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Dictionary of Lubricant Terms


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GAS ENGINE - internal combustion engine, either two or four stroke cylinder, powered by natural gas or LPG. Commonly used to drive compressors on gas pipelines, utilizing as fuel a portion of the gas being compressed.

GASOHOL - blend of 10% volume anhydrous ethanol (ethyl alcohol) and 90% unleaded gasoline.

GASOLINE - blend of light hydrocarbon fractions of relatively high anti-knock value. Finished motor and aviation gasolines may consist of the following components; straight-run naphthas, obtained by primary distillation of crude oil; natural gasoline, which is “stripped”, or condensed, out of natural gas; cracked naphthas, reformed naphthas; and alkylate. (See catalytic cracking). A high-quality gasoline has the following properties (1) proper volatility to ensure easy starting and rapid warm-up; (2) clean-burning characteristics to prevent harmful engine deposits; (3) additives to prevent rust, oxidation, and carburetor icing; (4) sufficiently high octane number to prevent engine knock.

GEAR - machine part which transmits motion and force from one rotary shaft to another by means of successively engaging projections, called teeth. The smaller gear of a pair is called the pinion; the larger, the gear. When the pinion is on the driving shaft, the gear set acts as a speed reducer; when the gear drives, the set acts as a speed multiplier. The basic gear type is the spur gear, or straight-tooth gear, with teeth cut parallel to the gear axis. Spur gears transmit power in applications utilizing parallel shafts. In this type of gear, the teeth mesh along their full length, creating a sudden shift in load from one tooth to the next, with consequent noise and vibration. This problem is overcome by the helical gear, which has teeth cut at an angle to the center of rotation, so that the load is transferred progressively along the length of the tooth from one edge of the gear to the other. When the shafts are not parallel, the most common gear type used is the bevel gear, with teeth cut on a sloping gear face, rather than parallel to the shaft. The spiral bevel gear has teeth cut at an angle to the plane of rotation, which, like the helical gear, reduces vibration and noise. A hypoid gear resembles a spiral bevel gear, except that the pinion is offset so that its axis does not intersect the gear axis; it is widely used in automobiles between the engine driveshaft and the rear axle. Offset of the axes of hypoid gears introduces additional sliding between the teeth, which, when combined with high loads, requires a high-quality EP oil. A worm gear consists of a spirally grooved screw moving against a toothed wheel; in this type of gear where the load is transmitted across sliding, rather than rolling surfaces, compounded oils and EP oils are usually necessary to maintain effective lubrication.

GEAR OIL (AUTOMOTIVE) - long-life oil of relatively high viscosity for the lubrication of rear axles and some manual transmissions. Most final drives and many accessories in agricultural and construction equipment also require gear oils. Straight (non-additive) mineral gear oils are suitable for most spiral-bevel rear axles (see gear) and for some manual transmissions. Use of such oils is declining, however, in favor of EP (extreme power) gear oils suitable for both hypoid gears (see gear) and for all straight mineral oil applications. An EP oil is also appropriate for off-highway and other automotive applications for which the lubricant must meet the requirements of Military Specification MIL-L-2105C.

GEAR OIL (INDUSTRIAL) - high quality oil with good oxidation stability, rust protection, and resistance to foaming, for service in gear housings and enclosed chain drives. A turbine oil or R&O oil is the usual gear oil recommendation. Specifically formulated industrial EP gear oils are used where highly loaded gear sets or excessive sliding actions (as in worm gears) is encountered. See gear.

GRAVITY - See Specific Gravity; API Gravity.

GREASE - mixture of a fluid lubricant (usually a petroleum oil) and a thickener (usually a soap) dispersed in the oil. Because greases do not flow readily, they are used where extended lubrication is required and where oil would not be retained. Soap thickeners are formed by reacting (saponifying) a metallic hydroxide, or alkali, with a fat, fatty acid, or ester. The type of soap used depends on the grease properties desired. Calcium (lime) soap greases are highly resistant to water, but unstable at high temperatures. Sodium soap greases are stable at high temperatures, but wash out in moist conditions. Lithium soap greases resist both heat and moisture. A mixed-base soap is a combination of soaps, offering some of the advantages of each type. A complex soap is formed by the reaction of an alkali with a high-molecular-weight fat or fatty acid to form a soap, and the simultaneous reaction of the alkali with a short-chain organic or inorganic acid to form a metallic salt (the complexing agent). Complexing agents usually increase the dropping point of grease. Lithium, calcium, and aluminum greases are common alkalis in complex-soap greases. Non-soap thickeners, such a clays, silica gels, carbon black, and various synthetic organic materials are also used in grease manufacture. A multi-purpose grease is designed to provide resistance to heat as well as water, and may contain additives to increase load-carrying ability and inhibit rust.

Lubriformance LLC, P.O. Box 1487, Canton, TX 75103