Vexpro 3 CIM ballshifter grinding

We got our chassis running today using two NEO motors on the 3 CIM gearbox. The wires were too short for one NEO so we ran one NEO and let the other spin as a generator. I expected that to be a non-issue, but we got a lot of grinding noise. Are the vexpro gearboxes picky about back driving like this, or do we have something suspect in the gearbox assembly?

While we don’t have pneumatics yet, we verified the shifter is properly engaged. It’s a WCD and both gearboxes have the same issue; one was built by our gearbox specialists and the other by a rookie getting trained at the same time.

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What do you mean that you let the other spin as a generator? Did you connect the leads for this motor to anything (including each other)? While I haven’t worked with these yet, if you connect the leads together through a direct connection or a small resistance, I expect the NEO to act as a brake. If the NEO leads are not connected (and preferably are insulated from everything else) and you are getting this result, then I would suspect a mechanical issue.

Good question. The leads of the 2nd NEO were open (in Anderson connectors not connected to anything). i agree, if they are shorted together they make a nice electronic brake. That’s called regenerative braking if you use it to recharge a battery.

It is most likely linked to the fact that you don’t have pneumatics installed yet. When we tested our gearboxes a few years back with low pressure pneumatics we experienced grinding.

Thanks, that’s a good clue. In 2018 we ran with an off-board compressor. I had to keep reminding the drive team to make sure they repressurized the pneumatic tanks before a match so they wouldn’t have this issue. We ended up doing a lot of offseason driving with low pressure and grinding was never an issue. Maybe we just got lucky, or this is less of an issue after the gearboxes are broken in.

Hi John,
Are these newly assembled gearboxes? Did the 2 halves seat together well without hardware or did they need to be compressed together? One thing to check would be that the 1/4" - 20 nuts are fully seated into the case and not touching the gears.
Good luck,

With pneumatics not being installed, the gearbox is not fully engaged, therefore trying to disengage it self. We had this issue last year.


We experienced the exact same grinding noise with the neos, and it came down to a bolt that was touching a gear (as suggested by silverD) that was being shredded into pieces by the new motors… You’re going to need to open the gearbox to see if this is the case, and if so, clean it up.

A quick tip for how we’re going to check from now on: If you can spin the wheels manually with the robot off, that means that gear is most likely not hitting a bolt.

Also, we’ve just run many experiments without pressure and have experienced no problems shifting manually by pulling/pushing the connector.

We experienced gears grinding and eventually seizing at our at our second event in 2017. We RPM matched both gearboxes to each other and a reasonable free speed based on the gearing, so we know that the plastic shells were not clamped too tight and no gears were rubbing on bolts. Along with that failure, the plastic shifting coupler broke and had about a quarter of an inch of play in it. We can’t be 100% certain of the cause but believe the shifting coupler breaking contributed to the gearbox chipping teeth and seizing. We ended up ordering new ones and machining a more secure shifting coupler and didn’t see failure for the remainder of the season.

I would believe that running with low pressure or no pneumatics could see a similar failure case to ours when our shifting coupling broke.

teams using ballshifters should keep this in mind when pushing your robot when it isnt pressurized, dont go to fast or it will try to engage and cause jerking/grinding

It’s good to check the simple things first – if you use screws that are too long in certain places in those gearboxes, they will crash into the side of a gear. Watch out for screws that go into the nuts in the pockets of the housings (on this drawing, view on the left, 4.625" vertically apart)

If that doesn’t fix it, I recommend backdriving them by hand to see how well-engaged the balls are (while powered down). You’ll also want to discharge your pneumatics and hold the shifting plunger in place by hand too. It’s important to leave the pneumatic cylinder in place for this, since its position limits the extents of the plunger.

When everything is working properly, it should take very little force to hold the drivetrain in gear – the balls should be fully engaged with the driven gear and be unable to exert axial force on the plunger. (see this diagram) If heavy acceleration causes the plunger to push or pull, it usually means the balls are in partial contact with the side or the rounded edge of the plunger. The fix is usually to adjust the axial position of the pneumatic cylinder. This is usually accomplished by removing the shifting coupler, adjusting the position of the nut on the pneumatic cylinder shaft, and reinstalling the coupler.

In rare cases, you may run out of travel and need to add washers between the standoffs that attach to the gearbox and the plate that holds the pneumatic cylinder.

The grinding in our 3-CIM ball shifters last year was this:

A little sloppiness during assembly and the ‘retained’ nuts were not retained. Worth cracking them apart to see if these, or something else, is causing the issue.


If our ball shifters are grinding, the nuts not being retained (as shown by James above) is usually the most common reason. The #2 reason is having star rings retaining the motor pinions grinding against the gears.


Bahaha. Us too!


Thanks all. We had a snow day today so we’ll get back in the shop tomorrow to check this out. I’ll let you know what we find. I believe the free spin is okay, so we’ll pressurize the shifter first.

Brittan - I’ve seen that free rolling too fast issue in the past. I wondered what that was. Thanks.

Well, after the Polar Vortex moved North, we finally got back into our shop yesterday. We found the grinding was actually belt slippage (lesson learned, don’t use 24T pulleys in your drive train! You need more teeth). It’s been about 3 years since we last had belt slippage issues; this mentor should have remembered that sound!

Today, we switched that section of the drive train over to #25 chain drive. Tomorrow we test.