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.
Browse Alphabetically

 A   B   C   D   E   F   G   H   I   J   K   L   M   N   O   P   Q   R   S   T   U   V   W   X   Y   Z    Home  
Paraffin – Any hydrocarbon identified by saturated straight (normal) or branched (iso) carbon chains; also called an alkane. The generalized paraffinic molecule can be symbolized by the formula CnH2n+2. Paraffins are relatively non-reactive and have excellent oxidation stability. In contrast to naphthenic oils, paraffinic lubricating oils have relatively high wax content and pour point, and generally have a high viscosity index (VI). Paraffinic solvents are generally lower in solvency than naphthenic or aromatic solvents.

Particulates – Particles made up of a wide range of natural materials (e.g., pollen, dust, resins), combined with man-made pollutant (e.g., smoke particles, metallic ash); in sufficient concentrations, particulates can be a respiratory irritant.

Polychlorinated Biphenyl (PCB) – A class of synthetic chemicals consisting of an homologous series of compounds, beginning with monochlorobiphenyl and ending with decachlorobiphenyl.  PCBs do not occur naturally in petroleum, but have been found as contaminants in used oil. PCBs have been legally designated as a health hazard, and any oil so contaminated must be handled in strict accordance with state and federal regulations.

pH – A measure of the acidity or alkalinity of an aqueous solution. The pH scale ranges from 0 (very acidic) to 14 (very alkaline), with a pH of 7 indicating a neutral solution equivalent to the pH of distilled water.

Phenol – A white, crystalline compound (C6H5OH) derived from benzene, used in the manufacture of phenolic resins, weed killers, plastics, disinfectants; also used in solvent extraction, a petroleum refining process. Phenol is a toxic material; skin contact must be avoided.

Phosphate Ester – Any of a group of synthetic lubricants having superior fire resistance. A phosphate ester generally has poor hydrolytic stability, poor compatibility with mineral oil, and a relatively low viscosity index (VI). It is used as a fire-resistant hydraulic fluid in high-temperature applications.

PNA (polynuclear aromatic) – Any of numerous complex hydrocarbon compounds consisting of three or more benzene rings in a compact molecular arrangement. Some types of PNAs are formed in fossil-fuel combustion and other heat processes such as catalytic cracking. 

Poise – A unit of measurement of absolute (or dynamic) viscosity. 

Polar Compound – A chemical compound whose molecules exhibit electrically positive characteristics at one end and negative characteristics at the other end. Polar compounds are used as additives in many petroleum products.  Polarity gives certain molecules a strong affinity for solid surfaces; as lubricant additives, such molecules plate out to form a tenacious friction-reducing film.  Some polar molecules are oil-soluble at one end and water soluble at the other end; in lubricants, they act as emulsifiers, helping to form stable-oil water emulsions. Such lubricants are said to have good metal-wetting properties.  Polar compounds with a strong attraction for solid contaminants act as detergents.

Polyglycols – Polymers of ethylene or propylene oxides used as a synthetic lubricant base.  Properties include very good hydrolytic stability, high viscosity index (VI), and low volatility. Used particularly in water emulsion fluids.

Polymer – A substance formed by the linkage (polymerization) of two or more simple molecules (called monomers) to form a single, larger molecule having the same elements in the same proportions as the original monomers (i.e., each monomer retains its structural identify). A polymer may be liquid or solid; solid polymers may consist of millions of repeated linked units. A polymer made from two or more dissimilar monomers is called a copolymer; a copolymer composed of three different types of monomers is a terpolymer. Natural rubber and synthetic rubbers are examples of polymers. Polymers are commonly used as viscosity index improvers in multigrade oils and tackifiers in lubricating greases.

Polyolefin – A polymer derived by polymerization of relatively simple olefins. Polyethylene and polyisoprene are important polyolefins.

Polyol Ester – A synthetic lubricant base formed by reacting fatty acids with a polyol (such as a glycol) derived from petroleum. Properties include good oxidation stability at high temperatures and low volatility. Used in formulating lubricants for turbines, compressors, jet engines and automotive engines.

Pour Point – The lowest temperature under which an oil will flow when cooled under prescribed conditions.

Pour Point Depressant – An additive used to lower the pour point of a petroleum product.

ppb – parts per billion.

ppm – parts per million.

Process Oil – An oil that serves as a temporary or permanent component of a manufactured product. Aromatic process oils have good solvency characteristics; their applications include proprietary chemical formulations, ink oils, and extenders in synthetic rubbers. Naphthenic process oils are characterized by low pour points and good solvency properties. Paraffinic process oils are characterized by low aromatic content and light color.

Pump – A mechanism through which force is applied to a liquid. There are two basic categories of pumps: positive displacement and centrifugal. Positive displacement pumps force liquid to flow in volumetric proportion to decreasing pump volume. Hydraulic systems are a primary application, wherein the hydraulic fluid functions as the lubricant. Positive displacement pumps can be divided into reciprocating and rotary. Reciprocating pumps use pistons, plungers, or diaphragms to increase and decrease volume.  Rotary pumps use a rotating device (gear, screw, or vane) to force liquid from the pump. Centrifugal pumps, also called kinetic pumps, differ from positive displacement pumps in that they provide uniform (non-pulsing) flow and adjustable flow velocity.  Movement is imparted to the liquid through centrifugal force created by a rotating impeller. There are two basic types of centrifugal pumps:  radial flow and axial flow.  In the former type, liquid enters the pump at the impeller’s axis of rotation and is forced outward by vanes. In the latter type, a propeller or screw on a rotating shaft moves liquid in the axial direction of the shaft.