Gears
The spur gears of the external or auxiliary gearbox gear train (fig. 7-4 and 7-5) are mounted between bearings supported by the front and rear casings which are bolted together. They transmit the drive to each accessory unit, which is normally between 5000 and 6000 r.p.m. for the accessory units and approximately 20,000 r.p.m. for the centrifugal breather,
All gear meshes are designed with ‘hunting tooth’ ratios which ensure that each tooth of a gear does not engage between the same set of opposing teeth on each revolution. This spreads any wear evenly across all teeth.
Fig. 7-4 An external gearbox and accessory units.
Spiral bevel gears are used for the connection of shafts whose axes are at an angle to one another but in the same plane. The majority of gears within a gear train are of the straight spur gear type, those with the widest face carry the greatest loads. For smoother running, helical gears are used but the resultant end thrust caused by this gear tooth pattern must be catered for within the mounting of the gear.
Gearbox sealing
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 contamination 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.
Materials
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
