Hydraulic Oil Filtration & ISO 4406 Contamination Codes (1500+ Word Technical Guide)

Oil contamination is responsible for 70–80% of all hydraulic failures. Pumps, valves, servos, cylinders and proportional systems fail prematurely due to abrasive particles, dirt and wear debris.

Understanding oil cleanliness is essential for hydraulic engineering.

This guide covers:

ISO 4406 codes
Beta Ratio (βx)
filter selection
contamination sources
online/offline filtration
recommended cleanliness levels

1. Why Oil Cleanliness Matters

Contamination causes:

pump wear
valve sticking
servo instability
increased leakage
reduced efficiency
overheating

Even a 5-micron particle is large enough to block a servo valve land.

2. ISO 4406 Contamination Codes

ISO 4406 uses three particle size ranges:

≥4 µm
≥6 µm
≥14 µm

Format:

ISO 4406: xx / yy / zz

Each increase by 1 equals doubling of particle count.

Example:
ISO 17/15/12 means:

64,000 particles ≥4µm
20,000 particles ≥6µm
2,500 particles ≥14µm

3. Recommended Cleanliness Codes

System Type	ISO Code
Mobile hydraulics	19/17/14
Industrial hydraulics	17/15/12
Proportional valves	16/14/11
Servo systems	15/13/10
High-precision systems	14/12/9

Servo valves require extremely clean oil.

4. Filter Efficiency: What Is Beta Ratio?

Beta Ratio:

βx = particles entering / particles leaving the filter

Example:

β10 = 200 → 99.5% efficiency

Efficiency = (1 − 1/βx) × 100

5. Types of Filters

5.1 Suction Filters

Coarse filtration (80–125 µm), protects pump, but must not be too restrictive.

5.2 Pressure Filters

3–10 µm, protects proportional and servo valves.

5.3 Return Filters

10–25 µm, keeps tank clean.

5.4 Offline Filtration (Kidney Loop)

Provides the highest oil cleanliness levels.

6. Contamination Sources

manufacturing debris
hose rubber fragments
seal wear
airborne dust
pump or motor wear
dirty new oil

New oil typically arrives at ISO 22/20/17, too dirty for modern systems.

7. Why Particles Are Dangerous

Three wear mechanisms:

Abrasive wear
Adhesive wear
Fatigue wear

Particles damage pumps, spools, cylinders and servo components.

8. How to Measure ISO 4406 Cleanliness

online particle counters
laboratory analysis
portable field analyzers

9. Filtration Strategies

9.1 Use Filtered Breathers

Prevents airborne dust from entering tank.

9.2 Install Offline Filtration

Maintains stable ISO code.

9.3 Filter New Oil Before Filling

New oil is not clean.

9.4 Select Correct Micron Rating

Servo → 3 µm
Proportional → 6 µm
Standard → 10–25 µm

9.5 Use Differential Pressure Indicators

Shows filter clogging.

9.6 Maintain Proper Tank Volume

Allows particles to settle and improves cooling.

10. Cost of Contamination

Dirty oil:

reduces pump life by up to 60%
increases valve failure by 4×
raises heat generation
lowers efficiency

Filtration cost is nothing compared to system damage.

11. Conclusion

Hydraulic oil cleanliness is essential for:

component life
system stability
energy efficiency
precision control

Key takeaway:

ISO 4406 defines cleanliness.
Beta Ratio defines filtration.
Offline filtration ensures long-term reliability.

This guide is part of the Sancoqhub advanced engineering series.