indication of a contaminated filter. Some filters have a
button to reset the switch after the filter has been
serviced; however, on other filters, the switch resets
automatically when the differential pressure is relieved.
If the filter is not properly serviced following the
contamination indication and the equipment is kept in
operation, the differential pressure continues to build.
At 100 ÷ 10 psi, the bypass valve will open and allow
the fluid to flow straight through, bypassing the filter
element (view (B) of fig. 10-31). But on this filter the
contamination indicator is to warn the operator that the
filter element is clogged. The equipment can then be
stopped before the bypass valve opens, thus preventing
contaminated fluid from being passed through the
In spite of the great variety of support equipment,
all hydraulic systemsfrom the simplest to the most
complex-operate according to the basic principles and
make use of the components discussed thus far in this
As a CM1 you are responsible for analyzing the
malfunctions of hydraulic equipment, ranging from the
simple jack to large earth-moving equipment. Thus, the
development, piece by piece, of a representative system
should assist you in analyzing any hydraulic system.
REPRESENTATIVE HYDRAULIC SYSTEM
Basically, any system must contain the following
units: PUMP, ACTUATOR, RESERVOIR, CONTROL
VALVE, and TUBING. Figure 10-32 shows a simple
system that uses only these essentials.
The flow of hydraulic fluid can be easily traced from
the reservoir through the pump to the selector valve.
With the selector valve in the position indicated by the
solid lines, the flow of fluid created by the pump flows
through the valve to the upper end of an actuating
cylinder. Fluid pressure then forces the piston down, and
at the same time, forces out the fluid on the lower side
of the piston, up through the selector valve, and back to
When the selector valve is rotated 90 degrees, the
fluid from the pump then flows to the lower side of the
actuating cylinder, thus reversing the process. The
movement of the piston can be stopped at any time
simply by moving the selector valve to the neutral
position (45-degree movement either way). In this
position, all four ports are closed and pressure is trapped
in both working lines.
Figure 10-32.-A simple hydraulic system.
The hydraulic system just described would be
practical if it were operated by a hand pump, such as a
system common to the engine installation/removal
stands and bomb trucks. However, since the illustrated
pump is a power-driven, constant delivery gear pump,
pressure builds up immediately to such proportions that
either the pump fails or a line bursts. Therefore, a
pressure relief valve is incorporated in the system to
protect it, as shown in figure 10-33. This valve is set to
Figure 10-33.-Hydraulic system with a relief valve