Page 79 - The Jet Engine

Accessory drives

which requires accessories specific to the propeller 'shear-neck', which is designed to fail and thus
reduction drive, but has the external gearbox located protect the other drives. This feature is not included
away from this area to receive the drive from the for primary engine accessory units, such as the oil
compressor shaft. pumps, because these units are vital to the running
of the engine and any failure would necessitate
Gear train drive immediate shutdown of the engine.
13. When space permits, the drive may be taken to
the external gearbox via a gear train (fig. 7-1). This 19. Since the starter provides the highest torque
involves the use of spur gears, sometimes incorpo- that the drive system encounters, it is the basis of
rating a centrifugal breather (Part 8). However, it is design. The starter is usually positioned to give the
rare to find this type of drive system in current use. shortest drive line to the engine core. This eliminates
the necessity of strengthening the entire gear train
Intermediate gearbox which would increase the gearbox weight. However,
14. Intermediate gearboxes are employed when it is when an auxiliary gearbox is fitted (para, 21) the
not possible to directly align the radial driveshaft with starter is moved along the gear train to allow the
the external gearbox. To overcome this problem an heavily loaded auxiliary gearbox drive to pass
intermediate gearbox is mounted on the high through the external gearbox. This requires the spur
pressure compressor case and re-directs the drive, gears between the starter and starter/driven
through bevel gears, to the external gearbox. An gearshaft to have a larger face width to carry the load
example of this layout is shown in fig. 7-1. applied by the starter (fig. 7-5).

External gearbox 20. When a drive is taken from two compressor
15. The external gearbox contains the drives for the shafts, as discussed in para. 7, two separate
accessories, the drive from the starter and provides gearboxes are required. These are mounted either
a mounting face for each accessory unit. Provision is side of the compressor case and are generally
also made for hand turning the engine, via the known as the 'low speed' and 'high speed' external
gearbox, for maintenance purposes. Fig. 7-4 shows gearboxes.
the accessory units that are typically found on an
external gearbox. Auxiliary gearbox
21. An auxiliary gearbox is a convenient method of
16. The overall layout of an external gearbox is providing additional accessory drives when the con-
dictated by a number of factors. To reduce drag figuration of an engine and airframe does not allow
whilst the aircraft is flying it is important to present a enough space to mount all of the accessory units on
low frontal area to the airflow. Therefore the gearbox a single external gearbox.
is 'wrapped' around the engine and may look, from 22. A drive is taken from the external gearbox (fig.
the front, similar to a banana in shape. For 7-5) to power the auxiliary gearbox which distributes
maintenance purposes the gearbox is generally the appropriate gear ratio drive to the accessories in
located on the underside of the engine to allow the same manner as the external gearbox.
ground crew to gain access. However, helicopter
installation design usually requires the gearbox to be CONSTRUCTION AND MATERIALS
located on the top of the engine for ease of access.
Gears
17. The starter/driven gearshaft (fig. 7-4) roughly 23. The spur gears of the external or auxiliary
divides the external gearbox into two sections. One gearbox gear train (fig. 7-4 and 7-5) are mounted
section provides the drive for the accessories which between bearings supported by the front and rear
require low power whilst the other drives the high casings which are bolted together. They transmit the
power accessories. This allows the small and large drive to each accessory unit, which is normally
gears to be grouped together independently and is between 5000 and 6000 r.p.m. for the accessory
an efficient method of distributing the drive for the units and approximately 20,000 r.p.m. for the
minimum weight. centrifugal breather,
18. If any accessory unit fails, and is prevented 24. All gear meshes are designed with 'hunting
from rotating, it could cause further failure in the tooth' ratios which ensure that each tooth of a gear
external gearbox by shearing the teeth of the gear does not engage between the same set of opposing
train. To prevent secondary failure occurring a weak teeth on each revolution. This spreads any wear
section is machined into the driveshafts, known as a evenly across all teeth.

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