flow in or out of the servo valve ports
electric solenoid and internal hydraulic pres-
leading to the OD box piston chambers.
sure to actuate the valve component
Therefore, pitch remains unchanged.
that controls oil flow through the valve.
The function of the control oil servo valve
The servo valve has four ports bored in the
is to pass control oil to the ahead or astern
mounting base. These ports are for incoming
side of the OD box piston. The OD box
control oil, outflow to the astern pitch side of the
piston, in turn, changes the pitch of the propeller
OD box piston, outflow to the ahead pitch side
blades.
of the OD box piston, and oil return to the
The control oil servo valve is electrically
hydraulic oil sump. These ports line up with
actuated by the servo valve controller circuit
passages drilled in the control valves manifold
card contained in the CRP electronics en-
block assembly on which the servo valve is
mounted.
closure. Two conductors carry the servo signal
from the electronics enclosure to a terminal
b o x mounted near the OD box in each
engine room. At the terminal box, the two
OD Box-Mounted Potentiometers
conductors from the electronics enclosure
are doubled. This provides a four-conductor
The linear feedback and shaped poten-
tiometers are mounted side by side on the
cable that carries the signal from the term-
t o p of the OD box. Each potentiometer
inal box to the servo valve. The four con-
d u c t o r s connect to the two coils of the
is housed in a rectangular box about 18
inches long, 1 l/2 inches wide, and 1 l/2
servo valve solenoid in a parallel configu-
inches tall. A shaft which moves the sliding
r a t i o n . Each coil has a dc resistance of
1 , 0 0 0 ohms. If an ohmmeter is used to
contact extends out of the forward end of
measure the total resistance of the solenoid,
each potentiometer box. A Y-shaped yoke
it should read 500 ohms when measured from
connects the ends of the potentiometer shafts
the two conductors that run between the
to the end of the follow-up rod that extends
electronics enclosure and the terminal box.
out the forward end of the OD box. The
The absolute value of the maximum voltage
follow-up rod provides a mechanical position-
that should appear on the servo valve so-
ing of the potentiometer shafts that corresponds
lenoid is approximately 7.2 volts.
to actual propeller pitch positions. The follow-
up rod also positions a mechanical pointer
The solenoid discussed in this manual is
along a calibrated scale mounted between
more correctly called a torque motor. The
the potentiometers. Slots in the potentiometer
coils are wrapped around a small plate called
mounting feet provide for longitudinal position
the armature. When a dc voltage is applied
adjustments when the zero pitch feedback
to the coils, the armature flexes in reaction
and readout signals are calibrated.
to the current flow within the magnetic
field. The magnetic field is established by
LINEAR POTENTIOMETER.--The linear
permanent magnets mounted in the torque
potentiometer provides the pitch feedback signal
motor housing. Direction of armature move-
to the servo valve controller card in the CRP
ment depends on the dc voltage polarity.
electronics enclosure. The resistance element is
The magnitude of armature movement depends
approximately 10 kilohms from end to end. The
on the strength of the dc voltage. The dc
term linear means that the resistance is evenly
voltage can be from 0 volts to 7.2 volts
distributed along the potentiometer body and
of either positive or negative polarity. A
that the voltage picked up by the sliding
small pilot valve is attached to the arm-
contact is proportional to its position along the
ature. When the armature is moved, it un-
potentiometer body.
balances the internal oil pressure and causes
t h e valve to rotate and open the ahead
SHAPED POTENTIOMETER.--The shaped
or the astern valve port. With zero volt-
age applied to the coils, the pilot valve
potentiometer generates the pitch readout
signal. The pitch readout signal is used to
is centered by internal oil pressure so that
display pitch on the five-point indicator at
the valve component also becomes centered
the PLCC and on the digital demand display
by the oil pressure. Oil is then unable to
8-43
