Lubricant Terms



Additive - a chemical added in small quantities to a petroleum product to improve certain properties. Such is the complexity of their action, that additives should not be incorporated indiscriminately, but should constitute well-defined components of the product’s original formulation, a formulation determined by research to yield best performance. Among the more common petroleum-product additives are: oxidation inhibitors for increasing the product’s resistance to oxidation and for lengthening its service life, rust inhibitors and corrosion inhibitors to protect lubricated surfaces against rusting and corrosion, demulsifiers to promote oil-water separation, V.I. improvers to make an oil’s viscosity less sensitive to changes in temperature, pour-point depressants to lower the pour point of petroleum products, oiliness agents, anti-wear agents, EP additives, etc., to prevent high friction, wear, or scoring under various conditions of boundary lubrication, detergents and dispersants to maintain cleanliness of lubricated parts, anti-foam agents to reduce foaming tendencies, tackiness agents to increase the adhesive properties of a lubricant, improve retention, and prevent dripping and spattering.

Anti-wear Agent - an additive that minimizes wear caused by metal-to-metal contact during conditions of mild boundary lubrication (e.g. stops and starts. Oscillating motion). The additive reacts chemically with, and forms a film on metal surfaces under normal operating conditions.

Auto-ignition Temperature - minimum temperature at which a combustible fluid will burst into flame without an extraneous ignition source. The auto-ignition temperature assumes only enough "fuel" to form and explosive mixture in the presence of air at atmospheric pressures. The auto-ignition temperature may vary considerably depending upon the conditions of the test. For petroleum products the conditions are outlined in ASTM D 2155. Auto-ignition temperature is not to be confused with flash or fire points, which are generally a few hundred degrees lower. As a generality for gasoline, the higher the octane number, the higher the auto-ignition temperature. For a diesel fuel, the lower the cetane number, the higher the auto-ignition temperature. Such correlations are very general and assume the same boiling points or ranges in each case. In general industrial practice, auto-ignition temperature has its greatest importance with process oils such as heat transfer oils or transformer oils, and with solvents such as are used in cooking resins.


Bearing Corrosion - chemical attack on bearing metal or on one of the metals in a bearing alloy caused by acids evolved during chemical deterioration of the oil. The acids may be mild organic acids from the oil itself or, more likely, the strong acids that result from breakdown of nitrogen or sulfur compounds, which can enter the oil from several sources.

Blow-by - seepage of fuel and gases from the combustion chamber of an internal combustion engine into the crankcase as the result of the high pressure differential, incomplete combustion, loose rings, etc.


Crude Oil - naturally occurring hydrocarbon fluid that contains small amounts of nitrogen, oxygen, and sulfur derivatives and other impurities. It is refined to yield finished petroleum products and petrochemical feedstocks. The hydrocarbon composition of crude oils varies widely. Two broad classifications are naphthenic and paraffinic. The nature of crude oil is related to its geographical location and it is usually named accordingly.

cSt - abbreviation of centistoke.


Demulsibility - test time required for a specified oil-water emulsion to break, ASTM D 1401. Demulsibility is thus a measure of a lube oil’s ability to separate from water, an important consideration in the lubricant maintenance of many circulating systems.

Detergent - an additive in crankcase oils generally combined with (and confused with) dispersant additives. A detergent chemically neutralizes acidic contaminants in the oil before they become insoluble and fall out of the oil forming sludge. Neutral or basic compounds are created which can remain in suspension in the oil. Dispersants operate to break up insoluble contaminant particles already formed. Particles are kept finely divided so that they can remain "dispersed" or colloidally suspended in the oil.

Dielectric Strength - minimum voltage required to produce an electric arc through an oil sample under standard conditions. ASTM D 877 or D 1816. Hence, a measure of the insulating (arc-preventive) properties of a transformer oil. A low dielectric-strength value may indicate contamination, especially water. Also called the breakdown voltage.


Emulsion - a mechanical mixture of two mutually insoluble liquids (such as oil and water). Emulsification may or may not be desirable, depending on circumstances. Highly refined straight mineral oils are inherently resistant to emulsification. Even after violent shaking, the oil separates and rises rapidly to the top of the water. This is true also of other oils formulated for good demulsibility, and it is a desirable characteristic of oils, such as circulating lube oils, that must separate from water readily. Demulsibility may be reduced, however, by the presence of oxidation products or other contamination that promotes emulsification and makes separation difficult. In other cases, emulsification is desirable. It can be effected by the addition of emulsifiers, polar compounds that are oil-soluble at one extremity, water-soluble at the other. Emulsifiers are used in products formulated to mix readily with water. The resulting emulsions my be of two types - oil-in-water ( in which case, the oil is in the inner phase) or water-in-oil emulsions are used where the oil, not the water, should contact the solid surfaces, as in rust preventives, fire-resistant hydraulic fluids, compounded steam cylinder oils, etc. Such emulsions are sometimes referred to as invert emulsions. Oil-in-water emulsions, on the other had, are used in cutting-oil mixtures because of the need for the cooling effect of the water.

EP Agent - an additive to improve the extreme pressure properties of a lubricant.

Ester - a compound generally formed by the reaction of an alcohol with an organic acid. For example, ethyl alcohol and acetic acid produce ethyl acetate (an ester) and water. Esters were among the earliest types of synthetic lube oils; they are still widely used in this application. Esters also find applications as solvents.

Extreme Pressure - see boundary lubrication, EP agent.


Falex Test - a method for determining the extreme pressure or anti-wear properties of oils and greases. A rotating pin, ¼" in diameter is clamped between 2 vee blocks in such a manner that load can be applied to the blocks. Initially, 4 line contacts are made between the pin and blocks but, with increasing load, area contacts are made. Wear can be measured by determining the width of the contact areas or the weight loss of pin and blocks.

Flash Point - minimum temperature (°F) of a petroleum product or other combustible fluid at which vapour is produced at a rate sufficient to yield a combustible mixture. Specifically, it is the lowest sample temperature at which the air vapour mixture will "flash" in the presence of an ignition source (small flame). Flash point may be determined by the following ASTM Methods: closed cup (covered sample container): D 93 "Flash Point by Pensky-Martens Closed Tester" for fuel oils - also for cutback asphalts and other viscous materials and suspensions of solids: open cup (uncovered sample container): D 92 "Flash and Fire Points by Cleveland Open Cup" for lubricating oils. As indicated, this last method provides also for the determination of a fire point. Fire point is the minimum sample temperature at which vapour is produced at a sufficient rate to sustain combustion. Specifically, it is the lowest sample temperature at which the ignited vapour persists in burning for at least 5 seconds. Since the fire points of commercial petroleum oils ordinarily run about 50°F above the corresponding flash point, they are often omitted from petroleum-oil data. Flash and fire points have obvious safety connotations - the higher the test temperature, the less the hazard of fire or explosion. Of comparable significance, however, is their value in providing a simple indication of volatility, where a lower flash point denotes a more volatile material. The dilution of a crankcase oil with fuel, for example, lowers the flash point. With hydrocarbon solvents, flash point can be roughly related to initial boiling point (IBP) by the formula: Flash Point = (0.79 X IBP) - 136, where all units are degrees Fahrenheit. Flash and fire points should not be confused with auto-ignition temperature, the temperature at which combustion occurs spontaneously (without and external source of ignition).

Floc Point (of refrigeration lubricant) - highest temperature at which waxy particles solidify to give a cloudy appearance to a mixture of 10% oil in Refrigerant 12. Hence flocculation. A low floc point is desirable for refrigeration lubrication, in which waxy particles might otherwise settle out. Not to be confused with cloud point, a similar test value determined for the oil, or fuel, without mixture with refrigerant.

Foaming - may occur when a liquid is intimately mixed with air. Foaming may result in reduced film strength of a lubricant because of the entrained air. Erratic and poor fluid power transmission due to foaming may cause poor operational performance. Test methods provide an indication of foaming tendency and are reported on empirical ratings.

Foam Inhibitor - an additive which causes foam to dissipate more rapidly. It promotes the combination of small bubbles into large bubbles which burst more easily.

Friction - resistance to motion offered by a surface or substance as a result of its contact with another surface or substance. Sliding (kinetic) friction is that which occurs between two solid bodies, while fluid friction is that which occurs between the molecules of a fluid in motion. Sliding friction is measured in units of the resistive force, while fluid friction is measured in terms of shear stress. Both types of friction can be wasteful of power and energy, and sliding friction causes wear. In other respects, however, their characteristics are diametrically opposite. Whereas sliding friction is independent of speed and area, fluid friction varies with speed and area. Whereas sliding friction is directly proportional to the load (force normal to the sliding plane), fluid friction is independent of load (fluid pressure). Fluid friction also decreases with lower fluid viscosity. In general, lubrication is the substitution of low fluid friction in place of high sliding friction and the resulting wear.


Gravity - Weight-per-unit-volume relationship. With petroleum products, this relationship may be expressed as specific gravity, the ratio of the weight of a volume of the product at a designated temperature to the weight of an equal volume of water - also at a designated temperature. The designated temperature is often 60°F in both cases. The higher the specific gravity, the "heavier" the material. Petroleum products may also be defined in terms of API gravity (ASTM D287) in accordance with the formula:

API Gravity (degrees) = _____141.5______ - 131.5

                                                Specific Gravity


Hence, the higher the API value, the "lighter" the material. Kerosene has an API gravity of about 41.4 and a specific gravity of about 0.82. A measurement related to specific gravity is density, the weight of a given volume at a specified temperature, as pounds per gallon at 60°F. While the specific weight of a petroleum product has little quality significance, it does have a bearing on freight rates and on fuel loads for aircraft and ships. For a given type of fuel, moreover, the higher the specific weight, the greater the heating value.


Hydrodynamic Lubrication - that which is effected solely by the "pumping" action developed by the sliding of one surface over another in contact with a lubricating oil. Adhesion to the moving surface draws the oil into the high-pressure area between the surfaces, and viscosity retards the tendency to squeeze the oil out. If the pressure developed by this action is sufficient to completely separate the two surfaces - as it ordinarily is - full-fluid-film lubrication is said to prevail. 


ISO - International Standards Organization.


Kerosene - a colourless hydrocarbon distillate with distillation range higher than gasoline but lower than furnace oil or diesel. Largest use is jet fuel. Other applications are as a solvent, and for heating and lighting.

Kinematic Viscosity - see viscosity.


Lubricity (of and oil) - a moderate load-carrying ability over and above that indicated by its viscosity. The property can be enhanced by additive treatment.


Miscible - mutually soluble. Water and alcohol are miscible; whereas water and petroleum oil are immiscible.


Neut Number - short for neutralization number: the specific quantity of reagent required to "neutralize" the acidity or alkalinity of a lube oil sample. Either of these characteristics - acidity or alkalinity - may be exhibited by an unused oil, depending on its composition. In addition, certain additives impart acidity, while alkalinity may be derived from the presence of detergents or of basic material added to control oxidation. In service, the oil will, in time, show increasing acidity as the result of oxidation and, in some cases, additive depletion. Though acidity is not, of itself, necessarily harmful, an increase in acidity may be indicative of oil deterioration, and neut number is widely used to evaluate the condition of an oil in service. The most common measurement is acid number. The specific quantity of KOH (potassium hydroxide) required to counterbalance the acid characteristics. How high an acid number can be tolerated depends on the oil and the service conditions: and only broad experience with the individual situation can determine such a value. Neut number is determined in accordance with the ASTM Method D 664 or D 974. The former is a potentiometric method, the latter, colorimetric. Values for total acid, strong acid, total base, and strong base can, where they exist, be obtained. Strong acid numbers are considered to be related to inorganic acids, such as those derived from sulfur, while the difference between the total and strong acid numbers is attributed to weak (organic) acids - possibly the products of oxidation. A total acid number (TAN) and total base number (TBN) can exist simultaneously, both representing components too weak to completely neutralize the other. When results are reported simply as "neut number" or "acid number" , a total acid number (TAN) is implied.


Organic Compound - compound of carbon. So called because of its association with living organisms. Inorganic compounds include those substances that contain no carbon.

OSHA - Occupational Safety and Health Administration. A U.S. Federal agency that conducts studies leading to occupational safety and health standards in all facets of plant facilities, operation, and procedure. One of their responsibilities is to publish a list showing the safety threshold level for every contaminant in a plant working atmosphere, usually stated in parts per million. These threshold levels in turn lead to a level of contaminant allowable in every raw material used in a plant. These composition limits take into account type of contact between raw material and worker, exposure time, ventilation, and working conditions.

Oxidation - a form of chemical deterioration to which petroleum products - like most other organic materials - are subject. The resistance of many petroleum products to oxidation, however, is very high. Oxidation usually involves the addition of oxygen atoms, and the result is nearly always one of degradation. It is accelerated by higher temperatures, the reaction becoming significant at temperatures above 160°F. For every 18°F rise, the rate of oxidation doubles. Oxidation is also promoted by the presence of catalytic metals, copper being particularly active in this latter respect. What is more, the peroxides that are the initial products of oxidation are themselves oxidizing agents. So the oxidation of petroleum products is a chain reaction: the farther is progresses, the more rapid it becomes. With fuels and lube oils, oxidation produces sludges, varnishes, gums, and acids, all of which are undesirable. Nevertheless, many oils, such as turbine oils, give years of service without need for replacement. Petroleum products that require a long service or storage life can be formulated to meet requirements by: 1. Proper selection of crude type. Paraffinic oils are noted for natural resistance to oxidation: 2. thorough refining, which removes oxidation-susceptible materials and allows greater response to inhibitors: 3. addition of oxidation inhibitors. Long service is also promoted by good maintenance practices - filtration, centrifuging, or other means of controlling contamination: limiting duration or intensity of high temperatures: eliminating the presence of air and of catalytic metals. For information on the prediction of an oil’s oxidation stability, consult this heading. For information on determining the degree of deterioration sustained by a used oil and hence, its suitability for further service, see neut number, IFT.

Oxidation Inhibitor - chemical added in small quantities to a petroleum product to increase its oxidation resistance and, hence, to lengthen its service or storage life. An oxidation inhibitor may combine with the peroxides formed initially by oxidation, thereby modifying them in such a way as to arrest their oxidizing influence. Or the inhibitor (a pssivator) may react with a catalyst either to "poison" it or coat it with an inert film.


Petroleum - term applicable to crude oil and the hydrocarbon products and materials that are derived from it.

Pour Point - is a widely used low-temperature flow indicator and is 5°F above the temperature to which a normally liquid petroleum product maintains fluidity. It is a significant factor in cold-weather start-up, but must be considered along with pumpability, the ease with which an oil pumps at low temperatures. Paraffinic oils contain wax which forms a honeycomb of crystals at low temperatures near the pour point. However, agitation by a pump breaks down this wax structure and allows paraffinic oils to be pumped at temperatures well below their pour point. Napthenic oils, on the other hand, contain little or no wax and reach their pour point through increase in viscosity: they cannot be pumped readily near the pour point. ASTM D 97 is used to determine pour point. See also pour depressant under additives. ASTM D 97 also provides for the determination of cloud point, the lowest temperature (°F) at which the sample becomes clouded by the formation of was crystals. Clouding is a characteristic only of paraffinic oils. It is a consideration in the evaluation of fuels whose filtration might be impaired by the plugging effect of wax crystals.


R&O - rust- and oxidation-inhibited. Term applied to highly-refined industrial lubricating oils formulated for long service in circulating systems, compressors, hydraulic systems, bearing housings, gear cases, etc. The finest R&O oils are often referred to as turbine oils.

Rust Inhibitor - a lubricant additive for protecting ferrous (iron & steel) components from rusting caused by water contamination or other harmful materials from oil degradation. Some rust inhibitors operate similarly to corrosion inhibitors by reacting chemically to form an inert film on metal surfaces. Other rust inhibitors absorb water by incorporating it into water-in-oil emulsion so that only the oil touches the metal surfaces.


Surfactant - additive that reduces surface tension of a liquid. With a petroleum oil, a surfactant may increase its affinity for metals and other materials.


Vapour Pressure - a measure of liquid’s volatility. (The vapour pressure of water at 212°F is one atmosphere.) Under the ASTM Method D 323 (Reid vapour pressure), it is the absolute vapour pressure exerted by a liquid at 100°F. The higher this value, the more volatile the sample and the more readily it will evaporate. Unlike distillation data, vapour pressure provides a single value that reflects the combined effect of the individual vapour pressure of the different petroleum fractions in accordance with their mole ratios. It is thus possible for two wholly different products to exhibit the same vapour pressure at the same temperature - provided the cumulative pressures exerted by the fractions are the same. A narrow-cut distillate, for example, may exhibit the same vapour pressure as that of a dumbbell blend, where the effect of heavy fractions is counterbalanced by that of the lighter ones. In conjunction with other volatility data. Reid vapour pressure plays a role in the prediction of gasoline performance.

Viscosity - measure of a fluid’s resistance to flow. It is ordinarily expressed in terms of the time required for a standard quantity of the fluid at a certain temperature to flow through a standard orifice. The higher the value, the more viscous the fluid. Since viscosity varies inversely with temperature, its value is meaningless unless accompanied by the temperature at which it is determined. With petroleum oils, viscosity is commonly reported in Saybolt seconds, Universal-SSU or SUS-or, for very viscous oils, in Saybolt seconds, Furol-SSF. For a given oil, the Saybolt Universal viscosity will run about 10 times that of the Saybolt Furol viscosity measured at the same temperature. Both values are determined in accordance with the ASTM Method D88. Kinematic viscosity is now widely used in connection with petroleum oils, and the unit is the stoke or - more generally - the centistoke (cSt). Less common are the Engler and Redwood viscosity scales, whose principal applications are outside of North America.

Viscosity Index (V.I.) - the measure of the rate of change of viscosity with temperature. This change is common to all fluids - some more, some less. Heating tends to make them thinner - cooling, thicker. The higher the V.I., the less the tendency for the viscosity to change. V.I. is determined by formula from the viscosities at 100°F and 210°F in accordance with the ASTM Test Method D 567 or D 2270. The latter test is required for V.I.’s above 100. High -V.I. oils are often preferred for service in which a relatively constant viscosity is desired under conditions of varying temperature. Some hydraulic systems require this property. Paraffinic oils are inherently high in V.I., and the V.I. of any petroleum oil can be increased by the addition of a V.I. improver. Naphthenic oils are inherently low in V.I. and aromatics are still lower - often having negative numbers.

Volumetric Efficiency - mass of air drawn into the cylinder of an operating internal combustion engine cylinder in relation to the mass that the cylinder could hold under static equilibrium conditions. Though this ratio is associated with efficiency, the term is a misnomer. The higher the volumetric efficiency, the higher the potential thermal efficiency, but 100% volumetric efficiency does not indicate that the output at any point equals the input. Volumetric efficiency is reduced by the air cleaner, carburetor throat, throttling, preheating - anything that tends to reduce the flow to the cylinder or otherwise to decrease the mass of air admitted. The effect is to lower cylinder pressures, and, hence, the thermal efficiency of the engine.

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