Strangest problem ever

Technical Technical Discussion
#1

Ok so some background:

Our team is using a kit frame with belt driven 6 inch wheels from a gear box (forget which kind) with 2 cims. Standard.

The first time we drove the robot we heard a rhythmic noise, couldn’t find the problem externally so we disassembled the gearbox, found a gear put in backward fixed it then put it back together. We ran it and we heard a noise again. This is where it gets strange. You can’t feel the resistance when turning it by hand but when driven slowly you feel the vibration and hear it. (The previous problem we could feel.) This is the strangest part of all. It has a rhythm that is pretty exact. It goes: 1, 2, pause, 1, 2, pause, 1, 2, 3, pause, pause, repeat. As far as we can tell the wheel is in the same spot when it makes the sound.

So that’s the problem, my questions are what could it be/how can we tell/fix it. And can we leave it and let it reveal itself to us later?

#2

Make sure you oriented the middle wheel correctly. Probably hear the screw heads grinding the side plate.

#3

If you post a video of the phenomenon that would probably help with diagnosing the problem.

I did hear a similar rhythm on last years robot and it did turn out to be some loose screw heads grinding against the outer plate. Now there is circular track about 1 mm deep in our outside plate. We made sure to avoid that problem this year.

#4

Looked for this, didn’t see it, will look again tomorrow.

#5

We’ve had similar problems in the past. One time a bolt head was grinding on the inner plate. Another time there was a wad of duct tape on a pulley. Most recently a problem in the programming somehow caused the motors to go out of sync.

#6

I remember having a problem like this last year. I
I can’t quite remember if it had a pattern to the noise, but what was happening is there was a problem with code and the 2 motors were running against each other. It probably isn’t your issue, but something you might want to check if other ideas don’t solve it.

#7

If you don’t see anything causing it, divide and conquer.

Try removing the wheels and running the gearbox. Then add just the direct drive wheel and try again. Then add one belt drive wheel and try again, and the third wheel and try yet again. These should tell you whether the sound is coming from the wheels and whether it’s the direct drive wheel or one of the others.

#8

If your gearbox is a shifting gearbox you’ll want to check that the shifter is either locked into one gear or hooked up to pneumatics for shifting. I don’t think you would get the rhythm you’re hearing from a disengaged shifter but that might be your problem.

#9

Sounds like some slop in the gear train. As long as everything is running smooth and with some lubricant, try ‘breaking the gearbox in’ by running the robot up on blocks at full speed for a ~15 minutes.

The rhythm you are hearing is probably nothing to worry about just some mechanical slop.

#10

The standard gear boxes in the kit are supposed to be “run in”, by being assembled dry (no lubricant) and driven, at varying speeds, ideally under load… ie. drive your robot chassis around, weighted down to 120 pounds. They may be LOUD. Failing that, run them at speed with the robot up on blocks.

After doing this, disassemble, clean (they may be lots of metal dust), lube and reassemble. This will make a huge difference in the noise level, and smoothness of the gearboxes.

We just did this to our two robots. Out of the 4 gearboxes, 2 were quiet and smooth even running dry, one was a bit rough, and one was horrible. The one that was horrible, actually had a visible pile of metal dust under the robot at the end of the run in process (we did the run in with the robot up on blocks). After the process all 4 were smooth and quiet.

The issue is that the stock gears and gearboxes are not precision machining at its best. There is a fair amount of slop and tolerance issues. Running them in dry, wears off the points of interference, allowing the gears to mesh better. If you try to run them in after lubricating them, it will take way longer to create the level of wear needed to smooth things out and will leave you with all this metal dust in your lubricant.

#11

Did you try unplugging the PWM to all but one motor and testing it?

In the past we’ve had a CIM that didn’t mount well and was misaligned. This caused too much pressure on the driven gear and all sorts of noise.

Cycle through the motors one at a time see/listen for the culprit.