Help Needed With Vex Ball Shifter

Hey all -

We need some help. We recently had to replace the Ball Shifters on our competition robot with some single speed dual cims due to the fact that we couldn’t get the shifters to work. I’m hoping that you might be able to help us get them working.

When we run them, we are getting a loud clicking sound and a lot of stuttering in the bot - especially when we try to turn. The same issue doesn’t exist with the single speed. We initially thought it was the scrubbing due to the fact that we only had four wheels, but we no longer do.

I know this isn’t a lot of description, but I’m not sure what other info to provide. If anyone could help walk me through this I’d be appreciative.

Best -

Need more data:

  • Number of motors on each side.
  • Total Ratio from motor to wheels (3 numbers, 2 with the shifter set up, 1 with the single speed set up)
  • Size of the wheels
  • Also, what type of wheels are we taking about (let me guess 8" Pneumatic Tires from AndyMark).
  • Can you tell us about the pneumatic system you are using to power the shifting actuators? Are you sure it is working right? Is there another air actuator that you have that is working properly?

Dr. Joe J.

Ok -

So - I’m going to post the info I have. I’ll be honest - I’m not sure how to calculate the gear ratios.

Ball Shifter is 3 cim Ball Shifter using 2 Cims with a 2.65 spread. The pinion gear attached is 12 tooth and the wheels have a 30 tooth gear on them.

The Single Speed is a WCP with 2 motors with a 44:30 ratio attached to the same gears on the wheels.

Wheels are 12 inch pneumatic taped to reduce friction.

As far as we can tell the air actuator is working correctly - we are using another to launch our catapult and that is working properly.

Let me know what else you need.


Does the clicking noise happen all the time? We got the same noise in our ball shifters in the offseason, but only when we tried shifting with low air. What pressure are you using to shift?

It clicks at random times. It just seems not to give it enough power but I can’t figure why not.

Vexpro 3CIM Shifter with 2.65 spread:
CIM Shaft=Motor to ShifterShaft1:

ShifterShaft1 to ShifterShaft2 = Output:
50:34 High Gear
34:50 Low Gear

ShifterShaft2 to Wheel

Overall Ratio (High Gear):
------------- = 7.1:1

Overall Ratio (Low Gear):
------------- = 18.8:1

WCP with 30:44:
CIM Shaft=Motor to ShifterShaft1:

ShifterShaft1 to ShifterShaft2 = Output:

ShifterShaft2 to Wheel

Overall Ratio:
------------- = 15.3:1

Plugging these numbers into JVN’s spreadsheet (again, I say, this spreadsheet should be required to buy a gearbox from AndyMark or Vex) with a 12" wheel you are going to have (adjusted top speed) of

Shifter Low Gear: 12fps
Shifter High Gear: 31.8fps
Single Speed: 14.8fps

That 31.8fps is MONSTER. WAY TOO FAST. Seriously. That is almost certainly your problem. I suppose that the clicking you heard was the CIRCUIT BREAKERS not the shifters. You should probably get ratios more like 19:1 in high gear and 50:1 in low gear with those 12" wheels.

Dr. Joe J.

Check the small plastic coupling between the cylinder and the shifter gear shaft. We’ve had bad issues with the outermost retaining ring staying seated when shifting from low to high. Our cylinder pulled the ring off and left the shaft in between low and high. The ratcheting noise we got (sounds similar to the clicking one you describe) is from our shifter “hanging” between gears since it cannot shift into high. It is difficult to see it under load, but if you can watch it shift with wheels off the ground, you may be able to eliminate the issue we had. As mentioned above, the pressure while shifting is also very important as that can be a possible cause as well.

After reading through all the blog post and looking at the calculations Dr. Joe did it is likely you are tripping your snap action breakers or your roborio is browning out.

The biggest thing that jumped out at me was when you switched from a 6 cim drive train to a 4 cim drive train you said it quit doing it. Your total amperage draw should have gone down when making this switch. You said it does it more when you try to turn, this is likely because you are drawing more power due to the increase friction on the wheels while turning. Our robot does the same thing with the ball shifters on a dead battery (10 volts). IF you look at your RSL (robot signal light) does it flicker as well when it happens? if so you are simply drawing to much electrical current at once for the system. The new gearbox with less motors just reduced your overall power required to run the drive train.

We have this problem on our practice bot also when in low gear. What is your pneumatic pressure? And are you using chain or belts?

Echoing these comments, I’ve run into issues with similar clicking noises when running the shifting pneumatics working pressure at ~40psi. The issue virtually disappears when the pressure is increased to 50-60psi

Its been said by more than a few others, but I am almost positive your problem is simply low pneumatic pressure to your shifting actuators.

The clicking noise you described comes when the shifting shaft is not in full engagement into one or the other gears, and the balls interact with the contours in the gears.

In addition, this diagnosis would cause the jitteriness you described when driving. If the shifting actuator is not getting its full pressure (60 psi), then the shifting cluster could easily start to disengage.

Check pressure regs etc. on the bot, and it should help.