create a maximum flow demand, ensuring that the fluid
condition of installed filter elements, and is the
is at near normal operating temperatures.
operating principle used in the differential-pressure or
l o a d e d - fi l t e r i n d i c a t o r s f o u n d o n m a ny fi l t e r
assemblies. Differential pressure indicating devices
have many configurations, including electrical
Hydraulic filter maintenance consists of filter
switches, continuous-reading visual indicators
element replacement only. You must be familiar with
(gauges), and visual indicators with memory. Visual
both replacement and general inspection procedures.
indicators with memory usually take the form of
magnetic or mechanically latched buttons or pins that
Replacement of hydraulic filter elements is
extend when the differential pressure exceeds that
normally a maintenance operation performed on a
allowed for a serviceable element. See figure 12-33.
periodic basis, but need for prior replacement may be
When this increased pressure reaches a specific value,
indicated during routine inspection. Hydraulic filter
inlet pressure forces the spring-loaded magnetic piston
assemblies in some aircraft and SE are equipped with
downward, breaking the magnetic attachment between
indicating devices (buttons or pins) that will extend
the indicator button and the magnetic piston. This
when the differential pressure across the filter exceeds
allows the red indicator to pop out, signifying that the
a predetermined value, indicating a loaded element.
element must be cleaned. The button or pin, once
Upon appearance of this indicator, it becomes
extended, remains in that position until manually reset
necessary to verify the condition of the filter element,
and provides a permanent (until reset) warning of a
and replace it if required. When checking or changing
loaded element. This feature is particularly useful
filter elements, also check the functioning of any
where it is impossible for an operator to continuously
monitor the visual indicator, such as in an aircraft.
Indications of a loaded filter must be verified to
Some button indicators have a thermal lockout device
confirm that release of the button or pin is due to a
incorporated in their design that prevents operation of
loaded filter and not a result of system mechanical
the indicator below a certain temperature. The lockout
shock or cold start. Verification is accomplished by
prevents the higher differential pressure generated at
resetting the indicator (manually depressing it) and
cold temperatures by high fluid viscosity from causing
operating the system at full power. If the differential
a false indication of a loaded filter element.
pressure indicator extends again during this test, the
Differential pressure indicators are a component
filter element should be replaced.
part of the filter assembly in which they are installed,
It is important that the applicable MIM be
and, as such, are normally tested and overhauled as part
consulted for specific filter element replacement
of the complete assembly. With some model filter
procedures. The following basic principles apply to
assemblies, however, it is possible to replace the
most replacement operations:
indicator itself, without removal of the filter assembly,
1. Removal of the filter bowl is the first step in
if it is suspected of being inoperative or out of
replacing the filter element. With most filter
calibration. It is important that the external surfaces of
assemblies, this operation usually consists of removing
button-type indicators be kept free of dirt or paint to
a lockwire and unscrewing the bowl from the filter head.
ensure free movement of the button.
In most filter assemblies, an automatic shutoff valve in
Indications of excessive differential pressure,
the head will prevent fluid loss from the system when
regardless of the type of indicator employed, should
the bowl is removed.
never be disregarded. All such indications must be
2. Once the bowl is removed, the fluid in it is
verified and action taken, as required, to replace the
discarded, and the bowl is cleaned of sediment by
loaded filter element. Failure to replace a loaded
flushing with clean, unused hydraulic fluid or dry
element can result in system starvation, filter element
cleaning solvent, P-D-680. It is important that
collapse, or the loss of filtration where bypass
c h l o r i n a t e d s o l ve n t s s u c h a s M I L - C - 8 1 3 0 2 o r
assemblies are used. Verification of loaded filter
1,1,1-trichloroethane are not used, as their residues may
indications is particularly important with button-type
have harmful effect on the system.
indicators, as they may have been falsely triggered by
mechanical shock, vibration, or cold start of the
3. The filter element is, in most instances, removed
system. Verification is usually obtained by manually
from the head by a gentle twisting and pulling motion.
Once removed, the surface of the element should be
resetting the indicator and operating the system to