Balls Popping

We have tested 2 balls in our ball shooter, and have popped 2 balls. We are using 2 green compliant wheels and it appears they are cutting the balls. Anyone else having this issue?

Check the bearings or collars for sharp edges since the green compliant wheels are very compressible, they may be interacting with other objects. Slow mo video is very helpful. (Not with more balls)

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@Team6928 How are the balls being damaged? Does it appear as a smooth slice like you cut it with a flat edged knife, does it look like a tear like the ball was pinched then pulled until it was damaged, does it look more like a blow-out with slightly melted edges, or does it look different then those choices?

It looks like it was sliced. It seems to be getting sliced by our intake wheels (melted by friction?). We are using 2 775pro motors with a 10:1 gearbox on our intake. We are not intaking from the ground, just from the human station.

Holy overkill batman! Why so much power?!


Willing to bet it’s those compliant wheels. We had the same issues until we swapped to a totally smooth surgical tubing roller.

We have never built an intake before, so we had/have no idea how much power is needed.

It seems the edge of the compliant wheels is rubbing the balls and cutting them. How did you use surgical tubing?

We switched over to a VexPro VersaRoller based system. We haven’t managed to pop another yet (though my saying that probably means we’ll pop one tonight).

we use 2 bag motors

As you already have this thing built with 775pros, and its a day to bag, it makes sense to see if you can use this at a lower throttle and get sufficiently rapid intake without cargo rupture. I’d suggest seeing if you can locally source some el-cheapo balls (e.g. from the dollar store) about the same diameter to start.

A bit of math: 775pro free speed is 18700 rpm, reduced 10:1, that’s 1870rpm, or 31.2 rotations per second. If you’re using 4" wheels, that’s about 32.6 ft/s. Shed about 10% for the gearbox and you’re near 30fps. The stall torque would be about 7.1Nm, or 62.8 lb-in, meaning the stall force from a 4" diameter (2" radius) wheel is about 31 lb! If you have a lower-reduction VP slice, that would let you reduce that cutting force while keeping the high speed. If you’re using 2" wheels, then everything is half as fast and twice as much force already!

You may also want to round the corners of your wheels by applying sandpaper/file/rasp to them while the wheels are spinning. Hang on to your file with both hands, and don’t forget safety glasses!

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We have some extra ring gear ratios, but not sure where to start. I realize at our current ratio we are running at roughly 1800 RPM full throttle. What RPM would you suggest to aim for?

With 2" wheels. One rotation is 6.28".
You can go pretty dang low for just picking up cargo.
You can drop it down to 2-3 revs per second or 120-180 rpm

agreed, also make sure the intake is spinning AWAY from the direction you are when applying the file/sandpaper

Ideally the linear speed of your intake should be double of your robot’s max translation speed, so if your robot moves top speed at 10 fps, your intake with 4 inch wheels should have a linear speed of 20 fps, so 20/4pi is about 15.71 rotations per second, or 942.48 rpm, and 18700 rpm of the 775 / 942.48 rpm is 19.84, so that is the ratio you would need running at full speed to hit the target, or about a 20 to one gear reduction, but you can also do this with code with your existing gearbox, as you don’t need that much torque, and you may want a slightly smaller reduction for a faster speed, and the fine tune what works best with code. * note this is an example, I would recommend running your own calculations as well for a given speed of the robot/ intake wheel size

Unless you’re planning to launch the balls more than a few inches into the goal, I’m thinking that 6 lb of stall force on each wheel should be plenty. Unless your robot is really fast, 20-24 ft/s should be good for a free speed. That is, a total of about 1/7 force*speed of what you have, so you would target running 775 pros at 1/√7 or ~0.4 throttle. For 4" wheels, 5:1 at 0.4 throttle would get you 30*10/5*.4 = 24 ft/s free speed and a stall force of 31*5/10*.4, or 6.2 lb. If 2" wheels, 3:1 would make sense.

We won’t actually be driving when loading cargo. We are only intaking from the human station. We have 4" compliant wheels, 2 on the top and 2 on the bottom. We have a set of blue (4) and a set of green (4), so we can mix them in any combination. So in your formula, we would be turning around 700 RPM, right?

Just to get the picture straight, the two above are on one shaft, and the two below are on another? Are they all driven from one motor/gearbox, or one per shaft, or one per wheel? I assumed there were two motor/gearbox combinations, whether left/right or top/bottom.

18700rpm / 5 * 0.4 ~ 1500rpm.

And as you probably already realize, if you round/shave down the corners of the wheels, you only need to cut down the inboard corners, which is where the wheels contact the CARGO.

As to which color/durometer wheel, that’s going to be a testing thing. If all four wheels engage the CARGO at the same time in pretty much the same way, it seems likely that having the same wheel in each location would be best.

For reference (in case OP is interested), we are also using the green AM compliance wheels (4" diameter) for our ball intake powered by 1 AM Redline motor & 12:1 HD sport gearbox and have had zero problems so far. At least 50 balls intaked this build season and zero pops.

2 wheels on 1 shaft and 2 on another. Each shaft is driven by its own gearbox/motor. We got the general idea for our set-up from the snow problem reveal video. Looking at their video again it seems their wheels are closer together than ours. Moving ours closer together could fix our issue.