Hydraulic Fluid Selection

Hydraulic equipment is comprised of many kinds of hydraulic appliances, and does its job by transmitting energy through a non-compressible medium. In general, petroleum-based fluid is used as a medium, and is called working fluid. Hydraulic equipment works under conditions of high pressure and high speed. Also, many kinds of materials are used for the equipment, and temperature of the fluid under operation and other conditions may change the performance of the working fluid. When these conditions are considered, the following characteristics are demanded of the working fluid.
(1) Viscosity is stable despite temperature fluctuations.
(2) Stable flow-ability must be reached under low temperature.
(3) Change in quality and performance is minimal under high temperature.
(4) Desirable oxidation stability is met.
(5) Shear-ability needs to be invariable.
(6) It does not corrode metals.
(7) It needs to prevent rust.
(8) Rubber and coating inside pipes must not be eroded.
(9) Fluid is non-compressible.
(10) Ability to elimination foam is high.
(11) It must have the maximum possible resistance to fire.

JIS specifications for hydraulic working fluid do not currently exist. But, specialized hydraulic working fluid with viscosity equivalent to turbine oil with additives in petroleum lubricant is most widely accepted.
Synthetic working fluid and water-based working fluid are selected when danger of fire is present because of leakage or blowout of the fluid. These working fluids have different characteristics from petroleum-based fluid; caution is needed for practical use. Also, because of the increasing emphasis on socially-sustainable materials, biodegradable, vegetable oil based working fluids can be employed. Figure 14.1 shows the classification of working fluid used in hydraulic equipment. Table 14.1 presents characteristics of each working fluid.

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