joemarshall said:*
As for the ratio, there's a limit to how high a gearing up you can get
from a single set of epicyclic gears (I think it may be 2:1) , as you
get closer to it, you have to use smaller and smaller and more
delicate gears for a given hub size, 1.5 is probably a good
compromise. *
Joe is right, the limit without using compound gears is less than 2:1
which can't actually be attained. At the 2:1 ratio, the diameter of the
planet gears is zero and the sun gear and ring gear are the same size.
Since gears have teeth they can be thought of as having built in integer
limits, that is they are not continuously variable. In the geared up
mode, the number of teeth on the ring (outer) gear and the number of
teeth on the sun (inner) gear BOTH must be evenly divisible by the
number of planet (small) gears. If S is the number of teeth on the sun
gear, R is the number of teeth on the ring gear, P is the number of
teeth on the planet gear and N is the number of planet gears then the
gear ratio will be 1+(S/R):1 and S/N and R/N must both be integers and,
finally, R=S+2P. Any gear ratio design must meet all of these
constraints.
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