Differential Planetary Gearset

by emmett, published

Differential Planetary Gearset by emmett Mar 8, 2013
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Now that you have a planetary gearset that functions as its own bearing, what else can you do with it? How about a large reduction in a small package: This drive gives a 91:1 gear ratio. It's much more convenient to print than the last version, and the only assembly required is placing the output ring onto the upper planets of the input.


The input is the sun gear, which has a hex opening in the bottom so you can turn it with a drill or attach it to something. The output is the whole upper ring, which has its own hex opening. You can customize this in many ways, and if you change the numbers of teeth, the resulting gear ratio will be printed on the side of the input ring (which is hopefully visible in the render).

The torque capability of this gearset is not amazing, though it could probably be improved by changing some of the parameters like thinkness and twist. If any of you come up with an application for it, I'd love to see it in action.

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Printed on Hephestos Prusa i3 with PLA; the first prints are a bit tight and despite working with many parameters in slic3r I did noit succeed yet; 2 of the cockwheels are nearly glued to the outer ring while the middle and the other 2 seem to be work fine

however downloaded the openSCad file and believe that changing the
// clearance between gear teeth
tol=0.15; to slightly higher value will do the job

thx for a intersting model to get kinds excited in the worl of mechanics and 3D

can be used in various turrets as friction of bearing prevents turning of turret due i.e. wind. same goes for various windows and door opening gearboxes. the more planets one uses the more friction becomes. one could even tweak attack angle to make it easier to rotate in one of directions ...

Hi there. Thanks for a great model! Looks very promising as part of a project I'm working on so this could be a great kickstart. However I wondered if I could ask a (possibly) dumb question. I've tried a couple of prints using the standard and fine resolutions on a makerbot replicator 2 (PLA) and am getting the same problem each time. 2 (opposite) planets are printing well seperate and snug, but the other 2 opposites are fusing to the outer ring. I've tried tinkering with the tol value and that does not seem to make much difference. Can you suggest anything I might try?

Assuming that you're using slic3r to slice it, try clicking the "split" button before slicing it.

I've just printed one, am I supposed to glue the reduction ring to the planetaries? When I spin the sun gear the planetaries don't transmit the torque to the reduction ring, they just spin inside it.

Hi Emmett, Another great model - I have made a couple, one I have scaled down by 0.8 and made into a mount for a nema 23 motor. On the whole they run pretty well, although the printer I am using uses support material and where the gears have a collar on them it puts support material up to this - which is fine apart from in between the gears and the outer ring where the tolerance is such it leaves imperfections which ultimately cause the gears to go stiff at this point.
My question is do the collars need to be there? will it pull the drive wheel tight if removed or just wear the center pin?
I have printed loads of your other custom planetary gears without the collar and they run absolutely perfect..
Thanks, Lewis

The collars are just there to support the upper gear teeth, which aren't aligned with the lower ones. It works great with no support and a slicer that can handle bridging. In your case it may be doing more harm than good. You're welcome to tweak the scad files to suit your needs.

I must be doing something wrong. When I slice it using Slic3r 0.9.10b tells me there is a hole near one of the edges, the middle gear, plus two of the other gears fail to render in the g-code preview. I tried the STL files you have posted and I also tried to change the clearance, number of teeth and a few other variables in OpenSCAD, Rending there looks fine, but no matter what I seem to use it will not slice correctly. Anyone have any ideas?

Hmm, not sure why it thinks it's nonmanifold, but my recommendation would be to run it through netfabb's cloud service. That usually cleans up errors of that kind.

Thanks - I'm pretty new to this and I didn't know about netfabb, but I did find a solution. I downloaded Slic3r (the 64-bit version) and created the gcode from the STL directly in it instead of loading the STL with Repetier-Host first. Then I loaded the gcode in Repetier-Host and it's printing as I type this.

Hi, thanks for this file, it can be very usefull, but in your scad file your using the write.scad file and there is any link to where find it. Could you help me ?
If what i intend to do work I'll glad to show it in action. :)

Sorry you got the shaft. You should've won the contest! The music box was too difficult to understand how to print it out so as to play a melody of your choice. Your designs are always difficult to design but very easy to print out.

Rookie question here. Could this be used to replace the compound gear train / box in a dynamo generator? Increase rotation instead of decrease?

In an ideal world, yes. However, for gearing up you really need minimal friction, which will be hard with printed parts anyway, but probably even worse here since it has no planet carrier. I bet you could make a derivative that would work though, especially if the ratio wasn't too high.

This is very similar to the split annulus ring design that is implemented in the Brabeast Fan module http://www.thingiverse.com/thing:56542http://www.thingiverse.com/thi..., except the ring gears have the different number of teeth rather than the planet gear halves. http://thingiverse-production.s3.amazonaws.com/assets/96/bf/38/7d/a6/Render_3.pnghttp://thingiverse-production.....

Planetary Fan Module for Brabeast
by Gyrobot

Yes, it's the same idea, though technically it needs to have fewer teeth on both the planets and the rings of the upper stage to mesh properly, but you can get away with a lot if the parts are made to be a bit loose.

Yes, and the diameter should also decrease to keep the gear module correct, but then everything gets mess(h)y.

Yep, and that's what we have here. It's just a little math...

Ou, this was on your sleeve. Nice!

And you've done it again Emmet. Maybe this thing could work in drill powered dust buster. Or maybe flip it int he other direction and use it to smooth out the motion on a stop motion camera panning rig.