Glossary of Terms


Abrasion: A wearing, grinding or rubbing away caused by friction usually involving the action of particles against or between surfaces.

Absolute 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). 1 centipoise equals .01 poise.

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: Specific to oil analysis, acidity denotes the presence of weakly and strongly acidic materials whose total concentration is usually defined in terms of the TAN (Total Acid Number).

Additives: An agent added to oils, fuels, and coolants to impart specific beneficial properties to the finished products. Additives create new fluid properties, enhance properties already present, and reduce the rate at which undesirable changes take place in a fluid during service.

Aeration: Combining or charging a fluid with a gas.

Air entrainment : The incorporation of air (bubbles) as a dispersed phase in a liquid. May result from mechanical means or by sudden environmental changes. The presence of 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.

Alkali: Any substance having basic as opposed to acidic properties. A more limited definition restricts the alkali to hydroxides of ammonium, lithium, potassium and sodium.

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.

API Engine Service Categories: gasoline and diesel engine oil quality levels established jointly by API, SAE, and ASTM, and sometimes called SAE or API/SAE categories; formerly called API Engine Service Classifications.

API Gravity: a gravity scale established by the American Petroleum Institute and in general use in the petroleum industry, the unit being called "the API degree", which is represented by the equation:

API gravity, deg = (141.5/specific gravity at 60/60ºF)-131.5

API base stock: base fluid for automotive engine oils. The API (American Petroleum Institute) currently designates five classes of base stocks. These stock types are independent of crude source or processing. Instead, they are based on physical characteristics of the base stock, which are then related to formulated engine oil performance


Babbitt: A soft, white, non-ferrous alloy of copper, antimony, tin and lead. Found in sleeve/journal bearing overlays.

Base: A material that neutralizes acids. Also, term referring to an oil additive containing colloidally dispersed metal carbonate used to reduce corrosive wear. Total concentration is usually defined in terms of the TBN (Total Base Number).

Base stock: A primary refined petroleum fraction or a selected synthetic material into which additives are blended to produce finished lubricants.

Bearing: A machine part which positions and supports load and movement through the action of fitted or formed surfaces. These surfaces move with respect to each other by sliding, rolling, reciprocating, or by combinations of these motions. Selected types are as follows:

Rolling element bearings

Deep groove ball
Cylindrical roller
Tapered roller
Spherical roller
Needle roller

Generally consist of two races (rings or raceways) with a set of rolling elements running in their tracks. The rolling elements are in the form of balls or various types of rollers. The outer race is located in a housing and the inner race on the shaft. In 'self-aligning' types, often one of the races is not fixed axially but is free to move to allow for limited shaft movement. In 'thrust' types, the races are configured to support additional angular contact for axial loading. In needle roller bearings, there is no inner race, no element separator, and the rolling elements turn very close together directly against the shaft.

Plain (sliding) bearings


Generally consist of a unmoving solid surface conforming to, and operating directly against, an opposing shaft or plate. Journals may be single-piece or in connected sections, and wrap around all or part of the shaft surface. Guide bearings align rotating or reciprocating parts, while thrust bearings prevent a shaft from moving endwise and support heavy directional loads.

Tilting pad thrust bearing

Characterized by a journal-like enclosure of several separately mounted pivoting pads mounted at right angles to the shaft.

Sector pad thrust bearing
(also known as Kingsbury bearing)

Characterized by a series of pivoted wedge-shaped segments mounted parallel to the shaft, against a circular thrust collar extending outward at right angles from the shaft.

Flat bearings

Various types of slides, guides and ways.

Blow-By: Passage of unburned fuel and combustion gases beyond the piston rings of internal combustion engines which results in fuel dilution and contamination of the crankcase oil.

Boiling Point : The temperature at which a substance boils or is converted into vapor by bubble formation within the liquid. It varies as pressure does.

Brookfield Viscosity : Apparent viscosity in cP (centipoise) as determined by Brookfield viscometer which measures the torque required to rotate a spindle at constant speed in oil of a given temperature. Basis for measuring low temperature viscosity of lubricants.

BS&W (Bottom Sediment and Water) : The material that collects in the bottom of storage tanks which is usually composed of oil, water, and foreign matter. Also called bottoms, or bottom settling and water.

Bushing: A fixed or removable cylindrical lining for an opening used to limit the size of the opening, resist abrasion, or serve as a guide.

Bypass filtration: Filtration approach in which only part of the total flow of a circulating fluid system passes through a filter at any given time, or approach in which a separate pump and filter combination operates in parallel to the main flow.


Calibration / Standardization : These two terms have a broad or narrow definition, depending on circumstances, and some sources use them interchangeably. Calibration connotes extensive or exacting measurements under specified conditions to determine how closely the output values of a measuring process or system compare to nationally or internationally traceable standards of verified known value. Once this measurement has been performed, any deviation from required performance specifications may be corrected by adjustment, repair, or replacement. Standardization connotes a procedure for making limited routine measurements and adjustments to the process or system in order to correct minor known variations such as those associated with changes in operating environment or working materials. Calibrations are normally performed at longer intervals (annual or semi-annual) set by instrument manufacturers, official standardizing agencies, or operating circumstances, while standardizations are typically done on a daily or weekly basis. Not all simple systems or processes are readily adjustable. For example, the contents of a liquid-in-glass thermometer are not readily adjustable. In such a case the calibration would only involve determining the thermometer’s ‘true’ reading as opposed to its stated reading and applying a correction factor, without the corresponding physical adjustments normally associated with the calibration process. For complex instrument systems used in modern oil analysis laboratories, both manual and electronic calibrations and standardizations are essential in maintaining acceptable performance.

Cam: A rotating or sliding eccentric part which passes motion to a roller or pin moving against its edge. Used to cyclically raise, lower, or otherwise move a part a specific distance, such as the action of a cam moving an engine's valves.

Carbon: A non-metallic element found in the native, or uncombined, form as graphite or diamond. Carbon is a constituent of all organic compounds including coal, petroleum, asphalt, etc. It also occurs in combined forms in many inorganic substances like carbon dioxide, limestone, etc.

Carbon ResidueCoked material that remains after oil has been exposed to high temperatures under controlled conditions.

Cavitation Damage : The degradation of a solid body resulting from its exposure to the sudden formation and collapse of low pressure bubbles in liquids by means of mechanical forces. This may include loss of material, surface deformation, or changes in properties or appearance.

Centipoise (cP): A standard reporting unit of absolute viscosity. 1 centipoise = 0.01 poise

Centistoke (cSt): A standard reporting unit of kinematic viscosity. 1 centistoke = 0.01 stoke

Cetane Number: The performance rating of a diesel fuel, expressed as a percentage of cetane that must be mixed with liquid methylnaphthalene to produce the same ignition performance as the diesel fuel being rated. This estimate is used if a standard test engine is not available, or if the sample is too small for an engine test.

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

Coolant: A fluid used to remove heat. This term has broad application in any process which generates heat as a result of frictional or combustion processes.

Corrosion: The decay and loss of a metal due to a chemical reaction between the metal and its environment. It is a transformation process in which the metal passes from its elemental form to a combined, or compound, form.

Cutting Fluid: Any fluid that assists with the cutting operation by cooling or lubricating the cutting tool or the material being cut.

Cylinder: A device that converts fluid power into linear mechanical force and motion. It usually consists of a moveable element such as a piston and piston rod, plunger rod, plunger or ram, operating with in a cylindrical bore.

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Density: The mass of a unit volume of a substance. Its numerical value varies with the units used.

Deposit: Process of a solid residue accumulating on a surface. For example, oil-insoluble materials resulting from oxidation and decomposition of lube oil and contamination from external sources and engine blow-by are deposited on machine or engine parts. Other examples are sludge, varnish, lacquer and carbon.

Detection Limit : Practically defined as the point where the signal level decreases to less than two to three times the noise level.

Detergent: In lubrication, either an additive or a compounded lubricant having the property of keeping insoluble matter in suspension thus preventing its deposition where it would be harmful. A detergent may also redisperse deposits already formed.

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

Dieseling: The continued running of a spark-ignited engine after the ignition is turned off. There are two basic causes of dieseling. The first is surface ignition which means combustion chamber surfaces remain hot enough to ignite fuel after the spark is terminated. The second is compression ignition where the conditions of temperature, pressure, fuel composition and engine idle speed allow ignition to continue.

Differential Pressure Indicator : An indicator that signals the difference in pressure between any two points of a system or a component.

Distillation: Process of driving gas or vapor from liquids or solids by heating then condensing the vapor for the purposes of separation, purification, or measurement.


Emulsion: Intimate mixture of oil and water which generally creates 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).

Environmental Contaminant : Material entering a system from an operating system's external surroundings, such as dust, air, moisture or chemicals.

Erosion: The progressive removal of a machine surface by cavitation or by particle impingement at high velocities.


Failure: A general term used to imply that a part or system has become completely inoperable, is still operable but is incapable of satisfactorily performing its intended function, or has deteriorated seriously to the point that it has become unreliable or unsafe for continued use.

Fatigue: A structural failure due to flexing caused by cyclic motions or cyclic differential pressures which do not exceed the tensile strength of the material.

Ferrography: A method of debris analysis that uses a high gradient magnetic field to attract, hold and deposit particles contained in a fluid. The resulting slide traps both magnetic and non-magnetic particles which are then microscopically examined for characteristics that reveal size, composition, mode of wear, and possible source.

Filter: Any device or porous substance used as a strainer for cleaning fluids by removing suspended matter.

Filtration: The physical or mechanical process of separating insoluble particulate matter from a fluid by passing the fluid through a medium that traps the insoluble particles.

Flash Point : The lowest temperature at which vapors arising from the oil will ignite momentarily when exposed to a flame.

Flow Rate : The volume, mass, or weight of a fluid passing through any conductor per unit of time.

Fluid: A general classification of physical state, including liquids and gases.

Fluid Compatibility : Assessment of a fluid mixture or one or more of its components to avoid or control undue effects on fluid properties, filters, seals or the system serviced with the mixture.

Foam: An agglomeration of gas bubbles separated from each other by a thin liquid film, which is observed as a persistent phenomenon on the surface of a liquid.

Friction: The resisting force encountered at the common boundary between two bodies when, under the action of an external force, one body, moves or tends to move relative to the surface of the other.


Gear: A cylindrical or conical part using a tooth or screw-based surface configuration to mechanically transmit power from one portion of a machine to another. Gear designs are based in part on the shaft alignment: parallel, angled, over-and-under, etc.

Selected gear types are as follows:



Spur gear

Has teeth on the outside of a cylindrical body that are straight and parallel to the axis of rotation.

Helical gear

Has teeth that spiral around the outside of a cylindrical body at an angle.

Internal gear

Has teeth on the inside of a hollow cylindrical shape.

Bevel or miter gear

Has teeth on the outside of a conical body. They may be straight cut (as in the plain bevel gear), or spiral cut (as in the spiral bevel gear). Both transmit motion between intersecting shafts at various angles.

Hypoid gear

Has teeth cut in spiral bevel pattern, but set on non-intersecting shafts crossing at a right angle (over-and-under).

Worm gear

Has threads that wrap around a cylindrical body.

Herringbone gear

Has two separate rows of adjoining teeth on the same gear, cut in the configuration of two connected helical gears with teeth angled in a V-shaped alignment.

Crown gear

Has teeth set in the rim perpendicular to the rotation plane of the gear.

Straight gear (rack and pinion)

Has a toothed bar into which a worm or spur type 'pinion' meshes, normally used to translate rotating motion into reciprocating motion.

Ratchet and pawl

Has a toothed wheel or bar which catches a 'pawl' (a mechanical device that allows rotation only in one direction).


Has a gearlike wheel which drives or is driven by a chain as opposed to direct mesh.


Gear Train : Group or sequence of gears which perform a desired mechanical transfer of power. Selected gear train types are as follows:




Gear train characterized by multiple selectable gear speed ratios and the ability to uncouple the gear train from the power source to permit starting and stopping the gear train without stopping the power source. Used in applications where varying speed/torque/output direction requirements must be satisfied by a single geared system.


Gear train characterized by the connection of two output shafts or axles in the same line, with an epicyclic (one or more parts travel around the circumference of another fixed or moving part) gear arrangement permitting one output shaft to revolve faster than the other.


Gear train characterized by a predominantly epicyclic arrangement, consisting of a series of planet gears rotating in a carrier between a central sun gear and an outer ring gear (of internal gear type). A planetary gearset may be configured in a variety of ways, depending on which part of the gearset is used for power input, which part is held stationary or braked, which part is used for power output, and the actual number and arrangement of sun and planet gears. Generally, the more planet gears the greater the torque capacity of the system.

Speed increaser or reducer

Gear train characterized by high to extremely high ratios of input to output speed, for the purposes of large-scale RPM increases or large-scale torque increases. Usually a single-speed gear ratio; if multispeed, they differ from transmissions in that they are shifted as often or as easily.

Gravimetric Analysis: A method of analysis whereby the dry weight of contaminant per unit volume of fluid can be measured showing the degree of contamination in terms of milligrams of contaminant per liter of fluid.

Grease: A lubricant composed of an oil or oils thickened with a soap, soaps or other thickener to a semi-solid or solid consistency.


Hydraulic Fluid : Fluid serving as the power transmission medium in a hydraulic system with specific properties tailored to the fluid's use in this application.

Hydraulics: Engineering science pertaining to liquid pressure and flow.

Hydrocarbon: A compound containing only hydrogen and carbon. The simplest hydrocarbons are gases at ordinary temperatures; but, with increasing molecular weight, they change to the liquid form and, finally, to the solid state. They form the principal constituents of petroleum.


Inhibitor: Any substance that slows or prevents such chemical reactions as corrosion or oxidation.

Insolubles: Residues of carbon or agglomerates of carbon and other material such as spent additives or oxidation by-products.

ISO Solid Contaminant Code: Index number code assigned on the basis of the number of particles per unit volume, allowing quick assessment of contamination.

ISO Viscosity Grade : Number indicating the nominal viscosity of an industrial fluid lubricant at 40°C (104°F) as defined by ISO Standard 3448. For example, an unused ISO 68 grade product would be expected to show a viscosity at 40°C of 68 cSt, plus or minus 10% (6.8 cSt). This pattern is maintained throughout the ISO viscosity grading system.

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Kinematic Viscosity: The ratio of the absolute viscosity to the density at the temperature of the viscosity measurement. The metric units of kinematic viscosity are the stoke and centistoke which correspond to the poise and centipoise of absolute viscosity.


Lubricant: Any usually oily liquid or solid of vegetable, animal, mineral or synthetic origin that reduces friction, heat, and wear when applied to the surfaces of moving parts.

Lubricity: Ability of an oil or grease to lubricate. Also called film strength.


Micron, Micrometer (µm): a unit of length. One micron = 39 millionths of an inch (.000039"). Contaminant size is usually described in microns. Relatively speaking, a grain of salt is about 60 microns and the eye can see particles to about 40 microns. Many hydraulic filters are required to be efficient in capturing a substantial percentage of contaminant particles as small as 5 microns.

Mineral Oil: Any petroleum oil, as opposed to animal or vegetable-based oils.

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. For example, multigrade SAE 1 OW-40 grade may be used where SAE 1 OW, SAE 20W, SAE 20, SAE 30, or SAE 40 grades are specified. These oils are made possible by improved refining processes and the use of polymer additives.


Neutralization Number: A measure of the total acidity or basicity of an oil. Includes organic or inorganic acids or bases or a combination thereof.

Nitration: Specific to internal combustion engines, the formation of nitrogen oxides (NOx) during fuel combustion. NOx escapes the combustion area via ring blowby and reacts with water in the crankcase to form nitrous acid (HNO2), which can degrade the oil and increase oil viscosity. Nitration is a particular problem in gas engines due to relatively high combustion chamber temperatures.


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 organic acids formed by oxidation are corrosive to metals. The process is accelerated by heat, light, metal catalysts and the presence of water, acids, or solid contaminants. The basic oxidation process for hydrocarbons is summarized as:

  • Organic peroxides form as the first reaction products;
  • Peroxides catalyze continued formation of alcohols, aldehydes, ketones, and organic acids;
  • Alcohols, etc. further oxidize to form high molecular-weight, oil insoluble polymers;
  • Polymers settle out as sludges, varnishes and gums.

Oxidation Stability: Ability of a substance to resist natural degradation upon contact with oxygen.

Particle: A minute quantity or solid fragment, not necessarily of uniform composition. Particle size, quantity, and characteristics (composition, shape, surface features, color, etc.) are all important factors considered in a total fluid analysis. A particle’s visual characteristics may be described generically. They may also have specific terms associated with them, especially when certain wear modes are under examination.

Particle Count: The number of particles present greater than a particular micron size per unit volume of fluid. Selected methods of particle counting are:

  • automated optical, where particles are directly sized and counted using the dispersion or blockage of light created by a solid particle passing between a light source and a sensor;
  • Image analysis, where stationary particles are directly sized and counted under the microscope by manual or automated systems;
  • Electrical resistance, which measures the volume of a particle as it passes through an orifice in an electrically conductive liquid;
  • Flow decay, where count and size data is extrapolated from a standard equation applied to the rate at which flow decreases through a screen as particles block it.

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: 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.

Poise (Absolute Viscosity): A unit of viscosity equal to the viscosity of a fluid that would require a shearing force of one dyne to move a square-centimeter area of either of two parallel layers of fluid one centimeter apart, with a velocity of one centimeter per second relative to the other layer, with the space between the layers being filled with the fluid in question. It is the ratio of the shear stress to the shear rate of a fluid, expressed in dyne seconds per square centimeter. 1 centipoise equals .01 poise.

Positive Crankcase Ventilation (PCV): System for removing blow-by gases from the crankcase and returning them through the carburetor intake manifold to the combustion chamber where the recirculated hydrocarbons are burned. A PC valve controls the flow of gases from the crankcase to reduce hydrocarbon emissions.

Pressure: Force per unit area, usually expressed in pounds per square inch.

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Rings : Circular metallic elements that ride in the grooves of a piston providing compression sealing during combustion. Also used to spread oil for lubrication.

Ring Sticking: Freezing of a piston ring in its groove in a piston engine or reciprocating compressor due to heavy deposits in the piston ring zone.

Rust: A corrosion product consisting of hydrated oxides of iron.


SAE Viscosity Number : System for classifying crankcase, transmission, and differential lubricants, according to their viscosities, established by the Society of Automotive Engineers. SAE numbers are used in connection with recommendations for crankcase oils to meet various design, service, and temperature requirements affecting viscosity only.

Silt: Contaminant particles 5 µm and less in size.

Silting: A failure generally associated with a valve when its movements are restricted due to small particles that have wedged in between critical clearances (e.g., the spool and bore).

Solid: Any substance having a definite shape which does not readily relinquish. More generally, any substance in which the force required to produce a deformation depends upon the magnitude of the deformation rather than upon the rate of deformation.

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

Spectrometry: Using the analysis of electromagnetic radiation (light) to determine trace elements and their concentrations in a sample. In the atomic spectrometry, techniques most commonly used for trace element analysis cause the decomposition of a sample by intense heat into a cloud of hot gases containing free atoms and ions of the element of interest. In general, there are four types of thermal sources normally used in analytical atomic spectrometry to perform this decomposition process: flames, furnaces, direct electrical discharges, and plasmas. Instruments which separate, isolate and measure light by wavelength in this way are called spectrographs or spectrometers.

  • Atomic absorption spectrometry shines a light of a wavelength characteristic of the element of interest through the hot vapor (usually created by a flame or furnace). Some of this light is then absorbed by the atoms of that element. The amount of light that is absorbed is measured and used to determine the concentration of that element in the sample.
  • Optical emission spectrometryuses electrical discharges or radio-frequency stimulated plasmas to bring the sample to temperatures high enough to disassociate the sample into atoms and introduce significant amounts of excitation and ionization through atom-to-atom collisions. Once the atoms or ions are in this excited state, then they decay to lower states by radiating light. The intensity of the light is measured at specific wavelengths and used to determine the elemental concentrations. For example, two popular types of spectrometers use the emission principle:
    • Rotating disc emission(RDE) spectroscopy, where the electric arc struck between a rod and a rotating disc or between two rotating discs provides the energy source;
    • Indutively coupled plasma (ICP) spectroscopy, where a high-temperature discharge is generated by flowing a conductive gas (for example, argon) through the magnetic field generated by a radio-frequency load coil that surrounds the tubes carrying the gas. This highly energetic source stimulates the necessary light emission from the elements being analyzed.

Stainless Steel : Any of several steels containing 12% to 30% chromium as the principal alloying element; the steels usually are passive in aqueous environments.

Synthetic Lubricant: A lubricant produced by chemical synthesis rather than by extraction or refinement of petroleum to produce a compound with planned and predictable properties.

Sulfated Ash: The ash content of fresh, compounded lubricating oil as determined by ASTM Method D 874. Indicates level of metallic additives in the oil.

Synthetic Oil: Lubricant produced by synthesis rather than by extraction or refinement.


Turbidity: The degree of opacity of a fluid.


Valve: A device which controls fluid flow direction, pressure, or flow rate.

Valve Lifter: A component in engine designs that uses a linkage system between a cam and the valve it operates. The lifter typically translates the rotational motion of the cam to a reciprocating linear motion in the linkage system. Sometimes called a "cam follower."

Varnish: A thin film deposit resulting from the oxidation and polymerization of fuels and lubricants. Similar to but softer than lacquer, and not removable by mechanical wiping.

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

Viscosity: 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 inversely with temperature, its value is meaningless until the temperature at which it is determined is reported.

Viscosity, SUS: Saybolt Universal Seconds (SUS) is the time in seconds for 60 milliliters of oil to flow through a standard orifice at a given temperature. This viscosity system is still in limited use, although it has been generally replaced by the ISO grading system.

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. ISO and SAE are the two systems most often encountered in oil analysis applications.

Viscosity Index (VI): Empirical unitless number indicating the effect of temperature change on the kinematic viscosity of an oil. The higher the viscosity index, the smaller the relative change in viscosity with temperature.

Viscous: Possessing viscosity. Frequently used to imply high viscosity.

Volatility: Expression of evaporation tendency. The degree and rate at which a liquid vaporizes under set parameters of temperature and pressure. The more volatile a petroleum liquid, the lower its boiling point and the greater its flammability. Changes in liquid stability may result in reduced volatility.


Wear: The attrition or rubbing away of the surface of a material as a result of mechanical action. There is some difference of opinion on types of wear.


ZDDP: Common abbreviated term for an antiwear additive found in many types of hydraulic and lubricating fluids, specifically zinc dialkyldithiophosphate.

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