replace the lockwire between the filter case and
1
the head assembly.
METAL-TYPE FILTER ELEMENTS.--The
IN
OUT
two most common types of metal filter elements are the
3
sintered bronze and the stainless steel. The sintered
bronze element consists of minute bronze balls joined
together as one solid piece, but it still remains porous.
The process of joining the balls together is known as
the sintered process. Like the micronic filter described
2
previously, filters using the sintered bronze elements
are equipped with a bypass valve. If the element
4
becomes clogged, the bypass valve will open, allowing
t h e f l u i d t o b y p a s s t h e fi l t e r e l e m e n t w h e n a
predetermined pressure differential exists. The
sintered bronze element can be cleaned, tested, and
ASf08022
reused several times. The stainless steel filter element
1. Filter head
3. Relief bowl
may be of corrugated, sintered, or mesh construction.
2. Filter element
4. Filter bowl
Figure 8-22.--Full flow, bypass hydraulic filter.
WIRE MESH FILTER ELEMENTS.--A wire
mesh filter element is made of a fine wire mesh
Hydraulic fluid enters the filter assembly through
(screen) and is usually used where the fluid enters
and/or leaves a container or component (view A of
the inlet port in the body and flows around the filter
figure 8-18). The size of wire mesh openings vary with
element inside the filter bowl. Filtering takes place as
the particular filter element, but normally a wire mesh
the fluid passes through the filter element and into the
filter element removes only the larger particles of
hollow core, leaving dirt and impurities deposited on
contamination from the fluid.
the outside of the filter medium. The filtered fluid then
flows from the hollow core, through the outlet port, and
A wire mesh filter element can be reused. It should
be removed, cleaned, and reinstalled at scheduled
continues on through the system.
intervals or when it becomes dirty. Replace it when it
The bypass relief valve in the body allows the fluid
cannot be properly cleaned or is damaged.
to bypass the filter element and pass directly into the
outlet port if the filter element becomes clogged. In
Multiple Filtering Elements
many micronic filters, the relief valve is set to open
when the differential in pressure exceeds 50 psi. For
Normally, only one element is used in each filter;
however, some equipment use two or more elements to
example, if the pressure at the filter inlet port is 90 psi
obtain the desired degree of filtration. In this
and the pressure at the outlet drops below 40 psi, the
arrangement, the filter elements are usually designed
bypass valve opens and allows the liquid to bypass the
so that one fits inside the other. The elements may be of
element.
the same type or of a different type. An example of
equipment that uses this filter arrangement is the
Contamination Indicators
portable hydraulic test stand (in final filtration stage)
used on an aircraft.
Hydraulic systems in support equipment that is
used to service aircraft hydraulic systems use a
Bypass Characteristics
non-bypass filter, so some type of contamination
indicator must be used to prevent damage to the
A full flow, micronic, bypass filter is shown in
hydraulic system if these filters become clogged. A
figure 8-22. This filter provides a positive filtering
pressure switch, connected across the filter to sense the
a c t i o n . H ow eve r, i t o ff e r s r e s i s t a n c e t o f l ow,
pressure differential, actuates at a preset pressure
particularly when the element becomes dirty. For this
(usually 100 psi) and causes a warning device to
reason, a full flow filter usually contains a bypass
operate; this indicates that the filter is clogged. The
valve, which automatically opens to allow the fluid to
warning device may be a button popping up on the
bypass the element when the flow of fluid is restricted
because of contamination buildup on the element.
filter or control panel, a warning light illuminating at
8-18
