The hydraulic oil contained in hydraulic systems is subject to pollution due to solid contaminants. The most common are:
- Insoluble oxides
- Rock powder
- Welding residue
Most solid contaminants are less than 10 microns in size. Studies have shown that bearing life is significantly extended if hydraulic oil is filtered to retain solid contaminants as small as 3 microns. Few benefits are obtained, however, if filtration is performed between 10 and 40 microns.