Hydraulic fluids

Hydraulic fluids


After reading this chapter, the student will be able to:

• Understand and explain the primary functions of a hydraulic fluid

• Detail the various characteristics of hydraulics fluids

• List the different types and categories of hydraulic fluids used

• Understand the problems encountered with hydraulic fluids

• Compare the common properties of the different hydraulic fluid types.


The working fluid is the single most important component of any hydraulic system. It serves as a lubricant, heat transfer medium, Sealant and most important of all, a means of energy transfer. These fluid functions can be clearly understood with the help of the diagrams illustrated below. Fluid characteristics play a critical part in determining the equipment performance and life. Hydraulic fluids are basically non-compressible in nature and can therefore take the shape of any container. This tendency of the fluid makes it exhibit a certain advantage in the transmission of force across a hydraulic system. Use of a clean, high-quality fluid, is an essential prerequisite for achieving efficient operation of the hydraulic system. Although early hydraulic systems employed the medium of water for transferring hydraulic energy, there are serious limitations attached to it such as:

• Its relatively high freezing point (water freezes at 0 °C or 32 °F when the pressure is atmospheric)

• Its tendency to expand when frozen

• Its corrosive nature

• Its poor lubrication properties

• Its capacity to dissolve more oxygen leading to phenomena such as oxygen pitting.

This has necessitated the development of modern fluids designed specifically for application in hydraulic systems, a detailed discussion of which will follow.

Although hydraulic fluid types vary according to application, the four common types are:

1. Petroleum-based fluids which are the most common of all fluid types and widely used in applications where fire resistance is not required.

2. Water glycol fluids used in applications which require fire resistance fluids.

3. Synthetic fluids used in applications where fire resistance and non­ conductivity is required.

4. Environment-friendly fluids that end up causing minimal effect on the environment in the event of a spill.

As discussed earlier, hydraulic fluids have the four essential primary functions of power transmission, heat dissipation, lubrication and sealing to accomplish which, they should possess the following properties:

1. Ideal viscosity

2. Good lubricity

3. Low volatility

4. Non-toxicity

5. Low density

6. Environmental and chemical stability

7. High degree of incompressibility

8. Fire resistance

9. Good heat-transfer capability

10. Foam resistance and most importantly

11. Easy availability and cost-effectiveness.

It is quite obvious that no single fluid can meet all the above requirements and it is therefore essential that only the fluid that comes closest to satisfying most of these requirements be selected for a particular application.

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