Hydraulic Fluids

3/9/21, 9:36 PM
Hydraulic Fluids

We can imagine hydraulic fluid in the system is as blood in our body. The main purpose of the fluid in a hydraulic system is to transmit power through the system from the pump at one end to the actuator at the other. Fluid has two main properties: it needs to be able to flow freely and be incompressible. The ability of a liquid to flow freely is measured by a property known as its viscosity. Viscosity is related with temperature and fluid becomes thinner when temperature increased.

Generally accepted method for hydraulic fluids is the International Organization for Standardization (ISO) viscosity grade (VG) number, which defines a fluid’s approximate viscosity in centistokes (cSt) at a temperature of 40 ⁰C. For hydraulic systems, viscosity range is between ISO VG15 – ISO  VG100. For comparison, viscosity of water is around 1 cSt. Hydraulic fluid selection in the system depends on working climate and ambient temperature.

The viscosity of a fluid will affect the internal frictional losses within a system. The higher the viscosity the harder it is to push the fluid along pipes, hoses, etc., so in this respect a low viscosity fluid is beneficial. However, secondary purpose of the fluid is to lubricate the components of the system, in particular pumps and motors, which are subject to high Mechanical loads and speeds. A fluid that is too thin (low viscosity) may not be able to prevent metal to metal contact between components which will lead to rapid wear. Properties of ideal hydraulic fluid;

  • Incompressible
  • Free flowing
  • Good lubricator
  • Wide temperature range
  • Good material compatibility
  • Chemically stable
  • Non-flammable
  • Non-polluting
  • Non-hazardous
  • Low cost

For fluid classification there are ISO 6743-4 and DIN 51524 standards. Fluid types are indicated with “H” letter.

DescriptionISO 6743-4DIN 51524
Straight oil with no additivesHHH
HH plus anti-oxidant and anti-corrosion additivesHLHL
HL plus anti-wear additivesHMHLP
HLP plus detergent additives HLPD
HM plus VI improversHV, HRHVLP
HM plus anti stick-slip additivesHG 

It has big importance to select proper fluid and maintain its condition for hydraulic systems. We will mention what effects Hydraulic oil contamination do and measurement of oil cleanliness in our following posts. Stay tuned!

Part of this article is from the book “An Introduction to Practical Hydraulic System Maintenance”

Images and table are used for training purpose only