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SDC failure. The engine compressor bleed air selector
valve opens automatically if SDC output falls below 10
psi. Simultaneously, a spring-operated check valve
opens. When closed, this valve prevents pressurized air
from going to the engine compressor when the SDC is
operating. Each engine provides bleed air to its engine
inlet to prevent ice formation and to keep the engine
cooling louvers closed while the engine is operating.
The bleed air used in these last two instances is independent
of the pressurized air system.
2.39.3 External Air Source Receptacles. The receptacle
for external air source connection (fig 2-46) is on
the lower portion of No. 1 engine nacelle. When the air
source is connected and operating, a normally closed
check valve to the pressurized air system opens to allow
air to pressurize the manifold.
2-55
TM 1-1520-238-10
2-56 Change 6
Section VII. POWER TRAIN SYSTEM
2.40 POWER TRAIN.
The power train (fig 2-32) transmits engine power to
the rotors and transmission-mounted accessories. The
power train includes two engine nose gearboxes, two
shafts to the main transmission, the main transmission,
main rotor drive shaft, tail rotor drive shafts, intermediate
gearbox, tail rotor gearbox, and APU drive
shaft, and couplings.
2.40.1 Engine Nose Gearboxes. The engine nose
gearboxes are mounted on the nose of each engine.
They reduce nominal engine speed and change the
angle of drive. Both nose gearboxes have self-contained
pressurized oil systems with provisions to ensure limited
operation if a total loss of pressurized lubrication
occurs. These self-contained systems feature a pressure
pump driven by the gearbox output shaft, filter with
impending bypass filter indicator and filter bypass capability,
high-pressure relief valve, and sump. Mounted
on the nose gearbox output drive is an axial-type fan
that draws air through the gearbox fairing and past
cooling fins on the gearbox. The input drive shafts have
flexible couplings that require no lubrication. Externally
accessible accessories include a filler, breather, oil
level sight gage, chip detector/temperature sensor,
pressure transducer, and temperature probe.
a. Engine Nose Gearbox Caution Light Provisions.
High oil temperature, low oil pressure, and the
presence of chips are detected in each gearbox. The pilot
is alerted to the condition by six caution segments:
OILHOT NOSE GRBX 1, OIL HOT NOSE GRBX 2,
OIL PSI NOSE GRBX 1, OIL PSI NOSE GRBX 2,
CHIPS NOSE GRBX 1, and CHIPS NOSE GRBX 2.
Either the ENG 1 or ENG 2 segment will simultaneously
illuminate on the CPG caution/warning panel
(fig 2-44).
2.40.2 Main Transmission. The main transmission
combines the two engine nose gearbox inputs and provides
drive to the main rotor, tail rotor, accessories, and
rotor brake disc. Two overrunning clutches at the main
transmission permit either engine to be disengaged
from the transmission during autorotation. The main
transmission reduces the rpm input to the main rotor,
tail rotor, and the rotor brake disc. The main transmission
is mounted below the main rotor static mast base
which allows its removal without removing the upper
controls, mast, hub, or blades. The main rotor drive
shaft is designed to carry torque loads only. The rotor
hub is on a static mast which carries vertical and bending
loads. The drive shaft rotates inside the static mast.
The main transmission has primary and accessory
drive trains. The primary drive train, through three
stages, changes the angle and speed of the power drives
to the main rotor, tail rotor, and rotor brake. Overrunning
clutches allow the APU to drive the transmission
accessory gearbox when the engines are not operating.
The transmission accessories consist of two alternating-
current generators, two hydraulic pumps, and a
shaft driven compressor. A magnetic pickup measures
main rotor rpm.
a. Main Transmission Lubrication. The main
transmission has two independent oil systems. Each
system has its own sump, pump, filter and heat exchanger.
Oil level sight gauges are located in the transmission
housing at each oil sump. These systems are
not totally independent in the usual sense because during
normal operation, the oils will mix. If oil loss occurs
in either sump or in either heat exchanger, the diverter
(float) valve will seal off that sump to prevent a total
loss of oil. There are provisions throughout the transmission
so that even with a total loss of oil there will be
limited lubrication. Each oil filter has a bypass capability
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