Fluid Basics (Part 2)


  • Identify popular hydraulic fluid additives
  • Examine various properties of fluid, and discuss how each can impact hydraulic operation
This is part two of a two-part exposé on hydraulic fluid basics.
Click here to go to part one of this lesson.


High on the list of important functions for hydraulic fluid is its ability to lubricate the components in the system.

Lubrication is used to decrease the friction between moving surfaces that come into contact with each other.

This reduces wear and helps prevent excessive heat buildup.

At a microscopic level, even the smoothest, most polished metal surfaces appear jagged and spiky.

These spikes are called asperities.

Not touching!

The gold standard in lubrication is known as full-film lubrication.

This is when the fluid is able to completely cover the exposed surfaces and is thick enough to ensure that the opposing asperities never come into contact with each other.


If the fluid film is too thin and asperities are allowed to make occasional contact with each other, you have a condition called boundary lubrication.

Past the boundary is the no man's land known as unlubricated. We do not go there.

A fluid's lubricating capabilities are tied to its viscosity.

If the viscosity is too low, the fluid will be too thin to create a full film condition.

But if the viscosity is too high, the fluid will be too thick to even flow between the opposing surfaces.

Fortunately, additives can be used to improve the fluid's lubricating capabilities.

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We hope you enjoyed Fluid Basics (Part 2)

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