When that happens, the equipment should be stopped
the control panel, a buzzer or horn sounding, or the
and the filter serviced (cleaned or replaced).
motor (driving the hydraulic pump) secured.
Contamination indicators are also used on some
NOTE: It is important to remember that cold
bypass filters. The full flow, bypass, electrical-
hydraulic fluid can produce a false pressure indication.
indicating hydraulic filter (shown in figure 8-23) is
To prevent needless changing of filters, fluid should be
used in many hydraulic systems of modern support
at operating temperature for a true indication of
equipment. This filter uses one or a combination of the
contamination. Some filters, such as that shown in
contamination indicators previously described.
figure 8-23, have a button to reset the switch after the
filter has been serviced. However, on other filters, the
Under normal conditions the fluid enters the inlet
switch resets automatically when the differential
of the filter, passes through the filter element, and
pressure is relieved.
leaves the filter through the outlet. As the fluid passes
through the filter element, impurities are deposited on
If the filter is not properly serviced following the
the outside of the element. As the deposits accumulate,
they cause a differential pressure to build up between
in operation, the differential pressure continues to
the inlet and outlet of the filter. The differential
build. At 100 ±10 psi, the bypass valve will open and
pressure is sensed across the contamination indicator
allow the fluid to flow straight through, bypassing the
switch, causing the red indicator button to pop up.
SWITCH RESET BUTTON (SWITCH
CLOSES WHEN DIFFERENTIAL
PRESSURE EXCEEDS 70 + 10 PSI
SWITCH IN CLOSED POSITION
INDICATING A CLOGGED FILTER
RELIEF VALVE OPENS
AT A DIFFERENTIAL
100 + 10 PSI
(CLOGGED FILTER ELEMENT)
Figure 8-23.--Full flow bypass, electrical indicating hydraulic filter.