Page 81 - The Jet Engine

Accessory drives

Fig. 7-5 An external gearbox with auxiliary gearbox drive.

employed. This prevents oil from the gearbox Materials
entering the accessory unit and also prevents con- 27. To reduce weight, the lightest materials possible
tamination of the gearbox, and hence engine, in the are used. The internal gearbox casing is cast from
event of an accessory failure. The use of an air blown aluminium but the low environmental temperatures
seal results in a gearbox pressure of about 3 lbs. per that an external gearbox is subjected to allows the
sq. in. above atmospheric pressure. To supplement a use of magnesium castings which are lighter still.
labyrinth seal, an 'oil thrower ring' may be used. This The gears are manufactured from non-corrosion
involves the leakage oil running down the driving resistant steels for strength and toughness. They are
shaft and being flung outwards by a flange on the case hardened to give a very hard wear resistant
rotating shaft. The oil is then collected and returned skin and feature accurately ground teeth for smooth
to the gearbox. gear meshing.

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