CONSTRUCTION AND MATERIALS,Gears

CONSTRUCTION AND MATERIALS
Gears


23. 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

Fig. 7-4 An external gearbox and accessory units.

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.

24. 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.




25. 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