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.


Hoses, o-rings and seals can break down and become contaminants, especially once they are damaged.

Polymers will fail if they are exposed to higher pressures or higher oil temperatures than they are intended for.


Water vapor is a common contaminant in hydraulic systems.

Humid air drawn in through the breather condenses into water.
Rust corrosion can occur if this water is allowed to remain in the system.

Tiny rust (iron oxide) particles are extremely hard and can easily score, or jam, a valve.


Silica, also known as quartz or silicon dioxide, is a very hard and sharp edged mineral particle.

It is commonly found in the dust on a concrete floor or a roadway.

Dirty Filler Cap


Fibers may come from wood, cardboard, paper, shop towels, rags, etc.

Shop towel fibers found in oil sample (100x magnification)
Brand new flow control valves, covered in cardboard package fibers!


Many, if not most, of these contaminant particles are incredibly tiny, yet can still have a huge impact on a hydraulic system!

Contaminants are often measured in microns (μm).

A micron is:

  • one millionth of a meter
  • one thousandth of a millimeter
  • 0.00003937 of an inch
The average quarter is about 24,000 microns wide
Picture taken with a microscope (100x) showing particles only a few microns wide

The width of a human hair is around 70 microns and objects under 40 microns are too small for human eyesight to detect.

Even humans with good eyesight!

But many hydraulic components have clearances of 10 microns (or less!)

A particle that is a boulder in terms of the system might remain invisible to the naked eye.

Ingression Paths

How is all of this contamination getting into the system in the first place?
Some contaminants are created within the system through normal wear and tear, but many are introduced from the outside world.

New Fluid

Sometimes the jug/drum/tote in which the new fluid arrives is already contaminated.

The refinery or the container the oil was stored in may be dirty, or the container could be contaminated by multiple careless openings/closings.

Filling Process

A careless filling procedure without proper filtration and cleaning will drag contaminants in along with the new oil.


Older style breather/filler caps have very porous mesh (40 microns and larger is typical).

They are not up to the task of keeping contamination out.

If someone has been shortcutting the filling process by dumping fresh fluid in through the breather port, it becomes an excellent entry point for contamination.

Tank Gasket

The tank gasket can become misaligned, or a bolt hole can become stripped.

If the gasket is not being pinched tight along the entire reservoir rim, it becomes an entry point where dirty air completely bypasses filtration.

Quick Couplers

Quick couplers are used as test points, accessory attachments, and tank filling points.

If they are not spotless when you plug into them contamination will be introduced into the system.

Cylinder Rod Scratch/Gouge

While the cylinder is extended, dirt packs into the scratches/gouges and sticks to the oil film.

As the cylinder retracts, the dirt/dust in the rod scratch ducks under the wiper seal and gets a free ride into the oil of the rod end chamber.

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