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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Saponification Number – The number of milligrams of potassium hydroxide (KOH) that combine with one gram of oil under conditions specified by test method ASTM D94. Saponification number is an indication of the amount of fatty saponifiable material in a compounded oil. Caution must be used in interpreting test results if certain substances such as sulfur compounds or halogens are present in the oil, since these also react with KOH, thereby increasing the apparent saponification number.

Saybolt Universal Viscosity
– The efflux time in Saybolt Universal Seconds (SUS) required for 60 milliliters of a petroleum product to flow through the calibrated orifice of a Saybolt Universal viscometer, under carefully controlled temperature, as prescribed by test method ASTM D88. In the petroleum industry, this method has largely been replaced by the kinematic viscosity method ASTM D455.

Saponification – The alkaline hydrolysis of fats to form a soap; more generally the hydrolysis of an ester by an alkali with the formation of an alcohol and a salt of the acid portion. In situ saponification is the tradi¬tional method of making soap-type grease thickeners.

Scoring – Distress marks on sliding metallic surfaces in the form of long, distinct scratches in the direction of motion. Scoring is an advanced stage of scuffing.

Scuffing – Localized distress marks on sliding metallic surfaces, appearing as a matte-finished area rather than as individual score marks.

Seal Swell (Rubber Swell) – The swelling of rubber (or other elastomer) gaskets, or seals, when exposed to petroleum, synthetic lubricants, or hydraulic fluids. Seal materials vary widely in their resistance to the effect of such fluids. Some seals are designed so that a moderate amount of swelling improves sealing action.

Semisynthetic – A metal-removal fluid typically composed of a translucent micro-emulsion of water, chemicals and a small percentage of oil. A lubricant consisting of a blend of conventional mineral oil and a synthetic hydrocarbon.

Shear Rate – The rate at which adjacent layers of a fluid move with respect to each other, usually expressed as reciprocal seconds (also see shear stress). When the fluid is placed between two parallel surfaces moving relative to each other:

shear rate  (seconds)-1 =
relative velocity of surface (meters/second)
distance between surfaces (meters)

Shear Stress – The frictional force overcome in sliding one layer of fluid along another, as in any fluid flow.  The shear stress of a petroleum oil or other Newtonian fluid at a given temperature varies directly with shear rate (velocity). The ratio between shear stress and shear rate is constant; this ratio is termed viscosity. The higher the viscosity of a Newtonian fluid, the greater the shear stress as a function of rate of shear.  In a non-Newtonian fluid, such as a grease or a polymer-containing oil (e.g., multigrade oil), shear stress is not proportional to the rate of shear. A non-Newtonian fluid may be said to have an apparent viscosity, a viscosity that holds only for the shear rate (and temperature) at which the viscosity is determined.

SI –  The international system of units developed and maintained by the Gen¬eral Conference on Weights and Measures. This is the modern metric system based on the fundamental units of meters, kilograms, and seconds (MKS). The fundamental units of the older (cgs) metric system are centimeters, grams and seconds.

Soluble Oil – A metal-removal fluid typically composed of a stable milky emulsion of water, oil, emulsifiers and other functional additives. Commonly used where cooling is of primary importance.

Solvent – A material with a strong capability to dissolve a given substance. The most common petroleum solvents are mineral spirits, xylene, toluene, hexane, heptane, and naphthas. Aromatic-type solvents have the highest solvency for organic chemical materials, followed by naphthenes and paraffins. In most applications, the solvent disappears usually by evaporation after it has served its purpose. The evaporation rate of a solvent is very important in manufacture.

Solvent Extraction – A refining process used to separate components (unsaturated hydrocarbons) from lube distillates in order to improve the oil’s oxidation stability, viscosity index, and response to additives. The oil and the solvent extraction media are mixed in an extraction tower, resulting in the formation of two phases: a heavy phase consisting of the undesirable unsaturates dissolved in the solvent and a lighter phase consisting of a high quality oil with some solvent dissolved in it. The phases are separated and the solvent recovered from each by distillation.

Spectrographic Analysis (Elemental Analysis)
– A technique for detecting and quantifying metallic elements resulting from wear, contamination, or additives. The oil sample is energized to make each element emit or absorb a quantifiable amount of energy, which indicates the element’s concentration in the oil.

Spindle Oil – A low-viscosity oil of high quality for the lubrication of high-speed textile and metalworking (grinding) machine spindles. In addition to the rust and oxidation inhibitors needed for prolonged service in humid environments, spindle oils are often fortified with antiwear additives to reduce torque load and wear, especially at start-up.

Statistical Process Control (SPC)
– The use of control charts to track and eliminate variables in repetitive manufacturing processes in order to ensure that the product is of consistent and predictable quality. If a chart reveals only chance variations that are inherent in the system, the process is said to be in a state of “statistical control.” If the chart reveals variations traceable to changes in equipment, procedures or workers, the process is said to be “out of control.”  Statistical process control differs from statistical quality control in that the former monitors manufacturing process parameters and the latter monitors product quality parameters.

Stick-Slip Motion
– Erratic, noisy motion characteristic of some machine ways, due to the starting friction encountered by a machine part at each end of its back-and-forth (reciprocating) movement. This undesirable effect can be overcome with a way lubricant, which reduces starting friction.

Straight Mineral Oil – Petroleum oil containing no additives. Straight mineral oils include such diverse products as low-cost, once-through lubricants and thoroughly refined white oils.  Most high-quality lubricants, however, contain additives.

Straight Oil – A metal-removal fluid typically composed of mineral or vegetable oil or esters and functional additives. Commonly used where lubricity is of primary importance.

Sulfated Ash – Ash content determination by ASTM D874 in which the oil is burned and treated with sulfuric acid. Indicates level of metallic additives in the oil.

Sulfonate – A hydrocarbon in which a hydrogen atom has been replaced with the highly polar (SO2OX) group, where X is a metallic ion or alkyl radical. Petroleum sulfonates are refinery byproducts of the sulfuric acid treatment of white oils. Sulfonates have important applications as emulsifiers and chemical intermediates in petrochemical manufacture, and substituted sulfonates are widely used as corrosion inhibitors. Synthetic sulfonates can be manufactured from special feedstocks rather than from white oil base stocks.

Sulfur – A common natural constituent of petroleum and petroleum products. While certain sulfur compounds are commonly used to improve the EP, or load-carrying properties of an oil, high sulfur content in a petroleum product may be undesirable, as it can be corrosive and create an environmental hazard when burned.  For these reasons, sulfur limitations are specified in the quality control of fuels, solvents, etc. 

Surfactant – Surface-active agent that reduces interfacial tension of liquid. A surfactant used in a petroleum oil may increase the oil’s affinity for metals and other materials.

Synthetic Fluid
– A metal-removal fluid composed of a transparent solution of chemical lubricants (typically glycols or esters) in water with functional additives.

Synthetic Lubricant – A lubricating fluid made by chemically reacting materials of a specific chemical composition to produce a compound with planned and predictable properties; the resulting base stock may be supplemented with additives to improve specific properties. Many synthetic lubricants (also called synlubes) are derived wholly or primarily from petrochemicals; other synlube raw materials are derived from coal and oil shale, or are lipochemicals (from animal and vegetable oils). Synthetic lubricants may be superior to petroleum oils in specific performance areas. Many exhibit higher viscosity index (VI), better thermal stability and oxidation stability, and low volatility (which reduces oil consumption). Most synlubes offer longer service life and, in some cases, better biodegradability than conventional lubricants. Consequently, they are increasingly being used in industrial and automotive applications. Individual synthetic lubricants offer specific outstanding properties: phosphate esters, for example, are fire-resistant, diesters have good oxidation stability and lubricity, and silicones offer exceptionally high VI. Polyalphaolefins are versatile lubricants with low pour points, and excellent thermal and oxidation stability; they have good compatibility with petroleum lubricants and most seals used with petroleum lubricants. Most synthetic lubricants can be converted to grease by adding thickeners.  Because synthetic lubricants are higher in cost than petroleum oils, they are used selectively where performance or safety requirements may exceed the capabilities of a conventional oil.