Three hidden killers of a hydraulic system
Clear oil does not mean clean oil. The most common hydraulic failures in industrial settings are often not caused by poor component quality, but by neglected oil management.
KILLER 1
Particle contamination | Contamination level NAS 9 or above
Invisible metal chips, rubber fragments and outside dust continuously grind valves and sealing surfaces, expanding clearances and increasing internal leakage. Ultimately, component life drops sharply and failure arrives earlier than expected.
KILLER 2
Excess water | Water above 0.1%
Water destroys the load-bearing capacity of the oil film. Lubrication collapses, metal surfaces come into direct contact and the wear rate doubles. It also accelerates oil oxidation and microbial growth, further contaminating the entire hydraulic circuit.
KILLER 3
Trapped air | Incomplete system bleeding
Bubbles collapse instantly under high pressure—the cavitation effect—creating local impact pressure that erodes metal surfaces. They also destabilize hydraulic pressure, increase output vibration and reduce positioning accuracy.
NAS contamination levels: the numbers you need to know
The NAS (National Aerospace Standard) contamination level classifies the number of particles in every 100 ml of oil. The smaller the number, the cleaner the oil. The following are reference levels for industrial applications:
| NAS level | Applicable system | Risk level |
|---|---|---|
| NAS 4 – 6 | Precision servo systems, proportional valves | ✔ Best |
| NAS 7 – 8 | General industrial hydraulic systems (Ashun internal standard) | ✔ Recommended range |
| NAS 9 – 10 | Warning range; component life begins to decline significantly | ⚠ High risk |
| NAS 11 and above | Severe contamination; shutdown may occur at any time | 🚨 Dangerous |
ASHUN INTERNAL STANDARD · FACTORY STANDARD
Ashun: no exception for our own equipment
Daily inspections use Hydac equipment, and oil on the factory test stands is maintained below NAS 8 throughout.
- Daily Oil Inspection: Hydac professional oil-analysis instruments are used to regularly test contamination levels on all hydraulic test stands. This is not a slogan; it is an internal discipline carried out every day.
- Maintained Below NAS 8: All hydraulic oil on the factory test stands is kept below NAS 8 throughout, ensuring that every product accepted on the test stand is tested in an environment held to the same high standard as the customer's site.
Oil-management support provided by Ashun
Recommend suitable oil specifications and filtration precision, selecting the correct viscosity grade and additive formula for the duty.
Design the correct return path and breather system to prevent air and water from entering the hydraulic circuit at the source.
Provide oil-management recommendations and regular inspection plans to extend the life of the entire hydraulic system.
The harsher the environment, the more stable Ashun performs.
This is not a slogan—it is the acceptance standard for every unit we ship.
Frequently Asked Questions
What consequences can hydraulic-oil contamination cause?+
Hydraulic-oil contamination accelerates component wear, shortens seal life, causes valves to stick and destabilizes system pressure. Above NAS 9, component life declines significantly; above 0.1% water, lubrication collapses; trapped air causes cavitation and increased vibration.
What is the NAS contamination level? Below what level should a hydraulic system be controlled?+
NAS levels classify the number of particles in the oil per 100 ml; the smaller the number, the cleaner the oil. General industrial hydraulic systems are recommended below NAS 8; precision servo systems are recommended below NAS 6. Ashun factory test stands are maintained below NAS 8 throughout.
Why is water in hydraulic oil dangerous?+
When water exceeds 0.1%, the load-bearing capacity of the oil film drops sharply, lubrication collapses and direct metal-to-metal contact increases wear. It also accelerates oxidation and microbial growth, contaminating the entire hydraulic system.
How can I tell whether hydraulic oil needs replacement or filtration?+
Use professional instruments such as Hydac equipment to regularly test contamination levels. Clear oil does not mean clean oil; particles, water and air bubbles must be detected with instruments. Regular sampling and analysis is the most reliable preventive-maintenance method.
