Hydraulic oil filtration accuracy standard

The filtration accuracy standard of hydraulic oil depends on the working pressure of the hydraulic system, the sensitivity of the components and the cleanliness requirements of the oil. Different industries and application scenarios have different standards for the cleanliness of hydraulic oil. The following is a detailed classification and guide:

1. Basic requirements for filtration accuracy of hydraulic systems

The filtration accuracy of hydraulic oil is usually expressed in microns (µm). The smaller the value, the higher the filtration accuracy.

Recommended filtration accuracy for common hydraulic systems:

| Hydraulic system type | Recommended filtration accuracy (µm) | Applicable scenarios |

|------------------------|----------------------|----------------------------------|

| Low-pressure hydraulic system | 10~25 µm | Agricultural machinery, general engineering machinery |

| Medium-pressure hydraulic system | 5~10 µm | Injection molding machine, machine tool, general industrial equipment |

| High-pressure hydraulic system | 3~5 µm | Servo system, precision hydraulic equipment |

| Ultra-high pressure/servo system | 1~3 µm (even ≤1 µm) | Aerospace, precision machine tool, electro-hydraulic servo valve |

2. International cleanliness standard (ISO 4406)

The cleanliness of hydraulic oil is usually expressed in ISO 4406 standard, in the format of XX/XX/XX (such as 18/16/13), which respectively represent:

- Number of particles above 4µm

- Number of particles above 6µm

- Number of particles above 14µm

3.Typical hydraulic system cleanliness level requirements

|                  System type                      | ISO 4406 target level |                         Applicable equipment                                    |

|-----------------------------------------|------------------------|------------------------------------------------------------------ |

| General hydraulic system                   | 20/18/15                   | Excavator, forklift                                                                     |

| High-performance hydraulic system | 18/16/13                   | CNC machine tools, injection molding machines                    |

| Servo/precision hydraulic system      | 16/14/11 or higher   | Aviation hydraulics, high-precision electro-hydraulic control |

> Note: Servo valves are most sensitive to contamination and usually require β₅≥200 (filtration ratio, i.e. 5µm particles are reduced by 99.5% after filtration).

4. Filter selection criteria

(1) Filter accuracy (µm)

- Coarse filter: 25~100 µm (for pre-filtration, to protect the pump).

- Fine filter: 3~10 µm (for protecting valve components).

- Ultra-fine filter: 1~3 µm (for servo systems).

(2) Filtration ratio (β value)

- β₃=75 means that the filter can intercept 98.7% of 3µm particles (calculation formula: β=1/(1-filtration efficiency)).

- Servo systems usually require β₅≥200 (99.5% filtration efficiency).

(3) Dirt holding capacity

- The total amount of pollutants that the filter can absorb before clogging affects the replacement frequency.

- Ordinary hydraulic system: 10~25 µm, ISO 20/18/15.

- High pressure/precision system: 3~5 µm, ISO 18/16/13 or higher.

- Servo system: 1~3 µm, ISO 16/14/11, β₅≥200.

Selecting the appropriate filtration accuracy can extend the life of hydraulic components and reduce failures. It is recommended to adjust according to the requirements of the equipment manufacturer and the actual working conditions.