Double reduction with spur gears ahead of bevel gears

(Fig. 7.18) With a pair of helical gears providing the first gear reduction before the crownwheel and pinion, a high mounted and compact final drive arrangement is obtained. This layout has the disadvantage of the final gear reduction and thus torque multiplication is transmitted through the crownwheel and pinion bevel gears which therefore absorbs more end thrust and is generally considered to be less efficient in operation compared to helical spur type gears. The first stage of a double reduction axle is normally no more than 2:1 leaving the much larger reduction for the output stage.

Fig. 7.18 Final drive spur double reduction ahead of bevel pinion

Fig. 7.19 Final drive spur double reduction between crownwheel and differential

Double reduction with bevel gears ahead of spur gears (Fig. 7.19) A popular double reduction arrangement has the input from the propellor shaft going directly to the bevel pinion and crownwheel. The drive is redirected at right angles to that of the input so making it flow parallel to the half shafts, the first stage gear reduction being deter mined by the relative sizes (number of teeth) of the bevel gears. A helical pinion gear mounted on the same shaft as the crownwheel meshes with a helical gear wheel bolted to the differential cage. The combination of these two gear sizes provides the second stage gear reduction. Having the bevel gears ahead of the helical gears ensures that only a proportion of torque multiplication will be constrained by them, while the helical gears will absorb the full torque reaction of the final gear reduction.

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