News & Events

» System Cleanliness – Contamination & Analysis

The performance of hydraulic systems can be impacted by the presence of contaminants in the hydraulic fluid.

But how does contamination arise, and how can we monitor and quantify it so that we can effectively manage it and minimise any potential damage to an hydraulic system?

An overview of contamination

Definition in fluid power

Any material, foreign to a system’s fluid that has the potential to damage components within that system.

System Cleanliness - Contaminants

Contaminants can be solids, liquids or gasses.

System Cleanliness - Contaminants in Valve

What is contamination?

There are 7 different classes of contamination:

  1. Abrasion – Hard particles causing wear
  2. Fatigue – A repeated stressing on a surface
  3. Erosion – Particles striking at high velocity
  4. Corrosion – Chemical degradation
  5. Adhesion – Particles sticking to a surface
  6. Cavitation – A vacuum being induced on a fluid
  7. Aeration – Gas bubbles within a fluid

What causes contamination?

There are 3 main causes of contamination:

  1. Manufacturing
    Contaminants can be built-in to a system during manufacture. Hoses, tubes and other components are dirty when fitted
  1. Ingress
    Contaminants may ingress into a system. They may gain entry through seals or tank breathers
  1. Internal generation
    Contaminants will be generated within the system. As systems work, they will self contaminate. These particles must be filtered out of the system

What contamination causes…

System Cleanliness - Degradation of Equipment

The 5 principal effects of contamination

  1. Intermittent failures – Systems behaving erratically
  2. System degradation – Loss of system performance due to wear
  3. Catastrophic failures – Sudden total destruction of components
  4. Leakage – Fluid boss. both internally and externally
  5. Safety risks – Unexpected operation or non-operation

Measuring contamination

How small is a micron?

When assessing contamination in hydraulic systems, the unit of measurement used is

MICRONS (µ).  A micron is VERY small, and is equal to:

1 millionth of a metre. 0.000001m

10 thousandth of a centimetre. 0.0001cm

1 thousandth of a millimetre. 0.001mm

Common items measured in microns

Grain of sand: 120 microns

Grain of salt: 100 microns

Human hair: 75 microns

White blood cell: 25 microns

Red blood cell: 10 microns

System Cleanliness - Red Blood Cells

Micron sized objects in hydraulic systems

In hydraulics, the clearances between moving parts can be very small, for example:

Gear pump

0.5µ to 5µ clearance between the gear and housing

Vane pump

0.5µ to 1µ clearance on vane tip

5µ to 13µ clearance on vane sides

Piston pump

0.5µ to 5µ clearance on lens plate

5µ to 40µ clearance on pistons


10µ to 250µ clearance on rod to gland

System Cleanliness - Cylinders

So far we have learned:

  • There are various types of contamination
  • Contaminants are measured in microns (µ)
  • Hydraulic clearances are very fine
  • Small sized contaminants can damage a system just as well as large particles

How do we measure and gauge contamination?

We have learned that contamination can be very small solid particles. We also understand that the clearances between components in hydraulic systems can be very small also.

The level of system contaminants must be accurately measured, quantified and recorded.

Then we need to identify methods and products for removing these contaminants from systems.

We will now investigate the practice of measuring and analysing contamination.

This is called fluid analysis.

Fluid Analysis

Sample collection

A fluid sample is collected, usually through a test point and test hose. For more information on test points, see RYCO R1620 in the accessories section of the catalogue.

The sample is usually 100ml, and it is analysed in two ways; chemical analysis and spectral analysis.

System Cleanliness - Fluid Analysis

The intention is to find out the following:

  • What chemicals are present,
  • How many solid particles are within it, and
  • What is the size of the particles

The 100 ml fluid sample is fed through a portable particle testing machine.

This machine will count the number of particles of certain sizes, and display the results in a graphical form on the screen.

System Cleanliness - Particle Testing Machine

A print out can be made of the results, and they can also be sent to a PC in Excel format for record keeping.

Typical results of the particle count look like this:

Typical Spectral Analysis Results:

  • 6240 particles over 4µ
  • 96 particles over 6µ
  • 48 particles over 10µ
  • 26 particles over 14µ
  • 5 particles over 25µ
  • 0.2 particles over 50µ

The results of a spectral analysis are easy to understand, but the are difficult to express.

How would you describe these results to someone?

How would you judge if the results are acceptable or not?

How would manufacturers express maximum allowable levels of contamination?

The have been several methods developed over the years for assessing fluid cleanliness, including the commonly used ISO4406, plus NAS 1638 (1964), SAE AS4059 (1963) and Mil Std 1236A.

Want more information?

Read part 2 of our series of articles on System Cleanliness – Ratings and Filtration to learn more about the commonly used cleanliness rating systems, and how to filter fluids to reduce the impact of fluid contamination.

And to help you choose the right hydraulic hose for your project, download our 15 Keys To Selecting The Right Hose guide now.