The return line is the critical interface in the hydraulic circuit that determines how clean the system remains. This is where production residues, abrasion particles and contaminants enter the hydraulic tank. Without consistent filtration, pumps, valves and cylinders would be directly contaminated with dirt, which has significant consequences: increased wear, uncontrolled functions, reduced service life. Return filters from ARGO-HYTOS meet these challenges with a sophisticated system design that permanently ensures the cleanliness class of the hydraulic oil.

ARGO-HYTOS has been producing tank-mounted return filters with integrated ventilating filters since 1971. And this innovation continues to set standards today. The filters perform several tasks simultaneously:

• They protect components from contamination.
• They prevent oil foaming through correctly guided return flow.
• They increase the operational reliability of the entire system.

Thanks to their modular design, integrated bypass valves and variable connection solutions, they can be flexibly integrated into a wide variety of hydraulic concepts – whether in mobile machines or stationary systems.

What is a return filter?

A return filter is a special filter. It is installed in the return line of hydraulic systems to clean the returning hydraulic oil before it is returned to the tank. From a technical point of view, it is a working filter that operates in full flow.

This means that the entire return flow volume passes through the filter element without any partial flow being bypassed. Its main task is to reliably remove dirt particles such as metal abrasion, seal residues, production residues or externally introduced contaminants.

The return filter thus acts as the last barrier before the oil storage tank and prevents contaminated medium from re-entering the circuit. A return filter thus fulfils two key protective functions. On the one hand, it protects downstream components such as pumps, control blocks and valves from wear. On the other hand, it protects the system from long-term contamination, which would increase uncontrollably due to circulating particles.

Modern return filters consist of

• a filter housing,
• a replaceable filter element (usually made of glass fiber fleece, paper or metal mesh)

and additional functions such as integrated tank ventilation, clogging indicator or oil separator complement the functional design.

Compared to suction or pressure filters, the return filter operates at lower pressures. This allows finer filter elements to be used without causing cavitation in the inlet or excessive pressure losses in the system. The result is high oil cleanliness with low flow resistance.

Which parameters are crucial for return filters?

The size of a return filter must never be determined solely based on the pump capacity. The decisive factor is the maximum volume flow that can occur in the system return. Depending on the application, this can exceed the pump flow rate many times over. Particularly in the case of differential cylinders with a large surface ratio between the piston and piston rod sides, the return flow can be up to four times greater than the pump flow.

A practical example to illustrate this: a cylinder with a diameter of 80 mm and a piston rod with a diameter of 56.57 mm generates a return flow of 80 l/min with a stroke of 800 mm – with a pump flow rate of only 40 l/min. If this maximum value is underestimated when designing the filter, dangerous back pressures can occur in the system.

Excessive back pressure in the return flow not only endangers the pump, but also the seals of hydraulic motors or pistons. This is particularly critical when leaking oil lines are connected. The permissible back pressure must not overload the respective sealing systems of the components, such as shaft seals or piston seals. This can also result in the unintentional extension of cylinders, for example due to back pressure effects.

The installation position of the return line is also crucial for stable return of the oil to the tank. The oil outlet must always be below the oil level to prevent foaming. If the return line is extended into the interior of the tank, a distance of at least two to three times the pipe diameter from the tank bottom should be maintained. This prevents settled particles from being stirred up and stabilizes the flow.

What are the advantages of the ARGO-HYTOS return filter concept?

ARGO-HYTOS return filters are designed not only to perform the filtering process, but also to fulfil several system-relevant tasks at the same time. The concept focuses on the complete cleaning of the entire return volume using the so-called full-flow principle.

In return flow filters, the hydraulic oil generally flows from the outside to the inside through the filter element. In the RFI series of return flow filters, however, the flow direction is reversed – the oil flows from the inside to the outside. This calms the oil flow. In conjunction with large-dimensioned outlet openings, this results in very low outlet velocities of less than 0.5 m/s. The oil enters the tank close to the surface and below the oil level, which improves air separation and reduces the tendency for oil foaming.

Another advantage of the RFI series is its compact design. The filter inlet is located at the bottom of the filter bowl and can be connected either from below or via a side pipe bend. Only the cover remains on the top of the tank – a plus in terms of clarity and access during maintenance work.

The filter cover can be opened without special tools. Depending on the series, an integrated holder ensures precise guidance of the element. Depending on the filter type, the bypass valve can also be integrated into the filter element itself. This prevents sedimented dirt from entering the tank from the housing when the bypass is opened.

Depending on the application, the filter modules can be equipped with a variety of additional functions, such as:

• Integrated tank ventilation filter with 2 micrometer filter fineness
• Oil separator to prevent spillage losses
• Optical or electrical clogging indicators
• Pipe extensions for guided oil return

The available filter elements cover a wide range of fineness from 5 to 100 micrometers. Depending on requirements, they are available in glass fiber fleece, paper or stainless-steel versions.

The advantages of ARGO-HYTOS return filters at a glance

• Full-flow filtration to protect pumps and valves by systematically removing dirt particles
• Smooth return flow thanks to sophisticated flow control and large outlet openings
• Easy maintenance thanks to removable bowl, tool-free cover and integrated indicators
• In certain series, the bypass valve is integrated into the filter element. This design directs the bypass flow past the element and prevents contaminants deposited at the bottom of the housing from mixing back into the oil flow.
• Modularity for every system requirement: pipe extension, outlet diffuser, second return connection and much more.

What types of return filters does ARGO-HYTOS offer in detail?

The ARGO-HYTOS return filter portfolio covers a wide range of applications. It ranges from compact solutions for small volume flows to high-performance filters for industrial hydraulic systems with up to 960 liters per minute. The range distinguishes between tank-mounted filters and variants for line and tank installation.

Both versions have a removable filter bowl. During servicing, this is removed together with the element, which significantly reduces the risk of recontamination from dirt deposits in the housing.

In the lower performance range, for example, the E 043 type offers a compact solution for applications up to 35 l/min with a G½ thread connection. The filter is equipped with an aluminum housing and plastic cover and can be optionally supplemented with a visual or electrical clogging indicator.

Variants such as type E 072 and type E 103 are available for medium volume flows. Both are designed for tank installation and are optionally equipped with a ventilation filter, oil dipstick and integrated oil separator.

For applications with higher return flow requirements, the range includes the E 222, which can handle return flows of up to 220 l/min. Additional functions such as a second return connection or a filling filter can be integrated.

The E 703 type is used in the high-performance range. This variant is designed for maximum flows of up to 900 l/min. It has flange connections and large steel filter bowls and can be equipped with an outlet diffuser and several return connections.

In the new RFI series, such as the RFI 206, the oil flows from the inside to the outside through the filter element. Large outlet openings keep the outlet velocity low and promote the degassing of air. The entire filter insert is located in the tank cap, with only the cover remaining visible. This ensures a compact design, a tidy tank surface and makes servicing much easier.

The RFT-150 is also available as a robust tank mounting variant. It is designed for return flow rates of up to 180 l/min and combines full-flow filtration with a compact 4-hole mounting. The lower part of the housing, including the filter element, can be completely removed for servicing, which reduces downtime and prevents recontamination.

All filter types are characterized by modular expandability, high ease of maintenance and robust materials. They are tailored to the specific requirements of mobile machinery and industrial hydraulic systems.

What are the specific benefits of using a return filter for your system?

A correctly dimensioned return filter is a key element for trouble-free operation. Its effectiveness is not only evident in clean oil. Significantly extended maintenance intervals, lower energy consumption thanks to friction-optimized components and demonstrably increased system availability are also good arguments.

For OEMs, this means:

• Lower complaint rates
• More stable process quality
• Measurable increase in the value of your own machine

For operators, this means:

• Predictable maintenance
• Reduced downtime
• Long-term stable hydraulic system

Discover ARGO-HYTOS return filters now

Would you like to learn more about optimal return filtration in your hydraulic system? Or do you need assistance with dimensioning and selecting a suitable return filter? Our experts will be happy to help you. Visit our contact page and arrange a consultation. With ARGO-HYTOS, you can rely on decades of expertise in filtration technology – for durable, efficient and safe hydraulic systems.