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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Ferrography – A method of particle analysis using precision magnets to strip iron-laden and other susceptible particles from a used lubricating oil for study; results indicate extent of equipment wear and likelihood of imminent failure. Direct-reading ferrography uses optical sensors to measure the density of particles collected and the ratio of large particles to small (fatigue-related catastrophic failure generally is characterized by generation of particles larger than 10-15 microns). Analytical ferrography employs microscopic and photographic evaluation of wear particles. The test provides in-depth analysis of particle composition (e.g., steel, copper, bronze) and type of wear (e.g., corrosion, metal-to-metal contact).

Fire Point – The temperature rating at which a lubricant will catch fire.

Fire-Resistant Fluid
– A lubricant used especially in high-temperature or hazardous hydraulic applications such as steel mills and underground mining. Three common types of fire-resistant fluids are (1.) water-petroleum oil emulsions, in which the water prevents burning of the petroleum constituent; (2.) water-glycol fluids; and (3.) non-aqueous fluids of low volatility such as phosphate esters, silicones, and halogenated hydrocarbon-type fluids.

Flash Point
– The minimum temperature at which a fluid will support instantaneous combustion (flash) but before it will burn continuously (fire).

Flow Improver – An additive designed to modify wax crystal growth, thereby lowering the pour point and improving low temperature fluidity.

Foaming – A frothy mixture of air and a petroleum product (e.g., lubricant, fuel oil) that can reduce the effectiveness of the product, and cause sluggish hydraulic operation, air binding of oil pumps, and overflow of tanks or sumps. Foaming can result from excessive agitation, improper fluid levels, air leaks, cavitation, or contamination with water or other foreign materials. Foaming can be inhibited with an antifoam agent.  The foaming characteristics of a lubricating oil can be determined by blowing air through a sample at a specified temperature and measuring the volume of foam, as described in test method ASTM D892.

Fretting – A form of attritive wear resulting from small-amplitude oscillations or vibrations that cause the removal of very finely divided particles from rubbing surfaces (e.g., the vibrations imposed on the wheel bearing of an automobile when transported by rail car). With ferrous metals, the wear particles oxidize to a reddish, abrasive iron oxide, which has the appearance of rust or corrosion, and is therefore sometimes called fretting corrosion; other terms applied to this phenomenon are false brinelling (localized fretting involving the rolling elements of a bearing) and friction oxidation. Generally, lubricants will not prevent fretting, but they can alleviate the problem to varying degrees. ASTM D4170 is used to determine the fretting wear protection quality of greases, but it cannot distinguish between fretting wear and false brinelling.

Friction – The resistance to the motion of one surface over another. The amount of friction is dependent on the smoothness of the contacting surfaces, as well as the force with which they are pressed together. Friction between unlubricated solid bodies is independent of speed and area.  The coefficient of friction is obtained by dividing the force required to move one body over a horizontal surface at constant speed by the weight of the body. Coefficients of rolling friction (e.g., the motion of a tire or ball bearing) are much less than coefficients of sliding friction. Sliding friction is thus more wasteful of energy and can cause more wear. Fluid friction occurs between the molecules of a gas or liquid in motion, and is expressed as shear stress. Unlike solid friction, fluid friction varies with speed and area. In general, lubrication is the substitution of low fluid friction in place of high solid-to-solid friction. 

Friction Modifier – An additive designed to affect the frictional properties of rubbing surfaces.

Full Fluid-Film Lubrication – The presence of a continuous lubricating film sufficient to completely separate two surfaces, as distinct from boundary lubrication. Full fluid-film lubrication is normally hydrodynamic lubrication, whereby the oil adheres to the moving part and is drawn into the area between the sliding surfaces, where it forms a pressure, or hydrodynamic, wedge. A less common form of full fluid lubrication is hydrostatic lubrication, wherein the oil is supplied to the bearing area under sufficient external pressure to separate the sliding surfaces.