Filtration and Contamination


  • List common types of contamination
  • Discuss the ISO 4406 method for coding contamination
  • Examine filters and filtration strategies

Why Worry?

Why is contaminated hydraulic fluid even a concern?
Modern hydraulic systems do not tolerate contamination well.
Excessive contamination will make reliable performance virtually impossible.

Older hydraulic systems featured lower operating pressures, simple gear pumps and lever operated directional valves. Particle contaminants may have caused some degradation, but this seldom caused intermittent or catastrophic failure.

Modern hydraulic systems feature valves with close tolerances between parts, and variable current solenoids that use minimal force.

This means that particle contaminants can easily interfere with normal operations.

For example, particle contamination can cause a number of issues with a modern directional control valve.

These include:

  • Valve stuck closed
  • Valve stuck open
  • Being stuck in a partially open position
  • Exhibiting a stick-slip behavior (jerky motion)

Contaminant Particle Types

There are 5 common types of particle contamination.

  • Metals
  • Elastomers/Polymers
  • Rust
  • Silica
  • Fibers

There are two other types of contamination, microorganisms and carbon, but they are pretty rare.


Metal slivers can enter a system in a number of different ways.

When hose is cut and not properly cleaned before crimping and sending to work.

Hose Wire

Leftover filings from the drilling and cutting of the reservoir.

Wear metals from all moving parts in contact with other surfaces.
This is especially common in gear pumps.

Gear Scuffing

Cavitation causing pitting in pumps.

It is possible to get metal shavings from any moving metal components.
The obvious suspects include hydraulic motors, cylinders, and spools, but metal shavings can come from anywhere that erosion (washout) is taking place.
Don't rule out non-moving components like manifolds and fittings.

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