Weird Behavior - Vex Pro Ball Shifters

Team 4155 is using ball shifters as a ratcheting/neutral gear mechanism as a part of our catapult system like many other teams on CD are doing. However, the piston controlling which set of gears is engaged is behaving in a way I wasn’t expecting.

Basically, whenever our motor is activated and begins to wind our winch, the shifter unexpectedly shifts to the neutral gear almost immediately. We noticed that this problem can be delayed by providing more PSI to the piston itself (much more than 30 psi).

If you work the gearbox with your hand, it seems that rotating the axle on which the winch is mounted is somehow “pulling” the piston into the gearbox and switching into our neutral gear. You have to physically hold the shifter to keep this from happening.

Is this normal behavior for the ball shifters? I do not have any previous experience with them and was wondering if someone could provide some information in order to help us troubleshoot.

Are you sure the ball shifter is assembled correctly? It sounds a bit like when you’re putting the ball shifter in gear, you don’t have full engagement of the balls. Check to see that the nut on the piston is positioned correctly.

If you take a look in the CAD model, the plunger that forces the balls into the inside of the gear is sort of wedge-shaped. If you didn’t extend it fully, it might be gripping the gear a bit with friction that later slips under a more aggressive ball load. This may explain why adding pressure helps a bit but doesn’t solve the problem (more force, more friction).

We are also using a ballshifter in a ratcheting winch - as a caution, we had issues getting the gearbox to shift into neutral under load with the stock pneumatic piston.

Hope this helps.

Thanks for the idea. Are you referring to the nut that eventually is housed inside the coupler?

What did you end up using instead of the stock cylinder?

This is SUPER CRITICAL!!! We tried to use ball shifters last year to use our drive motors to climb up the tower, However we were less than 1/16 out on our dimensions which resulted in the shifters clicking to neutral every time there was a load applied.

For getting the shifter to shift into neutral this year we used 2 of the ball shifters in parallel and they provide enough force to shift to neutral while the winch is under load

Yes - if I recall correctly, tightening or loosening this nut can change how the coupler assembles relative to the piston.

We ended up using two of the stock pancake cylinders in parallel with a 3D printed coupler (graciously made available by user tag_groff of 4265 here ). This required us to mount the standoffs for the cylinder in new holes in the ball shifter, and make a plate to hold the pancake cylinders side-by-side.

Depending on the forces in your winch, these might still have trouble shifting. For TESTING purposes, my understanding is you can run these pistons up to 120 PSI to see if they have enough force to shift. This is equivalent to a single 1.5" bore cylinder at 60 PSI, which you could order after ship and swap out at competition.

Note that I’m just a guy on the internet - check all the safety stuff (pneumatic component ratings especially) before trying anything out of spec like this. Also, since this is well out of the expected usage of the ball shifter, be prepared for possibility of these big forces damaging the ball shifter over time. With our brief testing of 2 cylinders at 60 PSI, we haven’t encountered any issues… yet.

We have had the problem of keeping the shifter engaged. We learned it is based off of the nut on the cylinder as well as stand off length.
If you open up the shifter and look at the shaft without the gear or balls in. Pull on the coupler all the way back to see how far it pulls on it.
Pulling to far will ha e the problem of not disengaging to close will ethier break the coupler or pull the cylinder a bit and release early. We found at the stand off length of 1(5/8) is good with an 1/8 inch mounting plate instead of .09.
Hope this helps.

Even though our team has moved away from a winching catapult, I am really glad to see that the work I did for this mod is being utilized by other teams. Thank you very much.

If the problem is solved by upping the pressure above 30 PSI, why not just crank it up to 60? We’ve had problems with out Vex Pro Ball Shifters not shifting correctly when they don’t have full pressure. There’s probably enough separation forces in the shifter that you need that extra pressure from the piston to keep it locked in gear.

No, thank you! That little tweak took us from not firing to firing.

We plan to put a sticker on the coupler to acknowledge the borrowed design work - Would you prefer your name, team number, Chief handle?

In normal use, this is just not true. The ball shifter is set up such that when the shifter is in each position the balls are sitting on the flat area of the plunger. This means there is no separation force at all.

As some of the people using this as a pull back device, the normal force between the flat area of the plunger and the balls and the side of the ball pocket in the gear at high (externally applied) torque conditions, the plunger does not want to disengage.

The critical dimension on the layout drawing on the vexpro.com web site must be met if you are using your own gearbox.

Also, I have found that the SMC solenoids are not that happy at anything under 40 psi.
Paul

another thing to remember is the radius of the spool you are using to wrap a strap around if that is your way of pulling. We over looked the diameter of the spool and use a 2 inch diameter spool. If it is 1 inch diameter than you are halfing the torque put on the balls allowing more force for releasing and using the cylinder set up.

Gearbox is functioning as it should now. We opened up our gearbox and made sure that the cylinder was pushing and pulling at the correct distance. Also, we fabricated some more exact spacers to input onto the hex shaft where our wrench is mounted. It seems like both of these modifications fixed the problem.

Even without applying any PSI, the shifter is no longer pulled into the gearbox.

Thanks for all of the suggestions!

team number please :slight_smile: