High precision speed reducer using rope (capstan drive)

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Alan Timm

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Jun 13, 2024, 3:58:31 PM6/13/24
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A really great introduction to designing and using a rope-based gear reduction also known as the capstan drive.  You may have already seen this in use during Christoph Kohstall's March presentation on his Kind Humanoid project.

https://www.youtube.com/watch?v=MwIBTbumd1Q

Test+Stand.jpg




Sergei G

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Jun 13, 2024, 8:46:19 PM6/13/24
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Does this one qualify? 😉

BTW, why "ropes"? using steel cables makes better sense. Local Home Depot carries some. As for reliability - mine are working mercilessly since May 2020, and I had only one failed part (which I then reprinted with higher fill).


And a linear version:

Thanks for the video link, Alan - that's a real nice actuator they made. 

Best Regards,
-- Sergei

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Chris Albertson

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Jun 13, 2024, 9:18:44 PM6/13/24
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Did you watch the video?  The guy who made it tried steel cables and they failed badly.  I think he already knew they would not work but wanted to show what would happen and then shows us how to select the bast cable.   

The reason steel fails is related to the reason he does not get the same “gear ratio” as the basic geometry would predict.     Cables do not bend like beams.  Beams are roughly symmetric and compress one side and put the other side in tension.   Cables are VERY poor at compression. and reduce their length when bent.

  Actually the guy is a mechanical engineering student and knows the math. It has to do with the fact the steel cables don't have the required flexibility.




On Jun 13, 2024, at 5:46 PM, Sergei G <msg...@hotmail.com> wrote:

Does this one qualify? 😉

BTW, why "ropes"? using steel cables makes better sense. Local Home Depot carries some. As for reliability - mine are working mercilessly since May 2020, and I had only one failed part (which I then reprinted with higher fill).

<image.png>

And a linear version:

Thanks for the video link, Alan - that's a real nice actuator they made. 

Best Regards,
-- Sergei

From: rssc...@googlegroups.com <rssc...@googlegroups.com> on behalf of Alan Timm <gest...@gmail.com>
Sent: Thursday, June 13, 2024 2:58 PM
To: RSSC-List <rssc...@googlegroups.com>
Subject: [RSSC-List] High precision speed reducer using rope (capstan drive)
 
A really great introduction to designing and using a rope-based gear reduction also known as the capstan drive.  You may have already seen this in use during Christoph Kohstall's March presentation on his Kind Humanoid project.

https://www.youtube.com/watch?v=MwIBTbumd1Q

<Test+Stand.jpg>




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Sergei G

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Jun 15, 2024, 12:05:12 PM6/15/24
to Chris Albertson, gestalt73, RSSC-list
"The guy" (actually, his name is Aaed Musa) is a talented youtuber, and his task was to demonstrate the benefits of the ropes over the steel cable, which he did by taking a too thick cable and making it fail in about 3600*4 cycles. Someone could take the rope and make it fail under time, UV, oil residue and abrasive dust. There is also some effect of rope flattening that might cause problems. Rope has its place, no judgement here.

For us, mortals, using steel cable over unobtanium European rope is just a more practical choice, even if it requires replacing the cable every 10 years. I've done it and actually that worked for me quite well.

As for the student's math knowledge - I'd give him that, we all been there and done that when graduating. His way of emphasizing it with mesmerizing talking hands and flashing math equations on the screen was truly genius. So sad I do not very easily submit to hypnosis... 

Overall, let's wish Aaed Musa all the best and subscribe to his channel. Let's enjoy the achievements of our fellow roboticists together!

Best Regards,
-- Sergei

"In theory, there is no difference between theory and practice; but in practice, there is."

From: Chris Albertson <alberts...@gmail.com>
Sent: Thursday, June 13, 2024 8:18 PM
To: Sergei G <msg...@hotmail.com>
Cc: gestalt73 <gest...@gmail.com>; RSSC-list <rssc...@googlegroups.com>
Subject: Re: [RSSC-List] High precision speed reducer using rope (capstan drive)
 

Chris Albertson

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Jun 26, 2024, 1:57:13 PM6/26/24
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For us, mortals, using steel cable over unobtanium European rope is just a more practical choice,

Steel will fail very quickly, as shown in the video,   The reason is the compression on the “back” side of the cable that faces the drum.  If you MUST use metal, then wire would be a better choice.  OK you can make thr steel last longer by using much larger diameter drums.   But in a real robot you want the smalled drums you can make.   Dyneema or Kevlar is what you want.  But Dyneema also has VERY low friction, almost like Teflon.  In fact it is so slipperly you will not be able to make normal knows hold, that will slide apart

The stuff he used is easy to get.   You can buy it at any descent marine hardware store like West Marine.  You can also buy it from Amazon. But the best deal is at REI.  They offer 1mm diameter by 50 feet for about $10.   But this stuff is of “core/mantle" construction.  Which means the core is parallel Dyneema and the cover is an abraision resistant poly.   It is very easy to remove the cover and pull the core out.   The cover adds no strength and is just there for handling.

In sailbot racing removing the cover from the section of rope that is in free air and don’t need to be abraision resistant is a common thing.   It lightens the rope and improves the aerodynamics.    

I can walk to a  West Marine store and get this stuff but if you don’t live near the ocean there are no marine hardware stores, REI has it cheap enough and will ship it to you.



Sergei G

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Jun 26, 2024, 2:19:23 PM6/26/24
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Good info, Chris - thanks! Pretty affordable at ~$10 for 50 ft, 1mm and 1.8mm thickness available.

REI doesn't seem to have it in store here, $60 order minimum for free shipment to home, or free to the store. There are plenty of "river/lake marine" shops here though, people do tons of boating.

I am curious to try these ropes for something in the future. Let's see if anybody on this forum would get it to work too. That's the only way to see if theory holds.

Meanwhile I'll stick with steel cables for lawn mowing. I hope all those metal birds in the sky will do so too. Somehow they don't seem to fail as easy in real life as on YouTube... Practical engineering, I guess.

Best Regards,
-- Sergei

From: Chris Albertson <alberts...@gmail.com>
Sent: Wednesday, June 26, 2024 12:57 PM

To: Sergei G <msg...@hotmail.com>
Cc: gestalt73 <gest...@gmail.com>; RSSC-list <rssc...@googlegroups.com>
Subject: Re: [RSSC-List] High precision speed reducer using rope (capstan drive)
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