Sometimes I hate the algorithm and sometimes it shows me cool new robotics / mechanics / gadgets and makers. Aaed Musa has been working on something called a “Capstan Drive” which is a rope driven alternative to gears. By removing gears and teeth and replacing them with rope you cut down on noise, eliminate backlash, high torque, low inertia, and low cost – with the major costs being low range of movement and a vertical path for the rope to travel over. Aaed’s video is well worth a watch and blog well worth reading. But… if you want to get a sense of how the Capstan drive works…
The benefit of a planetary gear is that it’s a very vertically compact method for increasing rotational speed at the cost of complexity. With a Capstan Drive (I don’t know if this is supposed to be capitalized) the rope needs to be wrapped around the thinner shaft several times to prevent slippage. As Aaed notes:
One question that I had when first exploring this reducer was “why doesn’t the rope slip if it’s just wrapped around the smaller drum?”. The answer to that question lies in the capstan equation. With each turn of rope on a drum, the amount of friction increases exponentially. With 3-5 turns of rope, there is enough friction for slipping to not be an issue.
Aaed indicated he was using Dyneema DM20 cord as it has almost no stretch to it. I wonder if something like fishing line would work?
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My 2c.
It’s very cool what he is doing but its not a direct replacement for planetary gears.
You don’t have as much of a worry about backlash which is what the capstan drive is very good at.
And the capstan drive is limited in angle of rotation.
Plus you prefer inline than side by side.
Still very cool though … ;)
What intrigued me was the design simplicity – no need for gears, just a couple of tubes. Maybe with channels for the rope to coil. Plus, I don’t need/want a large angle of rotation on the big gear. The outer handle should only make a half revolution at most to make sufficient rotations to the central spiral core. I was trying to figure out how a Capstan drive could be used for concentric rotation, rather than side by side. I feel like it’s possible, but stupid work got busy and I haven’t been able to through enough brain computing cycles at it. I should be done with the bulk in a few hours – then I can turn back to the really important things in life. :)