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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Vapor Pressure – Pressure of a confined vapor in equilibrium with its liquid at a specified temperature; thus, a measure of a liquid’s volatility.

Varnish – A hard coating formed from oil oxidation products that bakes on to surfaces during high-temperature operation of automotive engines and industrial machinery. Varnish can accelerate cylinder wear. Varnish formation can be reduced with the use of a detergent-dispersant and an oxidation inhibitor in the oil.

Viscosity – A fluid’s resistance to flow. The common metric unit of absolute viscosity is the poise, which is defined as the force in dynes required to move a surface one square centimeter in area past a parallel surface at a speed of one centimeter per second with the surfaces separated by a fluid film one centimeter thick. For convenience, the centipoise (cP)one one-hundredth of a poiseis the unit customarily used in the petroleum industry. Laboratory measurements of viscosity normally use the force of gravity to produce flow through a capillary tube (viscometer) at a controlled temperature. This measurement is called kinematic viscosity. The unit of kinematic viscosity is the stoke, expressed in square centimeters per second. The more customary unit is the centistoke (cSt)one one-hundredth of a stoke. Kinematic viscosity can be related to absolute viscosity by the equation: cSt = cP * fluid density. In addition to kinematic viscosity, there are other methods for determining viscosity, including Saybolt Universal viscosity, Saybolt Furol viscosity, Engler viscosity, and Redwood viscosity. Since viscosity varies inversely with temperature, its value is meaningless unless the temperature at which it is determined is reported.

Viscosity Index (VI) – An empirical, unitless number indicating the effect of temperature on the kinematic viscosity of an oil. Liquids change viscosity with temperature, becoming less viscous when heated; the higher the VI of an oil, the lower its change in viscosity with temperature. The VI of an oil (with known viscosity at 40*C) is determined by comparing the oil with two standard oils having an arbitrary VI of 0 and 100, respectively, and both having the same viscosity at 100°C as the test oil. The following formula is used, in accordance with test method ASTM D2270:

where L is the viscosity at 40°C of the 0-VI oil, H is the viscosity at 40°C of the 100-VI oil, and U is the viscosity at 40°C of the test oil. There is an alternative calculation, also in ASTM D2270, for oils with VIs above 100. The VI of paraffinic oils is inherently high, but is low in naphthenic oils, and even lower in aromatic oils (often below 0).  The VI of any petroleum oil can be increased by adding a viscosity index improver. High-VI lubricants are needed wherever relatively constant viscosity is required at widely varying temperatures.

Viscosity Index Improver – A polymeric additive designed to increase the viscosity index of an oil. Also known as a viscosity modifier. 

Viscosity-Temperature Relationship – The manner in which the viscosity of a given fluid varies inversely with temperature. Because of the mathematical relationship that exists between these two variables, it is possible to predict graphically the viscosity of a petroleum fluid at any temperature within a limited range if the viscosities at two other temperatures are known. The charts used for this purpose are the ASTM Standard Viscosity-Temperature Charts for Liquid Petroleum Products, available in 6 ranges. If two known viscosity-temperature points of a fluid are located on the chart and a straight line drawn through them, other viscosity-temperature values of the fluid will fall on this line; however, values near or below the cloud point of the oil may deviate from the straight-line relationship.

Volatility – Expression of evaporation tendency. The more volatile a petroleum liquid, the lower its boiling point and the greater its flammability.