Return filters for clean hydraulic oil

Hydraulic filters are essential to ensure the reliable operation of hydraulic systems. Their main function is to remove dirt particles and contaminants from the hydraulic fluid. Since around 80% of premature hydraulic failures are caused by contaminated pressure fluid, hydraulic filters protect against excessive wear, extend the service life of hydraulic components, reduce downtime and maintenance costs, and ensure high machine reliability.

• Internal contamination: Caused by abrasion, rubber particles from hoses, or aging residues in the pressure fluid, often due to high temperatures.
• External contamination: Enters the system from outside, e.g., during assembly, through leaking tanks, or insufficient air filters.

• Surface filters: Thin mesh (e.g., metal, cellulose, plastic, or paper), usually pleated. They trap particles on the surface. Typical ratings: 25, 60, 90 µm.
• Depth filters: Fluid passes through a porous structure (e.g., fiberglass, cellulose, metal fleece). Particles are trapped within deeper layers. Microfiber fleece enables filtration down to 1 µm. Paper filters are the most cost-effective option (10–25 µm).

Return filters are the most commonly used hydraulic filters and are installed in the return line circuit directly on the tank or in the return line. They clean the hydraulic fluid of dirt particles before it flows back into the tank.

Advantages:

• Cost-effective and efficient
• Easy maintenance and filter replacement
• Option to include clogging indicator
• Fine filtration capabilities
• No risk of pump cavitation

Disadvantages:

• Bypass valve required
• During bypass operation (e.g., cold start), contaminants can enter the system unfiltered
• Contamination is only captured during the return flow

• Suction filters: Installed directly in the tank, protect the pump from coarse contamination. Require large filter surfaces, bypass valve, and indicator.
• Pressure filters: Located after the pump in the pressure line, protect sensitive valves. Very fine filtration, but robust and expensive.
• Tank breather filters: Prevent particles from entering through the tank ventilation, essential for system cleanliness.

• Maximum flow rate (Qmax): e.g., 42, 52, or 138 l/min.
• Filter element & filter rating: Microfiber or paper, absolute (abs.) or nominal (nom.) filtration, e.g., 10 µm abs., 25 µm nom.
• Bypass valve: Opening pressure (e.g., 1.75 bar ±10%) provides protection during cold start or clogging.
• Maximum working pressure: Load capacity of the filter housing (e.g., 8–12 bar).
• Installation type: Tank-mounted or in-line return filter.

A correctly selected return filter reliably protects hydraulic systems in industry, agriculture, and construction. It prevents premature wear, system failures, and unnecessary maintenance costs. Specifically tailored filter solutions maximize system performance and ensure operational safety – both in stationary plants and in mobile machinery.

A bypass valve serves as a safety element in a hydraulic filter. It opens when there is a high pressure drop across the filter element, e.g., during cold start or severe contamination. Typical opening pressure: around 1.75 bar. This allows fluid to continue flowing through the system, albeit unfiltered. It protects the pump and prevents interruption of oil supply, although particles may enter the system in the process.