The actuating unit converts fluid pressure into
useful work. The actuating unit may be an
actuating cylinder or a hydraulic motor. An
actuating cylinder converts fluid pressure into
useful work by linear/reciprocating-mechanical
motion. A hydraulic motor converts fluid pressure
into useful work by rotary-mechanical motion.
Look at figure 4-12, You can trace the flow
of hydraulic fluid from the reservoir through the
pump to the selector valve. With the selector valve
in the #1 position, fluid flow created by the pump
is through the valve to the right-hand end of the
actuating cylinder. Fluid pressure then forces the
piston to the left. As the piston moves left, fluid
on that side of the piston flows out, up through
the selector valve, and back to the reservoir. With
the selector valve in the #2 position, fluid from
the pump flows to the left side of the actuating
cylinder, reversing the process. You can stop
piston movement at any time by moving the
selector valve to neutral. In this position, all four
ports close, and pressure is equal in both working
Figure 4-12.-Basic hydraulic system, hand-operated pump.
lines.
You can see how hydraulic systems work by
farm, in industry, aboard ship, and many other
looking at this basic system. You can make
places as well as in aircraft. Regardless of its
additions to it to provide additional sources
application, each hydraulic system has a minimum
of power, operate additional cylinders, make
of four components--reservoir, pump, selector
operation more automatic, or increase reliability.
valve, and actuating unit--plus lines through
These additions are all made on the framework
which the fluid flows. Figure 4-12 shows a basic
of the basic hydraulic system diagram shown in
hydraulic system, with the four essential
components and their relationship within the
system.
ELECTROHYDRAULIC SYSTEMS
The reservoir provides storage for a supply of
fluid for operation of the system. It replenishes
Basically, there are two types of electrically
the fluid of the system when needed, provides
controlled, hydraulically operated components.
room for thermal expansion, and normally
Selector valves that start, stop, or change the
provides a means for bleeding air from the system.
direction of a fluid flow (similar to a switch in
an electrical circuit), and control valves that
The pump creates a flow of fluid. The pump
schedule fluid flow (much like a potentiometer
in this system is hand operated; however, aircraft
controls current flow). Each of these components
systems have engine-driven or electric-motor-
is manufactured in varying degrees of complexity,
driven pumps. There are two types used in naval
depending on its use. Each manufacturer uses its
aircraft--the piston type and the gear type. One
own identification for its components, such as
aircraft may incorporate both types due to the
transfer valve, engagement valve, servo valve, etc.
requirements of the various hydraulic systems.
Aircraft hydraulic systems normally operate
at a pressure of 3,000 PSI; therefore, each
The selector valve directs the flow of fluid.
component must be capable of operating at this
These valves actuate either manually or by
pressure. You should observe all applicable safety
solenoid; they may operate directly, or they may
precautions when working with high-pressure
operate indirectly with the use of mechanical
hydraulic or pneumatic systems.
linkage.
