Additive –A material added to change or enhance properties, characteristics, or performance.
Adhesion –The attraction or joining of two materials, such as a lubricating grease and a metal.
Age Hardening –An increase in consistency (hardening) over time (also see Thixotropy).
Anhydrous –A lubricating grease without water (as determined by ASTM D 128).
Antioxidant (oxidation inhibitor) –An additive that retards oxidation of lubricants.
Anti-wear Agents –Additives or their reaction products, which form thin, tenacious films on highly loaded parts to prevent metal-to-metal contact.
Apparent Viscosity –The ratio of shear stress to rate of shear of a non-Newtonian fluid. Apparent viscosity of most greases depends on shear rate and temperature (see ASTM D-1092).
Appearance –A general term relating to characteristics which are observable only by visual inspection (see Bloom, Bulk Appearance, Color, Luster and Texture).
Base Stock –The base fluid, usually a refined petroleum fraction or a selected synthetic material, into which additives are blended to produce finished lubricants.
Bleeding –Separation of liquid lubricant from a grease.
Blending (see Compounding) –Blending is the process of mixing fluid lubricant components for the purpose of obtaining desired physical properties.
Bloom –The typical blue or green surface color of a grease when viewed by reflected daylight.
Boundary Lubrication –Lubrication between two rubbing surfaces without the development of a full fluid lubricating film. It occurs under high loads and requires the use of antiwear or extreme-pressure (EP) additives to prevent metal-to-metal contact.
Bright Stock –A heavy residual lubricant stock with low pour point used in finished blends to provide good bearing film strength, prevent scuffing, and reduce oil consumption. Usually identified by its viscosity (SUS at 210°F or cSt at 100°C).
Bulk Appearance –Appearance of an undisturbed grease surface. Bulk appearance is described by:
Bleeding – free oil on the surface(or in the cracks of a cracked grease).
Cracked -surface cracks.
Grainy – composed of small granules or lumps of constituent thickener.
Rough – composed of small irregularities.
Smooth – relatively free of irregularities.
Cavitation –Voids produced as a result of reduced pressure in a grease-dispensing system causing the grease to stop flowing due to suction of the system.
Channeling-1 –Grooves in a lubricating grease formed by passing rolling elements. Unworked grease in the shoulders of the channel serves as a seal and reservoir.
Channeling-2 –The low-temperature change in structure of a flow-type lubricating grease that prevents flow due to gravity.
Coefficient of Friction –Coefficient of static friction is the ratio of the tangential force initiating sliding motion to the load perpendicular to that motion. Coefficient of kinetic friction (usually called coefficient of friction) is the ratio of the tangential force sustaining sliding motion at constant velocity to the load perpendicular to that motion.
Cohesion –Molecular attraction between grease particles contributing to its resistance to flow.
Color –The predominant hue of lubricating grease (such as amber, brown or perhaps green, red, or blue for dyed grease) and intensity (light, medium, or dark) when viewed to eliminate Bloom.
Complex Soap –A soap crystal or fiber formed usually by co-crystallization of two or more compounds. Complex soaps can be a normal soap (such as metallic stearate or oleate), or incorporate a complexing agent which causes a change in grease characteristics usually recognized by an increase in dropping point.
Compounding (see Blending) –The mixing or otherwise combining lubricant components with other components for the purpose of securing chemical and/or physical properties not usually obtainable by blending of fluid lubricant components alone.
Consistency (Hardness) –The resistance of a lubricating grease to deformation under load. Usually indicated by ASTM Cone Penetration, ASTM D-217 (IP 50) or ASTM D-1403.
Corrosion –The wearing away and/or pitting of a metal surface due to chemical attack (see Fretting).
Corrosion Inhibitor –An additive that protects lubricated metal surfaces from chemical attack by water or other contaminants.
Dispensibility –The ease with which a grease can be delivered through its dispensing system to the point of application (see Pumpability and Feedability).
Dropping Point –The temperature at which grease becomes soft enough to form a drop and fall from the orifice of the test apparatus of ASTM D566 (IP 132) and ASTM D 2265.
Dry Film Lubricant –Low shear-strength lubricants that shear in one particular plane within its crystal structure (such as graphite, molybdenum disulfide and certain soaps).
Elastohydrodynamic Lubrication (EHD) –A lubricant regime characterized by high unit loads and high speeds where the mating parts (usually in rolling-element bearings) deform elastically causing an increase in lubricant viscosity and load-carrying capacity.
EP Additive (Extreme Pressure Agent) –Lubricant additive that prevents sliding metal surfaces from seizing under extreme pressure conditions.
Evaporative Loss –The loss of a portion of a lubricant due to volatization (evaporation). Test methods include ASTM D 972 and ASTM D 2595.
Extreme Pressure Property (EP) –That property of a grease that, under high applied loads, reduces scuffing, scoring and seizure of contacting surfaces. Common laboratory tests are Timken OK Load (ASTM D 2509 and ASTM D 2782) and Four Ball Load Wear Index (ASTM D 2596 and ASTM D 2783).
False Brinelling –Wear, similar in appearance to Brinnell depressions, caused by the small-amplitude vibration or oscillation of rolling elements while contacting a bearing race. Under these conditions, localized fretting occurs when asperities weld, are torn apart, and form wear debris which may be subsequently oxidized. The wear debris cannot rapidly escape (because of the small amplitude of motion) and becomes an abrasive that accelerates the wear process. (See Fretting)
Feedability –The ease with which a grease flows within a dispensing pump.
Fiber –Grease soap thickeners occur in fibrous form. Most soap-thickener fibers are microscopic and the grease appears smooth; if the fiber bundles are large enough to be seen, the grease appears fibrous. Although the most common fibrous grease contains sodium soap thickener, not all sodium base greases are fibrous (see Appearance and Texture).
Fibril –A tiny fiber barely visible even with an electron microscope. Fibers may be formed by bundles of fibrils that collect together.
Filler –Materials, such as talc, pigments, and carbon black, that increase a lubricant’s bulk or density. Fillers can have various effects (or no effect) on the lubricating properties of a grease.
Film Strength –The ability of a lubricant film to withstand the effects of load, speed, and temperature without breaking down or rupturing.
Fluid Friction –Friction between the molecules of a gas or liquid in motion, expressed as shear stress. Unlike solid friction, fluid friction varies with speed and area.
Wear characterized by the removal of fine particles from mating surfaces. Fretting is caused by vibratory or oscillatory motion of limited amplitude between contacting surfaces. (See False Brinelling.)
Friction –The force resisting relative motion between two bodies. Friction depends on the smoothness of the contacting surfaces, as well as the force with which they are pressed together.
Homogenization –The intimate mixing of a grease to produce a uniform dispersion of components.
Hydrodynamic Lubrication –The formation of a continuous lubricating fluid film between mating surfaces of sufficient pressure to prevent contact. Film formation depends on shape of the surfaces and their relative motion.
Hydrophilic –Characterized by an affinity for water and capable of uniting with or dissolving in water.
Characterized by an incompatibility with water and not capable of uniting or mixing with water.
Incompatibility –When a mixture of two greases shows physical properties or service performance which are markedly inferior to those of either of the greases before mixing, they are incompatible. Performance or properties inferior to one of the products and superior to the other may not be due to incompatibility.
Induction Period (Grease Oxidation) –The time during which grease oxidation proceeds at a relatively slow rate (see ASTM D-942).
Inorganic Thickener (see Non-Soap Thickener)
Insolubles –Grease components that are insoluble in the prescribed reagents in an analytical procedure (such as ASTM D 128).
Lubricating Grease –A solid to semifluid dispersion of a thickening agent in a liquid lubricant containing additives (if used) to impart special properties.
Lubrication –Control of friction and wear by the introduction of a friction-reducing film between moving surfaces in contact. May be a fluid, solid, or plastic substance. (See Boundary Lubrication, Elastohydrodynamic Lubrication, Hydrodynamic Lubrication.)
Luster –Reflected light intensity from a grease; its sheen or brilliance. Luster is either:
Bright – reflects light with a relatively strong intensity. Dull – reflects light with a relatively weak intensity. A high water content or certain thickeners and fillers may give a grease a characteristic dull luster.
Naphthenic –A type of petroleum fluid derived from naphthenic crude oil containing a high proportion of closed-ring methylene groups.
Newtonian Behavior –A lubricant exhibits Newtonian behavior if its shear rate is directly proportional to the shear stress. This constant proportion is the viscosity of the liquid.
Newtonian Flow –Occurs in a liquid system where the rate of shear is directly proportional to the shearing force. When rate of shear is not directly proportional to the shearing force, flow is non-Newtonian.
NLGI Number –A scale for comparing the consistency (hardness) range of greases (numbers are in order of increasing consistency). Based on the ASTM D 217 worked penetration at 25°C (77°F).
Non-Newtonian Behavior –The property of some fluids, and many plastic solids (including grease), of exhibiting a variable relationship between shear stress and shear rate.
Non-Soap Thickener –Specially treated or synthetic materials (not including metallic soaps) dispersed in liquid lubricants to form greases. Sometimes called Synthetic Thickener, Inorganic Thickener, or Organic Thickener.
Oiliness Agent –A material which forms an adsorbed film to reduce friction.
Organic Thickener (see Non-Soap Thickener)
Oxidation –Occurs when oxygen attacks petroleum fluids. The process is accelerated by heat, light, metal catalysts and the presence of water, acids, or solid contaminants. It leads to increased viscosity and deposit formation.
Oxidation Inhibitor –Substance added in small quantities to a petroleum product to increase its oxidation resistance, thereby lengthening its service or storage life; also called Antioxidant.
Oxidation Stability –Resistance of a petroleum product to oxidation and, therefore, a measure of its potential service or storage life. Several test methods are in use including ASTM D 942 (IP 142).
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.
Penetration –A measure of consistency (hardness) based on an inverse penetration measurement (the softer the consistency, the higher the penetration number). Based on ASTM D 217 (IP 50) and similar standardized methods.
Plasticity –The ability of a solid material to permanently deform without rupturing under the application of force. (Plastic flow differs from fluid flow in that the shearing stress must exceed a yield point before any flow occurs).
Pumpability –The flow characteristics of a grease that permit satisfactory delivery from lines, nozzles, and fittings of grease dispensing systems and subsequent lubrication of moving components.
Rheology –The deformation and/or flow characteristics of grease in terms of stress, strain, temperature and time (commonly measured by Penetration and Apparent Viscosity).
Saponification –The formation of a metallic salt (soap) due to the interaction of fatty acids, fats, or esters generally with an alkali.
Scuffing –Abnormal wear due to localized welding and fracture. It can be prevented through the use of antiwear, extreme-pressure, and friction modifier additives.
Shearing –Relative slipping or sliding between one part of a substance and an adjacent part. Shearing in a solid involves cutting or breaking of the crystal structure; in a fluid or plastic, shearing does not necessarily destroy the continuous nature of the substance.
Shear Rate –The rate of slip within a flowing substance. See ASTM D-1092.
Shear Stability –The resistance of a grease to changes in consistency (hardness) during mechanical working.
Shear Stress –The stress (force per unit area) tending to cause shearing. The ratio of shear stress to the shear rate in a fluid is its viscosity.
Slumpability (see Feedability)
Soap (See Thickener, Complex Soap, Saponification)
Squeeze Film Lubrication –The development of fluid pressure sufficient to support a load between surfaces thickly coated or flooded with lubricant and rapidly moving toward each other. Because of viscosity (or apparent viscosity), the lubricant cannot immediately flow away from the area of contact. Squeeze-film lubrication occurs between gear teeth and between wrist pins and their bushings, for example.
Synthetic Grease –A grease that contains a liquid lubricant that is not a mineral oil.
Synthetic Lubricant –Lubricating fluid made by chemically reacting materials of a specific chemical composition to produce a compound with planned and predictable properties.
Synthetic Thickener (see Non-Soap Thickener)
Texture –The texture of a grease is observed when a small portion of it is pressed together and then slowly drawn apart. Texture can be described as:
Brittle – ruptures or crumbles when compressed
Buttery – separates in short peaks with no visible fibers
Long Fibers – stretches or strings out into a single bundle of fibers
Resilient – withstands a moderate compression without permanent deformation or rupture
Short Fibers – short break-off with evidence of fibers
Stringy – stretches or strings out into long fine threads, but with no evidence of fiber structure
Thickener –The structure within a grease of extremely small, uniformly dispersed particles in which the liquid is held by surface tension and/or other internal forces.
Thixotropy of a lubricating grease is manifested by a decrease in consistency (softening) as a result of shearing and an increase in consistency after shearing is stopped.
Tribology –Science of the interactions between surfaces moving relative to each other, including the study of lubrication, friction, and wear.
Viscosity –A measure of a fluid’s resistance to flow.
Water Resistance –The resistance of a lubricating grease to adverse effects due to the addition of water to the lubricant system. Water resistance is described in terms of resistance to washout due to submersion (see ASTM D 1264) or spray (see ASTM D 4049), absorption characteristics and corrosion resistance (see ASTM D 1743).
Wear –Damage resulting from the removal of materials from surfaces in relative motion. Wear is generally described as:
Abrasive – removal of materials from surfaces in relative motion by a cutting or abrasive action of a hard particle (usually a contaminant)
Adhesive – removal of materials from surfaces in relative motion as a result of surface contact (galling and scuffing are extreme cases)
Corrosive – removal of materials by chemical action.
Yield –The amount of grease (of a given consistency) that can be produced from a specific amount of thickening agent; as yield increases, percent thickener decreases.
Yield Point (Yield Value or Yield Stress) –The minimum shear stress producing flow of a plastic material.