1:246 Gearbox

by emmett, published

1:246 Gearbox by emmett Mar 28, 2011
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38316Views 6980Downloads Found in Engineering


Have you ever wanted to turn a CD-ROM motor into a winch? I decided to see how much gear reduction could be fit into a small space, so I came up with this differential planetary arrangement. This is a well-known design, but now you can use the parameterized OpenSCAD version to customize this to your needs and include it in your mechanical designs.

The version shown here is less than 60mm in diameter and 13mm thick, but achieves a 1:246 gear reduction, all with teeth printed on a TOM. The input is the small shaft to the sun gear, the output is the smaller ring gear, and the larger ring gear is to be held fixed. If you stacked two of these units you would have a ratio of 1:60516!


Print one each of the three STLs. The sun gear's shaft hangs though the hole in the larger ring gear. The three planets are placed around the sun gear (they have one more tooth on the bottom than the top), and their orientation is important. Each planet has exactly one place where the tooth on top is exactly above a tooth on the bottom. These must all point the same direction when the gears are assembled (i.e. along the x-direction). The way fourgears.stl is made, the planets are all lined up like this, so the fill should help with the orientation (make the fill lines parallel). Finally, place the smaller ring gear on top. It should slide right on if the planets are oriented properly.

The OpenSCAD file is commented and parameterized for easy tweaking. It will output the gear ratio you get based on the number of teeth you use. It is also set up to visualize through animation: just compile the assembly, then select view->animate. I'd recommend setting steps to more than 2000 or else it will be jerky. This file makes use of Greg's involute gear script, and it's called through the MCAD library.

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Hi could you please post the original files for editing?

Comments deleted.

anyone else having trouble slicing this with slic3r? The planets/sun combination doesn't seem to slice too well.

Might the different planetary teeth and of course ring and sun gears be converted

to DXF for output to a laser cutter? The planets could then be glued together ???

thanks - hope you can post.

Yes, OpenSCAD can output DXFs, so you can use the scad file to make whatever planetary arrangement you like. I don't have access to a laser cutter, so I'll leave the details to you.

How'd you drive the brushless DC motor?

You mean the CD-ROM motor? I was planning to hook it up like http://www.thingiverse.com/thing:4881http://www.thingiverse.com/thi..., but perhaps that only works with older ones.

CD-Rom Motor Pump
by Skimbal

I made a little spreadsheet that helped me find the right parameters to make the reduction ratio 1:60, and by stacking two of these between an escapement and a power source, I think we can make a really compact and solid printable clock.

I made a little animated gif of the 1:60 gearbox running thru one revolution of the input (say, 60 seconds) with two hands attached to the input and output. :-)

Beware when opening this and generating the gcode in replicatorg. emmett...in openscad you should fix your z height so that it doesn't place the 2 rings below the build platform. I just ruined a belt because I didn't click "put on platform". So just a warning to anyone else printing the two rings...make sure you generate the gcode with it on the build platform.

Oh no, I'm sorry! I always have the "bottom" module activated in SF, which does the same thing as "put on platform", so I never thought about the z-height of the STL. I hope someone puts a height safety check in RepG one of these days...

In RepG 25 they will add that feature. ;)

I love and want to use it.

what are the number of teeth? I count 41 on the upper ring and 42 the lower.

I count 17 on the upper planet - but fail on the lower planet: 16 or 18?

and how is the gear-ratio calculated: R1/R2*P2/P1??

Yes, and the lower planet has 18. Look at the SCAD file (it's plain text) and you'll see how the numbers of teeth are related and also the formula for the gear ratio (which is actually a bit more complex than your formula).

Which numbered ring is the larger and which the smaller?

Ring1 is larger and Ring2 is smaller.

This is really cool. I really need to make some gears for a project I am working on and am not very good at openscad. I was wondering if someone could help me out by answering a few questions?

  1. The script keeps looking for a gear module that I obviously don't have. I downloaded GregFrost's involute gear script, I have it in the openscad directory as well as the file for this 'thing'. It still can't find the gear module. What do I have to do to get it to reference greg front involute g
    ear script's gear module?

  2. This planetary gearbox as it is, is almost perfect. I just need it a little smaller. I am willing to sacrifice gear ratio or whatever I would need to do to achieve a planetary gear box about half the size of this one. Is this possible to print on a ToM? All I am trying
    to do is slow the release of energy from a clock spring I have in a printable wind up case with key that I have already created. I need the gearbox to be smaller to fit in the device I am making. I plan to post the whole thing when I am done, it is kind of neat too!


Ok scratch the 1st question. I figured it out. I downloaded the most recent version of the involute gear script which had a wildly different filename. I've figured that much out.

Trying to work on some smaller gear boxes now with the script.

Any pointers or tips for what I am trying to do would still be much appreciated! :]

Thanks again!

To make it half the size, just make d1 half as big. To get printable teeth, you'll want to reduce their numbers as well. 6 teeth is probably about the minimum on a gear, so I'd reduce np1 until the teeth look like a reasonable size to you. You can also change the various thicknesses (t, t1, s, td), though I'd recommend setting them all to multiples of your layer thickness to make sure SF doesn't do something stupid.

Sweet! :)

We can now print "Machine With Concrete" as an existential reminder of our mortal coil:


More Arthur Ganson:


Instead of a concrete block it can be attached to the 3D printer itself!

Very nice demo piece. I printed it on a Thing-O-Matic. I had to scale the 4 gears down to 95% to get them to fit.

Yeah, I imagine different printer profiles will need to adjust the tolerance a little. The 'real' way is to change the backlash parameter in the OpenSCAD code, but I'm sure these gears probably won't notice a 5% change in circular pitch. Nice print.

How should backlash be changed? bigger? smaller?

If the gears are too tight to spin freely, increase backlash; if they're too loose, decrease it. 0.1 mm can actually make quite a bit of difference.


Totally sweet.