is normally located directly ahead of the engine and is fixed to isolation
mounts on structural support assemblies of airframe structure. It is joined to
the cold end of the engine through a drive shaft and serves the following
Provides angle change to the drive and speed
reduction by a train of spiral bevel gears and one/ two – stage planetary gears
in order to drive the rotor mast.
A free wheel clutch in the input drive quill
coupling disengages to allow the main rotor and gear train to turn freely when
the engine is stopped or is idling below rotor driving speed, as in auto
A secondary gear train drives the tail rotor
through a shaft supported by hanger bearings or support bearings.
Power is transmitted through the 42 and
90-degree gearboxes to the tail rotor.
42-degree gearbox change angle of drive only and
provides a gear ratio of 1:1.
f. The 90 – degree gearbox changes angle
of drive and provides a gear reduction.
The transmission also drives the tail rotor
drive quill, the rotor tachometer/hydraulic pump quill and the cock pit blower
quill as accessories on the transmission.
9.1 Transmission and Drive System
A lift – link and five pylon isolation mounts
attach the transmission to the fuselage structure. The forged steel lift – link
is connected between the forward underside of the transmission support case and
a fuselage beam, to give direct single point suspension from transmission to
the main fuselage. Four main mounts support the pylon through the transmission
support case with the aft pair restrained by friction dampers. The fifth
isolation mount is centered on a support fitting, bridged across the rear side
of the pylon support structure, with its eyebolt attached to a welded beam,
extending between the aft legs of support case.
Statically, the transmission supports the
weight of the mast, the rotating controls, and the main rotor. In flight, the
transmission along with the mast is the main and only link between the main
rotor and the airframe. Output reduction ratios (Table 9.1), expressed as
revolutions of each driven unit per engine output revolution, are as follows.
Rotor Drive Shaft
pit Air Blower
9.1 Reduction ratios
After further reduction in the 90-degree
gearbox, the tail rotor turns at 1650 rpm (0.25 engine rpm). Engine rpm (6600)
is reduced to main rotor rpm (324) by internal transmission gear reduction.
This reduction is accomplished by a system of bevel gears and a two – stage
planetary, for a total reduction of 20.37:1 for the transmission.
Operation is accomplished by the engine being
connected to the transmission by the main drive shaft. The main drive shaft,
commonly called the short shaft, connects the engine to the freewheeling clutch
portion of the transmission input quill. The input quill pinion drives the
horizontal spiral bevel gear, providing the first stage of reduction. The
horizontal spiral bevel gear in turn drives the cockpit air blower and vertical
shaft. Through the vertical shaft, torque is transmitted up to the two stage
planetary gears and to the mast.
(Figure 9.2)The transmission is divided into
five sections: top case, planetary gear case, main case, support case and
accessory drive/sump case. The planetary gear case, are made of magnesium. The
entire inner surface of the transmission is coated with heat-applied epoxy.
9.2 Main Transmission
case. The top case
(Figure 9.3), also called the upper case, is the support for the main rotor
mast when installed, or for the lifting adapter to transport the transmission
when necessary. Also, the top case houses a vent that relieves internal
pressure with in the transmission.
9.3 Top case
Gear Case. The planetary gear case, or ring gear case,
is the only transmission case assembly made of steel. It houses the first and
second stage planetary gear sections which give the transmission the gear
reduction to the main rotor mast.
Housed with the planetary gear case are two
auxiliary oil jets. At no time should any attempt be made to tighten the mount
bracket for the jets. The bolts for the bracket are factory – torqued and have
nuts with cotter keys through them.
Figure 9.4 Planetary
CASE. The main
case (figure 9.5) of the transmission has the provisions for the main input
quill and output blower quill. The left side of the case has a cover plate
installed which will allow access to the internal bearing that the main input
quill shaft seats when installing a new quill in the transmission.
9.5 Main case
Drive/ Support case. The accessory drive/sump case houses the
tail rotor output quill, transmission hydraulic pump/ tachometer drive quill
and transmission oil pump. Also, within the sump case is the oil level sight
gauge and a chip detector. For the oil system the sump case has the internal
oil filter and the oil pump inlet screen.
9.6 Accessory Drive
transmission contains a varied variety of quills. The quills maybe replaced
individually without shimming.
Input Quill. The
main input quill (3, figure 9.2 and figure 9.7) is located on the aft side of
the transmission. The engine transmits power to the transmission through the
main driveshaft and the main input quill. A free wheel (one way) clutch located
in the main input quill operates automatically, engaging to allow engine to
drive rotor or disengaging the idling engine during auto rotational descent.
Figure 9.7 Main
Tail Rotor Drive Quill. The tail rotor drive quill (figure 9.8)
is located in the aft side of the transmission sump case. The forward tail
rotor driveshaft is attached to a splined coupling which is part of the tail
rotor drive quill.
Figure 9.8 Tail
Rotor Drive Quill
Pump and Tachometer Drive Quill. The
hydraulic pump and tachometer drive quill (9, figure 9.2) is located on the
right side of the transmission sump case. The quill has pads for two hydraulic
pumps and the rotor tachometer generator. It is shown in figure 9.9.
Figure 9.9 Hydraulic
Pump And Tachometer Drive Quill
Drive Quill. The
fan drive quill (10, figure 9.2 and Figure 9.10) is located on the forward side
of the transmission. This quill transmits power from the transmission input bevel
gear to drive the air distribution blower (fan).
Figure 9.10 Fan
Drive Quill. The
alternator drive quill (5, figure 9.2 and figure 9.11) is located on the left
side of the transmission. The purpose of this quill is to take power from the
input bevel gear of the transmission to drive the alternator.
Figure 9.11 Alternator
Case. The support case of the transmission allows the
transmission to be attached to the airframe at six different positions.
Rotor Mast Assembly. The
main rotor mast assembly (Figure 9.12) is a tubular steel shaft fitted with two
bearings, which support it vertically in the transmission. Mast driving splines
are engaged with transmission upper stage planetary gear providing counterclockwise
rotation as viewed from above. Splines on upper portion of mast provide
mounting for main rotor and control assemblies.
9.12 Main Rotor Mast Assembly
Main Drive Shaft
A main driveshaft with crowned tooth couplings
is installed between an adapter on engine output shaft and the freewheel
coupling of the transmission input drive quill. Two clamp sets, of split V-band
type, hold the mating curvic splined faces of couplings in secure contact.
Flexibility of couplings is provided by sliding an inner coupling in splines of
an outer coupling to accommodate movement of transmission on pylon mountings. A
spring in each coupling assists centering of shaft during operation and tends
to hold shaft assembly in place if clamps are removed during maintenance.
Figure 9.13 Main
Tail Rotor Drive Shaft
Five driveshaft sections (Figure 9.17) transmit
power from the transmission to the tail rotor through two gearboxes. The shaft
sections are identical and are supported by three hanger assemblies on the
tailboom and engine deck.
9.17 Tail Rotor Driveshaft
Rotor Driveshaft Hanger Assembly. Three hanger assemblies
connect and support tail rotor driveshaft along top of tailboom and above the
engine deck. Each assembly consists of couplings on a short, splined shaft,
mounted through a single-row sealed ball bearing in a ring-shaped hanger
equipped with two mounting lugs for attachment on a support fitting (figure
9.18 Tail Rotor Driveshaft Hanger Assembly
The intermediate gearbox 15, (figure 9.19) is
located on the tailboom at the base of the vertical fin. The gearbox provides a
forty-two degree change in direction of tail rotor driveshaft. It consists of a
case with a gear quill in each end. The case is fitted with a breather-type oil
filler cap, an oil level sight gage and a drain plug
equipped with a chip detector which activates warning lights on the pilot and
gunner caution panels and the miscellaneous controls panel when excessive metal
particles contamination occurs. The input and output quills have flexible
couplings for attachment of drive-shafts. Access is provided by a cover with
9.19 Intermediate Gearbox
Figure 9.20 Damage
Limits Intermediate Gear Box
Gearbox Quills. The
intermediate gearbox input and output quills consist of a pinion gear bearing
mounted in a sleeve. Each quill has a flexible coupling for attachment of drive
shafts. The output quill has an oil collector cone installed on the inboard end
of the pinion gear.
Tail Rotor Drive Gearbox
Description. A gearbox (Figure 9.25) at top of tailboom
vertical fin provides ninety-degree change in direction of drive and speed
reduction between the input driveshaft and the output shaft on which the tail
rotor is mounted. The gearbox consists of mating input and output gear quill
assemblies set into gear case provided with a breather-type oil filler cape oil
level sight gage, and a drain plug with a chip detector. The input quill has a
flexible coupling for attachment of driveshaft. Control linkage is attached on
the left side, with a control rod extending through the rotor shaft.
9.25 Tail Rotor Drive Gearbox