reliability of the entire system depend upon adequate
Continuous filtration of hydraulic fluid during
system operation is necessary to maintain system
cleanliness. You should use filters that have fine
pores or openings to allow hydraulic fluid to pass but
that are small enough to trap contaminant particles.
Hydraulic filter elements are rated in several ways.
The absolute filtration rating is the diameter in
microns of the largest spherical particle that will pass
through the filter under a certain test condition. This
rating is an indication of the largest opening in the
filter element. The mean filtration rating is the
measurement of the average size of the openings in
the filter element. The nominal filtration rating is
usually interpreted to mean the size of the smallest
particles of which 90 percent will be trapped in the
filter at each pass through the filter. Figure 7-31
shows a typical filter arrangement in a hydraulic
Filters may be located within the reservoir, the
pressure line, the return line, or any other location
where they are needed to safeguard the hydraulic
system against contaminants. Their location in the
system and other design criteria determine their shape
The filter assembly is composed of three basic
units. The units are a head assembly, a bowl, and a
Figure 7-31.Typical filter arrangement in hydraulic system.
filter element. See figure 7-32.