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Gear Lubricants. The American Petroleum Institute gives three definitions of gear lubricants, as follows:

1. Regular-type gear lubricant. “This term designates gear lubricants generally suitable for use in automotive transmission and in most spiral-bevel and worm-gear differentials.”

2. Worm-type gear lubricant. “This term designates gear lubricants generally suitable for use in truck-type worm-gear axles under severe conditions of service.”

3. Mild-type E.P. (extreme pressure) gear lubricant. “This term designates gear lubricants having load-carrying properties suitable for many automotive transmissions and spiral-bevel differentials under severe conditions of speed and load.”

The SAE viscosity grade for gear lubricants is given in Table 5–2.

In general, SAE 140 is recommended for high-temperature summer operation, SAE 90 for moderate-temperature conditions, and SAE 80 for use under severe low-temperature conditions. Manufacturers of tractors and equipment usually recommend the grade of lubricant to use for any particular gear-drive mechanism.

Viscosity. The viscosity of an oil is a measure of its fluidity or flow-ability at definite temperatures. The viscosity is usually determined at temperatures of 100 and 210°F. An oil to be tested is placed in a viscosimeter or the Saybolt apparatus, consisting of a specially shaped vessel equipped with a standard size orifice, or opening, at the lower end. This vessel is surrounded by a liquid bath for temperature control. Thermometers are placed in both the oil and the bath liquid. The orifice is kept closed with a cork until the two liquids have been heated to 100 or 210°F. The cork is removed and the oil permitted to flow into a graduated flask. The time, expressed in seconds, required for 60 cubic centimeters of oil to flow through the orifice is called the viscosity of the oil at the temperature tested. The viscosity at other temperatures can be calculated. A special viscosity-index chart has been worked up for the chemist. The effect of viscosity on lubricating a bearing is shown in Fig. 5–4.

TABLE 5–2. SAE CLASSIFICATION OF GEAR LUBRICANTS

* Lubricant manufacturer responsible for maintaining satisfactory minimum.

† Lubricant manufacturer responsible for maintaining satisfactory maximum.

FIG. 5–4. Effects of viscosity in lubricating a bearing. (John Deere.)