It is well-known that current running through a bearing surface/ball-bearing can cause damage, by micro-arcing between the balls and the races as a current goes from the rotor to the stator. I was curious if anyone had any failed electric motor bearings lying around that I could have, to cut up and see what caused them to fail. Has anyone else every looked for this sort of failure?

   I note that the low radial load but relatively high shaft speed makes this a good application for ceramic bearings, unlike an IC engine, where the biggest problem seems to be the radial load, particularly on the rear bearing, leading to chipping or Brinnelling.

      Brett

Current induced bearing failure is usually in very large induction motors. That's the only place I've heard of it. These motors use wipers to bypass the current around the bearing. Our motor bearings typically fail due to brinelling from impact with the ground or otherwise. Back in the day of spur gearboxes some setups could exceed the radial load and lead to very short life. Some bearings you get in cheap motors are garbage to begin with.

We had problems with bearing in past. Especially with Scorpion motors, but also AXIs. They produced squeaking sound after some hours of run. After communication with maker of AXI motors we tried better bearings (what pushed price up), but it did not solve problem completely. Later we tried also ceramic and hybrid bearings what made that sound even worse and coming earlier.

Later we found that reason is probably combination of 2 blade props rattling in corners and constant RPM which hits some harmonic frequency of open bell of outrunner, so we tried to push RPM 1000rpm up (that is why we use only 5" pitch props and all of them 3 blade and AXI motors made us stator with 13 turns of winding). Later we did hollowed props with lower precession and now I do not remember any problems with bearing on AXI motors. That tells me that probably static radial loads are not high, but if it vibrates, it can damage bearing soon. I remember those small bearings inside the stator were totally black when I replaced them. If outer diameter is only 10mm and inner diameter 6mm, how large are those balls? It is simply too much for them.

So what I wanted to say, that I think if that bearing fails, it is probably mechanical load. Now I can make minimally 2 years on one motor without visible problem. And I fly lot ... or better that time I flew lot, because this year I have hardly 100 flights 

Another issue might be that the shafts are (relative to IC) very small in diameter.
So under the large-ish processional loads during turns the shaft flex allows the inner bearing race to twist, further increasing loads on the balls.

That's a good point seeing that the motors are made for RC planes with a huge turning radius.
So the solution would be to put the motor farther back and support the prop with proper bearings spaced farther apart.
The carcass of an old ball bearing engine would be a good starting point for this device.

  I stumbled across this thread and have a question. What is the typical fit of the motor bearings, which is, I think, what they call the "C" rating for how tight the components fit together and how free the bearings turn. In my work experience we had bearings that were all over the range in printing presses and bindery equipment that had to spin at high RPM and low speeds, high and low load factors. Some carried huge amounts of radial load. Lubrication ranged from factory applied grease to chilled oil that flowed through the bearings that were under heavy loads. Are the motor bearings typically factory sealed and lubricated? The one motor I have taken apart was on a club member's model that had seized in flight, and was making piercing screeching noises just before doing so. We swapped out the motor and I took the old one apart just to check it out, and found the motor full of debris. All sorts of junk from bug parts, to gravel, seeds (I think) and something had got between the armature and the magnets and broke one of those free. I gave the motor back to him to rob parts from it but don't remember what the bearings looked like, but I did check them to see if the spun freely and they did. I think all this junk gets sucked into the motors, especially when spooling up on the ground and while taking off,  and can cause issues. They have to have cooling air.  As far as arcing, I wonder if replacing the grease in the bearings with dielectric grease would offer any positive results if arcing does prove to be an issue. I wonder what kind of amperage we are talking about at that point when the motor is at full operating RPM ?
 
  Type at you later,
   Dan McEntee