viscosity selection

in selecting a lubricant of the proper viscosity is based on three operating factors: speed, temperature, and pressure. The faster the parts move, the easier it is to maintain full-fluid-film lubrication; therefore, a heavier load can be carried with a lighter oil. Also, higher speeds increase fluid friction, resulting in power loss; such friction is less with a low-viscosity oil. Conversely, slow-moving parts require a heavier oil, since a full-fluid film is more difficult to maintain at low speeds. High operating temperatures cause an oil to thin out (see viscosity- temperature relationship); a high-viscosity oil is needed to compensate for this effect. At low ambient temperatures, an oil tends to thicken, which calls for a low-viscosity oil that will flow at cold temperatures. If temperature varies substantially, an oil with a high viscosity index should be considered (see multi-grade oil). High pressures, or loads, tend to squeeze the oil out of the bearing, which calls for the greater film strength of a high-viscosity oil (see lubricity). In summary, high speed/low-temperature/low-pressure conditions call for a low-viscosity oil; low speed/high-temperature/high-pressure conditions call for a high-viscosity oil. The best general practice is to use the lightest oil possible to minimize power loss.

 

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