rise to a predetermined pressure, the valve
automatically returns to the neutral position and,
consequently, to open-center flow.
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.
