is the fluid serving as the power transmission medium in
a hydraulic system. The most commonly
used fluids are petroleum oils,
synthetic lubricants, oil-water emulsions, and
water-glycol mixtures. The principal requirements of a premium hydraulic
fluid are proper viscosity, high
viscosity index, anti-wear protection (if
needed), good oxidation stability,
adequate pour point, good
demulsibility, rust inhibition (see
rust inhibitor), resistance to
foaming, and compatibility with seal materials.
Anti-wear oils are frequently used in compact, high-pressure, and
high-capacity pumps that require extra lubrication protection. Certain
synthetic lubricants and water-containing fluids are used where fire
resistance is needed. Synthetic lubricants also are used in
extreme-temperature conditions.
Also see
fire-resistant fluid, pump.