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located on the underside of the engine to allow
ground crew to gain access. However, helicopter
installation design usually requires the gearbox to be
located on the top of the engine for ease of access.
17. The starter/driven gearshaft (fig. 7-4) roughly
divides the external gearbox into two sections. One
section provides the drive for the accessories which
require low power whilst the other drives the high
power accessories. This allows the small and large
gears to be grouped together independently and is
an efficient method of distributing the drive for the
minimum weight.
18. If any accessory unit fails, and is prevented
from rotating, it could cause further failure in the
external gearbox by shearing the teeth of the gear
train. To prevent secondary failure occurring a weak
section is machined into the driveshafts, known as a
’shear-neck’, which is designed to fail and thus
protect the other drives. This feature is not included
for primary engine accessory units, such as the oil
pumps, because these units are vital to the running
of the engine and any failure would necessitate
immediate shutdown of the engine.
19. Since the starter provides the highest torque
that the drive system encounters, it is the basis of
design. The starter is usually positioned to give the
shortest drive line to the engine core. This eliminates
the necessity of strengthening the entire gear train
which would increase the gearbox weight. However,
when an auxiliary gearbox is fitted (para, 21) the
starter is moved along the gear train to allow the
heavily loaded auxiliary gearbox drive to pass
through the external gearbox. This requires the spur
gears between the starter and starter/driven
gearshaft to have a larger face width to carry the load
applied by the starter (fig. 7-5).
20. When a drive is taken from two compressor
shafts, as discussed in para. 7, two separate
gearboxes are required. These are mounted either
side of the compressor case and are generally
known as the ’low speed’ and ’high speed’ external
gearboxes.
Auxiliary gearbox
21. An auxiliary gearbox is a convenient method of
providing additional accessory drives when the configuration
of an engine and airframe does not allow
enough space to mount all of the accessory units on
a single external gearbox.
22. A drive is taken from the external gearbox (fig.
7-5) to power the auxiliary gearbox which distributes
the appropriate gear ratio drive to the accessories in
the same manner as the external gearbox.
CONSTRUCTION AND MATERIALS
Gears
23. The spur gears of the external or auxiliary
gearbox gear train (fig. 7-4 and 7-5) are mounted
between bearings supported by the front and rear
casings which are bolted together. They transmit the
drive to each accessory unit, which is normally
between 5000 and 6000 r.p.m. for the accessory
units and approximately 20,000 r.p.m. for the
centrifugal breather,
24. All gear meshes are designed with ’hunting
tooth’ ratios which ensure that each tooth of a gear
does not engage between the same set of opposing
teeth on each revolution. This spreads any wear
evenly across all teeth.
Accessory drives
69
25. Spiral bevel gears are used for the connection
of shafts whose axes are at an angle to one another
but in the same plane. The majority of gears within a
gear train are of the straight spur gear type, those
with the widest face carry the greatest loads. For
smoother running, helical gears are used but the
resultant end thrust caused by this gear tooth pattern
must be catered for within the mounting of the gear.
Gearbox sealing
26. Sealing of the accessory drive system is
primarily concerned with preventing oil loss. The
internal gearbox has labyrinth seals where the static
casing mates with the rotating compressor shaft. For
some o! the accessories mounted on the external
gearbox, an air blown pressurized labyrinth seal is
Accessory drives
70
Fig. 7-4 An external gearbox and accessory units.
employed. This prevents oil from the gearbox
entering the accessory unit and also prevents contamination
of the gearbox, and hence engine, in the
event of an accessory failure. The use of an air blown
seal results in a gearbox pressure of about 3 lbs. per
sq. in. above atmospheric pressure. To supplement a
labyrinth seal, an ’oil thrower ring’ may be used. This
involves the leakage oil running down the driving
shaft and being flung outwards by a flange on the
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