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• # Help needed - Planetary gear

Discussion in 'Calculations' started by OLarsen, Apr 22, 2015.

1. ### OLarsenNew Member

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Apr 2015
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Hi

I was wondering if anybody could help me figure out some gears I have to make.
I have never worked with gears before and I am hopelessly stuck.

I have attached a simple drawing to explain what I need.

I am constructing a planetary gear where gear A will be the driving gear.
Gear A needs to be turned 80 complete revolutions for gear D to do 1 complete revolution.

http://postimg.org/image/oaeytu6wh/

Is it possible to do this just using A-B + C-D exchange? (C+D preferably in one part)

Like I said, I have never worked with or learnt about gears, so I am completely lost with the math on this.

Could anybody help me figure out the number of teeth + ratio between the exchanges? If it is not possible, adding another exchange might be possible.

I really hope somebody could point me in the right direction with this.

2.
3. ### LochnagarWell-Known Member

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Hi Larsen,

You have indicated this question is about planetary gears - but the picture you have posted does not look much like a planetary gear system - instead it looks more like a multi-shafted gear reduction system.

However, I have included a link below - that you might find helpful - since it explains the four basic components of a planetary gear system (assuming it is a planetary gear system you are looking for);
1. The sun gear
2. The planet gear
3. The ring gear
4. The planet gear carrier
Included in the link is the computation of the gear ratio. As you may be aware, you can have multi-stage planetary gear boxes too.

https://woodgears.ca/gear/planetary.html

Hope this helps.

4. ### K.I.S.S.Well-Known Member

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Lochnagar,
A good link to a very nice article - thanks. I'm in the somewhat indeterminate process of designing my own watch (which will use a visible planetary arrangement as part of the final drive). Maybe I'll find the time to complete it in the next life...

5. ### JefflucasartMember

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Oct 2014
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Planetary Gearing

Larson,
if I am interpreting your drawing right, with the BC shaft just rotating and not orbiting and D just showing a cross section of just one side of the outer ring gear, then the solution is not too tough. If A has 80 T (teeth), B could have 320 T, C could have 15 T, and D would then have 300 T. B would turn one fourth of A and D would turn one twentieth of BC; one twentieth of one fourth being one eightieth (80:1).
Things only get complicated if you start orbiting the BC shaft.
Number of teeth can vary just as long as the ratios stay the same or the resulting combination of ratios sums up to the same. Could for example have one 16th of one 5th, but for spur on spur it is probably better to stay around 4:1 or less.
That said, you can increase the spur gear ratio if you don't have to transmit high torques because then you can have smaller teeth, which means more teeth per diameter and your mesh angles don't become too severe. For example, one might have ratios that produce one tenth of one eighth if your torque is light.

Last edited: Apr 29, 2015

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