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Fixing
the sun gear, and using the planet carrier and ring as input and output
shafts gives another overall gear ratio: Gear
Ratio2 = {(NS / NR) + 1}.
A
final ratio may be obtained by fixing the ring and allowing the sun gear
and the planet carrier to rotate. For this configuration the overall gear
ratio is:
Gear Ratio3 = {(NR
/ NS) + 1}. The number of teeth
on the gears is constrained by the meshing requirement so that: NR
= (NS + 2NP ).
Also:
{(NR + NS) / Number
of Planets} = an Integer
In
an actual epicyclic box, dog clutches select the gears by fixing together
various gearbox components. This system is compact, can easily provide
three or four speeds, and is enclosed so that dirt is not an issue in its
performance. The diagram shows a Sturmey Archer three-speed hub with the
middle gear engaged i.e. the ring gear is locked to the input shaft that
is driven by the sprocket. |
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