Glossary of Lubrication Terms

This glossary is designed to help the understanding of some of the terms used in Tribology and Lubrication Engineering. Quickly look up a definition or explanation for a topic.
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Elastohydrodynamic (EHD) Lubrication – A lubrication phenomenon occurring during elastic deformation of two non-conforming surfaces under high load. A high load carried by a small area (as between the ball and race of a rolling contact bearing) causes a temporary increase in lubricant viscosity as the lubricant is momentarily trapped between slightly deformed opposing surfaces.

Elastomer – A rubber or rubber-like material, both natural and synthetic, used in making a wide variety of products such as seals and hoses. In oil seals, an elastomer’s chemical composition is a factor in determining its compatibility with a lubricant.

Electrical Insulating Oil – A high-quality, oxidation-resistant oil refined to give long service as a dielectric and coolant for electrical equipment, most commonly transformers. Insulating oil must resist the effects of elevated temperatures, electrical stress, and contact with air, which can lead to sludge formation and loss of insulation properties. It must be kept dry, as water is detrimental to dielectric strength; the minimum voltage required to produce an electric arc through an oil sample, as measured by test method ASTM D877.

Elastomer (Seal) Compatibility
– The qual¬ity of a lubricant to remain in contact with an elastomer without significantly affecting the chemical and physical properties of either. Immersion tests at elevated temperatures are commonly used to evaluate compatibility. Changes in volume and hardness (Durometer) are most often deter¬mined in lubricant laboratories; rubber labo¬ratories usually run additional tests such as tensile strength and elongation.

Emulsifier – An additive that promotes the formation of a stable mixture, or emulsion, of oil and water. Common emulsifiers are metallic soaps, certain animal and vegetable oils, and various polar compounds (having molecules that are water-soluble at one extremity of their structures and oil-soluble at the other).

Emulsion – A two-phase liquid system in which small droplets of one liquid are immiscible in, but uniformly dispersed throughout, a second, continuous phase. Generally of a milky or cloudy appearance, emulsions may be of two types: oil-in-water (where water is the continuous phase) and water-in-oil (where water is the discontinuous phase). Oil-in-water emulsions are used as cutting fluids because of the need for the cooling effect of the water. Water-in-oil, or invert, emulsions are used where the oil, not the water, must contact a surfaceas in rust preventives, non-flammable hydraulic fluids, and compounded steam cylinder oils; such emulsions are sometimes referred to as invert emulsions. Emulsions are produced by adding an emulsifier. Emulsibility is not a desirable characteristic in certain lubricating oils such as crankcase or turbine oils that must separate from water readily. Unwanted emulsification can occur as a result of oxidation productswhich are usually polar compoundsor other contaminants in the oil.

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

EP Oil – A lubricating oil formulated to withstand extreme pressure (EP) operating conditions.

– A chemical compound usually derived from the reaction of an organic or inorganic acid with an alcohol.