Adhesion
The force or forces between two materials in contact, such as lubricating grease and a metal substrate, that causes them to stick together.
Age Hardening
Increase in consistency (hardness) of a lubricating grease during storage.
Anhydrous
Without water, for example, a lubricating grease in which no water is detected by ASTM D128. Calcium 12-hydroxystearate grease is called anhydrous calcium soap grease.
Anti-Wear (AW) Additive
An additive used to protect lubricated surfaces from contacting one another under moderate to high loads, as in the elastohydrodynamic lubrication regime. Antiwear additives function by forming an adsorbed molecular layer on metal parts, thus keeping the surfaces separated. For more highly loaded applications, extreme pressure (EP) additives are required.
Antioxidant (Oxidation inhibitor)
An additive used to slow the degradation of lubricants by oxidation. Oxidation is degradation caused by chemical reactions with oxygen. These reactions change the chemical composition, alter the properties and shorten the service life of a lubricant.
API Groups I-V
The lubricating fluid in a grease can be mineral oil (derived from petroleum), a synthetic fluid, or a vegetable based fluid. Lubricant base fluids are divided into five groups, defined by the American Petroleum Institute (API) as follows:
In Europe, ATIEL defines similar categories to the API Groups, but in addition has classified poly(internal olefins), i.e., PIOs, as Group VI.
Apparent Viscosity
The apparent viscosity of grease is the ratio of shear stress to shear rate as measured by ASTM D1092 or other techniques. It is a function of temperature and shear rate. A graph of apparent viscosity versus shear is used to predict the pressure drop in a grease distribution system under steady-state flow conditions at constant temperature.
Appearance
Characteristics of a lubricating grease that are observed by visual inspection: Bloom, Bulk Appearance, Color, Luster and Texture.
Applied Shear Stress
The force applied per unit area of fluid to sustain flow.
Asperity
An asperity is a microscopic "bump" or "peak" on a solid surface. Asperities are present on virtually every solid surface due to the machining processes used to make bearings, gears, etc.
ASTM D02.G0
ASTM International (formerly the American Society for Testing and Materials) is an international standards organization. Committee D02 covers petroleum products, liquid fuels, and lubricants. Subcommittee D02.G0 covers lubricating grease.
ATIEL – The Technical Association of the European Lubricants Industry
The Technical Association of the European Lubricants Industry, ATIEL is a nonprofit association that represents leading European and international engine oil manufacturers and marketers. ATIEL promotes consensus on key technical, product stewardship and sustainability issues. A major focus is the performance of engine oils including wear protection, deposit control, fuel economy and CO2 emissions.
Biodegradable
Biodegradable lubricants rapidly decompose to water, carbon dioxide, and minerals due to the action of naturally-occurring microorganisms or enzymes present in soil or groundwater. Readily biodegradable lubricants undergo ≥60% decomposition by mass, and inherently biodegradable lubricants undergo somewhat less (between 20 and 60%), in OECD 301 testing. They are typically used in environmentally sensitive applications.
Bleeding
The separation of liquid from a lubricating grease, also referred to as “oil separation.”
Blending
Blending is the process of mixing components to produce a mixture with desired properties.
Bloom
The surface color (usually blue or green) of a lubricating oil or grease when viewed by reflected daylight at an angle of about 45 degrees from the surface. Bloom is associated with the absorption of ultraviolet light in the oil and may not be visible if the sample is viewed using artificial light.
Boundary Lubrication
See Lubrication.
Bulk Appearance
Visual appearance of grease when the undisturbed surface is viewed in an opaque container. See also Texture. Bulk Appearance should be described in the following terms-
Cavitation
Formation of a void due to reduced pressure in lubricating grease or oil. Cavitation of a lubricating grease in a dispensing system can prevent the grease from flowing.Certification Marks
See NLGI Certification Marks.Certified Lubricating Grease Specialist (CLGS)
Certification an a CLGS indicates that an individual possesses a defined level of expertise in the field of lubricating grease as defined by NLGI. Certification is awarded after an individual passes a two-hour exam that consists of 120 questions about lubrication fundamentals and grease types, selection, manufacturing, applications, maintenance, testing, etc.Channeling
The tendency (usually desirable) of a channel to form when grease is worked in a bearing, leaving shoulders of unworked grease that serve as a seal and a reservoir of oil. Also the tendency of liquid lubricants and certain lubricating greases to form a plastic structure sufficiently strong to resist flow under gravitational forces at low temperatures. Similar, but not identical to, the pour point of liquid lubricants, channeling is measured with empirical tests such as Method 3456.2 in Federal Test Method Standard No. 791 (D).Coefficient Of Friction (Dynamic)
The dynamic COF is the ratio of the tangential force that is needed to maintain relative motion between two contacting surfaces to the perpendicular force (applied load) holding them in contact.Coefficient Of Friction (Static)
The static coefficient of friction (COF) is the ratio of the tangential force that is needed to start relative motion between two contacting surfaces to the perpendicular force (applied load) holding them in contact.Cohesion
Attractive forces between molecules in a substance. For example, cohesion between molecules in grease contribute to its resistance to flow.Cold SETT
See Sett.Colloid
A substance that consists of microscopically dispersed insoluble particles that are suspended throughout another substance, typically in a fluid. Lubricating grease is a colloidal system of thickener particles or fibers in oil. (see also Thickener).Color
(of lubricating grease) - The shade and intensity observed when lubricating grease is viewed under conditions that eliminate Bloom. For example, lubricating grease in an opaque container such as a metal package can be observed under reflected light from a position approximately perpendicular to the surface. Grease color can also be observed with transmitted light by placing the sample on a transparent plate. It is important to indicate the method used to determine the color of grease. Colors of lubricating greases are best described in terms of the predominant hue such as amber, brown (or perhaps green, red, or blue for dyed grease) with a qualifying adjective describing intensity in terms of light, medium, or dark. (of lubricating oil) - That shade shown when viewed under transmitted light only. Usually lubricating oil colors are obtained by viewing under specified conditions in test equipment. Several such methods are available, the most widely used being ASTM D1500, which describes the colors in terms of numbers.Complex Soap
A soap thickener wherein the soap crystal or fibers are formed by co-crystallization of two or more compounds, , i.e., the primary soap and complexing agents such as metal salts of organic acids or inorganic salts. The complexing agent modifies the grease characteristics and usually increases the dropping point.Cone Penetrometer
A device described in ASTM D217 and D1403 (ISO 2137) that measures the consistency (hardness) of lubricating grease. The device consists of a freely moving cone and shaft assembly, a pedestal for placing the grease sample below the cone, a locking device to stop the motion of the cone and shaft, and a scale for measuring the depth to which the cone penetrates the grease.Consistency (Hardness)
The resistance of a lubricating grease resists deformation under the application of force. Consistency characterizes the plasticity of a solid in much the same way that viscosity characterizes a fluid. Grease consistency is usually measured by cone penetration according to ASTM D217 (ISO 2137).Corrosion
The gradual destruction and/or pitting of a metal surface due to chemical reactions with the environment. The most common form of corrosion is caused by oxidation of metal, i.e., or electrochemical reactions of metal with oxygen and/or aggressive ions. (See also Fretting).Corrosion Inhibitor
An additive used in grease to prevent metal corrosion. Test methods for corrosion prevention properties of grease include D1743, D5969, and D6138 for ferrous metals and D4048 for copper. Different types of chemistries are used to protect different types of metals.DIN – Deutsches Institut für Normung e. V
Deutsches Institut für Normung e. V., the German Institute for Standardization, is a nonprofit organization that develops and publishes standards on a wide variety of topics. Headquartered in Berlin, Germany, DIN is Germany's national standards organization. DIN standards are synchronized with the rest of the world.Dispensability
The ease with which grease may be transferred from its container to its point of application. Mostly used in discussion of grease dispensing systems, where it includes both the properties of pumpability and feedability.Dropping Point
Refers to the temperature observed in a laboratory test where a small quantity of lubricating grease is heated until a drop of material separates from the grease. That temperature is adjusted according to ASTM D 2265. The dropping point is not the melting point of lubricating grease. Dropping point is used in many grease specifications. However, this test has very limited relevance to service performance. Dropping point should not be used to determine the upper operating temperature of a grease. To estimate how grease performs at high temperature, certain bearing life tests may be helpful.Dry Film Lubricant
Dry or solid lubricants form films that reduce friction without the need for oil. Dry lubricants are often used in locks, certain bearings, and applications at high temperatures and oxidizing conditions. Examples include graphite, molybdenum disulfide, boron nitride, PTFE (polytetrafluoroethylene) and certain soaps.Dynamic Friction
Also called kinetic friction, the resistance to motion of contacting bodies as they move relative to one another.Dynamic Viscosity
The ratio between the applied shear stress and rate of shear of a liquid. The dynamic viscosity can be measured by placing the liquid between two parallel plates, and measuring the force required to move one plate while holding the other plate fixed.Elastohydrodynamic Lubrication
See LubricationEnvironmentally Acceptable Lubricant or Environmentally Advantaged Lubricant (EAL)
A lubricant meeting application or regulatory requirements on toxicity, bioaccumulation, biodegradability, and, in certain circumstances, renewable content.Evaporation Loss
That portion of a lubricant that volatilized in use or in storage. Typical test methods are ASTM D972 and D2595.Extreme Pressure (EP) Additive
Extreme pressure (EP) additives enhance the load carrying capacity of a grease under boundary lubrication conditions. EP additives chemically react with the metal surface under conditions of high load and elevated temperature, forming a chemically bonded lubricating film.Extreme Pressure Property (EP)
The ability of a lubricant to reduce scuffing, scoring and seizure that can occur when highly-loaded moving surfaces are in contact. High loads are applied to moving surfaces that are in contact. Commonly used laboratory test measurements of the EP level of greases are the Timken OK Load (ASTM D2509, 4-Ball Weld and Load Wear Index (ASTM D2596), and EP by SRV Step Load (ASTM D5706).False Brinelling
Localized fretting that occurs when the rolling elements of a bearing vibrate or oscillate with small amplitude while pressed against the bearing race. The mechanism proceeds in stages: 1) asperities weld, separate, and form wear particles, which may then be oxidized; 2) the wear particles cause abrasive wear. The resulting wear is in the form of depressions that appear similar to Brinell depressions obtained with static overloading. (See also Fretting).Fatigue
Fatigue refers to the initiation and growth of cracks due to cyclic loading.Feedability
The ability of lubricating grease to flow under suction in a dispensing pump at a rate at least equal to pump delivery capacity. When the feed of grease is not satisfactory, cavitation can occur at the inlet to the dispensing pump. In such cases, feedability can often be improved by the use of follower plates.Fiber
Soap thickeners form microscopic fibers in lubricating grease. Some soaps crystallize in the form of threads, which are 20 or more times as long as they are thick. (Most soap fibers are microscopic in size, so that the grease appears smooth to the eye). Some greases have a fibrous appearance when fiber bundles are large enough to be seen by the unaided eye. The most common fibrous lubricating greases contain sodium based thickeners, although not all sodium base greases are fibrous. (See also Appearance and Texture).Fibril
An extremely small fiber, barely visible even at maximum magnification of an electron microscope. Fibrils may collect in bundles to form larger fibers.Filler
A material added to a grease to increase bulk or density. Examples of fillers are talc, pigments, and carbon black.Film Strength
The ability of a film of lubricant to resist rupture due to load, speed, and temperature.Follower Plate
A plate fitted to the top surface of lubricating grease in a container and designed to assist delivery of grease to the inlet of the dispensing system.Food Grade
A term used casually to refer to lubricants certified by NSF International for use primarily in food and beverage, personal care product, and pharmaceutical processing applications.Food Machinery Lubricant
Any lubricant that has received H1designation from NSF for use “above the line” in manufacturing and packaging food, beverage, personal care, and pharmaceutical productsFour-Ball Weld Point
In ASTM D2596, the applied load at which the lubricant can no longer prevent metal-to-metal contact, and the standard steel balls used in the test weld together.Fretting Corrosion
Under conditions of fretting wear, fretting corrosion occurs between a bearing inner ring and shaft or the outer ring and housing. The corrosion occurs at points where the fit is too loose. When the fit is too loose, the metal oxide film can be worn away, allowing oxygen to attack the underlying metal. The build-up of corrosion products can cause severe bearing damage, including cracking of bearing rings.Fretting Wear
A form of wear caused by vibratory or oscillatory motion of limited amplitude and characterized by the removal of fine particles from the rubbing surfaces. Fretting wear is often followed by localized oxidation, hence the term Friction Oxidation. (See also False Brinelling and Fretting Corrosion).Friction
The force resisting relative motion between two surfaces that are in contact.Friction Oxidation
See Fretting Wear.Gas to Liquids (GTL)
Refers to the conversion of gaseous hydrocarbons, such as natural gas, to liquid synthetic fuels and lubricant base stocks. The GTL lubricant base stocks are classified as API Group III.Gel
An elastic, jelly-like material that resembles a solid but flows like a liquid. Most gels are dispersions of liquids in networks of colloidal particles or polymers.Grease
See Lubricating Grease.Grease Worker
A standard laboratory-scale device used to apply shear to a grease. The grease worker consists of a metal cup of standard dimensions, a cover fitted with a handle, and a shaft attached to a metal plate drilled with 61 x ¼-inch holes. The cup is filled with grease, the cover is closed, and the plate is pushed through the grease sample. The device can be manually or mechanically driven. For ½-scale and ¼-scale penetrations, the equipment is progressively smaller. The equipment details can be found in ASTM D217 (full-scale) and ASTM D1403 (small-scale) and ISO 2137.Grease Worker Cup
A machined metal cup that holds grease for working. It is also the cup that is used to hold the grease for penetration measurement. A full-scale worker cup holds about one pound (450grams) of grease. See ASTM D217 for the details of the cup dimensions. Smaller scale (½- and ¼-scale) worker cups are detailed in ASTM D1403.H1/H2/H3/3H/HX1
Designations defined by NSF International (Ann Arbor, Michigan) for finished lubricants and components used in food-processing applications. H1 lubricants must be colorless, odorless, tasteless, non-toxic and meet other criteria; they are certified for incidental food contact. H2 lubricants may be used in food processing but only in situations where food contact cannot occur. H3, also known as soluble or edible oils, are used to clean and prevent rust on hooks, trolleys, and similar equipment. 3H lubricants are allowed to be in direct contact with food. HX1 ingredients must be used to formulate H1 lubricants.Herschel-Bulkley
A rheological model describing the relationship between applied shear stress and shear rate for a non-Newtonian fluid such as lubricating grease.Homogenization
The process of very thoroughly mixing the components in a mixture and applying intensive shear to improve the dispersion of components in a mixture. Grease is homogenized to disperse the thickener, improve the bulk appearance, improve the yield, and reduce the cost of the product.Hybrid Thickener
A grease thickener where a metallic soap is used with a non-soap thickener. Examples include urea complex thickener where a urea and a calcium complex soap are used together and calcium sulfonate complex where a fatty acid is reacted with residual lime to form a mixed calcium sulfonate-soap grease.Hydrated Soap
A soap that has water associated with its structure. A typical example is a water-stabilized calcium soap grease, which owes its stability to hydrated calcium soap.Hydrodynamic Lubrication
See Lubrication.Hydrophilic
Having an affinity for water; capable of uniting with or dissolving in water.Hydrophobic
Repelling water, incapable of uniting with or dissolving in water.Hydrostatic Lubrication
See Lubrication.Incompatibility
Two lubricating greases are incompatible when a mixture of the products has physical or performance properties that are inferior to those of the individual greases. Physical or performance properties inferior to one of the products and superior to the other may be due to simple mixing and would not be considered as evidence of incompatibility.Induction Period (Grease Oxidation)
A period of time during which oxidation occurs at a relatively slow rate. At the end of the induction period, the oxidation rate may become more rapid. Methods of measurement of induction time include ASTM D942 and D5483.Inorganic Acid
An acid that does not contain a carbon chain. Inorganic acids, such as boric acid, are sometimes used as complexing agents in complex soap greases. Contrast with Organic Acid.Inorganic Salt
The reaction product of an alkali with an inorganic acid. Inorganic salts, such as lithium borate, are sometimes found in complex soap greases.Inorganic Thickener
See Non-Soap Thickener.Insolubles
Components of a lubricating grease that are insoluble in the prescribed solvents in an analytical procedure such as ASTM D128. The analytical procedure should be indicated when specifying insolubles. Additional identifying analytical tests are required to determine the nature and composition of insolubles, which may consist of fillers, additives, certain types of thickeners, or impurities.ISO
The International Organization for Standardization, a non-governmental, independent organization with a membership of 166 national standards bodies, is based in Geneva, Switzerland. It has developed and published more than 24,000 international standards covering almost all aspects of technology and manufacturing.Kinematic Viscosity
The resistance of a liquid to flow under gravity. Kinematic viscosity can be measured directly, as in ASTM D445, where it is measured by the time required for a volume of liquid to flow under gravity through a calibrated glass capillary tube referred to as a viscometer. Standard temperatures of 40°C and 100°C (104 °F and 212 °F) are typically used. Kinematic viscosity can also be calculated as the ratio of the dynamic viscosity to the density of the liquid.Load Wear Index
An index of the ability of a lubricant to minimize wear at applied loads in the 4-Ball EP Test (ASTM D2596). The applied load is increased step-wise, until welding (seizure) occurs. The load wear index is calculated based on the wear scars of the 10 highest non-seizure loads. See ASTM D2596 for further details.Lubricant
Any material that is used or applied to surfaces in order to reduce friction and/or wear that can occur when two surfaces are in contact and undergo relative motion. Lubricants are available in various forms: liquids, greases, dry films, and coatings.Lubricating 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. An alternative definition (Vold and Vold, 1952) states, “A grease is a lubricant that has been thickened in order that it remain in contact with the moving surfaces and not leak out under gravity or centrifugal action, or be squeezed out under pressure.”Lubricating Grease Structure
The physical arrangement of thickener particles or fibers in a lubricating grease. The nature, form, and stability of this arrangement determine the appearance, texture, and physical properties of the grease.Lubrication
The use or application of a material (lubricant) to reduce friction and/or wear that can occur when two surfaces undergo relative motion while in contact under an applied load. Friction and lubrication performance depend upon the relative speed of the surfaces, the lubricant viscosity, and the applied load. There are four major lubrication regimes or types of behavior.- Boundary Lubrication - The lubricant film is too thin to form a fluid layer that completely separates two surfaces. Asperities on the surfaces collide. Friction and wear depend upon the presence of chemical additives that adsorb and form molecular layers on the surfaces.
- Elastohydrodynamic Lubrication - Elastohydrodynamic or EHD occurs in rolling element bearings and certain gears when very high loads are concentrated on small surface areas. Under these conditions, the surfaces deform elastically or flatten to increase the surface area that bears the load. Lubricant is trapped between the surfaces, and its viscosity can increase under the pressure. As a result, the lubricant is able to form a hydrodynamic film and separate the surfaces.
- Hydrodynamic Lubrication - Under appropriate conditions, the relative motion of two sliding surfaces causes a continuous fluid film to form and completely separate the surfaces. This requires a balance between the sliding speed, the applied load, and the lubricant viscosity. Fluid Film Lubrication is another name for this type of lubrication.
- Mixed Lubrication – A lubrication regime where the load is supported partially by a fluid film and partially by the surfaces in contact
- Hydrostatic Lubrication - Lubricant is supplied under pressure to a plain bearing. This applied pressure forces the lubricant to form a continuous fluid film that completely separates the surfaces. Hydrostatic lubrication occurs typically during start-up of plain bearings. Hydrodynamic lubrication becomes effective when plain bearings are in motion.
Luster
The intensity of light reflected by lubricating grease, i.e., its sheen or brilliance. Luster should be described as follows:- Bright - Reflects light with a relatively strong intensity.
- Dull - Reflects light with a relatively weak intensity. Some greases with a high water content may have a dull luster. Certain thickeners and fillers give a grease a characteristic dull luster.
Macroscopic
Visible to the unaided human eye, not microscopic, where the particles are at least 40 μm or 0.0015 in. in size.Mechanical Stability
See Shear Stability.Metallic Soaps
Metallic soaps are the most common thickeners used in lubricating greases. Materials such as NaOH (sodium hydroxide), LIOH (lithium hydroxide), Ca(OH)2 (calcium hydroxide), and Al(OH)3 (aluminum hydroxide) contain basic or alkaline hydroxide groups (OH-). These materials are mixed with organic fatty acids in oil and heated to prepare grease. The OH- groups react with acidic H+ ions on fatty acids such as stearic acid. This reaction produces thickener fibers plus water as a by-product. This reaction is known as saponification.Microscopic
Not visible to the unaided human eye, smaller than 40 μm or 0.0015 in. in size.Newtonian Behavior
Simple liquids are said to be Newtonian when the applied shear stress is directly proportional to the shear rate. In other words, the viscosity (applied shear stress/shear rate) of a Newtonian fluid is constant and does not depend on shear stress or shear rate at constant temperature.NLGI Certification Marks
NLGI licenses certification marks for use on labels of products that satisfy performance criteria and pass laboratory evaluations managed by NLGI. In 1989, GC/LB Certification Marks were designed and standardized by NLGI and ASTM (D4950) to meet requirements for lubrication of automotive wheel bearings and chasses and then written into many original equipment manufacturer (OEM) specifications. Chassis lubricants are designated L, and wheel bearing lubricants are designated G. There are two performance classifications for chassis greases (LA and LB), and three for wheel bearing greases (GA, GB, and GC). The automotive industry is in general agreement that the highest performance classifications (LB and GC) are suitable for service relubrication of vehicles. In 2020, NLGI introduced the HPM (High Performance Multiuse) Certification Marks. In addition to the general HPM Certification Mark, there are additional specifications and Marks for lubricating greases with enhanced water resistance (WR), corrosion resistance (CR), load carrying ability (HL), and low temperature performance (LT).NLGI Grade
A numerical scale for classifying the consistency of lubricating greases, based on the ASTM D217 worked penetration at 25°C (77°F). NLGI Grades are also referred to as NLGI Consistency Numbers or NLGI Numbers. In order of increasing consistency (hardness):| NLGI Grade | Worked Penetration Range, 25 °C (77 °F) |
| 000 | 445-475 |
| 00 | 400-430 |
| 0 | 355-385 |
| 1 | 310-340 |
| 2 | 265-295 |
| 3 | 220-250 |
| 4 | 175-205 |
| 5 | 130-160 |
| 6 | 85-115 |