775 Veraplanetary Possible ground to frame?

Hardware:

775 pro with a Versaplanetary gearbox bolted to a piece of aluminum stock through the side mount holes on the versaplantary. The motor controller is a PWM Spark Max.

When the motor runs in one direction, it runs fine.

When it runs in the opposite direction, it acts like it’s fighting itself. Another motor in the same index subsystem runs funky as well.

If we unbold the versa planetary from the frame, the problem goes away.

We already swapped to a different, new 775 motor. Issue seemed like it was fixed, but is recurring again.

Could there be an electrical issue that goes through the gearbox into the frame? How does that figure since we swapped out for a new motor?

Any help would be greatly appreciated.

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If anyone knows how i could troubleshoot this with a multimeter, I’d love the advice.

Disconnect the motor, then put the meter on the Ohms scale Ω and read how many ohms from one of the wires on the motor, to the motor housing. If it is infinite, all is well, If there is a number, it’s a bad motor.

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Have you accounted for the 775 diameter? It is slighter bigger than the versaplantery and could be wrenched up in a direction causing the funky directional control. I’d recommend adding a .25 spacer to get some distance for where you’re attaching to the frame. I think it’s plausible because you said it seems to work fine when it is unbolted.

I’ll try this tomorrow, thank you!

The motor is mounted perpendicular to the frame. The only thing touching the frame is the gearbox.

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Do I just disconnect one of the wires and test that or both?

Is it possible that there is a short between one of the motor leads and the chassis of the motor right where the leads are soldered on? It’s super easy to do and it causes some very weird behavior.

You only have to disconnect one wire, before testing. I would disconnect both, though, just to be sure there is no other path from the motor windings to the motor housing.

One thing to check is if the bolting to the frame is causing the stages of the Versaplanetary to not align properly. Check to make sure that each stage of the gearbox is sitting nicely flush against the adjacent stage(s) on all four side - if not, loosen up the two long #8 bolts that hold it together and adjust. The other diagnostic you could do would be to bolt it to the fame but don’t tighten the bolts much. If it works fine then, it suggests that the holes in the plate don’t align perfectly with the holes in the gearbox.

The 775 is supposed to have a smaller diameter (1.744”) than the width of the gearbox (1.75” is the side-mount dimension) but they are really close. If something funny is going on there, I could see the motor being pushed on by the mounting which could also cause the issue. Also, if you happen to be using version 1 of the gearbox from a few years back, it didn’t have the side-mount bosses that it does now, and it definitely had this problem, so check that out.

It doesn’t figure. Years ago there was a batch of motors with internal shorts to the motor case but that was before the 775 Pro existed. I would put the chances of finding two 775 Pro motors with identical internal shorts at zero.

I’ve seen plenty of mechanisms that spun freely in one direction but got bound up in the other. My money is on that.

If you have an issue when the motor is electrically connected to the frame, then you really have two issues since if the frame is isolated as it is supposed to be, it wouldn’t effect the motor regardless. I have poor luck finding shorts in motors with a DVM. Try wiring the gear box to the chassis and see if that replicates the problem. With the gear isolated from the chassis measure the voltage between the motor and the chassis while running the motor in both directions. You should get the same somewhat random readings in both directions.
I suspect you either have trash in the gearbox or you are putting the gearbox is a slight bind.

What if you leave it bolted to the frame, but disconnect the output shaft so it can spin freely? Or bolt it to the frame in a different location, with nothing touching the output shaft? If the output shaft supported with other bearing(s) that might be bad or misaligned?

It sounds more like mechanical binding somewhere to me, based on two successive 775Pro’s exhibiting the same behavior. With the gearbox output disconnected, try moving your mechanism by hand in both directions, passing through the full range of motion several times each. If that all seems good, with no binding anywhere, do the same thing with the gearbox. Keep in mind your overall gear ratio - if it’s a 15:1 gearbox, spin it at least 30 times in each direction so the motor moves through two full revolutions.

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I agree it’s likely a mechanical problem. The OP asked how to check for a short from the motor windings to the housing, and I think it’s a worthwhile check, just to be sure it’s not the problem.

Back when we had that problem with the other 775 motors years ago, I easily found a short in one of our motors, using an ohm meter. I really doubt that’s what’s going on here, but the ohm meter test is so easy (if you understand how to do it), it might be worth spending the minute or two it takes to perform the test and eliminate that possibility, just so you can then move on to fixing the real problem.

That’s exactly what I was thinking Mr Forbes. We’ve ruled out software. I wanted to know how to test the motor and wiring with a multimeter to troubleshoot that component as well.

My guess is that the bolt is going too far into the gearbox and causing an issue. If we back off the bolt it seems to fix it.

I will be going through the suggestions here later today and will see what I can find out. Knowing how to systematically troubleshoot a problem is such an essential skill.

Thank you for helping.

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Turns out, the chain was too tight.

When I was unscrewing the bolt holding the gearbox to the frame the problem would go away. I think this is because it allowed the aparatus to move and give a bit more slack to the chain. After slightly repositioning and breaking new chain, the system ran well.

Thanks for all the help.

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