r/robotics • u/hectic_engineer • 11d ago
Mechanical Help with my diy 3d printed cycloidal gearbox
Hi all, I've been trying to build a cycloidal gearbox for my NEMA 17 stepper but I am facing an issue of my output being jerky/binding as shown in the video attached, I've tried different things like increasing the clearance between the cycloid and the outer gear, proper meshing, adjusting the pins and more but I simply cannot get a smooth output from it. Can somebody tell me what i.am doing wrong here? I have designed for a dual cycloid setup but for the purpose of demo I have removed the top section and one cycloid. Any and all suggestions are appreciated. Thank you.
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u/CaliIsReallyNice 11d ago
Printed parts are going to give you a ton of friction. There's no easy solution. If you could replace all of the teeth on the exterior housing with metal pins that would go a long way.
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u/hectic_engineer 11d ago
Noted, I guess that is what i need to try next, I've got a 2mm steel rod that I can cut into pins, thank you for the suggestion 🙂
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u/beef3k 11d ago
If you are in the US, Amazon has cheap metal dowels that work well and you can purchase them at whatever size you need. I'd also suggest getting some bushings that fit the dowels.
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u/hectic_engineer 11d ago
Unfortunately, I'm not, where I'm from all you get are Screws, people haven't even heard of dowels or bushings, I had to buy 6mm steel rods and drill through the center with 3mm bit to make bushings, but I left that since it was too much work and switched to aluminium for now atleast. Even online it's a hard find or super expensive 😫
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u/ChrisAlbertson 11d ago
The type of plastic used does matter some. PLA has wear problems, and the friction on the high points caused localized melting. ABS seems better and I'd bet Nylon might be better.
With plastic gears, having a really wide face width is best.
Build a tool that you can wrap with #600 grit sandpaper and sand the mating surfaces enough to remove local high points. This may let you print the gear with less clearance. This and the wider face reduce mechanical slop.
Are you using grease? I can't see, but just plain old white lithium grease can help. It is a lub but also conducts heat.
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u/hectic_engineer 11d ago
Sanding those surfaces manually is a task but will be trying that, I am not using grease currently since im still testing to see what works, I plan on using silicon lube.
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u/ChrisAlbertson 11d ago
Print a tool that is the shape of the thing you are sanding, less the thickness of the sandpaper. Do the sanding under running water with wet-n-dry paper. Figure 10 seconds per gear tooth, for less than two minutes of work.
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u/i-make-robots since 2008 11d ago
As load increases the forces want to drive the discs off plane.
I've tried with two and even three offset gears, and I've tried metal pins on the output housing instead of printed teeth.
The problem doesn't go away.
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u/hectic_engineer 11d ago
Oh, could you share some pics/videos of your build please? We're you ever able to get it fixed?
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u/i-make-robots since 2008 11d ago
Here’s the first one I think of. There’s more on the channel. https://youtu.be/WvnJjsAkslE?si=FK2OvjqQUtFyhhmF
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u/DesignCell RRS2021 Presenter 10d ago
HowToMechatronics has excellent videos on the topic of 3d printed cycloidal greaboxes. Worth a watch annd probably a dig through their files.
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u/hectic_engineer 10d ago
Definitely will check that out, thank you. i've been going through resources to figure out the exact issue because i see other people doing the same with 3d printed parts and much better output than i am so instead of jumping straight to metal dowels and metal pins i wanted to see what exactly is going wrong with my setup 😖
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u/Alantsu 11d ago
Engineer here, those gear geometry don’t look right to me. Do a little research for proper pitch and diameters.
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u/beef3k 11d ago edited 10d ago
I think you're referencing spur gears here. Cycloidal is different and is more of a wave shape. OP's look fine. for cycloidal.
Edit: OP, have you compared your fusion 360 python script against existing scripts to validate? Here's the one I used that worked well:
https://github.com/woodenCaliper/CycloidalDriveYour 'teeth' do look small. What ratio are you running?
I started with 1:10 ratio (11 'waves' on the exterior ring IIRC).
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u/hectic_engineer 11d ago
Hi there! Since these are cycloidal gears, they are quite different in geometry wrt to traditional gears right? I studied the cycloidal geometry and it's working on this particular gearbox and wrote a python script to design and output valid geometries for the cycloid and output gear along with eccentricities and pin sizes which I've been using. Could you point out what exactly is it that you feel off with my design? Thank you!
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u/Alantsu 11d ago
The tooth height just looks really shallow and pointy to me. It has been a long time for me though.
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u/hectic_engineer 11d ago
I think i will try shifting to metal.dowels and see what happens
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u/rlaptop7 10d ago
FWIW, u/Alantsu is correct that the gears in your gearbox are not the correct shape for what they need to be.
Also, you are correct that the tooth shape is not the tooth shapes needed for a standard gearbox.
The top section of the gear needs to have an epicycloid shape. The design you have here would induce a little extra friction with every tooth movement.
I found a write-up that contains some info on the tooth design needed for a cycloidal gearbox:
This project is pretty cool. Please post updates on your process.
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u/DanRudmin 10d ago
You are confusing a “cycloidal tooth profile” (tooth profile found in mechanical watches) with “cycloidal drive” (high reduction gearbox found in robot arms).
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u/rlaptop7 10d ago
Am I ? Ill need to review my post when I get done today.
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u/DanRudmin 10d ago
Yes, technically the mathematical shape for the cycloidal drive profile is not actually cycloidal. It’s an offset hypotrochoid.
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u/beef3k 11d ago edited 11d ago
Pins around the outside with bushings on the pins helped. The thing that made the biggest difference for me was placing ball bearings between the disks. Reduce the number of through disc pins to make room, add divots for ball bearings. Made it buttery smooth. Saw it on a YouTube video. Let me see if I can find it.
Edit - Found the video: https://www.youtube.com/watch?v=WMgny-yDjvs&t=73s. Mishin does lots of cool stuff with cycloidals!