I’ve been thinking about possible ways to implement a system that allows movement in one direction but resists it for the opposite direction. I want to try designing a ratchet-like mechanism but have no idea how to even approach the design. any help is appreciated.
A simple approach is to use a COTS ratchet and pawl, e.g. ratchets | McMaster-Carr, or the planetary modular ratchet stages such as the VersaPlanetary ratchet. The very cheap solution (used by some FRC teams’ climbers in past years) is to just get a 1/2" ratcheting socket wrench, put the 1/2" on a hex shaft, and have the wrench push against a crossbeam.
If you want to actually CAD a ratchet and pawl and machine it yourself, the main thing to think through is how the pawl disengages from the ratchet when under load. You want that movement to be tangent to the surface, you don’t want the pawl to need to “push up” on the load applied by the ratchet. In the other direction (engaging the pawl), you similarly want it to slide into place. This will result in a raked sawtooth profile like what you see in the COTS solutions. I did a CAD of a large pneumatically-actuated ratchet and pawl for FRC back in 2010, I’ll see if I can dig that up.
We cut off a 1/2" wrench and attached it to the end of a pneumatic cylinder. We could then extend the cylinder and lock the hex axle in place or retract it to allow the axle to turn. It worked surprisingly well.
If you start at this post and scroll down you’ll see lots of solutions that teams have homebrewed to do this over the years as well - AndyMark Fall 2021 / Winter 2022 Product Launch - #24 by pfreivald
Our team used the versa ratchet and rev servo for our climb this off season and this worked really well together. Together with a rev bracket it all bolted together as an assembly. Lots of reuse potential.
Back in 2020, FRC teams cleaned out Greenville’s entire stash of 1/2 inch ratchet wrenches during the competition. 5892 started the competition with something else (I think it was a smaller ratchet wrench or 3d printed ratchet? @Weldingrod1 ) that resulted in our robot rapidly accelerating towards the ground in a fairly unfavorable manner.
The 2020 Everybot documentation shows how they used an off the shelf, 1/2" Flex Head Ratcheting Combination Wrench. It can also be seen in their reveal video. We emulated this set up quite successfully.
Actually, we weren’t the only ones buying up the 1/2" ratcheting wrenches in Greenville
I think the shock load as the ratchet caught up after the climb is what did it in; there was a fair amount of rotation before it hit the stop.
The rebuild used a 3/4" x 1/2" hex tube to allow a beefier wrench to fit.
If you want a ratchet the suggestions in this thread are all great.
Recently I’ve gained a preference for friction breaks or gearing slow enough such that the motor can hold up system in brake mode, as the NEO and Falcon brakes are quite potent compared to what we had with brushed motors.
Not having a one way device makes it match easier on drivers if they miss a hang, and is all around easier on the team since robots don’t need to be mechanically reset as much per match or when practicing.
I like this idea. Anything that removes complexity from the robot is a huge win in my book.
I’m curious if there are any numbers (theoretical or empirical) on the amount of breaking force to expect from these motors in break mode. Even the right order of magnitude would be helpful (.01 ft-lbs, .1 ft-lbs, 1 ft-lbs, etc…).
Is it enough that, with typical1 gearing, it could be used to hold a 140lb robot after a climb, or is relying solely on the motors more so for mechanisms with smaller loads (e.g. wrist/elevators that manipulate game pieces)?
typicalWith enough gearing, even the smallest braking force can be used to hold arbitrary large loads. By typical, I mean the robot could still climb in a reasonable amount of time (a couple of seconds at most).
Why gear slower when you can add more motors at the same ratio? taps head
If we did this, how would we know whether we are hurting the motor or not? Can we assume that the worst that would happen if we get the ratio wrong is that the robot would slip? Or would it hurt the motors?
Depends on what you’re doing really. We ended up with everything from “disappearing” brushes to smoke clouds in 2018, mostly because we didn’t care enough to characterize appropriate current limiting, and wanted the option of killing some motors if it meant a successful climb.
Depends on your exact ratios; something smart like a Falcon will probably just scream at you while slipping down. In theory you’re slowly damaging the enamel on the stator coils, but meh.
New strategy: Climb, then weld the motor into one solid piece of metal to prevent back-sliding. Very clever!
Trouble with that strategy is the heat is being applied in the wrong place. It will generate a lot of smoke as it burns up insulation, but won’t lock the bearings until much later.
@Bart_Kerfeld it happens that I -also- wanted to know something about NEO brake mode, so I tested it! However, I have not tested the power-off mode when set for “brake”. Might have to set it up again
I cant remember which year but sometimes end game rule required sustained climb for a few seconds after t=0 so depending on rules, you may not have power for motor braking.
Brake mode is persistent in disabled (or at least it can be).