form of lubrication in which the shape and relative motion of the sliding surfaces causes the formation of a fluid film having sufficient pressure to separate the surfaces. See Full-fluid-film lubrication.

Hydrolysis
Breakdown process that occurs in anhydrous hydraulic fluids as a result of heat, water, and metal catalysts (iron, steel, copper, etc.)

Hydrolytic Stability
The ability of a lubricant to resist chemical decomposition (hydrolysis) in the presence of water.

Hydrostatic Lubrication
A form of lubrication in which the lubricant is supplied under sufficient external pressure to separate the opposing and static (non-moving) surfaces by a fluid film.

Immiscible
Incapable of being mixed without separation of phases. Water and petroleum oil are immiscible under most conditions, although they can be made miscible with the addition of an emulsifier.

Infrared Spectroscopy
An analytical method using infrared absorption for assessing the properties of used oil and certain contaminants suspended therein. See FTIR.

Inhibitor
An additive that improves the performance of a petroleum product through the control of undesirable chemical reactions.

Insolubles
A test for contaminants in used lubricating oils, such as test method ASTM D893. In this method, the oil is first diluted with pentane, causing the oil to lose its solvency for certain oxidation resins, and also causing the precipitation of such extraneous materials as dirt, soot, and wear metals. These contaminants are called pentane insolubles. The pentane insolubles may then be treated with toluene, which dissolves the oxidation resins (benzene was formerly used). The remaining solids are called toluene insolubles. The difference in weight between the pentane insolubles and the toluene insolubles is called insoluble resins.

Interfacial Tension (IFT)
The energy per unit area present at the boundary of two immiscible liquids. It is usually expressed in dynes/cm (ASTM D971.)

Invert Emulsion
Water-in-oil emulsion, typically containing 40% water, utilized as a fire- resistant fluid; oil is the outer or continuous phase of an invert emulsion, in contrast to the normal (oil-in-water) emulsion, where water is the outer phase.

ISO Cleanliness Code (ISO 4406)
A coding system used in defining the quantity of solid particles in the fluid used in a given hydraulic fluid power system. ISO Codes relate to the ≥4 µm, ≥6 µm and ≥14 µm particle size ranges.

ISO Viscosity Grade
A designation corresponding to the mid-point of a viscosity range expressed in centistokes (cSt) at 40°C. For example, a lubricant with an ISO grade of 32 has a viscosity within the range of 28.8-35.2 cSt, the mid-point of which is 32.

Karl Fischer Titration
A titration method in analytical chemistry that uses coulometric or volumetric titration to determine trace amounts of water in a sample. Previously ASTM D1744, now commonly ASTM D6304.

Kinematic Viscosity
A measure of viscosity derived from the time taken for a fixed volume of oil to flow through a capillary tube. Common units are mm2/s or centistokes (cSt).

Lacquer
A deposit resulting from the oxidation and polymerization of fuels and lubricants when exposed to high temperatures. Similar to, but harder, than varnish.

Laminar (reworked) Particles
Particles are large and thin and are most likely due to thicker wear particles having been squeezed through a rolling contact.

Load-carrying Capacity
A property of a lubricant to form a film on the lubricated surface, which resists rupture under given load conditions. Expressed as maximum load the lubricated system can support without failure or excessive wear.

Load-Wear Index (LWI)
A measure of the relative ability of a lubricant to prevent wear under applied loads; it is calculated from data obtained from the Four Ball EP Method. Formerly called mean Hertz load.

Lubricant
Any substance interposed between two surfaces in relative motion for the purpose of reducing the friction and/or the wear between them.

Lubricity
Ability of an oil or grease to lubricate; also called film strength.

Micrometer (µm)
See Micron.

Micron
A unit of length equal to one millionth of a meter or 39 millionths of an inch (.000039"). Contaminant size is usually described in microns. A micron is also known as a micrometer, and exhibited as µm

Microscopy
A method of particle counting which measures or sizes particles using an optical microscope.

Mineral Oil
An oil derived from a mineral source, such as petroleum, as opposed to oils derived from plants and animals.

Miscible
Capable of being mixed in any concentration without separation of phases; e.g., water and ethyl alcohol are miscible.

Molybdenum Disulfide
A black, lustrous powder (MoS2) that serves as a dry-film lubricant in certain high-temperature and high-vacuum applications. It is also used in the form of pastes to prevent scoring when assembling press-fit parts, and as an additive to impart residual lubrication properties to oils and greases. Molybdenum disulfide is often called moly or molysulfide.

Multigrade Oil
An oil meeting the requirements of more than one SAE viscosity grade classification, and may therefore be suitable for use over a wider temperature range than a single-grade oil.

Naphthenic
A type of petroleum fluid derived from naphthenic crude oil, containing a high proportion of closed-ring methylene groups.

Neutralization Number
A number used as a measure of the acidic or basic constituents. This term is ambiguous and now obsolete. (See Acid Number and Base Number)

Newtonian Fluid
A fluid with a constant viscosity at a given temperature regardless of the rate of shear. Single-grade oils are Newtonian fluids.

Nitration
nitration products are formed during the fuel combustion process in internal combustion engines. Most nitration products are formed when an excess of oxygen is present. These products are highly acidic, form deposits in combustion areas and rapidly accelerate oxidation.

NLGI
National Lubricating Grease Institute is a trade association whose main interest is grease and grease technology. NLGI is best known for its system of rating greases by penetration.

NLGI Consistency Grades
A simplified system established by the National Lubricating Grease Institute (NLGI) for rating the consistency of grease.

Nominal Filtration Rating
an arbitrary micrometer value indicated by a filter manufacturer. Due to lack of reproducibility this rating is deprecated.

Non-Newtonian Fluid
A fluid, such as a grease or a polymer-containing oil (e.g., multi-grade oil), in which shear stress is not proportional to shear rate.

Oxidation
The chemical combination of a substance with oxygen. All petroleum products are subject to oxidation, with resultant degradation of their composition and performance. The process is accelerated by heat, light metal catalysts (e.g., copper), and the presence of water, acids, or solid contaminants.

Oxidation Inhibitor
An additive that increases oxidation resistance, thereby lengthening the service or storage life of a lubricant; also called anti-oxidant. An oxidation inhibitor may work in one of these ways: (1) by combining with and modifying peroxides (initial oxidation products) to render them harmless, (2) by decomposing the peroxides, or (3) by rendering an oxidation catalyst inert.

Paper Chromatography
A method which involves placing a drop of fluid on a permeable piece of paper and noting the development and nature of the halos, or rings, surrounding the drop through time. The roots of this test can be traced to the 1940s, when railroads used the "blotter spot" tests.

Paraffinic
A type of petroleum fluid derived from paraffinic crude oil and containing a high proportion of straight chain saturated hydrocarbons. Often susceptible to cold flow problems.

Particle count
A test quantifying the number of particles present greater than a particular micron size per unit volume of fluid, often stated as particles per milliliter.

Patch test
A method by which a specified volume of fluid is filtered through a membrane filter of known pore structure. All particulate matter in excess of an "average size," determined by the membrane characteristics, is retained on its surface. Thus, the membrane is discolored by an amount proportional to the particulate level of the fluid sample. Visually comparing the test filter with standard patches of known contamination levels determines acceptability for a given fluid.

pH
A measure of alkalinity or acidity in water and water-containing fluids. pH can be used to determine the corrosion-inhibiting characteristic in water-based fluids. Typically, pH > 8.0 is required to inhibit corrosion of iron and ferrous alloys in water-based fluids.

Polar compound
A chemical compound whose molecules exhibit electrically positive characteristics at one extremity and negative characteristics at the other. Polar compounds are used as additives in many petroleum products. Polarity gives certain molecules a strong affinity for solid surfaces; as lubricant additives (oiliness agents), such molecules plate out to form a tenacious, friction-reducing film. Some polar molecules are oil-soluble at one end and water-soluble at the other end; in lubricants, they act as emulsifiers, helping to form stable oil-water emulsions. Such lubricants are said to have good metal-wetting properties. Polar compounds with a strong attraction for solid contaminants act as detergents in engine oils by keeping contaminants finely dispersed.

Polyglycols
Polymers of ethylene or propylene oxides used as a synthetic lubricant base. Properties include very good hydrolytic stability, high viscosity index (VI), and low volatility. Used particularly in water emulsion fluids.

Polymerization
the chemical combination of similar-type molecules to form larger molecules.

Polyolefin
A polymer derived by polymerization of relatively simple olefins. Polyethylene and polyisoprene are important polyolefins.

Polyol Ester
A synthetic lubricant base, formed by reacting fatty acids with a polyol (such as a glycol) derived from petroleum. Properties include good oxidation stability at high temperatures and low volatility. Used in formulating lubricants for turbines, compressors, jet engines, and automotive engines.

Pour Point
The lowest temperature under which an oil will flow when cooled under prescribed conditions.

Pour Point Depressant
An additive which retards the adverse effects of wax crystallization, and lowers the pour point.

PPM
Parts per million is used to expression concentration, generally by weight, as in mg/kg.

Pumpability
The low temperature, low shear stress-shear rate viscosity characteristics of an oil that permit satisfactory flow to and from the engine oil pump and subsequent lubrication of moving components.

Quenching Oil
A high-quality, oxidation-resistant petroleum oil used to cool metal parts during their manufacture, and is often preferred to water because the oil's slower heat transfer lessens the possibility of cracking or warping of the metal. A quenching oil must have excellent oxidation and thermal stability, and should yield clean parts, essentially free of residue. In refining terms, a quenching oil is an oil introduced into high temperature vapors of cracked (see cracking) petroleum fractions to cool them.

R&O Oil
Rust-and-oxidation inhibited oil. A term applied to highly refined industrial lubricating oils formulated for long service in circulating lubrication systems, compressors, hydraulic systems, bearing housing, gear boxes, etc. The finest R&O oils are often referred to as turbine oils.

Rerefining
A process of reclaiming used lubricant oils and restoring them to a condition similar to that of virgin stocks by filtration, clay adsorption or more elaborate methods.

Rheology
The study of the deformation and flow of matter in terms of stress, strain, temperature, and time. The rheological properties of a grease are commonly measured by penetration and apparent viscosity.

Rolling Contact Fatigue
Damage process in a triboelement subjected to repeated rolling contact loads, involving the initiation and propagation of fatigue cracks in or under the contact surface, eventually culminating in surface pits or spalls.

Rolling Oil
An oil used in hot- or cold-rolling of ferrous and non-ferrous metals to facilitate feed of the metal between the work rolls, improve the plastic deformation of the metal, conduct heat from the metal, and extend the life of the work rolls. Because of the pressures involved, a rolling oil may be compounded or contain EP additives. In hot rolling, the oil may also be emulsifiable.

Rust Inhibitor
A type of corrosion inhibitor used in lubricants to protect surfaces against rusting.

Rust Prevention Test
A test for determining the ability of an oil to aid in preventing the rusting of ferrous parts in the presence of water.

Saturation Level
The amount of water that can dissolve in a fluid.

Saybolt Universal Viscosity (SUV) or Saybolt Universal Seconds, (SUS)
The time in seconds required for 60 cubic centimeters of a fluid to flow through the orifice of the Standard Saybolt Universal Viscometer at a given temperature under specified conditions. (ASTM D88.)

Semisolid
Any substance having the attributes of both a solid and a liquid. Similar to semiliquid but being more closely related to a solid than a liquid. More generally, any substance in which the force required to produce a deformation depends both on the magnitude and on the rate of the deformation.

Shear Rate
The rate at which adjacent layers of a fluid move with respect to each other, usually expressed as reciprocal seconds (also see shear stress). When the fluid is placed between two parallel surfaces moving relative to each other.

Shear stress
The frictional force overcome in sliding one "layer" of fluid along another, as in any fluid flow. The shear stress of a petroleum oil or other Newtonian fluid at a given temperature varies directly with shear rate (velocity). The ratio between shear stress and shear rate is constant; this ratio is termed viscosity of a Newtonian fluid, the greater the shear stress as a function of rate of shear. In a non-Newtonian fluid such as a grease or a polymer-containing oil (e.g. multi-grade oil) shear stress is not proportional to the rate of shear. A non-Newtonian fluid may be said to have an apparent viscosity, a viscosity that holds only for the shear rate (and temperature) at which the viscosity is determined.

Soluble Oil
A metal removal fluid typically composed of a stable milky emulsion of water, oil, emulsifiers and other functional additives. Commonly used where cooling is of primary importance.

Solvency
The ability of a fluid to dissolve inorganic materials and polymers, which is a function of aromaticity.

Specific Gravity
The ratio of the weight of a given volume of liquid to the weight of an equal volume of water.

Spectrographic Analysis
A technique for detecting and quantifying metallic elements resulting from wear, contamination, or additives. The oil sample is energized to make each element emit or absorb a quantifiable amount of energy, which indicates the element's concentration in the oil. (See Emission Spectroscopy)

Spindle Oil
A light-bodied oil used principally for lubricating textile spindles and for light, high-speed machinery.

Static Friction
The force just sufficient to initiate relative motion between two bodies under load. The value of the static friction at the instant relative motion begins is termed break-away friction.

Sulfated Ash
The ash content of fresh, compounded lubricating oil as determined by ASTM D874. Indicates level of metallic additives in the oil.

Surfactant
Surface-active agent that reduces interfacial tension of a liquid. A surfactant used in a petroleum oil may increase the oil's affinity for metals and other materials, or help reduce foam stability.

Synthetic Lubricant
A lubricating fluid made by chemically reacting materials of a specific chemical composition to produce a compound with planned and predictable properties; the resulting base stock may be supplemented with additives to improve specific properties. Many synthetic lubricants -- also called synlubes -- are derived wholly or primarily from petrochemicals; other synlube raw materials are derived from coal and oil shale, or are lipochemicals (from animal and vegetable oils). Synthetic lubricants may be superior to petroleum oils in specific performance areas. Many exhibit higher viscosity index (VI), better thermal stability and oxidation stability, and low volatility (which reduces oil consumption). Most synlubes offer longer service life and, in some cases, better biodegradability than conventional lubricants. Consequently, they are increasingly being used in industrial and automotive applications. Individual synthetic lubricants offer specific outstanding properties: phosphate esters, for example, are fire resistant, diesters have good oxidation stability and lubricity, and silicones offer exceptionally high VI. Polyalphaolefins are versatile lubricants with low pour points, and excellent thermal and oxidation stability; they have good compatibility with petroleum lubricants and most seals used with petroleum lubricants. Most synthetic lubricants can be converted to grease by adding thickeners. Because synthetic lubricants are higher in cost than petroleum oils, they are used selectively where performance or safety requirements may exceed the capabilities of a conventional oil.

Tackifier (Tackiness Additive or Agent)
A high molecular weight, fluid polymer added to greases to improve adhesiveness.

Thermal Stability
The ability of a fuel or lubricant to resist oxidation under high temperature operating conditions.

Thermography
The use of infrared thermography whereby temperatures of a wide variety of targets can be measured remotely and without contact. This is accomplished by measuring the infrared energy radiating from the surface of the target and converting this measurement to an equivalent surface temperature.

Thixotropy
The tendency of grease or other material to soften or flow when subjected to shearing action. Grease will usually return to its normal consistency when the action stops. Thixotropy is also an important characteristic of drilling fluids, which must thicken when not in motion so that the cuttings in the fluid will remain in suspension.

Tribology
the science and technology of interacting surfaces in relative motion, including the study of lubrication, friction and wear. Tribological wear is wear that occurs as a result of relative motion at the surface.

Vapor Pressure
Pressure of a confined vapor in equilibrium with its liquid at specified temperature thus, a measure of a liquid's volatility.

Vapor Pressure-Reid (RVP)
measure of the pressure of vapor accumulated above a sample of gasoline or other volatile fuel in a standard bomb at 100°F (37.8°C). Used to predict the vapor locking tendencies of the fuel in a vehicle's fuel system. Controlled by law in some areas to limit air pollution from hydrocarbon evaporation while dispensing.

Varnish
A hard coating formed from oil oxidation products, that bakes on to surfaces during high-temperature operation of auto-motive engines and industrial machinery. Varnish can accelerate cylinder wear. Varnish formation can be reduced with the use of a detergent-dispersant and an oxidation inhibitor in the oil.

Viscometer or Viscosimeter
an apparatus for determining the viscosity of a fluid.

Viscosity
A measurement of a fluid's resistance to flow. The common metric unit of absolute viscosity is the poise, which is defined as the force in dynes required to move a surface one square centimeter in area past a parallel surface at a speed of one centimeter per second, with the surfaces separated by a fluid film one centimeter thick. In addition to kinematic viscosity, there are other methods for determining viscosity, including Saybolt Universal Viscosity (SUV), Saybolt Furol viscosity, Engier viscosity, and Redwood viscosity. Since viscosity varies in inversely with temperature, its value is meaningless until the temperature at which it is determined is reported.

Viscosity, Absolute
The ratio of the shearing stress to the shear rate of a fluid. It is usually expressed in centipoise.

Viscosity, Kinematic
The absolute viscosity divided by the density of the fluid. It is usually expressed in centistokes.

Viscosity, SUS
Saybolt Universal Seconds (SUS), which is the time in seconds for 60 milliliters of oil to flow through a standard orifice at a given temperature. (ASTM D88)

Viscosity Grade
Any of a number of systems which characterize lubricants according to viscosity for particular applications, such as industrial oils, gear oils, automotive engine oils, automotive gear oils, and aircraft piston engine oils.

Viscosity index (VI)
An empirical, unitless number indicating the effect of temperature on the kinematic viscosity of an oil. Liquids change viscosity with temperature, becoming less viscous when heated; the higher the VI of an oil, the lower its change in viscosity with temperature. The VI of an oil -- with known viscosity at 40°C -- is determined by comparing the oil with two standard oils having an arbitrary VI of 0 and 100, respectively, and both having the same viscosity at 100°C as the test oil.

Viscosity Index Improver
A polymeric additive designed to increase the viscosity index of an oil. Also known as a viscosity modifier.

Volatility
An expression of evaporation tendency. The more volatile a petroleum liquid, the lower its boiling point and the greater its flammability.

Way Lubricant
A lubricant for the sliding ways of machine tools such as planers, grinders, horizontal boring machines, shapers, jig borers, and milling machines. A good way lubricant is formulated with special frictional characteristics designed to overcome the stick-slip motion associated with slow-moving machine parts.

White Oil
Light-colored, usually highly-refined mineral oils frequently employed in pharmaceutical and medicinal preparation and used as bases for creams, salves and ointments. White oil lubricants are used where color and/or environmental concerns are important.

ZDDP
An antiwear additive found in many types of hydraulic and lubricating fluids. Zinc dialkyldithiophosphate.