Hydraulic Filter

A hydraulic filter is a component in hydraulic systems that removes contaminants such as particles, abrasion, water, and oxidation byproducts from the hydraulic fluid. It thus protects pumps, valves, and cylinders from wear and failure. Up to 80 percent of all hydraulic damage can be attributed to contaminated oil, which underscores the importance of filtration.

Basics and Functioning of Hydraulic Filters

Hydraulic filters operate on the principle of mechanical separation. The hydraulic fluid flows through a filter medium that traps particles above a certain size. Design types vary considerably depending on the filter’s position in the circuit, the type of filter medium, and the required cleanliness class. However, the primary function of every hydraulic filter remains the same: to improve oil purity to the extent that the system’s sensitive components can operate reliably and with minimal wear.

Particles are removed through various mechanisms. Large particles are retained by sieving at the surface of the filter medium. Smaller particles adhere to the fibers of the filter medium through adsorption or inertial effects. The combination of these mechanisms determines a filter’s separation efficiency and contaminant holding capacity.

Filter Types and Their Position in the Hydraulic Circuit

The choice of hydraulic filter depends largely on where it is located in the circuit. Each position places its own demands on pressure resistance, filter fineness, and flow capacity.

Suction filters

Suction filters are located in the suction line between the tank and the pump. Their main function is to protect the pump from coarse contaminants that could be drawn in from the tank. Since they operate in a vacuum, their pressure drop must be minimal to prevent differential pressure in the pump. Typical filter fineness ranges from 25 to 100 µm. Suction filters are often designed as simple mesh or belt filters and are cleanable in many manufacturers’ models.

Discharge Filters

Pressure filters are installed on the discharge side behind the pump and protect the most sensitive components of the system, such as servo and proportional valves. They operate under full system pressure and must therefore have a particularly sturdy housing made of steel or cast iron. The filter fineness ranges from 3 to 25 µm, with high-pressure filters capable of operating at pressures up to 420 bar. Pressure filters are typically designed as in-line, flanged, or replaceable filters and feature integrated bypass valves.

Return Filters

Return filters are located in the return line upstream of the tank and clean the oil before it returns to the reservoir. They act as the final barrier against contamination and prevent wear debris and particles from the circuit from returning to the tank. Typical filter ratings range from 3 to 10 µm. Return filters operate in the low-pressure range up to approximately 20 to 63 bar and are often available in in-tank or duplex configurations, which allow for filter changes during operation.

Bypass Filters

Bypass filters, also known as side-stream filters, are installed in parallel with the main circuit. They divert a small portion of the flow—about 5 to 10 percent of the total flow rate—and pass it through a particularly fine filter stage. With filter finenesses ranging from 1 to 5 µm and additional water separation, they ensure continuous oil purification that goes beyond the performance of the main-flow filter. Bypass filters are indispensable, particularly in systems with high oil purity requirements, such as servo-hydraulic systems.

Breather Filters

Often overlooked but equally important: Breathing filters prevent ambient air containing dust and moisture from entering the hydraulic oil as the tank expands and contracts. They operate passively and filter the incoming air to a fineness of typically 1 to 10 µm.

Filter Media: Surface Filters and Depth Filters

The filter media is the heart of every hydraulic filter. There are essentially two operating principles, which differ significantly in terms of design, performance, and reusability.

Surface filters

Surface filters trap particles on a defined, screen-like surface. They typically consist of stainless steel wire mesh or metal fiber nonwovens with geometrically defined pores. Their strength lies in their sharp separation boundary and cleanability. They can be backflushed and reused multiple times. However, their dirt-holding capacity is limited because particles accumulate on the surface as a filter cake, which increasingly restricts flow. Typical filter fineness ranges from 10 to 250 µm.

Depth Filters

Depth filters consist of a three-dimensional fiber mesh, usually made of glass fiber, cellulose, or polyester. They trap particles inside the media, not just on the surface. As a result, they achieve a significantly higher dirt-holding capacity and a more gradual increase in differential pressure. Depth filters are disposable elements and must be replaced once saturated. With filter finenesses ranging from 2 to 25 µm, they cover the range that is critical for protecting sensitive hydraulic components. Glass fiber media achieve beta values above 200 according to ISO 16889, which corresponds to a separation efficiency of over 99. 5 percent.

Filter Fineness and Separation Efficiency

Filter fineness alone says little about the actual performance of a hydraulic filter. The key factor is the separation efficiency—that is, the percentage of particles of a specific size that the filter medium retains. The ISO 16889 standard defines the beta ratio for this purpose: A beta value of 200 at 10 µm means that out of 200 particles of this size, only one passes through the filter. The higher the beta value, the more reliable the separation.

A distinction must be made between nominal and absolute filter fineness. Nominal fineness describes a guideline value, while absolute fineness specifies the particle size at which the filter medium achieves a defined beta value. When designing a hydraulic filter, the absolute filter fineness according to ISO 16889 is always the determining factor.

Contamination Classes According to ISO 4406

The cleanliness of hydraulic oil is classified according to ISO 4406. This standard specifies the number of particles per milliliter in three size classes: greater than 4 µm, greater than 6 µm, and greater than 14 µm. For example, an oil rated 18/16/13 contains particle counts corresponding to those respective class levels. The lower the numbers, the cleaner the oil.

The required cleanliness class depends on the sensitivity of the components in the system. Servo and proportional valves require classes such as 16/14/11 or better, while standard hydraulic systems can operate with 18/16/13. High-precision drives may require cleanliness classes of 15/13/10 or lower. The hydraulic filter must be designed to reliably maintain the target class under operating conditions.

Bypass Valve and Differential Pressure Monitoring

A bypass valve in the hydraulic filter opens when the differential pressure across the filter element exceeds a defined threshold. This occurs when the filter medium becomes saturated with particles and the flow resistance becomes too high. The valve diverts the oil past the filter, thereby preventing the pressure in the system from rising to an unacceptable level or the pump from cavitating. The downside: The oil passes through unfiltered, which is why the opening of the bypass valve is a clear signal that the filter needs to be replaced.

Differential pressure monitoring is therefore a key tool for condition monitoring. Manual or electronic differential pressure switches indicate how clogged the filter element is. Visual contamination indicators with color-changing or pop-up indicators provide maintenance personnel with a quick visual cue. Electronic sensors can transmit the differential pressure to the system control system, thereby enabling predictive maintenance.

Damage Caused by Inadequate Filtration

Inadequate filtration causes cumulative damage that builds up over months and then suddenly leads to failure. Particle contamination blocks fine control orifices in proportional valves, causes increased wear on pumps and cylinders, and can lead to valve sticking. Water in the oil accelerates additive degradation, promotes corrosion, and alters viscosity. Oxidation byproducts form resins and deposits that clog cooling channels and orifices.

The consequences range from increased energy consumption and uncontrolled movements to complete system shutdown. A properly sized hydraulic filter significantly reduces this failure rate and lowers the system’s lifecycle costs.

Maintenance and Servicing

Regular maintenance of hydraulic filters is crucial for system reliability. Filter elements must be replaced in a timely manner before the bypass valve opens. Replacement intervals depend on the manufacturer’s specifications, operating hours, and the actual level of contamination. Differential pressure monitoring helps determine the optimal replacement time.

In addition to filter replacement, regular oil analysis is part of proper maintenance. It provides information on particle content, water content, viscosity, and additive condition. The filter housings themselves also require attention: condensate drain plugs on pressure filters must be operated regularly, and breather filters on the tank should be inspected during every oil change.

Standards for Hydraulic Filters

Several standards govern the testing, classification, and use of hydraulic filters:

• ISO 16889: Defines the multipass test for determining filter performance and the beta ratio. It is the primary standard for evaluating filter elements.
• DIN ISO 11170: Specifies the test procedures for filter elements and supplements ISO 16889 with requirements for conducting the tests.
• ISO 4406: Classifies the cleanliness of hydraulic fluids and forms the basis for establishing target values in filter design.
• DIN ISO 4413: Sets safety requirements for hydraulic systems, including specifications for filtration and oil purity.