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Time：2026-01-07 13:22:05 Reading volume：
Hydraulic oil filtration is a key factor in hydraulic system reliability and service life. Its primary goal is to maintain oil cleanliness, ensuring stable operation, high efficiency, and reduced component wear. Filtration requirements should be defined across cleanliness standards, filtration accuracy, system layout, and maintenance practices.
Cleanliness grade is the most important indicator of hydraulic oil contamination, expressed by particle concentration at different sizes.
Common Standards
ISO 4406 (e.g., 18/16/13): Particle counts ≥4 μm, ≥6 μm, and ≥14 μm per milliliter
NAS 1638: Still referenced in some industrial and aerospace applications
Typical Requirements
High-pressure / servo systems: ISO 15/13/10 or cleaner
Medium-high pressure systems: ISO 18/16/13 or 17/15/12
Low-pressure / general systems: ISO 21/19/16 or lower
Always follow the equipment manufacturer’s specified cleanliness level.
Filtration accuracy defines the smallest particle size a filter can effectively remove, measured in microns (μm).
Absolute filtration: ≥98% removal efficiency (recommended for critical systems)
Nominal filtration: Partial efficiency with no unified standard
Servo / proportional valve systems: 3–5 μm (absolute)
High-pressure systems (>21 MPa): 5–10 μm
Medium-pressure systems (7–21 MPa): 10–15 μm
Return line filtration: 15–25 μm
Suction filtration: 100–150 μm (pump protection only)
Effective contamination control requires proper filter placement:
Pressure line filtration – Protects precision valves
Return line filtration – Captures wear debris before oil returns to tank
Suction line filtration – Prevents large particles from entering pumps
Offline (bypass) filtration – Continuous high-precision oil cleaning for large or critical systems
Oil filling filtration – Mandatory, as new oil often contains contaminants
Filtration must control contaminants without degrading oil properties:
Moisture content: ≤1000 ppm (≤500 ppm for precision systems)
Air control: Prevents foaming and loss of stiffness
Physicochemical stability: No change to viscosity, additives, or acid value
Temperature and viscosity matching: Filtration system must suit real operating conditions
Differential pressure monitoring for timely filter replacement
High dirt holding capacity to extend filter life
Material compatibility between oil, seals, and filter media
Flow rate matching to avoid excessive pressure loss
Monitor oil cleanliness using particle counters
Establish a target cleanliness level based on system criticality
ｒｅｐｌａｃｅ filters based on differential pressure or oil analysis
Perform system flushing during oil changes or overhauls
Prevent contamination entry via sealed tanks and clean breathers
The goal is to keep solid particles, water, and air below target limits, reducing wear, preventing valve sticking, stabilizing performance, and extending the life of both oil and components—ultimately improving system reliability and operating economy.
In real applications, always rely on the manufacturer’s technical manual and oil analysis data for scientific maintenance decisions.
