High Reduction Ratio Planetary Drive

by mkeveney, published

High Reduction Ratio Planetary Drive by mkeveney Sep 17, 2012

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63718Views 12809Downloads Found in Engineering


This model illustrates an interesting planetary drive variation that produces a very low gear ratio.

Click the video link to see it in action:



The video shows how it works and includes some construction tips.

You'll probably want to make the Tube cutting tool as well:


I started designing this in metric units, but was only able to find brass tubing in imperial sizes, so I simply used both measurement systems. If you live in a metric part of the world, you should be able to adapt to locally available materials by drilling out the bearing holes as necessary

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How is the 16:1 part calculated?

The video (@0.38) only appears to show the sun, planet and annular stage and does not show the algorithm for calculating the final stage.

Feb 9, 2017 - Modified Feb 9, 2017
mkeveney - in reply to bmcdanel

Good question. I don't have the generalized formula handy; I'll post if I can find it. Can anyone point us to an online reference?

Meanwhile, perhaps my informal explanation will help: In this model, the fixed ring is 30 and the driven ring is 32. In one revolution of the carrier, 30 teeth of the outer gear are carried around. Since the gear has 32 teeth, it's carried forward by two teeth, or 1/16th of a turn.

Both rings must be a multiple of the number of planets, (two in this case) since they must mesh with all planets at once, assuming the planets are evenly arranged about the center. Also, as fma pointed out below, this mechanism only works due to a bit of deliberate 'slop' in the gear train, allowing the same planets to mesh with rings of different tooth counts. It would not work if the tooth count difference was more than a few teeth. Emmett's version here http://www.thingiverse.com/thing:7390, is a more flexible design.

1:246 Gearbox
by emmett

Does this kind of gearing reduction result in the same increase in torque as other reduction drive methods?
What size brass tube do you use in this model, I want to find somewhere online to order it from.

Does this kind of gearing reduction result in the same increase in torque as other reduction drive methods?

Yes, though 3D printed parts have their limits, of course.

What size brass tube do you use in this model, I want to find somewhere online to order it from.

Sizes are in the fifth image posted above. I used K&S brand tubing, available at good hardware stores in the US. Also online hobby dealers. Their site: http://www.ksmetals.com/

Comments deleted.

Perhaps I am missing it, but what is the input:output ratio? Thanks!

64:1. The Video explains it.

Thanks. I was viewing via my mobile and the video was not playing. sorry for that

Vraiment super,
Moi qui aime bricoler après impression, j'ai aimé le coupe tube aussi!
Super conception.

Is it possible to rotate yellow gear to speed up sun gear ? Please help ! If not can you suggest me another lower ratio which I can try . Thanks a lot in advance

I think there would be too much friction to drive it backwards. A more conventional gear train might be a better bet

Thank you !

could this be set up with a 1:60 ratio? It could make for a pretty awesome clock gear setup

By changing the fixed ring to 28 teeth and the output ring to 30, this mechanism gives a 60:1 ratio. With 3D printed gears, I think there will be far too much friction to drive it 'backwards' as clocks typically do.

Look up the 'Strutt Epicyclic' clock. I'm not sure if that one is exactly what you were thinking, but it's interesting nonetheless.

Hello, Im trying to work on this on CAD to make some minor changes for one of my designs. Was wondering if you knew why the DXF scaling is messed up? the ring gears are about 6' in diameter.

Welcome to the DXF "standard." It seems that no two CAD programs produce perfectly compatible files.

In this case, I think I designed that piece in millimeters; your CAD program probably assumed each unit was an inch; thus my 70 mm piece becomes about 6 feet.

See if your CAD program has any options for 'units' when importing. If not, simply scale the drawing down after importing. As long as you scale proportionally, you ought to be OK.

Also, note that the DXFs only have 2D outlines--I only included them later, at someone else's request. You'll have to do quite a bit more work to reproduce the 3D models. Unfortunately, I designed this thing a long time back, using a trial version of Autodesk Inventor, I think. I no longer have the original source files; just the STLs and DXF files you see here.

Good luck!

I figured it out. I'm self taught on CAD so i had to scale the image to my size. the DXF file was a huge help. Thank you so much

Nov 24, 2014 - Modified Nov 24, 2014

I just found a terrific animation that very clearly illustrates the principle of this mechanism:


See the fourth step in the explanation, particularly.


this is a very interesting mechanism. i would like to try it. can u just tell me that how u calculated that second speed reduction, i.e. 16:1 . Em not getting it. Please help me out. Thank you!!!

Any chance of also posting DXF file output from the stl program for us poor laser cutter folks?   I believe this could also be fab'd using acrylic or even wood...  thx, GMT   Great video on utube BTW !!!

I used a few different tools to develop the models.  I think I can get the gear profiles and perhaps a few basic outlines exported to DXF, but you'll have to take it from there.

I'll try to get something posted this weekend.

 That would be terrific... I do know that it can't be exact but with enough of the basic gearing done to dxf.. then others can re-create the "extra stuff" thx

Okay. Profiles.dxf has the same info as the drawing illustrating hole sizes.  I think that ought to get you off to a good start
Let us know what you come up with!

 thx a bunch... will let you know GMT

I thought I'd add my comments and video here, too. The mechanism runs pretty smoothly. I had to do a bit of filing to get it to fit together, and despite your warning I ended up snapping one of the dovetails trying to remove it after a too-tight test fit. I used a bit of superglue to fix the break, and also to secure the handle. I found it easier to line up and insert the little bushings by sticking them on a small round detail file to get them started straight.

Here's my video: http://youtu.be/0i59XkmR7y8http://youtu.be/0i59XkmR7y8 Thanks for the great design! I think the kids' classmates will have a good time figuring this out!

Thanks for the update.  Love the video!

Well, I printed one. It turns out that the last hobby shop in the area (that doesn't just deal in RC stuff) closed down a couple years ago, and no one else seems to have the brass tube. :/ So I ordered them online. Hopefully by next weekend I'll be able to assemble it, and I'll post a picture when I do... 

Sep 21, 2012 - Modified Jan 2, 2016
mkeveney - in reply to CodeCreations

Ah, sorry you had to resort to that.  If anyone else has the same problem, be sure to call the hobby shops, craft stores and hardware stores... the RC hobby shops used to be your best bet, but I guess now it's all prefab.  Ask if they have a "K&S Metal Center" rack.  It's not terribly hard to find at least in the US:

I've heard of this being called a differential planetary gear set. You can find my parametric version from last year here: http://www.thingiverse.com/thing:7390http://www.thingiverse.com/thi.... It was just a proof of concept; making a desktop model is a good idea.

1:246 Gearbox
by emmett

Very nice! I decided to make mine in part because I hadn't found an example on Thingiverse. Thanks for pointing yours out!

Awesome! I'm going to try it -- seems like it would be a great model to bring into the kids' school, too. Thanks for the design and the very informative video!

Very Nice, especially the documentation! Keep up the good work!

very impressive. I am starting to print it NOW.

Beautiful !!!

I like the way the things are put together (and choice of colors) and I do love the dial !!

Ah, good to hear from you aubenc. Your 'tiny planetary gear' set is one of the things that inspired me to make this one!

Interesting concept. But this means that the module of the second ring does not exactly match the sun module, does it? It guess it works fine with plastic gears, but it would not work as well with metal gears (ar at least with a lot of friction)... In fact, this is a mix between a standard planetary gears and harmonic drive ;o)

Yes, strictly speaking, the planet gears should have a step in the middle, with a slightly different diameter where they intersect the driven ring, and the sun gear should only intersect the half that meshes with the stationary ring. In this model, the difference is well below the manufacturing tolerance of the gears.

Yes, it does function much like the harmonic drive. I'm not sure whether this drive has a proper name, but it's been around a long time. I first discovered it in a hobby-motor reduction drive when I was a kid.

Very nice :)

I think "Split annulus, compound planet, epicyclic gears" might be correct: http://en.wikipedia.org/wiki/Epicyclic_gearing#Galleryhttp://en.wikipedia.org/wiki/E...
(Although your planets aren't actually compound due to the tolerance as you mentioned)

Ah, yes that does look like it. I missed it on my first read of that page. Thanks for pointing it out!