Setting Oil Cleanliness by ISO 4406 and NAS 1638

What does the ISO 4406 cleanliness code mean?

ISO 4406 describes the particle load of an oil with a three-part code such as 18/16/13. Each number is the count of particles per millilitre larger than 4 µm(c), 6 µm(c) and 14 µm(c). The (c) suffix means the count was taken with a particle counter calibrated to ISO 11171.

Each code number maps to a range: code 18 means 1,300 to 2,500 particles per ml, code 16 is 320 to 640, code 13 is 40 to 80. Crucially, one code number higher means roughly double the particle count, because the scale is logarithmic.

How does NAS 1638 relate to ISO 4406?

The older US standard NAS 1638 (now superseded by SAE AS4059) states cleanliness with a single class number from 00 to 12. It counts particles in fixed size bands such as 5 to 15 µm or 15 to 25 µm. Many machines and datasheets still use it, so converting to ISO 4406 matters.

There is no exact conversion because the two standards count differently. As a practical rule of thumb, a NAS class maps to a mid-range ISO value plus a fixed offset. The table below shows common approximations used in the field.

Which cleanliness do pumps, valves and servo systems need?

The required cleanliness follows the most sensitive component in the circuit. Servo valves with clearances under 5 µm need far cleaner oil than rugged gear pumps. Always set the target class by the most critical part, not by the average of the system.

• Servo and control valves: ISO 16/14/11 to 17/15/12, filter beta rated at 5 µm.
• Proportional valves: ISO 18/16/13, since the clearances are slightly larger.
• High-pressure axial piston pumps: ISO 18/16/13 for long service life.
• Gear and vane pumps: ISO 20/18/15 is usually sufficient.
• Standard cylinders and directional valves: ISO 20/18/15 to 21/19/16.

How do you reach and monitor the target class?

You reach the target through the right filter fineness, expressed as the beta ratio to ISO 16889. A filter with beta5 >= 1000 removes 99.9 percent of all particles from 5 µm upward. You monitor the condition with laboratory particle counters or online with a permanently installed sensor.

• Take the oil sample under pressure at a running, turbulent point.
• New oil is not automatically clean - often ISO 21/19/16 from the drum, filter before filling.
• Offline or kidney-loop filtration lowers the class permanently without loading the main flow.
• Document trends: a sudden rise signals wear or a filter breakthrough.

Frequently asked questions