Fluid Basics (Part 1)


  • List common base stocks used for hydraulic fluid
  • Analyze the various aspects of viscosity, and discuss how each can impact hydraulic operation

What is Hydraulic Fluid?

What exactly is hydraulic fluid?
And why are there so many different kinds!?
Hydraulic fluid is the life blood of a hydraulic system.
It dissipates heat, lubricates moving parts and, of course, transmits force from the pump to the actuators.
Hydraulic fluid is a blend of liquids and additives. It can be oil-based or water-based, and it can be natural or synthetic.

Each formulation has advantages and disadvantages, so choosing the best mix requires an understanding of your system's specific needs.

Base Stocks

The base stock of a hydraulic fluid makes up the bulk of what the fluid actually is.

Roughly 99% of the fluid is made up of the base stock, with the remaining 1% consisting of additives.

That just seems like a good way to get dirty...

Hydraulic fluid base stocks come in three basic types.

  • Petroleum (mineral oil) based
  • Bio-degradable (vegetable oil) based
  • Fire-resistant

Petroleum based fluids are the original, and still by far the most popular choice in hydraulic fluids.

These fluids are preferred because of their relatively low cost and high availability.

They also tend to have good viscosity properties, are good lubricants, and work well with many additives.

Bio-degradable fluids are used in situations where the environmental impact of a fluid leak is a concern.

A food processing facility would not want petroleum oil leaking into the food!

The most common bio-degradable fluid base stocks are made from canola oil.

While these fluids generally perform well, they are not as versatile as petroleum based fluids.

They break down faster due to oxidation, do not function well at low temperatures, and do not combine well with some additives.

Also, they are more expensive than petroleum based fluids.

Fire-resistant hydraulic fluids (FRHFs) sacrifice performance to gain additional protection against burning.

This is especially important in situations where there are readily available ignition sources, for example, in a foundry or steel mill.

These fluids come in a few different varieties.

  • Water-in-oil emulsions: a petroleum based fluid with water purposely mixed into it
  • Oil-in-water emulsions: a water-based fluid with petroleum added to it to improve its performance
  • Water glycol: a solution consisting of water dissolved into a glycol
  • Phosphate esters: synthetic fluids based on phosphoric acid and an alcohol
  • Polyol esters: synthetic fluids based on a fatty acid and an alcohol

The different formulations provide different levels of resistance to burning.

Some will not be able to start burning at all, and may even provide limited fire extinguishing properties.

Others can light on fire but will not be able to sustain burning. Once the ignition source is removed, the fluid stops burning.

Each variety has different trade-offs.

For example, hydraulic systems using fluids containing water suffer from higher rates of corrosion and oxidation.

And hydraulic systems using fluids containing esters need special seals and gaskets.

Fire-resistant hydraulic fluids tend to be among the most expensive types of hydraulic fluid available.

This is just a preview!

Become a member to get immediate access to the rest of this lesson, and all the other great content on LunchBox Sessions.

Join Now

Already a member? Log In
Not ready to join? Back to the menu.

We hope you enjoyed Fluid Basics (Part 1)

Vacuum Pressure
Drain Pressure
Low Pressure
Medium Pressure
High Pressure
Lowest Voltage
Medium Voltage
Highest Voltage
Magnetic Field
Check Your Console