The motion between gear teeth as they go through mesh is a combination of sliding and rolling. The type of gear, the operating load, speed, temperature, method of application of the lubricant, and metallurgy of the gears are all important considerations in the selection of a lubricant.

Industrial gearing may be enclosed, where the gears and the bearings that support them are operated off the same lubricant system; or open, where the bearings are lubricated separately from the gears themselves.

Due to the high sliding contact encountered in enclosed worm and hypoid gears, lubricant selection for these should be considered separately from lubrication of other types of enclosed gears.

As with all equipment, the first rule in selecting a gear lubricant is to follow the manufacturer’s recommendation, if at all possible. In general, one of the following types of oils is used:

Rust- and Oxidation-Inhibited (R & O) Oils.
R & O oils are high-quality petroleum-based oils containing rust and oxidation inhibitors. These oils provide satisfactory protection for most lightly to moderately loaded enclosed gears.

Extreme-Pressure (EP) Oils.
EP oils are usually high-quality petroleum-based oils containing sulfur- and phosphorus-based extreme-pressure additives. These products are especially helpful when high-load conditions exist and are a must in the lubrication of enclosed hypoid gears.

Compounded Oils.
These are usually petroleum-based oils containing 3 to 5 percent fatty or synthetic fatty oils (usually animal fat or acid less tallow).They are usually used for worm gear lubrication, where the fatty content helps reduce the friction generated under high sliding conditions.

Heavy Open-Gear Compounds.
These are very heavy bodied tarlike substances designed to stick tenaciously to the metal surfaces. Some are so thick they must be heated or diluted with a solvent to soften them for application. These products are used in cases where the lubricant application is intermittent.

A number of gear lubrication models and viscosity selection guides exist. In the United States, the most widely used selection method employs the American Gear Manufacturers Association (AGMA) standards. Under its specifications for enclosed industrial gear drives, the AGMA has defined lubricant numbers, which designate viscosity grades for gear oils.

Open gears operate under conditions of boundary lubrication. The lubricant can be applied by hand or via drip-feed cups, mechanical force-feed lubricators, or sprays.

Heavy bodied residual oils with good adhesive and film-strength properties are required to survive the relatively long, slow, heavy tooth pressure while maintaining some film between applications of lubricant. Several PC software programs exist to aid in lubricant selection to reduce wear, scuffing, and pitting of gear-tooth surfaces.

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