Long story short, we found that one of our 393 motors (which was on the drive system) had a tremendous amount of resistance to turning in one direction, but a normal amount when turning in the other direction.
I’ve taken the motor cover off (both top and bottom) and none of the internal gears have any visible problems.
Is this 393 completely broken, or is there something I could try to revive it?
Is there anything glaringly obvious that would have caused this?
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The drive has 2 motors on each side: a 393 and a 3-wire. The 393’s and 3-wires are driving the same axle, but the 3-wire has a clutch, so if the motors were spinning at different rates, the clutch would pop. So far, the clutches don’t pop unless we do something extreme (like driving continuously at full speed into the wall).
this happened to a few of our motors too!
one was a HS and one was a 3-wire motor
this isnt just “normal” resistance
this feels like the entire motor is filled with syrup (going in one direction)
and there is no physical difference when we take it apart
i think its still somewhere in the “dead motor box” waiting to be dismantled for parts
Same issues with lots of our motors. Once this happens they are stuffed! It’s something in the motor itself, not the gearing . This seems to be the way these motors fail. Our team, in New Zealand have a loathing for these motors. Its a case of hold your breath, cross your fingers and hope they don’t fail in the middle of a match. Actually, something happens, usually like a stall - even a minor stall (which they are supposed to be able to handle), and then they’ll last about another 10 minutes and wala! they seem to fail on cue. They have been so unreliable it isn’t funny. Try having a whole box full of wasted money. I think “Paul” from vex took a whole box full back to the US for testing after visiting our RWC event in October.
Yeah, one of ours had the same problem, it sort of freezes up when you turn it in one direction. We noticed that, even before it stopped working, it sort or “jerked” when you changed direction, and then it just slowly began becoming slower and slower. Its now totally stopped, so we just put a 3 wire motor on. Its not as good, but at least it won’t die on us.
If anyone is interested in selling these broken 2-wire motors, our program can use them for demonstration/instruction (voltage, current, back EMF type lessons). We also need some housings and parts for our non-competition projects.
we cannot spend big money for them, but it might be better than sitting in a bone pile forever and can then allow you to buy more VEX goodies.
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We recently had the same problem with one of our motors on our drive train the night before our competition, though I don’t know what caused it. We also haven’t tried running it by itself, just by pushing it it is a lot different
To try extend the life (and efficiency) of the 393 and 269 motors, grease the internal gears every so often, you will find after many hours use the original grease will not be adequate to protect the gears, leading to excessive wear.
I would also imagine if you are overheating your motors/pushing the limits of them then this should be checked very frequently. When you take the motors off and run them you can hear the difference between a well greased motor and one that has lead a hard life.
We’ve had two do this. One that was somewhat abused throughout a competition, and a brand new one that had little driving time on it.
Last night the second brand new one died, so I took it apart. It was adequately greased inside, and found no chipped teeth, or any excessive visible wear on the gears. I did find that the DC motor inside would softly “click” when turned by hand in one direction.
It’s happened to my team too. Make sure that there is enough grease in the internal gears and there is nothing stuck between the teeth of them. If all else fails you might have to replace the motor, as the actual motor which moves all the internal gears might be busted up. You can, however, continue using the housing and gears.
How far did you take it apart?
Did you open the motor can to look at the brushes?
DC brush motors are old technology and cheap.
“switched reluctance” motors are new technology, no brushes, no rare-earth magnets, just semiconductor h-bridges and computer chips. The Dyson airblade hand-dryer uses one that spins up to 100,000 rpm. If this tech gets fanned out to the point that Vex can get it (hopefully at lower rpm), we won’t have these kinds of problems.
(We’ll have different kinds.)
We have noticed that some of our motors (269’s in general), seem to turn faster one way than they turn the other way. They also seem to be consistent about how fast they turn one way vs the other, across all motors. They are two-wire, so we have tried switching the polarity (red to black, black to red), but they still give us the same problem; it’s not based on which direction we tell them to go, it’s the absolute direction they are going, so this tells me it’s not software. This also happens if no axle is in the motors; you can actually hear the difference in speed of the motors going one way vs the other. It’s this a related issue?
My understanding that “higher-speed in one direction” is common in cheap DC brushed motors, and is one reason untuned tank-drive doesn’t go straight.
It would be an interested discussion with a judge for a “design/think” type award, to have designed a robot where either:
all the drive motors face the same direction
one side has an extra reversing gear, so that all drive motors operate CW for forward
Similar for arm motors, although this eliminates the common Motor–axle–Motor setup.
i think in more instances of the issue, it is because of access friction on one side or another. the motors themselves have very little differences in my experience. to check if a side has “too much friction” we pull the axles out 1" so they would not be connected to the motors. and then we quickly spin the wheels with our hands. if they continue spinning for X amount of seconds, then its good. if not, then we see whats the problem and try again. the majority of the time, i see robots with HS chain that are too tight