I have been trying to figure out what the gear ratio for a planetary
gear is when the sun gear is held stationary and the input and output
are the Planet carrier and Ring gear. I think the reason I can't find
anything that talks about that specific scenario is because it is always
a 2 to 1 ratio and for that reason probably never be used. Is that
correct or does it change with different size Sun, Planet and Ring gears?

--
Chris W
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I have been trying to figure out what the gear ratio for a planetary
gear is when the sun gear is held stationary and the input and output
are the Planet carrier and Ring gear. I think the reason I can't find
anything that talks about that specific scenario is because it is always
a 2 to 1 ratio and for that reason probably never be used. Is that
correct or does it change with different size Sun, Planet and Ring gears?

On Sun, 24 Feb 2008 11:55:44 -0600, Chris W <1qazse4@cox.net> wrote:

I have been trying to figure out what the gear ratio for a planetary
gear is when the sun gear is held stationary and the input and output
are the Planet carrier and Ring gear. I think the reason I can't find
anything that talks about that specific scenario is because it is always
a 2 to 1 ratio and for that reason probably never be used. Is that
correct or does it change with different size Sun, Planet and Ring gears?


Hmmm....it's been a while.
It's easy to show that that the planet will count twice as many ring
teeth as it counts sun teeth. And then the ratio starts looking like
2 X # sun teeth / # ring gear teeth. But the extra piece in the
formula that I have often forgotten is the 1 extra turn per
revolution as the planet carrier circles the sun. ,,,,But I am too
preoccupied at present to look up this formulation, which
features a "1 plus" in there somewhere....

Sorry

Brian W

I guess I wasn't clear in my question. In a planetary gear there are
three different parts of the gear that can rotate about the center of
the assembly, the sun gear, the ring gear and the planet carrier. In
order for the assembly to be useful, one of the three has to be held
stationary. So far the 2 replies have addressed the configuration where
the planet carrier is held, and the configuration where the ring gear is
held. What I want to know is what happens when the sun gear is held
stationary. From what I have read so far, my guess is that regardless
of the gears, that ratio will always be 2 to 1. Can anyone confirm that?

On Mon, 25 Feb 2008 01:57:48 -0600, Chris W <1qazse4@cox.net> wrote:

I guess I wasn't clear in my question. In a planetary gear there are
three different parts of the gear that can rotate about the center of
the assembly, the sun gear, the ring gear and the planet carrier. In
order for the assembly to be useful, one of the three has to be held
stationary. So far the 2 replies have addressed the configuration where
the planet carrier is held, and the configuration where the ring gear is
held. What I want to know is what happens when the sun gear is held
stationary. From what I have read so far, my guess is that regardless
of the gears, that ratio will always be 2 to 1. Can anyone confirm that?


Hi Chris, I pointed you to a URL with an animated diagram and formula
of the sun gear fixed case. Take a look.