Galling: surface condition on one or both mating surfaces where excessive friction results in localized welding with subsequent spalling and a further roughening of the surface.

Gas oil: partially refined liquid petroleum distillate having an intermediate viscosity between that of kerosene and that of lubricating oil.

Gas turbine: burns a liquid or gas fuel, producing a pressurized gas that passes through a turbine-powered shaft. The turbine also powers the compressor that supplies the air at the required pressure.
Gate valve: a valve with a sliding gate that opens the entire area of the pipe.

Gears: machine elements that transmit motion by means of successively engaging teeth.

Gear oils: the variety of specially formulated oils for all types of gears and operating conditions; AGMA, SAE, the military and industry have their own requirements, some of which overlap. In general, gear oils are carefully formulated and highly refined, with additives tailored to the application in question.

Gear pump: a versatile, positive-displacement pump in which fluid is propelled from the intake to the discharge by being trapped in the gear casing, in the space between the teeth of the rotating gear.

Gear shield/gear compound: a highly adhesive lubricant, formulated with asphaltic compounds or polymers, for once-through use on gearing like open gears.

Gel (grease): a solid, elastic mixture of a colloid and a liquid possessing a yield point and a jellylike texture.

Gel permeation chromatography: a size-exclusion chromatography method that separates fluid components by molecular weight (e.g., size and shape) (see chromatography).

GL-4,5,6 service: transmission and axle lubricants classified according to SAE J308.

Gland (follower): adjustable follower that compresses packing in a stuffing box.

Glycerine/glycerol: syrupy liquid obtained by saponification of certain natural fats and oils; a by-product of the manufacture of fatty acids or their salts (soaps), correctly termed glycerol.

Glycol: a class of polyol compounds like ethylene glycol, commonly used to lower the freezing point of water; especially useful in antifreeze. Formulated with 35-50% water, they function as fire-resistant fluids.

Gouging (gears): describes a condition which may develop when gear teeth lack hardness. It is usually limited to the bottom or lower part of the tooth surface, or it may occur when gears are fitted too tightly, or if there is some interference between the driving and driven gears. It is sometimes the result of no tip relief (rounding off the sharp edges at the top of the tooth).

Graphite: a form of carbon available as natural or synthetic material: natural graphite is either flaky or amorphous, synthetic graphite is crystalline; it is processed to colloidal size and abrasive contaminants are removed from the natural form. Graphite is used as a solid lubricant, in dry form or mixed with oil or grease.

Grease: lubricant composed of an oil or oils thickened with a soap, soaps or other thickener to a semifluid to solid consistency. May also contain other additives.

Grinding oils: oils formulated for grinding service where grinding wheel life, metal surface finish, etc., are important.

Gum: sticky, rubbery deposit, black or dark brown in color, resulting from the oxidation of lubricating oils or from unstable constituents in gasoline that deposit during storage or use.