Crown wheel and pinion
In all manual automotive gearboxes, except those designed specifically for motor
racing or other uses where noise is not a problem, the crown wheel and pinion
usually consists of either a pair of spiral or hypoid gears. Both the spiral and hypoid
bevel gears have certain advantages over each other, all of which must be seriously
taken into account when a new design of gearbox or transmission is being initiated.
Comparison of the two types of bevel gears and their advantages can be listed as
follows :
1. Noise. The ability to lap the entire tooth surface of a hypoid gear, as there is
lengthwise sliding motion between the mating teeth at every point, generally results
in smoother and consequently quieter running gears.
2 Strength. Due to the offset required in a pair of hypoid bevel gears, the crown
wheel and the pinion have different spiral angles, which results in the two gears
having the same normal pitches. It is usual to design the pinion with a coarser
transverse pitch than the crown wheel; this results in a larger pinion diameter than
for the corresponding spiral bevel pinion. The amount of the enlargement is
dependent upon the amount of the pinion offset, and results in the following
advantages:
(a) a better bending fatigue life than that of the corresponding spiral bevel gears
(b) the use of a larger shaft or shank diameter on the hypoid pinion
But it must be realized that with low gear ratios, the use of hypoid gears may result in
very large'diameter pinions and therefore it may prove advantageous to use a spiral
bevel design in such situations. These factors must be fully and carefully investigated
at the initial design stages.
3 EfJiciency. The efficiency of both hypoid and spiral bevel gears can be very high,
although the efficiency of hypoid gears is slightly less than that of the equivalent
spiral bevel gears, due to the increase in the sliding motion between the mating teeth.
Efficiencies as high as 99% have been obtained with spiral bevel gears, as against the
96% obtained with hypoid bevel gears when tested on the same rig in laboratory
conditions. This efficiency is dependent upon the following:
(a) the amount of the hypoid offset |
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发表于 2007-5-5 00:35:21
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