Hydraulic oil filters are classified by filter media into three types: surface-type filters, depth-type filters, and magnetic filters. They remove solid contaminants through two mechanisms: direct interception and adsorption.
The primary function of a hydraulic oil filter is to filter the hydraulic fluid. Contaminants inevitably enter hydraulic systems from various sources:
Once mixed into the hydraulic fluid, these contaminants circulate with the oil and cause damage throughout the system, seriously affecting normal operation. They can jam or block the tiny clearances between moving parts of hydraulic components (measured in microns), as well as throttle orifices and gaps; they destroy the oil film between moving parts, score clearance surfaces, and increase internal leakage, reducing efficiency and increasing heat generation; they also accelerate chemical reactions in the oil, causing the fluid to degrade.
According to production statistics, more than 75% of hydraulic system failures are caused by contaminants in the hydraulic fluid. Maintaining oil cleanliness and preventing contamination are therefore essential to the proper operation of hydraulic systems.
| Item | Specification |
| Filtration rating | 1–100 μm, filtration ratio X ≥ 100 |
| Max. working pressure | 21 MPa |
| Working media | Mineral hydraulic oil, phosphate ester, emulsion, water-glycol, etc. |
| Working temperature | -30°C to 110°C |
| Filter media | High-performance glass fiber media |
| Structural strength | 1.0 MPa, 2.0 MPa, 16.0 MPa, 21.0 MPa |
| Applications | Pressure-line filtration for hydraulic and lubrication systems, removing contaminants to ensure normal system operation |
The installation position of a hydraulic oil filter has a significant impact on contamination control. Filters can be installed in the suction line, pressure line, or return line as required, or arranged as a separate filtration system outside the main circuit.
To prevent contaminants from entering the pump, a suction filter is generally installed at the suction port or in the suction line. A suction-port filter is immersed at the bottom of the reservoir, where it is easily clogged and difficult to maintain. For this reason, cageless screen-type or wire-wound filters are typically used at the suction port.
Filters installed in the pressure line protect the hydraulic components downstream of the pump. As the main filter of the system, its filtration rating should meet the fluid cleanliness requirements of the system. Pressure-line filters have less restrictive pressure-drop limits and can therefore use high-precision elements. The maximum allowable pressure drop is generally 0.35–0.5 MPa, with an initial pressure drop of 0.07–0.14 MPa. Pressure-line filters must have sufficient strength to withstand the maximum system working pressure and frequent pressure fluctuations and peaks.
From an installation standpoint, installing a filter in the return line is generally considered ideal, because the filter removes both contaminants that enter the system and particles generated within the system before the fluid returns to the reservoir, supplying clean fluid to the system. However, flow pulsation caused by hydraulic motors and cylinders in the return line reduces filtration performance. This is especially true for large-volume cylinders: when a high-pressure return line is suddenly opened, the sudden pressure release creates a large instantaneous flow surge, which not only significantly reduces filtration efficiency but can also cause element rupture. Return-line filtration therefore works best in systems with stable operation. In addition, return-line filtration cannot remove contaminants that have already entered the reservoir, so special attention should be paid to cleaning the reservoir before operation and to taking effective measures to prevent contaminant ingress. High-precision filters can be used in the return line, with an initial pressure drop of generally 0.035–0.05 MPa and a maximum allowable pressure drop of 0.2–0.35 MPa.
