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Graphic Diagrams
The primary purpose of a graphic (schematic) diagram
is to enable the maintenance person to trace
the flow of fluid from component to component
within the system. This type of diagram
uses standard symbols to show each component
and includes all interconnecting piping.
Additionally, the diagram contains a component
list, pipe size, data on the sequence of
operation, and other pertinent information. The
graphic diagram (fig. 12-3) does not indicate the
physical location of the various components, but
it does show the relation of each component
to the other components within the system.
Figure 12-3.—Graphic diagram of LST 1182 class hydraulic steering gear.
Notice that figure 12-3 does not indicate the physical
location of the individual components with
respect to each other in the system. For example,
the 3/4-inch, solenoid-operated, 4-way valve
(10) is not necessarily located directly above the
relief valve (26). The diagram does indicate, however,
that the 4-way valve is located in the working
line, between the variable-displacement pump
and the 1-inch rotary selector valve, and that
the valve directs fluid to and from the rotary actuator.
Combination Diagrams
A combination drawing uses a combination of
graphic, cutaway, and pictorial symbols. This drawing
also includes all interconnecting piping.
FLUID POWER SYSTEMS
A fluid power system in which the fluid in the system
remains pressurized from the pump (or regulator)
to the directional control valve while the
pump is operating is referred to as a closed-center system.
In this type of system, any number of
subsystems may be incorporated, with a separate
directional control valve for each subsystem.
The directional control valves are arranged
in parallel so that system pressure acts equally
on all control valves.
Another type of system that is sometimes used in
hydraulically operated equipment is the open-center system.
An open-center system has fluid flow
but no internal pressure when the actuating mechanisms
are idle. The pump circulates the fluid from
the reservoir, through the directional control valves,
and back to the reservoir. (See fig. 12-4, view
A.) Like the closed-center system, the open-center system
may have any number of subsystems, with
a directional control valve for each subsystem. Unlike
the closed-center system, the directional control
valves of an open-center system are always connected
in series with each other, an arrange-ment in
which the system pressure line goes through
each directional control valve. Fluid is always
allowed free passage through each control valve
and back to the reservoir until one of the con-trol valves
is positioned to operate a mechanism. When
one of the directional control valves is positioned
to operate an actuating device, as shown
in view B of figure 12-4, fluid is directed from
the pump through one of the working lines to
the actuator. With the control valve in this position,
the flow of fluid through the valve to the
reservoir is blocked. Thus, the pressure builds up
in the system and moves the piston of the
Figure 12-4.—Open-center hydraulic system.
actuating cylinder. The fluid from the other end of
the actuator returns to the control valve through
the opposite working line and flows back to
the reservoir.
Several different types of directional control valves
are used in the open-center system. One type
is the manually engaged and manually disengaged.
After this type of valve is manually moved
to the operating position and the actuating mechanism
reaches the end of its operating cycle, pump
output continues until the system relief valve
setting is reached. The relief valve then unseats
and allows the fluid to flow back to the reservoir.
The system pressure remains at the pressure
setting of the relief valve until the directional
control valve is manually returned to the
neutral position. This action reopens the open-center
flow and allows the system pressure to
drop to line resistance pressure. Another
type of open-center directional control
valve is manually engaged and pressure disengaged.
This type of valve is similar to the valve
discussed in the preceding paragraph; however,
when the actuating mechanism reaches the
end of its cycle and the pressure continues to
One of the
advantages of the open-center system
is that the continuous pressurization of the system
is eliminated. Since the pressure is gradually
built up after the directional control valve
is moved to an operating position, there is very
little shock from pressure surges. This provides
a smooth operation of the actuating mechanisms;
however, the operation is slower than
the closed-center system in which the pressure is
available the moment the directional control valve
is positioned. Since most applications require
instantaneous operation, closed-center systems
are the most widely used.
HYDRAULIC POWER DRIVE SYSTEM
The hydraulic power drive has been used in
the Navy for many years. Proof of its effectiveness
is that it has been used to train and elevate
nearly all caliber guns, from the 40-mm gun
mount to the 16-inch turret. In addition to gun
mounts and turrets, hydraulic power drives are
used to position rocket launchers and missile
launchers, and to drive and control such equipment
as windlasses, capstans, and winches. In
its simplest form, the hydraulic power drive consists
of the following:
1. The prime mover, which is the outside source
of power used to drive the hydraulic pump
2. A variable-displacement hydraulic pump
3. A hydraulic motor
4. A means of introducing a signal to the hydraulic
pump to control its output
5. Mechanical shafting and gearing that
transmits the output of the hydraulic
motor to the equipment being
operated
Hydraulic power drives differ in some
respects, such as size, method of control,
and so forth. However, the
fundamental operating principles
are similar. The unit used in the following
discussion of fundamental operating principles
is representative of the hydraulic power drives
used to operate the 5"/38 twin mounts. Figure
12-5 shows the basic components of the
train power drive. The electric motor is constructed
with drive shafts at both ends. The forward
shaft drives the A-end pump through reduction
gears, and the after shaft drives the auxiliary
pumps through the auxiliary reduction gears.
The reduction gears are installed because
Figure 12-5.-Train power drive—components.
the pumps are designed to operate at a speed much slower
than that of the motor.
The replenishing pump is a spur gear pump. Its
purpose is to replenish fluid to the active system
of the power drive. It receives its supply of
fluid from the reservoir and discharges it to the
B-end valve plate. This discharge of fluid from the
pump is held at a constant pressure by the action
of a pressure relief valve. (Because the capacity
of the pump exceeds replenishing demands,
the relief valve is continuously allowing some
of the fluid to flow back to the reservoir.)
The sump pump and oscillator has a twofold purpose.
It pumps leakage, which collects in the sump
of the indicator regulator, to the expansion tank.
Additionally, it transmits a pulsating effect to
the fluid in the response pressure system. Oscillations
in the hydraulic response system help eliminate
static friction of valves, allowing hydraulic
control to respond faster. The
control pressure pump supplies high-pressure fluid
for the hydraulic control system, brake
pistons, lock piston, and the hand-controlled clutch
operating piston. The control pressure
pump is a fixed-displacement, axial-piston type.
An adjustable relief valve is used to limit
the operating pressure at the outlet of the pump.
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