Page 78 - The Jet Engine

Accessory drives

Fig. 7-3 An internal gearbox.
rotate to provide the same power. However, this accessory unit to be located in any desirable location
raises the internal stress and gives greater dynamic around the engine and decreased the power
problems which result in vibration. A long radial transmitted through individual gears, it necessitated
driveshaft usually requires a roller bearing situated a large internal gearbox. Additionally, numerous
halfway along its length to give smooth running. This radial driveshafts had to be incorporated within the
allows a rotational speed of approximately 25,000 design. This led to an excessive amount of time
r.p.m. to be achieved with a shaft diameter of less required for disassembly and assembly of the engine
than 1.5 inch without encountering serious vibration for maintenance purposes.
problems.
12. In some instances the direct drive method may
Direct drive be used in conjunction with the external gearbox
11. In some early engines, a radial driveshaft was system when it is impractical to take a drive from a
used to drive each, or in some instances a pair, of particular area of the engine to the external gearbox.
accessory units. Although this allowed each For example, fig. 7-1 shows a turbo-propeller engine

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