The reliability of hydraulic systems depends entirely on the cleanliness of the fluid used. Even the smallest particles in the oil can be enough to block valves, damage seals, or accelerate wear on pumps and cylinders. Filter elements ensure that dirt, abrasion, and foreign substances are reliably separated before they can cause damage.

However, they only work effectively if selection, installation, and maintenance are precisely coordinated. In this article, you will learn

• which criteria are decisive when selecting a filter element,
• how to monitor the condition of the filter correctly, and
• which maintenance intervals ensure the operational reliability of your hydraulic systems.

The objectives are to achieve consistently high oil cleanliness, maximum service life, and trouble-free plant operation.

Clean oil as basis for reliable hydraulic systems

In every hydraulic system, performance depends directly on the cleanliness of the fluid used. Filter elements perform the key task of removing solid particles from the oil, thereby protecting pumps, valves, and cylinders from premature wear. Even the smallest contaminants can cause friction, leaks, or loss of performance in fine control gaps, thereby impairing the entire process.

Filtration ensures that the required oil cleanliness is maintained and that the system remains fully functional. It therefore not only contributes to the longevity of the components, but also to the energy efficiency of the system. This is because a clean oil flow is more consistent, and pressure losses are reduced.

There are basically two types of filter elements in hydraulics:

• Depth filters
• Surface filters

Depth filters consist of a fiber mesh (made of glass or polyester fibers, for example) and separate particles within the filter structure. They are characterized by high retention capacity but cannot be cleaned.

Surface filters work with precisely defined mesh openings (usually made of metal or plastic mesh) that retain larger particles on the surface. This design allows for cleaning and multiple use.

How are filter elements constructed in hydraulics and how do they work?

Filter elements in hydraulics are complex components whose performance depends on the right combination of several layers. Each layer fulfills a specific task: pre-filter layers retain coarser particles, while finer layers separate the smallest contaminants. Support and protective fabrics stabilize the pleated structure, prevent the filter layers from sticking together, and ensure the free flow of oil.

This multi-layer design ensures that the oil flows evenly through the element. In addition, the differential pressure remains as low as possible throughout the entire life cycle. Mechanical stability is ensured by precisely bonded end edges that prevent fibers from escaping. Metal support tubes are laser-welded using modern processes to permanently strengthen the structure.

Important parameters for the operation of filter elements in hydraulics are differential pressure stability and flow fatigue resistance. Return filters can withstand differential pressures of over 10 bar, while pressure filters can withstand loads of more than 20 bar. With changing loads, the high fatigue resistance ensures that the filter element remains dimensionally stable even after millions of load cycles. In addition, it maintains its filtration performance consistently over its entire service life.

Selection criteria for the correct filter design

The selection of a suitable filter element is based on clearly defined technical parameters. The filter fineness, which specifies the particle sizes that are separated, plays a central role. It is specified in micrometers (approximately 5, 10, or 16 µm) and depends on the sensitivity of the components to be protected.

The beta value (β value) describes the ratio of the number of particles before and after the filter element. A beta value of 200 means that out of 200 particles of a certain size, only one particle passes through the filter barrier. Thus, β = 200 corresponds to a separation efficiency of 99.5%. This key figure enables an objective evaluation of the filter performance.

Closely related to this is the separation efficiency (η), which quantifies the filter's ability to retain particles of a defined size. Together with the beta value, it provides a basis for determining the oil cleanliness in the system.

Dirt holding capacity and service life are also crucial for practical operation. The dirt holding capacity describes how many particles the filter element can retain before the maximum permissible differential pressure is reached. The service life refers to the operating time up to this point. Both values depend on the volume flow, viscosity, and degree of contamination of the oil.

In addition, the differential pressure stability is important. It shows how resilient the filter structure remains when the back pressure increases. Insufficient stability can cause the filter medium to collapse and unfiltered oil to enter the circuit. Therefore, the operating conditions (pressure, temperature, and flow rate) should always be considered when making your selection.

What does maintenance and servicing of filter elements in hydraulics involve?

The performance of filter elements in hydraulics depends directly on consistent maintenance. The condition of the filter element can be determined by the differential pressure, which increases as the load increases. A clogging indicator on the filter signals when the preset limit value has been reached – either visually via a display window or electrically via a sensor signal.

For filters with a clogging indicator, the filter elements are replaced based on their condition. Replacement is only necessary when the indicator responds continuously at the normal operating point, i.e., at operating temperature and nominal volume flow. A brief signal when the oil is cold can be ignored, as the increased viscosity temporarily leads to higher pressure loss.

Filters without clogging indicators are serviced preventively. It is recommended that they be replaced after approximately 1000 operating hours or at least once a year. The same interval applies to ventilating filters, which are replaced rather than cleaned due to environmental influences.

Surface filters, on the other hand, can be cleaned and reused several times. To do this, they are treated in an ultrasonic bath for a few minutes or immersed in cleaning fluid for about 15 minutes. The loosened dirt is then removed from the outside. The element is then rinsed from the inside out with clean fluid and blown out with compressed air. This procedure can be repeated up to three times.

Suction filters without a seal to the environment can be cleaned regularly to ensure low differential pressures. Models with a seal, on the other hand, should be replaced at least every 2000 operating hours or every two years. This is because the seals lose their elasticity over time.

Safe operation through consistent maintenance

A reliable maintenance schedule ensures that filter elements perform their function without interruption. If a filter is changed too late, the differential pressure increases significantly. This can cause the integrated bypass valve to open, allowing unfiltered oil to enter the circuit – with corresponding consequences for sensitive system components.

The combination of preventive and condition-based maintenance allows optimal utilization of the dirt holding capacity while minimizing the risk of downtime. A permanently stable differential pressure is associated with consistent filtration performance and protects the system from unnoticed contamination.

During each maintenance check, seals, O-rings, and housing connections should be inspected and replaced if necessary. The filter system will only remain permanently sealed if the sealing surfaces are clean and mounted without tension. It is equally important to use approved spare parts, as only these can guarantee the specified filtration performance and mechanical stability. Replica components can deform under load or be subjected to uneven flow, which significantly reduces the filtering effect.

A regularly maintained filter system not only ensures oil cleanliness, but also the cost-effectiveness of the entire hydraulic system. Reduced downtime, extended oil change intervals, and a longer service life for components are direct results of proactive maintenance.

Your contact to our experts

The selection and maintenance of filter elements requires technical understanding and experience with the system parameters. ARGO-HYTOS supports you in defining suitable filter finenesses, determining maintenance intervals, and designing complete filtration concepts for hydraulic and lubrication systems.

If you want to increase the reliability of your machines and extend the service life of your hydraulic components, we recommend regular inspection of your filter system. Ensure long-term operational reliability through professional filter maintenance and the correct selection of your filter elements!

Contact the experts at ARGO-HYTOS for individual advice - for permanently clean oil and maximum system availability.