plugs (fig. 9-38) also are used in some strainers
to trap iron and steel particles carried by fluid.
Studies have indicated that even particles as small
as 1 to 5 microns have a degrading effect, causing
failures and hastening deterioration in many cases.
There will always be controversy over the exact
definitions of filters and strainers. In the past,
many such devices were named filters but
technically classed as strainers. To minimize the
controversy, the National Fluid Power Associa-
tion gives us these definitions:
FILTER - A device whose primary function
is the retention, by some porous medium, of
insoluble contaminants from a fluid.
STRAINER - A coarse filter.
To put it simply, whether the device is a filter
or a strainer, its function is to trap contaminants
from fluid flowing through it. The term porous
medium simply refers to a screen or filtering
material that allows fluid flow through it but stops
various other materials.
MESH AND MICRON RATINGS
Filters, which may be made of many materials
other than wire screen, are rated by MICRON
size. A micron is 1-millionth of a meter or
39-millionths of an inch. For comparison, a grain
of salt is about 70 microns across. The smallest
particle visible to the naked eye is about 40
microns. Figure 9-39 shows the relationship of
Figure 9-39.—Relationship of micron sizes.
9-27
