Desirable Properties of Hydraulic Fluids

Every power transmission system has some means or medium to convey power from its source to the application area. hydraulic fluid is used to convey power from a power source such as a motor or engine to the actuator. One point should be clear in mind that the hydraulic fluid as a working medium should not be compared with the fuel in the combustion engine, where the energy from the fuel is used to develop the power, whereas in the hydraulic system no internal energy of the fluid is used. It is used only as a medium to carry energy or power.

desirable properties of hydraulic fluid

Following are the important properties of hydraulic fluids:
  1. Viscosity
  2. High Viscosity Index
  3. Demulsibility
  4. Lubricity
  5. Resistance to oxidation
  6. Resistance to Foaming
  7. Compatibility
  8. Pour point
  9. Flash Point
  10. Film strength

Let us study each hydraulic fluid property one by one in detail.


Viscosity is the most important property of hydraulic fluid and is a measure of its internal friction. In non-scientific terms, we talk about treacle having high viscosity and water having low viscosity. Bother extremes are not desirable to the hydraulic system. A low viscosity fluid flows easily, but increases losses from leakage, whereas a viscous fluid seals well, but is sluggish and leads to energy and pressure losses around the system. An ideal hydraulic fluid must be between these extremes. This imposed the need of specifying viscosity in numbers.
There are basically two terms to specify viscosity, namely Absolute viscosity and kinematic viscosity.

Absolute Viscosity

Absolute viscosity also known as dynamic viscosity is defined as the force required to move a flat surface of a unit area at unit velocity when it is separated by a unit thickness.
S.I. unit of absolute viscosity is Pascal seconds (Pa-s).

Kinematic Viscosity

Kinematic viscosity is a function of dynamic viscosity. It is defined as the ratio of absolute viscosity and mass density of the fluid.
S.I. unit of kinematic viscosity is centistroke (cSt).
In order to pump the hydraulic fluid properly, the fluid must maintain low viscosity at higher temperatures. However, the hydraulic fluid should not be so viscous especially at lower temperatures that it cannot be pumped.

High Viscosity Index

Ideally, the viscosity of hydraulic oil should remain the same at all operating temperatures, but this is not true in practical cases, the viscosity of oil decreases with an increase in temperature. This rate of change of viscosity with temperature is defined as Viscosity Index.
In order to have satisfactory functioning of hydraulic systems, it is desired that the oil should have a higher viscosity index. It is possible to improve by adding a certain synthetic agent called additive.
To obtain the numerical indication of the degree, to which viscosity changes with change in temperature, two oils are taken as a basis for scale. Two oils may have the same viscosity at 100 degrees F, but may show entirely different viscosity at some higher temperature. The oil with a higher viscosity index will exhibit a minimum change in viscosity over a wide range of temperatures and vice-versa.


Emulsification is the process of oil getting mixed with water and forming a milky and foaming mixture. The demulsibility of hydraulic fluid is the property of the hydraulic fluid, which enables it to separate from moisture and to resist the emulsification. The use of oil with less demuslibility imposes the following problems:
  • Lower lubricating value and sealant properties.
  • Reduction in the service life of working surfaces.
  • Due to high water content in oil, it absorbs many contaminants, which accelerates wear and tear.

It is found that highly refined hydraulic oils are basically water-resistant in nature and have excellent demulsibility.


For the efficient lubrication of all the internal moving parts of hydraulic components, fluids must possess sufficient lubricity and surface adhesion. If the oil film breaks down as a result of insufficient film strength, the parts, which are moving with respect to one another come into direct contact resulting in excessive wear out and damage. Wearing of the parts which seal the fluids will lead to increased internal leakage and in case if pumps reducing the pressure developed.
Special additives are used to impact lubricity or film strength to oil, which is essentially organic chemicals. Such specially developed oils are called high-pressure fluids.

Resistance to oxidation or Aging

Resistance to oxidation or chemical stability is defined as the fluid’s ability to resist the oxidation and chemical changes that appear in the fluid due to the influence of high temperature and the catalytic effect of metals, particularly non-ferrous metals for long periods.
Oxidation has the following effects on the hydraulic system. When the oxidation of oil takes place, sludge is formed. A soon as a small amount of sludge is formed, the rate of formation generally increases more rapidly thereafter. As sludge is formed, certain changes in the physical and chemical properties of oil take place. The oil becomes progressively darker in color and heavier in viscosity.
The formation of sludges, gums, carbon, or other deposits that clog the valve openings, causes the valve spools and pistons to stick and results in poor functioning. A great difference in stability is found in different types of oil. Oil may break down and form sludge excessively after a 24 hour-run whereas another oil may continue to perform up to 1000 hours.
The resistance to oxidation also called the aging of hydraulic oil can be increased by the addition of oxidation blocking additives.

Resistance to foaming

Foaming is resulted due to the entertainment of air in oil. Under normal atmospheric conditions, a hydraulic fluid contains about 9% by volume of dissolved air. This will not lead to any reduction in the performance of the system. However, the ability to dissolve air increases with an increase in pressure and temperature.
If the drop of pressure occurs at any point in the system, the dissolved air (which has dissolved due to an increase in pressure) emerges out in the form of bubbles. This undissolved air, which is visible, as the foam is extremely dangerous for hydraulic systems and can lead to cavitation particularly in pumps and valves.
The cavitation results in
  • Material erosion due to localized high-pressure spots.
  • Pressure shocks and noises.
  • Undissolved air increases the compressibility of hydraulic fluid, which adversely affects the accuracy of cylinder displacement and travel.

The tendency of hydraulic oil to foam excessively can be greatly reduced by the use of anti-foam agents. These agents are usually synthetic organic chemicals that are added in proportions of varying from 0.001% to 0.5%.


The hydraulic fluid must be compatible with non-ferrous metals. It should also be compatible with seals and hoses.

Pour Point

In normal operation, pour point below the minimum temperature is expected.

Flash Point

In normal operation, the flashpoint should be above the maximum operating temperature.

Film strength

Some of the above-mentioned properties can be put in values by determining their indices with standardized procedures as well some of the above properties can also be influenced by suitable additives.

Functions of Hydraulic Fluid

Following are the main functions of hydraulic fluid:
  1. Power Transmission
  2. Lubrication
  3. Sealing
  4. Cooling

Power Transmission

The primary function of the hydraulic fluid is to transmit power from the pump to the hydraulic actuator (hydraulic cylinder or motor). In order to perform this task efficiently, it must be incompressible and flowable to respond to every minute change in applied force.


In addition to efficient power transmission, the hydraulic fluid should lubricate the moving parts. Such as sliding surfaces of pistons and spools, bearings, and switching elements.


It is practically difficult to provide perfect mechanical sealing at all places in the hydraulic system, hence oil should be thick enough to form a strong film to seal the close clearances against leakage.


The hydraulic fluid should carry and dissipate the heat generated due to various frictional losses in the system to keep the temperature of the system under control.

1 thought on “Desirable Properties of Hydraulic Fluids”

Leave a Comment