Learn the Properties of Hydraulic Fluids

Hydraulic Fluids

Looking for the best hydraulic fluid for your hydraulic machine? But don’t have complete knowledge to choose the best? Here we have listed the most important properties of hydraulic fluids which will make your job easier than ever. Hydraulic fluids are also known as hydraulic liquids. Before buying the best hydraulic fluid, one should need to know its consistency, components, thermal stability, hydrolytic stability, low chemical corrosiveness, and much more to make an informed decision. We have listed some most important properties of hydraulic fluid along with FAQs that would help you to know more. 

hydraulic fluids and properties
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A Guide to the Important Properties Of Hydraulic Fluids

Transmitting energy is the most important function of hydraulic fluid. So to get proper energy supply the following factors should be kept in mind to get better performance by hydraulic fluid :


The maximum and minimum temperatures needed to operate hydraulically and the maximum load of the system, tell us about the viscosity of that fluid. Most of the hydraulic fluid maintains low viscosity at the highest operating temperature. Sometimes when the temperature is low, it is less viscous than usual.

Hydraulic Fluid Characteristics
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Zinc dithiophosphate (ZDP) is one of the common chemicals used in hydraulic fluids. But in most hydraulic systems, ashless anti-wear fluids are being used. Nowadays, ZDP or other types of heavy metals are not used to prevent the wearing of fluids.


If the foam is present in the fluid, it is carried out along with the fluid which stops the system by jamming the pump, so foam should be removed from the fluid as soon as it is formed. It can be removed by covering the air leaks inside the system. Two general types of foam occur frequently —

  •     Surface foam – This foam formed on the fluid surface in a reservoir and form air which enters from the pipe
  •     Entrained air

Removing surface foam is a very easy and quick process that can be done by defoaming additives or by dissipating directly from the reservoir because of the reservoir design.

Though removing surface foam is quite an easy process, removing entrained air often attracts serious problems as it is drawn directly into the system. This may cause cavitation, a hammering action that can destroy parts of the system. It can be prevented by adding proper additives and base oils. 

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R & O

R & O inhibitors protect the liquid from metal to metal contamination. It also contains some anti-oxidation chemicals which help to increase fluid longevity.


The most important part to notice before buying hydraulic fluid is its corrosive nature. The hydraulic fluid can cause 2 kinds of corrosion to the hydraulic system—

  •     System rusting
  •     Acidic chemical corrosion.

We can see the system rusting when ferrous metal parts are carried away by the fluid. Most of the hydraulic fluids contain rust. To protect the system from rust and chemical corrosion, several additives are used which can resist the acidic attack on the systems and provide stability in the presence of water.

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Oxidation and thermal stability

Acids, sludge, and varnish are formed when oxidation takes place inside the fluids. This process of oxidation can also cause severe damage to the hydraulic system. Regulating the maximum temperature and thermal stability can cause to reverse the formation of fluid decomposition products. System design can reduce the oxidation process and thermal stability of the fluid.

Water retention 

The water retention capacity of a fluid is an important point to be noticed before buying. If huge amounts of water are found in hydraulic fluids it can be removed by regularly draining the reservoir. Sometimes demulsifiers are also added to the fluid to make the process of separation more speedy. The remaining water can be removed by the process of filtration from the hydraulic oil

Hydraulic Fluid Water retention
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Thermal stability

The normal temperature of the hydraulic system varies from 120°F and the maximum can reach up to 150°F. Temperatures between 180° to 200°F are quite normal in the case of some special hydraulic systems. Because of the system temperature, the fluid temperature also varies from time to time. All the systems cannot operate above 250°F because the thermal stability of the hydraulic fluid at that high temperature is not sufficient. In that case, the fluid undergoes rapid decomposition. Hence, checking the thermal stability of the hydraulic fluid is a prime factor.

Hydraulic Fluid Thermal Stability Tester
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Fluid life and disposable properties

Another two features that do not directly fall under the properties of hydraulic fluid but play important considerations during buying it- fluid life and disposable properties.

Depending on the consistency, maintenance, and disposal qualities, fluid can be measured. Non-toxic fluids increase their disability owing to their environment-friendly nature.

FAQs about Hydraulic fluid

  • How long is hydraulic fluid Good For?

The life of hydraulic fluid depends on its consistency, quality of oil, and potential contamination. Mostly good hydraulic fluid lasts more or less for 6 months.

  • How often should the hydraulic fluid be changed?

The hydraulic fluid must be changed every 1,000 hours. Changing the hydraulic fluid is a time-consuming job. The first thing after changing is draining it off.


  • What happens when hydraulic fluid is low?

A whirling mass of fluid or air takes up space in the reservoir and then it enters the pump. So the fluid level should be checked at every interval. If it has gone down the level, it should be filled immediately so that the air cannot fill that space.

  • What happens if the air enters a hydraulic system?

If the air goes in the hydraulic fluid, it starts to make a banging or knocking noise. When the system goes through high and low system pressures, it enters the hydraulic system. If the temperature goes above 180° F (82° C), it can damage the system and cause an overflow of the liquid

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  • What are the basic components of a hydraulic system?

The basic components of the hydraulic system without which the system will not work include the following:

  •     Reservoir.
  •     Filters.
  •     Shut Off Valves.
  •     Control Valves.
  •     Pressure Relief Valve.
  •     Hydraulic Fuses.


  • What are the 4 properties we should consider in choosing hydraulic fluids?

The best 4 properties of a hydraulic fluid need

  •     Thermal stability
  •     Hydrolytic stability
  •     Low chemical corrosiveness
  •     Constant viscosity 

Hydraulic fluid containing all these properties is eligible to run a hydraulic system very swiftly.

  • What does hydraulic fluid do to the skin?

The hydraulic fluid causes skin irritation, redness, dermatitis, eczema, and can even cause serious diseases like skin cancer. Skin exposure with the hydraulic fluid is common for workers. Continuous exposure to the fluid can also burn the skin and can cause permanent damage to the skin.

  • Can I use ATF instead of hydraulic fluid?

Yes! ATF (automatic transmission fluid) can be used instead of hydraulic fluid. Both formulas help to run moving parts and work as coolants in those systems. Thus, there is no problem if we use ATF instead of hydraulic fluid.


Hydraulic fluid is the blood of many automobile and stationary machines in factories. The important properties of hydraulic fluids should be kept in the mind before using them in any hydraulic system. A good quality hydraulic fluid will not only ensure the proper functioning of the machinery but also ensure the system’s health is in good hands and it lasts long. Lastly, before checking any specification, make sure the measurements and data are verified and accurate to run a lower risk of getting betrayed in terms of quality.

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