Harmonic Drive

by emmett, published

Harmonic Drive by emmett Jan 5, 2014

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53379Views 4889Downloads Found in Engineering


This is a 94.5 : 1 gearbox that is entirely preassembled and requires no support to print. It is a type of harmonic drive, which uses a flexible output ring to attain a high gear reduction. My design is unique (as far as I know) in that it also incorporates a planetary gearset for further reduction. Instead of a toothed belt for the flexible coupling, I use a crinkled shape that forms involute gear teeth on both the inside and outside. The edges of the teeth taper down in order to keep the belt from stretching. The herringbone teeth keep all the gears from falling out.

You can see a description and demo here: http://youtu.be/vzc53n0ssik


Like many of my designs, this is a true torture test of both slicers and printers. I actually delayed finishing this design until MakerWare was able to slice it reasonably well. Still, it's not for the faint of heart. The tolerance needs to be very small to keep it from falling apart, but that often causes the gears to stick together. The flexible ring is weak and may try to break when unsticking the gears from it. Be patient and it may yet work.

You can also use Customizer to make different gear ratios and adjust the tolerance and width to make it print better on your printer.

UPDATE: Nevermind, you can't use Customizer, because they haven't updated to the most recent release of OpenSCAD, which means this design doesn't get built correctly. You'll just have to download the .scad file and adjust the parameters in OpenSCAD directly.

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Would this work if you added another planetary gear to make it 3? That would make it much more centered. Then the deformed shape of the flexible gear would be a triangle. Just curious if that somehow changes how it works.

Truly impressive ! Is it backdriveable ?

I don't see how this is printable without support. There seems to be a big overhang!
Can somebody explain?

This is a really cool model! Keep it up! :)

Really cool item :-)

Dude, seriously. You are f'ing nuts. Amazing.

For some reason, Skeinforge created a raft at the symmetry plane of the herringbone gear. I'm going to try some different slicing settings to see what's up.

Sorry, quite new to 3D printing, but is this something I could just take the .stl, put in Makerware and print out using standard settings? Thanks in advance! Looks amazing

Yes, that's what I did. However, you bot has to be in pretty tip-top shape to print this well, as it's pretty difficult. You might want to start with something easier like my gear bearing before trying this.

Nice work. I love the way the gears are captured

Great concept - I made one - a bit fragile at 1:1 scale, but I can see that when scaled up it would be more robust. Trade off might be ratio v the efficiency loss of energy that's required to continually distort the harmonic idler.

Awesome! I Printed one and used a drill to turn it, perhaps a bit fast, and the gears came apart and I could not put them back together. So, I printed another one, and this time, turned them slowly.

I get such a happily inspired feeling when I see this sort of thing, so thank you for that :)

This is art. Seriously, art.

Featured! Congrat's!

cool, so as far as i can understand the difference in the number of teeth between the outer gear and the flexible gear translates to a number of teeth that turn per revolution. kind of like when you tune a guitar string using another string, you can actually hear the rise and decay of sound due to the overlapping pitches.

Yes, exactly. And the sound analogy is quite apt, which is where this gearbox gets its name. What you're describing is often called a beat frequency, and it's the basis of both this gearbox and its ancestor, my differential planetary gearbox.

Remarkable, as usual.

You're printing harmonic drive mechanisms while the rest of us are printing earbud wraps, and gnomes.... impressive!! LOL

Damnit, I also have only printed earbud wraps and alike, you know us too well.

I can count to potato.

Cura was not up for the task of slicing this, it ended in disaster when I printed it:) The problem seems to be with the thin walled "harmonic" part. Tried exploding the assembly for just slicing this one part without success. Anyways, cool design!

You might want to check out the one illuminarty made. He got Cura to work by scaling it up 12.5%.

I'll check it out, thanks. Keep up the good work!

Beautiful work, as usual, emmett. You might be interested to know that the "Micro Harmonic Drive" manufactured by Micromotion GmbH also uses planetary gears as the wave generator.

Ah, thanks for the head's up. I figured someone must have thought of that first...

Awesome! I had been wondering if it was possible to print a harmonic drive gearbox, but I hadn't considered a design like this. I do wonder how long the flexible gear will last under power.

Nice work Emmett, it looks like about 60:1 in the video for some reason?

Did you seriously count the revolutions? I'm pretty sure I did the math right... Keep in mind the ratio is from the sun gear turning, not the planets revolving.

Ah, that explains it, I was counting the planets, don't ask me why! I automatically counted without thinking and wondered why I didn't get to 94, so counted again.

Awesome work; I think I'll probably end up printing it, but designing a bearing holder with herringbone sleeves, seeing as the side to side movement makes the gears kind of clack.

Crazy. Interesting how much the sun gear can move sideways while still being held in meshing with the other gears. Does the drive still function when the sun gear isn't centered?

Yeah, but then it orbits. If you want it to spin smoothly it should be centered, though it kind of centers naturally when spinning.

I will be bored of life the day I would not +1 your designs! :)

Nice work man!

Very nice, excellent that it is a printed all at once design also.

This will be my next print. Did you print in PLA? I wonder if nylon wouldn't be a really good application for this print.

Yes, this is standard PLA (not the special flexible stuff or anything). I would love to see how it works with Nylon. Definitely post a pic if you succeed.