In brief
A hydraulic filter retains particles from the hydraulic fluid, protecting pumps, valves and cylinders against wear and malfunctions.
Contents
What is Hydraulic filter?
Contamination of the hydraulic fluid is considered one of the most common causes of failure in hydraulic systems. Particles from manufacturing, abrasion or the environment accelerate wear in the narrow clearances of pumps and valves and can cause spools to stick. Hydraulic filters retain these particles and thus safeguard the service life of the components.
By installation location, a distinction is made between suction filters upstream of the pump, pressure filters downstream of the pump to protect sensitive valves, and return line filters, which clean the oil flowing back to the reservoir. There are also breather filters on the reservoir and offline (bypass) filters. Characteristic values are filter rating, filtration efficiency, dirt-holding capacity and permissible differential pressure. Fluid cleanliness is specified according to ISO 4406 with a code of three numbers describing the particle count in three size classes.
A clogging indicator signals visually or electrically when the differential pressure across the element becomes too high. Many filters have a bypass valve that lets unfiltered oil through when the element is clogged; this protects the element, but not the system. Timely element replacement is therefore a key maintenance task.
In practice: maintenance and procurement
For replacement elements, the element designation, filter rating, filter media material, seal material and permissible differential pressure are required. An element with a different rating may fail to achieve the required oil cleanliness or clog too quickly; replacement elements should therefore be specified as equivalent.
Synonyms and abbreviations: hydraulic filter element · hydraulic oil filter · filter element · return line filter · pressure filter
Frequently asked questions
What is the difference between suction, pressure and return line filters?
Suction filters sit upstream of the pump and keep out coarse particles; they are usually coarse to avoid cavitation. Pressure filters are located downstream of the pump and protect sensitive components such as servo or proportional valves. Return line filters clean the oil before it flows back into the reservoir, thus maintaining the overall cleanliness of the system.
What does a cleanliness code such as 18/16/13 to ISO 4406 mean?
The three numbers stand for particle counts in three size classes per millilitre of fluid, measured at 4, 6 and 14 micrometres and above. Each number corresponds to a range in a standardised table; a value one step higher means roughly twice the number of particles. The required cleanliness is specified by the manufacturers of the most sensitive components in the system.
When does a hydraulic filter need to be changed?
The primary criterion is the clogging indicator, which responds when the differential pressure is too high. In addition, change intervals according to the operating instructions apply, for example after commissioning or after repairs. If an element is changed too late, the bypass valve may open and unfiltered oil may enter the system.