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Hypocycloidal gear with timing belts?

A project log for Strain Wave Gear with Timing Belts

Strain wave or harmonic drive gears are cool. As a cheap analog for metal or flex filament splined cup, try using timing belts.

Simon MerrettSimon Merrett 01/26/2018 at 03:429 Comments

Is this the next logical step? I was surprised by the torque efficiency of the hypocycloidal gear I made in the Hesitation, Repetition and Deviation log, so perhaps it's worth pursuing for a bit. Until today, in conversation with @Dan Royer I hadn't realised that a possible advantage of these hypocycloidal and strain wave gears is that when you want to DIY one you can design it so the output and fixed part are transferring torque relatively far from the axis compared to  a gear with a conventional shaft output. 

The setup above is just a concept tester, made with a 210mm (70 tooth) HTD 3M continuous belt, constrained within a printed PLA ring that has a toothed recess in the base to hold the belt in place and prevent it from rotating, relative to the outer ring. The back face of the outer ring is bolted to the NEMA 17 stepper motor housing in the usual four holes.

The belt teeth take the place of the ring pins in a conventional hypocycloidal gear. The cycloidal disc has 69 teeth, hence this is a 69:1 gear ratio.

This could be scaled up to a 100:1 with a belt length of 303mm and a diameter around 100mm. 100:1 would, if the components handle the forces, get the baseline NEMA 17 with 59 Ncm holding torque to the order of 20 Nm being targeted by #5+ Axis Robot Arm Study No.5. If the parts can't take it (which I imagine they won't) we could step up to HTD 5M and the diameter of the belt would be around 160mm.

Promise I'll get round to showing the strain wave gears soon. Let me know what you think of this approach in the comments.

Discussions

marc.janod wrote 05/18/2021 at 12:34 point

Hi Simon. great work!

Is there any way to test your last prototype ?

Are you ok sharing your 3d files ?

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Dan Royer wrote 01/26/2018 at 18:09 point

Nice!

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Paul Crouch wrote 01/26/2018 at 14:02 point

[oops, finger trouble!] I'm following with great interest as this has potential uses in my bot, which I'm yet to decide upon an actuation method. I'll hopefully be doing some more PAM testing over the weekend. Great work!

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Simon Merrett wrote 01/26/2018 at 19:57 point

Wheeled legs like Boston Dynamics' Handle robot? 

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Paul Crouch wrote 01/26/2018 at 21:49 point

Yep! :-)

Also seen in a lot of "mech" art and concepts.

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Simon Merrett wrote 01/26/2018 at 14:09 point

Ps, I love the concept of whegs. If you haven't seen it, this design had promise but sadly stalled  https://youtu.be/GqLrCPbBlT4

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Paul Crouch wrote 01/26/2018 at 14:23 point

Yeah, I follow robotic news quite closely. I’m actually mis-using the name I think. Mine will have wheeled-legs rather than having leg-wheels, but it is a convenient term.

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Simon Merrett wrote 01/26/2018 at 12:54 point

What's a PAM test? Good luck! Just looked at your #Droidbot 7 - Undefinable Whegged Robot and you may be interested in a geared BLDC I found and mentioned to @Dan Royer (https://hackaday.io/project/945-5-axis-robot-arm/discussion-99982)

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Paul Crouch wrote 01/26/2018 at 14:01 point

Thank you. PAM: pneumatic artificial muscle. I made my first in 2007 and keep coming back to them but they're not the easiest things to work with. I'm trying to get good reliable results so I know what I'm doing with Droidbot. I did have a quick look at those motors, have made note...

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