Hydraulic fluids are liquids that are composed of a variety of chemicals. They are used in automobile automatic transmissions, brakes, and power steering; fork lift trucks; tractors; bulldozers; industrial machinery; and airplanes.
Types of hydraulic fluids
The various materials in use as hydraulic fluids range from water to
inorganic salt solutions to water oil emulsions, synthetic and naturally
occurring organic materials. Water was the first hydraulic fluid used
during the early stages of industrial revolution. Presently, petroleum
based hydrocarbon type fluids are in wide use. A good hydraulic fluid
A hydraulic system has various components that contain surfaces that
are in close contact and move in relation to each other. A good
hydraulic fluid must protect against wear and separate & lubricate
Viscosity is a vital fluid property that varies with temperature and
pressure. Fluids having large changes of viscosity with temperature are
commonly referred as low viscosity index fluids and those having small
changes of viscosity with temperature are known as high viscosity index
Chemical and Physical Stability
The characteristics of a fluid should remain unchanged during an
extended useful life. Since many aspects of stability are chemical in
nature, the temperatures to which the fluid will be exposed is an
important criterion in the selection of a hydraulic fluid.
The hydraulic fluid should be inert to materials used in or near the
hydraulic equipment. If the fluid in anyway attacks, destroys, dissolves
or changes parts of the hydraulic system, the system may lose its
functional efficiency and may start malfunctioning.
Good Heat Dissipation
Pressure drops, mechanical friction, fluid friction, leakages, all
generate heat. The fluid must carry the generated heat away and readily
dissipate it to the atmosphere or coolers.
The flash point of a hydraulic oil is defined as the temperature at
which flashes will be generated when the oil is brought into contact
with any heated matter.
The hydraulic liquids are petroleum derivatives and thus for critical
applications, artificial or synthetic hydraulic fluids are used which
have high fire resistances. Various grades of fluids with high water
content are also available nowadays for oil hydraulic systems.
Prevent Rust Formation
Moisture and oxygen cause rusting of iron parts in the system that can
lead to abrasive wear of system components and also act as catalyst to
increase the rate of oxidation of the fluid. Fluids with rust inhibitors
minimize rust formation in the system.
Low in Volatility
The fluid should posses low vapor pressure or high boiling point
characteristic. The vapor pressure of a fluid varies with temperature
and hence the operating temperature range of the system is important in
determining the suitability of the fluid.
Low Coefficient of Expansion
The hydraulic fluid should have a low coefficient of expansion to
minimize the total volume of the system required at the operating