Most strainer material is of the mechanical type
Return Line Filters
which operates by trapping particles between
Return line filters (fig. 6-26) also can trap very
closely woven metal screens and/or disks, and
small particles before the fluid returns to the
metal baskets. The mechanical type of material
reservoir/tank. They are particularly useful in
is used mostly where the particles removed from
systems which do not have large reservoirs/tanks
the medium are of a relatively coarse nature.
to allow contaminants to settle out of the fluid.
Absorbent filters are used for most minute-
A return line filter is nearly a must in a system
particle filtration in fluid systems. They are made
with a high-performance pump, which has very
of a wide range of porous materials, including
close clearances and usually cannot be sufficiently
paper, wood pulp, cotton, yarn, and cellulose.
protected by an inlet line filter.
Paper filters are usually resin-impregnated for
strength.
FILTER/STRAINER MATERIALS
Adsorbent (or active) filters, such as charcoal
and Fuller's earth, are used mostly in gaseous or
The materials used in filters and strainers are
vapor systems. This type of filter material should
classified as mechanical, absorbent, or adsorbent.
not be used in hydraulic systems since they remove
essential additives from the hydraulic fluid.
CONSTRUCTION OF
FILTER ELEMENTS
Filter elements are constructed in various
ways. The three most common filter element
construction types are the surface-type (most
common), the depth-type, and the edge-type.
Surface-type filter elements (fig. 6-27) are
made of closely woven fabric or treated paper with
pores to allow fluid to flow through. Very
accurate control of the pore size is a feature of
the surface-type elements.
A depth-type filter element (fig. 6-28) is
composed of layers of a fabric or fibers which
Figure 6-26.--Return line filter.
provide many tortuous paths for the fluid to flow
Figure 6-28.--Depth-type filter element.
Figure 6-27.--Filler assembly using a surface-type element.
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