Hydraulic Filters
Essential components for maintaining hydraulic system integrity and performance
The Role of Filters in Hydraulic Systems
In hydraulic systems, due to internal formation or external intrusion, contaminants are inevitably present in the hydraulic oil. These contaminant particles not only accelerate the wear of hydraulic components but also block small holes in valves, jam spools, scratch seals, cause hydraulic valve malfunctions, and lead to system failures. Therefore, it is essential to remove impurities and contaminant particles from the hydraulic oil, especially when using high-quality universal hydraulic oil.
The most effective method to control the cleanliness of hydraulic oil, including universal hydraulic oil, is to use filters. The primary function of a filter is to filter hydraulic oil, controlling the cleanliness level of the fluid. Proper filtration ensures that universal hydraulic oil maintains its optimal properties and extends the service life of the entire hydraulic system.
Without adequate filtration, even the highest quality universal hydraulic oil will quickly become contaminated, leading to increased maintenance costs, reduced system efficiency, and potential catastrophic failures. Filters act as the first line of defense against these issues, protecting expensive components and ensuring smooth operation.
Contaminant Removal Process
Filters protect hydraulic systems by removing harmful particles from universal hydraulic oil, preventing component damage and system failures.
Performance Indicators of Filters
The main performance indicators of filters include filtration accuracy, flow capacity, and pressure drop, with filtration accuracy being the primary indicator. These parameters are crucial when selecting the right filter for specific applications, especially when dealing with different grades of universal hydraulic oil.
Filtration Accuracy
The working principle of a filter is to use a filter element with specific-sized pores to filter contaminants. Filtration accuracy refers to the maximum size of impurity particles (represented by the average diameter d of contaminant particles) that the filter can remove from the hydraulic oil. This is particularly important for maintaining the integrity of universal hydraulic oil in various operating conditions.
Currently used filters are classified into four grades according to their filtration accuracy:
- Coarse (d > 0.1mm)
- Ordinary (d > 0.01mm)
- Fine (d ≥ 0.001mm)
- Ultra-fine (d > 0.0001mm)
The principle for selecting filtration accuracy is that the size of filtered contaminant particles should be less than half the size of the sealing gap in hydraulic components. The higher the system pressure, the smaller the clearance between relatively moving parts in hydraulic components, and the higher the required filtration accuracy of the filter. This principle applies regardless of whether the system uses standard hydraulic oil or premium universal hydraulic oil.
| System Pressure | Recommended Filtration Accuracy | Typical Application with Universal Hydraulic Oil |
|---|---|---|
| Low Pressure (<7MPa) | 0.08mm | Agricultural machinery, simple hydraulic systems |
| Medium Pressure (7-21MPa) | 0.01-0.03mm | Industrial machinery, mobile equipment |
| High Pressure (>21MPa) | <0.005mm | Precision hydraulic systems, heavy machinery |
Flow Capacity
Flow capacity refers to the maximum flow rate of hydraulic oil, including universal hydraulic oil, that a filter can handle under specified working conditions without exceeding the allowable pressure drop. This parameter is critical for ensuring that the filter can keep up with system demands while maintaining proper filtration efficiency.
Pressure Drop
Pressure drop is the pressure difference between the inlet and outlet of the filter when oil flows through it. A lower pressure drop is generally desirable as it reduces energy loss in the system. The pressure drop characteristics must be considered when selecting a filter to ensure it works efficiently with the specific type of universal hydraulic oil used in the system.
Types of Hydraulic Filters
Mesh Filter
Mesh filters use metal screens as the filtering medium. These filters typically have a filtration accuracy of 0.08mm and a pressure drop of less than 0.06MPa. When used with high-quality universal hydraulic oil, they provide reliable basic filtration.
The advantages of this type of filter include a simple structure, good oil flow performance, and relatively high filtration accuracy, making it widely used in various hydraulic systems. They work particularly well with universal hydraulic oil in applications where cost-effectiveness is important.
However, mesh filters have some disadvantages, including difficulty in cleaning and low filter element strength. These limitations mean they are mostly used in medium and low-pressure systems where the universal hydraulic oil doesn't contain excessive contaminants.
Paper Element Filter
Paper element filters use filter paper as the filtering material. They consist of 0.35~0.7mm thick plain or corrugated microporous filter paper made of phenolic resin or wood pulp, wrapped around a perforated tinned iron frame to form a filter element, as shown in Figure 6-3. When using universal hydraulic oil, these filters provide excellent particle removal capabilities.
Oil flows from the outside of the filter element through the filter paper into the inside of the filter element, then flows out through the channels. To increase the filtering area of the filter paper, paper elements are almost always made in a folded design. This design allows for better flow rates even when using higher viscosity universal hydraulic oil.
This type of filter has two filtration accuracy specifications: 0.01mm and 0.02mm, with a pressure drop of 0.01~0.04MPa. Its advantages include high filtration accuracy, making it ideal for systems where maintaining clean universal hydraulic oil is critical.
The disadvantages are that it cannot be cleaned after clogging, requiring regular replacement of the paper element, and it has low strength. Paper element filters are generally used in precision filtration systems where the purity of universal hydraulic oil is paramount.
Sintered Filter
Figure 6-4 shows the structure of a sintered filter. This filter consists of an end cover (1), a housing (2), and a filter element (3). The filter element is made of granular steel powder through a sintering process. When used with universal hydraulic oil, sintered filters provide exceptional filtration performance in demanding applications.
The filtration process is as follows: hydraulic oil enters through port a, passes through the micropores between the copper particles into the interior of the filter element, and flows out through port b. This design ensures thorough cleaning of universal hydraulic oil even in high-pressure systems.
The filtration accuracy of sintered filters is related to the size of the micropores between the copper particles in the filter element. By selecting different particle sizes of powder and manufacturing filter elements of different thicknesses, various filtration accuracies can be achieved. This versatility makes them suitable for use with different grades of universal hydraulic oil.
The filtration accuracy of sintered filters ranges from 0.001mm to 0.01mm, with a pressure drop of 0.03~0.2MPa. The advantages of this type of filter include high strength, the ability to be made into various shapes, simple manufacturing, and high filtration accuracy—making it ideal for systems where universal hydraulic oil must remain extremely clean.
The disadvantages are that they are difficult to clean and metal particles can easily fall off. Sintered filters are often used in applications requiring precision filtration of universal hydraulic oil.
Selection of Filters
When selecting a filter, it is necessary to comprehensively consider the technical requirements of the hydraulic system and the characteristics of the filter. The choice of filter directly impacts the performance and longevity of the system, especially when using high-quality universal hydraulic oil that deserves proper protection.
System Working Pressure
The working pressure of the system is one of the main bases for selecting filter accuracy. The higher the system pressure, the higher the matching accuracy of hydraulic components, and the higher the required filtration accuracy. This principle applies regardless of whether the system uses standard hydraulic fluids or premium universal hydraulic oil. High-pressure systems with tight tolerances require finer filtration to prevent damage from even the smallest particles in the universal hydraulic oil.
System Flow Rate
The flow capacity of a filter is determined based on the maximum flow rate of the system. The rated flow rate of the filter must not be less than the system flow rate; otherwise, the pressure drop of the filter will increase, the filter will clog easily, and its service life will be shortened. However, the larger the rated flow rate of the filter, the larger its volume and the higher its cost. Therefore, an appropriate flow rate should be selected based on the system's requirements and the characteristics of the universal hydraulic oil being used.
Filter Element Strength
The strength of the filter element is an important indicator; different filter structures have different strengths. High-pressure systems or hydraulic circuits with large shocks should use filters with high-strength elements. This is particularly important when the system uses universal hydraulic oil in demanding applications where pressure spikes are common, as the filter must maintain integrity to continue protecting the system effectively.
Oil Type Compatibility
The filter material must be compatible with the type of hydraulic oil used in the system. This is especially important when using universal hydraulic oil, which may have different additive packages than specialized oils. Incompatible materials can lead to filter degradation, contamination of the universal hydraulic oil, and potential system failures. Always check that the filter is rated for use with universal hydraulic oil when selecting components for your system.
Installation of Filters
The installation of filters is determined according to the needs of the system. They can generally be installed in various positions as shown in Figure 6-5. The choice of installation position affects how effectively the filter can protect system components and maintain the cleanliness of the universal hydraulic oil.
Figure 6-5: Filter Installation Positions (a-e showing different configurations)
Suction Port of Hydraulic Pump
As shown in Figure 6-5a, installing a filter at the pump's suction port can protect all components in the system. However, due to the limitations of pump suction resistance, only mesh filters with low pressure drop can be used.
This type of filter typically has relatively low filtration accuracy. Particles generated by pump wear will enter the system, failing to fully protect other hydraulic components. Additional filters connected in series in the oil circuit are therefore necessary, especially when using high-quality universal hydraulic oil that deserves maximum protection.
Discharge Port of Hydraulic Pump
As shown in Figure 6-5b, this installation method can effectively protect all hydraulic components except the pump. However, because the filter works under high pressure, the filter element needs to have high strength.
To prevent hydraulic pump overload or filter damage caused by filter clogging, a clogging indicator or bypass valve is often installed next to the filter for protection. This setup is particularly effective for maintaining the cleanliness of universal hydraulic oil as it enters the high-pressure side of the system.
Return Oil Line
As shown in Figure 6-5c, the filter is installed in the system's return oil line. This method can filter out particles generated by the oxidation of the oil tank or pipe walls and wear of hydraulic components, ensuring the cleanliness of the hydraulic oil in the tank.
This protects the pump and other components. Since the return oil pressure is relatively low, the required filter strength does not need to be too high. This is an economical way to maintain the quality of universal hydraulic oil as it returns to the reservoir.
Branch Circuit
As shown in Figure 6-5d, the filter is mainly installed in the return oil line of the relief valve. This does not increase the pressure drop in the main oil line, and the filter flow rate can be less than the pump flow rate, which is more economical and reasonable.
However, it cannot filter all oil and cannot guarantee that impurities will not enter the system. This method is often used as a supplementary filtration method to maintain universal hydraulic oil cleanliness in specific parts of the system.
Independent Filtration System
As shown in Figure 6-5e, a hydraulic pump and filter are used to form an independent filtration circuit separate from the main system. This setup can continuously remove impurities from the system, ensuring system cleanliness.
This method is generally used in large hydraulic systems where maintaining optimal universal hydraulic oil quality is critical for operation and component longevity. The independent circuit can operate continuously or periodically to polish the universal hydraulic oil, removing contaminants that might otherwise accumulate and cause damage.
Proper selection and installation of hydraulic filters are crucial for maintaining system performance, extending component life, and ensuring the optimal functioning of universal hydraulic oil. By understanding the different types of filters, their performance characteristics, and appropriate installation locations, you can significantly improve the reliability and efficiency of hydraulic systems.
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