I’m building an FPV quad for a friend using a Hoverthings airframe (which went together beautifully, by the way) and a MultiWiiCopter Paris Aerial Photography control board. That’s a picture of the bird, over on the left.
Anyway, I was finished with assembly so I took it out for some initial flight testing. It did great: stable, agile and plenty of power. Then, a very bad thing happened, which I eventually recognized from my days racing RC cars – one of the motors had a malfunction where the shaft became detached from the bell housing.
Right away, I recognized that I was in trouble, because I heard a buzzing noise coming from the motor - which turned out to be the sound of the magnets rubbing up against the stator.
I had imagined that in this type of failure, the propeller would launch itself into the air, taking the bell housing and any prospect of controlled flight with it.
Fortunately, the magnets were strong enough to hold the bell housing in place until I was able to bring the bird in for a gentle landing. When I looked to see what happened, I could see that the set screw was loose and almost came out of the housing entirely.
This was a brand new 28mm 2217 outrunner motor. It came fully assembled and it seemed solid right out of the box.
What did I learn? Don’t trust it, even if it comes all put together straight from the factory.
From now on, I’m going to check all of the set screws myself to verify that they are in the correct position, and then apply thread lock to make sure they stay put. While I was making repairs, I took these photos to document the problem for you.
Here, you can see the set screw as I found it – very loose, almost ready to fall out of the motor...
When I removed the bell housing, I discovered that the shaft had recessed inside the motor because the set screw was no longer holding it in place.
In this picture, the shaft is back in the correct position. You can see the notch were the set screw rests, connecting the shaft to the bell housing.
All better now: the bell housing is back on the motor and the set screw is back in the correct position. I’ve applied a little thread lock to make sure it stays there.
By the way, I went back and checked the set screws on the other three motors. They were all perfect, and they all had thread lock so the screws stayed put.