Hydraulic Fluid Properties

Web Desk event Feb 27 2019 12:12PM visibility 354
Hydraulic Fluid Properties

Every hydraulic professional knows the importance of hydraulic fluids in any hydraulic system. The basic requirement for a hydraulic system is fluid. Fluids can be categorized into petroleum based and fire resistant types. The key functions of hydraulic fluids are power generation, lubrication, heat removal, and sealing. Fluids are selected for a particular application depending on the hydraulic fluid properties.
Hydraulic fluid properties can be subdivided into physical properties and chemical properties. Physical properties include those required for normal system operation and chemical properties include those required for providing stability for continuous operations.
A wrong selection of hydraulic fluid will cause severe system failures. But, it is impossible to select a fluid that satisfies all the properties. So, what we can do is select a hydraulic fluid that suits your hydraulic system. Certain additives like . pour point depressant, viscosity index improvers, defoamers(anti-foam additives), oxidation inhibitors, etc.. can be added with the fluid to obtain more stability. Some of the basic hydraulic properties are listed below.


Viscosity is the resistance to flow and it is considered as an important property of fluids. For a hydraulic system, the viscosity is inversely depended on the temperature. As the temperature increases, the viscosity will decrease and internal leakages occur. When the temperature decreases, the viscosity will increase and it will resist the flow of fluids. Using hydraulic fluids with suitable viscosity will reduce flow losses and provide precise control over the system.


Lubrication reduces the friction and protects the system components against wear. Lubrication property of the hydraulic fluid means creating a strong film with fluid that can’t be wiped out with the moving surface.

Minimum compressibility:

Most of the hydraulic system work under Pascal’s principle. So, the incompressible property of the fluid is essential for efficient operation. This property helps in generating a higher force by applying a small force on a larger area.


Demulsibility is the property of the fluid to release water. The presence of water in the system will create many contamination related issues and will result in corrosion and damage.

Fire Resistance:

We have both petroleum based and fire resistant hydraulic fluids. A high risk of fire is present in fluids that are used with coal mines and hot metal processing equipment due to high temperature. So, in such applications always prefer fire-resistant hydraulic fluids. Flashpoint, fire point and ignition temperature are the common characteristics to determine the fire resistance of fluid.

Foam Resistance:

Foaming occurs as a result of fluid contamination, poor system design, cavitation, etc. This can be a reason for the overflow of fluid from the reservoir.

Material compatibility:

To ensure long life and maximum performance, one needs to check the compatibility of fluid with the material(metals, rubber and elastomer components) the system use.

Heat Removal:

Continuous flow of fluids through the system will increase the temperature of both fluid and system components. This will create malfunctioning of the system components. So, the system requires good heat dissipation property.

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