|Fig. 7-5 An external gearbox with auxiliary gearbox drive.|
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 employed. This prevents oil from the gearbox entering the accessory unit and also prevents con-tamination 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 rotating shaft. The oil is then collected and returned
to the gearbox.
27. To reduce weight, the lightest materials possible are used. The internal gearbox casing is cast from
aluminium but the low environmental temperatures that an external gearbox is subjected to allows the
use of magnesium castings which are lighter still. The gears are manufactured from non-corrosion
resistant steels for strength and toughness. They are case hardened to give a very hard wear resistant
skin and feature accurately ground teeth for smooth gear meshing.