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» System Cleanliness – Ratings & Filtration

In order to monitor and quantify the cleanliness of the fluid in an hydraulic system, standard rating systems have been developed over the years.

These cleanliness ratings also allow us to measure the effectiveness of any filtration systems put in place to maintain the cleanliness of an hydraulic system.

Cleanliness Rating Systems

The cleanliness of a fluid can be measuring using a number of different standard classification systems, as outlined below.

ISO 4406

ISO 4406 describes a method by which to count and record particle sizes.

An average is taken by counting three size ranges.

  • Particles greater than 4 microns.
  • Particles greater than 6 microns.
  • Particles greater than 14 microns.

Because the number of particles within a system may be very high, the actual number is not used. Instead, a number is used to represent the value.

This “class” number is based on the following table.

The table below is the ISO 4406 codes.

Contamination - ISO4406 table

There has been two major editions of this standard, 1987 and 1999.

1999 is the current edition, and it should be used when possible.

ISO 4406: 1987 counted 2, 5 and 15 micron.

ISO 4406: 1999 counts 4, 6 and 14 micron.

In both versions of the standard, the class groups are set out so that the maximum number of particles in each class is double the figure of the class before it.

Hydraulic manufacturers are publishing cleanliness limits to ISO 4406.

While it is not the only method of counting particles, it is most commonly used method today as portable counting machines are becoming more affordable.

ISO 4406 ratings provide a system in which each ascending class number represents twice the contamination as the preceding class number.

System Cleanliness - ISO4406

NAS 1638 (1964)

The NAS class defines a maximum number of particles of a particular size per 100ml sample of fluid.

System Cleanliness - NAS 1638 Chart

SAE AS4059 (1963)

Similarly, the SAE class defines a maximum number of particles of a particular size per 100ml sample of fluid, using a different size classification system.

System Cleanliness - SAE AS4059 Chart

Comparison Chart

For any given fluid, the cleanliness can be measured with any of the available cleanliness rating systems, with comparative values as per the chart below.

System Cleanliness - Standards Comparison Chart

Using an appropriate fluid cleanliness rating system and fluid analysis, contaminants can be measured and the results quantified.

The next step is to choose a filtration product that will be able to effectively remove particles from the fluid.

Filtration of fluid

The finer the filtration the better the achieved cleanliness of the pressure fluid and the longer the life of the axial piston unit.

To ensure a reliable functioning of the axial piston uniti a minimum cleanliness of

20/18/15 to ISO 4406 necessary.

At very high temperatures of hydraulic fluid (195F (90C) up to max. 239F (115C)) at least cleanliness of

19/17/14 to ISO 4408 is necessary.

If above cleanliness classes cannot be met please consult us.

Filter placement

Generally, there are four areas of location for filters within a hydraulic circuit.

Full Flow Filtration:

  1. Suction filtration
  2. Pressure filtration
  3. Return filtration

Partial Flow Filtration:

  1. Charge or Off-Line filtration

In reality, most circuits have a combination of these types.


Suction filtration is placed in the suction line of a pump. The suction line is the line through which the pump draws fluid from a reservoir.

The filter can be placed within the reservoir, or externally, before the pump.

System Cleanliness - Filter Placement for Suction Filtration

Care must be exercised when designing a circuit with suction filtration, as incorrect selection or design can cause pump failure.

Often suction filtration is very coarse, at about 100 microns or greater.

In these cases, the item is not a filter as such, but a strainer.

See RHY-RHC0707AUS page 373


Pressure filtration is placed in the main circuit. If placed immediately after the pump, then the filter will protect the entire circuit.

System Cleanliness - Filter Placement for Pressure Filtration

Pressure filters must be able to withstand high pressures on both sides of the filter housing.

This makes them very robust, heavy and expensive. However, pressure filtration is very efficient for contaminant removal.

RYCO do not currently sell pressure filters.


Return filtration is very cost effective. Return filters can be of a light construction, as they are placed in the low pressure part of the circuit, where the fluid is returning to the reservoir at low pressure.

System Cleanliness - Filter Placement for Return Filtration

Charge or Off-Line Filtration

Offline filtration involves diverting the hydraulic fluid of a system through separate, external device that can remove contaminants from the fluid as it flows through the external filters.

Want more information?

Read part 1 of our series of articles on System Cleanliness – Contamination and Analysis to learn more about contamination, what causes it and the impact it can have on hydraulic systems, plus how we can monitor and quantify the degree of cleanliness of an hydraulic fluid.

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