Return filters

Return filters are suitable for tank mounting and remove contamination from the operating fluid flowing back into the oil tank. Another option for installing a return filter is to mount it inside the oil tank. Filtering the returning medium ensures that unavoidable contamination from the components of the hydraulic system does not enter the tank and contaminate the previously clean oil. Like all pressure filters, return filters are assigned a cleanliness class, which indicates how finely the filter operates. Due to the arrangement of the return filter, this type of filter cannot be used as a protective filter. However, unlike all types of pressure filters, return filters only have to withstand very low pressure, as the returning medium is never pumped back into the tank at high pressure in any application. In contrast to conventional pressure filters, the design of return filters is relatively simple.

Return filters when using a return flow diffuser

Return flow diffusers are often used in conjunction with return filters. These ensure that the returning oil flows into the oil tank in as calm a state as possible. A return flow diffuser primarily prevents foam formation and the stirring up of deposited dirt.

Design of return filters

The filter element in the return filter is realized using metal wire mesh. This makes the filter inexpensive, but it is only reusable to a limited extent. Depending on the requirements, different types of mesh are used. Examples include plain, twill, or Dutch weave. Other filters that use wire mesh as a filter element include protective filters or suction filters. Filters with wire mesh are suitable for filtering out impurities such as paint particles, abrasion from various hydraulic components, or small rubber particles from hoses. The filter element is replaceable in every design type.

Basically, return filters consist of a simple, one-piece housing with a screw-on cover. As a rule, there is a small clogging indicator on the housing of the return filter, which simplifies filter maintenance. A bypass valve is also fitted, which opens at around 3 bar in most applications.

Types of return filters

Depending on which hydraulic consumer is installed in the hydraulic system, a different filter fineness is required. In the following, conventional filter finenesses are assigned to some hydraulic consumers:

• 25 – < 63 µm filtration rating: Hydraulic cylinders and hydraulic gear systems
• 10 – < 25 µm filtration rating: Radial and axial piston systems
• 5 – < 10 µm filtration rating: Proportional directional valves and technology
• 3 – < 5 µm filtration rating: Servo valves

The fact that a return filter is installed in the return line, where the pressure is lowest, is primarily for economic reasons. Only at this point can a filter be used that has a lightweight housing and can therefore be built as cheaply as possible. In many hydraulic systems, a return filter represents the only existing system protection. Therefore, the importance of a careful selection of return filters must be emphasized in order to ensure maximum performance of the hydraulic system. The safety-related importance of this type of filter is therefore very high.

Comparison of different filter elements

In the following, some conventional filter elements are categorized with regard to their differential pressure stability

• Aquamicron (abbr. AM) 10 bar
• Betamicron (abbr. BN) 10 bar
• Stainless steel fleece (abbr. V) 210 bar
• Wire mesh (abbr. W / HC) 30 bar
• Paper fleece (abbr. P / HC) 10 bar
• Betamicron 3 (abbr. BN3HC) 25 bar