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.
Contaminants can be solids, liquids or gasses.
What is contamination?
There are 7 different classes of contamination:
Abrasion – Hard particles causing wear
Fatigue – A repeated stressing on a surface
Erosion – Particles striking at high velocity
Corrosion – Chemical degradation
Adhesion – Particles sticking to a surface
Cavitation – A vacuum being induced on a fluid
Aeration – Gas bubbles within a fluid
What causes contamination?
There are 3 main causes of contamination:
Contaminants can be built-in to a system during manufacture. Hoses, tubes and other components are dirty when fitted
Contaminants may ingress into a system. They may gain entry through seals or tank breathers
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…
The 5 principal effects of contamination
Intermittent failures – Systems behaving erratically
System degradation – Loss of system performance due to wear
Catastrophic failures – Sudden total destruction of components
Leakage – Fluid boss. both internally and externally
Safety risks – Unexpected operation or non-operation
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
Micron sized objects in hydraulic systems
In hydraulics, the clearances between moving parts can be very small, for example:
0.5µ to 5µ clearance between the gear and housing
0.5µ to 1µ clearance on vane tip
5µ to 13µ clearance on vane sides
0.5µ to 5µ clearance on lens plate
5µ to 40µ clearance on pistons
10µ to 250µ clearance on rod to gland
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.
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.
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.
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.
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