Without filtering, you would have an increase in leakage, piston clamps, switching time and valve failure. Dirt in the system will accelerate wear, which is equivalent to chain reaction.
The reliability of a hydraulic system depends strongly on the cleanliness of the system, that means on the filtering.
Depending on the arrangement within the hydraulic system, you can distinguish between suction, pressure and return flow filters.
The suction filter is arranged in the inlet of the pump. The oil from the container first passes through the filter element so that only filtered oil enters the hydraulic system.
Usually these suction filters are hard to access and the maintenance is difficult. Moreover, these filters may make the work of pumps difficult. Certain pumps (such as the axial piston pump) may not be used together with a suction filter. Suction filters are used to retain larger impurities, but they do not replace the return filter.
The filter fineness: usually > 100 μm.
The pressure filter is mounted in the pressure line of a hydro system. This can be at the pressure connection of the pump or in front of a sensitive control device, which has to be protected especially against pollution. High quality systems with proportional and servo control valves use these filters.
The return line filter is the most commonly used system filter. It is mounted within the return line. Oil is filtered before it flows back into the tank. You can get these filters to mount at the oil tank itself or directly within the piping.
Note: Display of soiling and protection against excessive pressure difference
The contamination can be indicated indirectly via the flow resistance of the filter element. The pressure acts on a small piston against the spring. The piston is pressed against the spring as the pressure increases (with increasing soiling).
If the system is to continue working even with a spoiled filter, a non-return valve is installed parallel to the filter. It opens when the pressure difference between the inlet and outlet at the filter exceeds the spring force (normal value approx. 3 bar).