The service properties of a hydraulic liquid

The liquid used in hydraulic systems are usually oils.  Only liquids which have specific qualities can be used in hydraulic systems.

The required properties of the hydraulic fluid.


Viscosity is the property by which a liquid resists motion between the different layers.  Hydraulic liquids should have an optimum value of viscosity.  If the viscosity is too low, there will be leakages from small gaps.  If the viscosity is too high, the liquids may not be able to flow through small passages and pipes.

Oxidation stability
This refers to the property of the oil to resist oxidation.  The oxidation of the oil can be reduced by adding special additives.

Viscosity Index
Viscosity Index refers to the change in viscosity to a change in temperature.  If the viscosity index is high, the oil will have a very small change in viscosity to a change in temperature.  A hydraulic system requires oils with high Viscosity Index

Flash Point
It is the temperature at which the oil flashes momentarily when it is exposed to a fire.  Oils in Hydraulic systems should have a high flash point.

Cloud Point
A Cloud Point is the temperature at which the oil turns to wax.  This is an important criteria for hydraulic systems which are used in cold climates.

The oil used in hydraulic systems should have excellent lubricating properties to minimize wear and tear.

Rust and Corrosion prevention
The oils chosen should be able to prevent rusting of components and prevent corrosion.