Understanding Oil Cleanliness Class per ISO 4406
More than 70 percent of all hydraulic system failures trace back to contaminated oil. ISO 4406 uses three numbers to describe how many particles per millilitre float in the fluid. Knowing the target class of your components and matching filtration to it clearly extends the life of pumps, valves and cylinders.
View hydraulics basicsHow is the ISO 4406 code structured?
ISO 4406 states oil cleanliness as a three-part code, for example 18/16/13. Each number is a cleanliness grade that describes the count of particles above a given size per millilitre. The first number refers to particles from 4 µm, the second from 6 µm and the third from 14 µm. The higher the number, the more particles and the dirtier the fluid.
Which target class does my component need?
The required cleanliness class follows the most sensitive component in the circuit. Servo valves and axial piston pumps with tight control clearances need very clean oil, while simple gear pumps are tolerant. Always use the maker's specification as the target and choose filtration one step cleaner to keep a reserve.
- Always size to the most sensitive component, not the average.
- The datasheet specification takes priority over rules of thumb.
- For longer service life, filter one class cleaner than required.
- New oil is not automatically clean and often sits at 21/19/16 or worse.
How do I measure and reach the cleanliness?
Cleanliness is determined by automatic particle counting per ISO 11500, either in the lab or with an inline particle counter on the running system. You reach the target class through the right filter rating, controlled top-up and protection against ingress of moisture and air. A filter's capture efficiency is given by the beta ratio per ISO 16889.
Frequently asked questions
What does an ISO 4406 code like 18/16/13 mean?
The three numbers give the cleanliness grades for particles from 4 µm, 6 µm and 14 µm per millilitre. 18/16/13 is a common target class for many hydraulic systems with piston pumps. The smaller the numbers, the cleaner the oil.
Which oil cleanliness do servo valves need?
Servo and control valves have very tight control clearances and typically need a class of 16/14/11 to 18/16/13. Follow the maker's specification and filter one step cleaner to keep a reserve against wear.
Is fresh oil from the drum automatically clean enough?
No. New oil often sits at 21/19/16 or worse because transport and storage add particles. Filter new oil through a filter unit while filling to reach the required target class.
Secure the right oil cleanliness
Our hydraulics specialists help with target class, filter selection and particle measurement for your circuit.
Clear target classes
Cleanliness values by component and maker data.
Clean fluids
Filter rating matched to the ISO 4406 target.
Longer service life
Less wear through a controlled particle load.
Expert advice
Hydraulics specialists support your design.