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widely employed on early engines to overcome gear
engagement difficulties at high speed.
7. To spread the load of driving accessory units,
some engines take a second drive from the slower
rotating low pressure shaft to a second external
gearbox (fig. 7-1). This also has the advantage of
locating the accessory units in two groups, thus
overcoming the possibility of limited external space
on the engine. When this method is used, an attempt
is made to group the accessory units specific to the
engine onto the high pressure system, since that is
the first shaft to rotate, and the aircraft accessory
units are driven by the low pressure system. A typical
internal gearbox showing how both drives are taken
is shown in fig. 7-3.
Radial driveshaft
8. The purpose of a radial driveshaft is to transmit
the drive from the internal gearbox to an accessory
unit or the external gearbox. It also serves to transmit
the high torque from the starter to rotate the high
pressure system for engine starting purposes. The
driveshaft may be direct drive or via an intermediate
gearbox (para. 14).
9. To minimize the effect of the driveshaft passing
through the compressor duct and disrupting the
airflow, it is housed within the compressor support
structure. On by-pass engines, the driveshaft is
either housed in the outlet guide vanes or in a hollow
streamlined radial fairing across the low pressure
compressor duct.
10. To reduce airflow disruption it is desirable to
have the smallest driveshaft diameter as possible.
The smaller the diameter, the faster the shaft must
Accessory drives
67
Fig. 7-2 Mechanical arrangement of internal
gearboxes.
rotate to provide the same power. However, this
raises the internal stress and gives greater dynamic
problems which result in vibration. A long radial
driveshaft usually requires a roller bearing situated
halfway along its length to give smooth running. This
allows a rotational speed of approximately 25,000
r.p.m. to be achieved with a shaft diameter of less
than 1.5 inch without encountering serious vibration
problems.
Direct drive
11. In some early engines, a radial driveshaft was
used to drive each, or in some instances a pair, of
accessory units. Although this allowed each
accessory unit to be located in any desirable location
around the engine and decreased the power
transmitted through individual gears, it necessitated
a large internal gearbox. Additionally, numerous
radial driveshafts had to be incorporated within the
design. This led to an excessive amount of time
required for disassembly and assembly of the engine
for maintenance purposes.
12. In some instances the direct drive method may
be used in conjunction with the external gearbox
system when it is impractical to take a drive from a
particular area of the engine to the external gearbox.
For example, fig. 7-1 shows a turbo-propeller engine
Accessory drives
68
Fig. 7-3 An internal gearbox.
which requires accessories specific to the propeller
reduction drive, but has the external gearbox located
away from this area to receive the drive from the
compressor shaft.
Gear train drive
13. When space permits, the drive may be taken to
the external gearbox via a gear train (fig. 7-1). This
involves the use of spur gears, sometimes incorporating
a centrifugal breather (Part 8). However, it is
rare to find this type of drive system in current use.
Intermediate gearbox
14. Intermediate gearboxes are employed when it is
not possible to directly align the radial driveshaft with
the external gearbox. To overcome this problem an
intermediate gearbox is mounted on the high
pressure compressor case and re-directs the drive,
through bevel gears, to the external gearbox. An
example of this layout is shown in fig. 7-1.
External gearbox
15. The external gearbox contains the drives for the
accessories, the drive from the starter and provides
a mounting face for each accessory unit. Provision is
also made for hand turning the engine, via the
gearbox, for maintenance purposes. Fig. 7-4 shows
the accessory units that are typically found on an
external gearbox.
16. The overall layout of an external gearbox is
dictated by a number of factors. To reduce drag
whilst the aircraft is flying it is important to present a
low frontal area to the airflow. Therefore the gearbox
is ’wrapped’ around the engine and may look, from
the front, similar to a banana in shape. For
maintenance purposes the gearbox is generally
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