Sun gear advice

Hopefully that worked and you will see a sun gear thingy of my design.
I need help - 1. designing a yoke/holder for the various planets i.e. how to measure the distance between planets depending on the number of planets etc. And 2. How to calculate the gearing/tooth numbers when using paired gears as in the second set of planets shown in the drawing

Thanx in advance Craig

I’m linking back to your original post - cuz you’ve got some links there that might help

For #2, a Google search for “Planetary Gear Ratio Calculator” comes with a a variety of links to help you calculate the ratios. In addition, you can look up Shigley’s Mechanical Engineering Design, which has all of the gearing calculations in it and is the standard for many ME training programs.

For #1 your frame design will depend on where you take out the output torque and where your input is. A simulator may be helpful to visualize the operation of the system as you have it designed and will help you determine what points to keep stationary and what points to keep fixed relative to each other.

I could do it for you, but my thought is that having it done for you doesn’t allow you to understand why.


If you ever figure it out then it would be awesome to see the final project! (and to re-create it, because I think the kids would love it!)

Give a man a fish… :smile:

Ordering the mechanical engineering book now. I have looked at several gear design calculators calculating a simple system is easy but when i go to the secondary planets as shown on the drawing that is a bit trickier. After that I want to play around with with gears are fixed and which are floating. So far my use of the Inkscape measuring tool has been unsatisfactory resulting in several misprints and attempts to measure with my caliper are also unsat. Buts that’s the challenge

By default Inkscape includes line width/thickness in its size calcs. The GF doesn’t - there’s no thickness to lines except the kerf and the GF runs down the middle of the line so you get half a kerf on either side of your part cut.

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Section 13.13 (5th edition) is the gear train section which will deal with what you need. It has been a while since I ran planetary calculations, but the online calculators should help you deal with the more complex parts once you know the theory behind it.
One thing to keep in mind is efficiency; you lose a little at every interaction, so if you are making a functional unit you have to run those calculations also. If it is just a show piece that won’t matter much.

Good luck!


I just found 9th edition online as a PDF:


By default.

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Wow, thank you all
I confess the engineering manual is way above my 70 year old brain skills although I think I once knew much of it
I did not find the bounding box where you suggested but am looking because I remember seeing it and not knowing what it did
Thanx again

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