As an offseason project, I’m considering developing a non-pneumatic shifter for the Vex Pro ball shifters. It would seem that the pneumatic cylinders supplied with the kit provide more than enough force to use reliably, and servo setups have been attempted a few times without success. Somewhere in the middle is the tipping point between sufficient force and not enough.
Has anyone collected data on how much force is required to engage the ball shifter under load?
Has anyone used a non-pneumatic means of shifting the ball shifters that they’d like to share?
I saw this idea in another thread, but along the same vein, would it be possible to use an electric solenoid in order to shift? I know FRC places a lot of regulations on the types of solenoids allowed, but if the force requirement isn’t excessive it may be possible.
What application will this ball shifter be for? I knew teams this year that used it for a winch mechanism, for which the pancake cylinder provided (3/4" bore, 1/2" stroke) was not enough force. I’m going to go ahead and assume drivetrain, though.
The main reason that a lot of teams use pneumatics is because they are very quick. Shifting with a window motor or servo wouldn’t be ideal since motors don’t store energy, so it could take a lot of time to shift. An added consequence of that would be offset between left and right gearboxes in shifting, something you don’t want.
We observed (anecdotally) that at our true top speed of 17.1ft/s (tested under 60fps camera) with a 3-CIM ballshifter, we had trouble shifting at less than 50-55psi. Our robot weight was about 135lbs; feel free to do the rest of the math.
I don’t know how much this helps, but our custom ball shifter engages correctly with less than 10 lbf pulling/pushing the shifter shaft. That being said we have a 45 deg taper on our shifter vs Vex’s ~60 deg (iirc) taper.
Actuator Details:
Bimba Flat 1
7/16" Bore, 3/8" Throw limited to ~.28"
Engaging from 45 to 60 psi
Out of curiosity, is there a need for being able to shift at top speed or was this done just as an experiment? It would seem that to be useful, a shifter will need to shift a) once low speed has maxed out and you want to go to high speed and b) in a pushing match in high speed, when you want to drop down to low speed to limit current. I can’t see a use case for downshifting from high speed at top speed.
Thats interesting, thanks for the info. What made you choose 45 degrees vs 60? Was your shifter also modified so that the balls were closer together? The Vex shifters are spaced apart by 0.5" so the full cylinder stroke travel is required.
Anecdotal, but our winch this year required 50+ PSI with the provided pancake cylinder in order to shift properly while under load. We did some playing around with working pressure in order to find the “sweet spot” for each of our mechanisms to avoid wasting too much air (mostly with our pickup arm, as that used a ton of air when compared to the pancake cylinder!)
Since it was my first time designing a ball shifter and I wasn’t sure the design decisions that led Vex to choose the 60 deg angle, I went with the 45 deg angle as a safer alternative. (As 192 prefers to use the smallest actuators possible.)
The ball cavity spacing is the same (I’ve never actually looked at that value before today), however the stroke is shorter because I designed our shifter shaft to engage in different positions along the shaft for each gear.
