Finding the right Gear Box

So I am currently making a exoskeleton that is just for one arm and i am in need of a planetary gearbox. My current needs for the gear box is for it to have a 40:1 gear ratio, as flat as possible because I want it to be directly on the joint of the exoskeleton. I’m also considering making my own gearbox with gears bought on Mcmaster Carr so i can make it specific to my needs. With the gearbox that i have designed the ring gears rotate while the planet gears are stationary, while as the ones i have found online, only the planet gears rotate, and if the gearbox has the ring gear rotating. it is only one stage. If any one can help i can give more detail or better description.
Current Design is about 83.5mm wide and 120mm in diameter, not including motor.

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Well you CAN have multiple ring gears rotating with the planetarys in place, but it is VERY difficult, hence no one does it.

Is there a reason the ring has to be the mover? From the drawing you provided I’m failing to see the advantage over a “traditional” planetary gearbox.

edit:
I see flatness is a concern for you. Planetary may not be well suited for you. Each layer adds thickness and gears can’t just be flattened or you will start stripping the teeth.

Have you look at cycloidal drives? or strain wave (Harmonic drive is the brand name)

edit again:
Welcome to CD!

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I guess i did not do a good job at researching because those look like they are perfect for what i need. This was very helpful, thank you.

For some background research, you can see Dr. Yoon Jeong’s PhD paper from UC Berkeley under Dr. Kazeroon*i:

*I’m biased here as I had the fortune of working under Yoon as an undergrad, but if you follow that rabbit hole it’ll surely lead you to other research as well.

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Food for thought. I have no idea if these are anywhere close to your power / torque requirements. But they are low profile:

Snow Blower Motor

Seat Motor

There is a strong possibility that a robotics team near you will be willing to donate either of these motors to your project.

You can do it with 2 stages. (we did a 401:1 that way. Sun1 is input Planets 1 and 2 are on the same planetcarrier and connected to each other. (We printed them a 1) . Ring2 is the output. Sun2 is just going for a ride and helps keep things stable - probably not needed if you don’t go plastics. Motor we used was a CIM. The 401:1 on partially worked as we had problems lining it up as there was no room for the fingers. You might also consider a harmonic or cycloidal one for a ratio like that The cycloidal should give you a nice flat surface - you will have to make the cycloid yourself though most likely.
gearing40.xls (9.5 KB)
You can play with gear sizes in that spread sheet until you get a desired reduction you can also see the math in the cells. Important thing is that stage 1 and 2 has the same number of planets and they are bolted/glued/welded together. So you might want to watch positioning or make it so that they are in sync (both ring and sun give an integer when divided by # of planets That would make it easier when you glue/weld/bolt the planet pairs together - unless you print them then you can print them as 1

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