We are planning on using bike brake cables to actuate a part of the intake / wrist this year.
Question: Does anyone have some renders, photos or CAD of a take-up spool for the brake cable?
Planning on mounting the spool onto a geared down Neo550 motor. We need around ~6" of take-up, so a smaller diameter spool but a bit more than a servo can handle in throw. Hence the spool.
Here are a couple of our models. We’re currently using the single wrap drum to actuate a ratechet & pawl. It’s designed for much less travel than you’re seeking. The drums are designed to bolt directly to an ultraplanetary output.
We tried a drum with a helical groove for reeving multiple wraps. Any loss in tension led to derailment. I would suggest going to a larger diameter drum for more travel in lieu of more wraps around the drum.
4087 Brake Cable Drums.zip (49.4 KB)
ETA: A photo & video of our system is here
Nice! Thanks. Do you find that you need the helical grove to manage the cable?
edit: yeah, the mass of a person on the timing belt forces the belt teeth to deform and slip. Add another belt, or a wider one, and that will go away. Not sure what happens to mass with two timing belts vs chain though.
We don’t know the forces required yet on the brake cable - it’s to move what is basically a wrist. (double roller intake with horizonal axles - not that much different than was was popular last year for both cones and cubes), but we need to change the pitch angle of that wrist between ground pickup, “hold” and amp / trap deposit.
We don’t think it’s much force though, but we will see.
We went away from any helical groove at all. Our current spool and cable arrangement only has ~1/4 of a wrap around the circumference of the drum. Any slack in the brake cable causes it to jump off of the drum and potentially kink the cable.
Our testing seems to indicate that constant tension in the system is very necessary, which is congruent with how bike brakes typically work (the brake lever’s torsion spring counterbalances the torsion spring on the brake caliper). We only have torsion springs on one end of our system, and have had promising results from limiting the possible range of travel to keep the cable on the drum. Our NEO550 is set to coast and has a very low current limit. Putting it in brake mode led to inconsistent ability of the torsion spring to return the system to the original state.
Another solution could be to build in a torsion spring on the drum/actuator end of the system, but that was a level of complexity we weren’t ready to delve into yet.
As a cyclist, yes, that makes a lot of sense - to keep constant tension on the cable.
We plan on “pulling” with the brake cable, and using a surgical tube on the other side of the intake plates to pull against the brake cable.
The math works out that a ~4" OD spool give us around 6" of take up at a 1/2 turn on the spool, so no overlap of the cable on the spool.
We will need to look carefully at the motor code so it won’t loose tension on the cable. Not much applied current, but yes, the Neo550 would need to be in brake mode basically all the time when it’s not pulling, so temperature management.
I don’t know if we have any really big servos that could do the job, as we can just increase the pulley OD a bit and keep the total throw under a servo.
Our application is a bit different. We only need on/off, not position control.
Looking forward to seeing what y’all come up with!
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