Any substance added to a lubricant to modify its properties. Typical examples are antioxidants, corrosion inhibitors, anti-wear, 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.

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 and properties 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 by solvent extraction processes, with a sulfur content of >0.03% and/or saturates <90%. The viscosity index is in the range from 80 to 120.
• Group II – Paraffinic mineral oil, typically produced by a combination of solvent extraction and catalytic processes, with a sulfur content of <0.03% and saturates >90%. The viscosity index is in the range from 80 to 120.
• Group III – Paraffinic oil produced by severe hydrocracking processes, with a sulfur content of <0.03% and saturates >90%. The viscosity index is >120. Group III oils are considered to be synthetic. Gas-to-liquid base oils are classified as Group III materials.
• Group IV – Polyalphaolefins. This group is dedicated only to polyalphaolefin (PAO) fluids. PAO is a synthetic lubricant base stock.
• Group V – This group covers all base stocks not covered elsewhere. It includes all synthetic fluids other than PAO, as well as naphthenic mineral oils.

The apparent viscosity of grease refers to flow under applied shear measured according to ASTM D1092. Apparent viscosity versus shear can be useful in predicting pressure drops 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 motive force per unit area for fluid flow.

An asperity is a microscopic “bump” or “peak” on a surface.

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.