Any substance added to a lubricant to modify its properties. Typical examples are antioxidants, corrosion inhibitors, anti-wear (AW), and extreme pressure (EP) additives.

The force or forces between two materials in contact, such as lubricating grease and a metal substrate, that causes them to stick together.

Increase in consistency (hardness) of a lubricating grease during storage.

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.

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.

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.

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:

• • Group I – Paraffinic mineral oil, typically produced from petroleum by solvent extraction processes, with a sulfur content of >0.03% and/or saturates <90%. The viscosity index (VI) ranges from 80 to 120.

• • Group II – Paraffinic mineral oil, typically produced from petroleum by a combination of solvent extraction and catalytic processes, with a sulfur content of <0.03% and saturates >90%. The viscosity index ranges from 80 to 120.

• • Group III – Paraffinic oil produced from petroleum by severe hydrocracking processes, with a sulfur content of <0.03% and saturates >90%. The viscosity index is >120. Group III oils are widely (but not universally) considered to be synthetic. Gas-to-liquid (GTL) base oils are classified as Group III materials.

• • Group IV – Polyalphaolefin (PAO) fluids are the sole member of Group IV. PAOs are synthetic lubricant base stocks that are synthesized by the polymerization of linear alpha-olefins.

• • Group V – This Group includes all base stocks not covered elsewhere. It includes all synthetic fluids other than PAOs. Group V includes naphthenic mineral oils, natural esters (vegetable oils), synthetic esters, silicone oils, and all other synthetic hydrocarbons (other than PAOs).

In Europe, ATIEL defines similar categories to the API Groups, but in addition has classified poly(internal olefins), i.e., PIOs, as Group VI.

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.

Characteristics of a lubricating grease that are observed by visual inspection: Bloom, Bulk Appearance, Color, Luster and Texture.

The force applied per unit area of fluid to sustain flow.

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 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.

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.