Hydraulic Fluid - Functions and Properties

Functions and Properties

The primary function of a hydraulic fluid is to convey power. In use, however, there are other important functions of hydraulic fluid such as protection of the hydraulic machine components. The table below lists the major functions of a hydraulic fluid and the properties of a fluid that affect its ability to perform that function:

Function Property
Medium for power transfer and control
  • Low compressibility (high bulk modulus)
  • Fast air release
  • Low foaming tendency
  • Low volatility
Medium for heat transfer
  • Good thermal capacity and conductivity
Sealing Medium
  • Adequate viscosity and viscosity index
  • Shear stability
Lubricant
  • Viscosity for film maintenance
  • Low temperature fluidity
  • Thermal and oxidative stability
  • Hydrolytic stability / water tolerance
  • Cleanliness and filterability
  • Demulsibility
  • Antiwear characteristics
  • Corrosion control
Pump efficiency
  • Proper viscosity to minimize internal leakage
  • High viscosity index
Special function
  • Fire resistance
  • Friction modifications
  • Radiation resistance
Environmental impact
  • Low toxicity when new or decomposed
  • Biodegradability
Functioning life
  • Material compatibility

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