Help inspecting/repairing a BLDC motor.

[ESjaavik]

I have a BLDC that is frozen. It has a brake, but it clicks when deenergized. Also if I forcefully turns it, much more torque than the 2Nm brake force needs to be applied. My suspicion is it has a frozen bearing. That about figures with the necessary torque.

I bought it from eBay, and would rather try to fix it than return it (freigth cost). And thus I don't know how it failed, it was sold as fully functional.

I would like to pull the front cover with the bearing and fixed part of brake that is attached to front cover. I know I have to apply deenergizing current to the brake while dismantling. But how can I keep the magnets from pulling up against the stator when I remove the front cover. I thought about suspending the rotor between centres while keeping the body fixed. I assume this will work as long as it's firmly fixed. That will let me inspect without doing any harm. But what if it is the front bearing, and it needs to be exchanged? I guess you see there is a problem. Do I contact a magician that knows the coupled ring trick?

Anyone out there that know how "the pro's" do this, or have a bright suggestion? Please indicate if you know or guess, but I will welcome good pot shots as long as I know that's what it is.

Why not chuck it, and try finding another? Well a 2Hp cont. 10Hp 20% motor with that low weigth would be as close to a perfect spindle motor as I can get on my budget. I'd spend some hours fixing it if I can.

[foamcutter]

ES,
This is a pot shot guess, so consider only after lots of thought on your part. I have never disassembled one like you have but from what you describe you need to hold the center armature shaft in the center so it doesn't hit the case walls while removing the front case plate? Can you rig a holding plate that would slide in behind the front cover plate while you have the armature and case fixed like say in a lathe on centers for the armature and figure out some way to hold the case in clamps or a vise or something. Loosen the front plate and slip a holding plate in behind the front plate once it is loose but not removed.This plate holds the case and shaft just like the front plate but allows you to take the whole motor case and all together off the fixed centers to remove the front plate? Don't know if that makes sense to you or for that matter if it would even work, but maybe it will spark an idea that will. Again I have not done what you are trying to do so do not follow this suggestion without assuring yourself it would work, I am not wanting you to ruin your motor. Just a thought maybe a good one maybe not. Ron

[arvidb]

There was a thread somewhere where a guy had similar problems; the kevlar wrapping around the rotor that keeps the magnets from flying apart from centrifugal forces had come loose. So he removed the rotor, removed the loose kevlar string, and managed to glue a new one in place. Lots of pictures too.

But I can't find it for the life of me! Been searching for half an hour! Anyone else recognise the thread I'm talking about?

Arvid

[ToyMaker]

arvib wrote: the kevlar wrapping around the rotor that keeps the magnets from flying apart from centrifugal forces had come loose. So he removed the rotor, removed the loose kevlar string, and managed to glue a new one in place. Lots of pictures too.

was that a 5bears project? he posts here as Swede.

robotic regards,

Tom
= = = = =
"We Americans deify our politicians and politicize our deities."
- - Fred Weiss

[ESjaavik]

Originally Posted by foamcutter This plate holds the case and shaft just like the front plate but allows you to take the whole motor case and all together off the fixed centers to remove the front plate? Don't know if that makes sense to you or for that matter if it would even work, but maybe it will spark an idea that will.

Actually it did. Maybe if I mount it between centres in the lathe and hold the case in a vice on the cross slide. Then I can pull off the end cover. Then support the axle stub using the fixed steady (if I can mount it in the narrow remaining space) before releasing the centre support of the tailstock. And of course I'll put all the blame on you if anything goes wrong!

I'll try to make a "dry run" and see if there is space for this. I'll go slow, as before I do anything, I'll see what reply I get from the seller. And we have nice weather now, so I have to get my paint bucket and climb the ladder.

ArvidB & ToyMaker: Yes, I remember that one, but can't find it either. The time limit on searches does not help. And maybe the searches does not cover articles from the "old forum".

Maybe I should say why I don't want the rotor to hit the stator:
1. The magnets are strong, so it may hit violently crushing or chipping the magnets. The debris can be damaging.
2. The remaining bearing will be forcefully bent because it will centre one end while the other goes off-centre. Thus the axle will not be parallel to the case. This will destroy that bearing and/or it's seating.
3. This same displacement may also damage the resolver mounted on the other end. I could remove it first, but would then have to re-align it before use.

Maybe I'm too careful, but if I can't fix it, at least I want to have learnt something new from trying. Thanks for input.

[HuFlungDung]

For sure, you would want to remove an resolver or encoder before disassembly. But, there is generally not so much room between the rotor and the stator that a significant "crash" will happen when you pull one endbell off. There is also a fair degree of clearance inside the ball bearings that will easily accomodate the temporary angular misalignment during disassembly. Its just another motor.

[ESjaavik]

Update:
After some thinking, I found that.
1. It's not the bearings. They don't behave that way when crashed.
2. Not the brake. It's not possible for it to give that much resistance to movement.
3. It's not the resolver. I took off the outer part to check it. That can be done without damaging it.

So, I was left with the rotor itself. And the cost of repair meant the motor was scrap anyway, so I took it apart. And sure enough, the string wound around the rotor to keep the magnets were broken. One magnet had forced it to break. I don't know how that can happen. As far as I can determine the magnet is not loose, it seems it has "grown" like a piece of rusting iron.

I sent the seller some pictures, and he promptly agreed to refund a good amount of the money. The deal was for motor and drive, and the drive works. I's a beast to tune though!

So I feel I got out of it with a bearable loss, and a spare resolver that I can use on a motor I have with an encoder for which I have no interface data (Fanuc).

[arvidb]

Could you post a pic here of that "grown" magnet? Just being curious...

Arvid

[foamcutter]

Have you thought of sanding the "grown magnet" back to it's origional size and shape? Might work might not, never tried it myself. See I have lots of ideas when someone else is doing the work and bearing the cost of success or failure. Just a thought Ron

[Al_The_Man]

It sound like the ferrite magnet has attracted metalic dust, either from rubbing on the armature itself or from outside, the dust particles would eventually build up and compact from the turning armature, you should be able to grind them off, carefull as you near the ferrite.
I have also succesfully cemented a loose stator magnet back in place by using one of the industrial 2-part adhesives.
Al

[ESjaavik]

Al: What did you use to replace the fibre wire that is wound around the magnets to keep them in place?

I don't know what it is, but guess it is glass fibre. Kevlar would be black I guess?

Maybe if I get bored, I'll try. For the time being I just stuck the rotor up on one of my steel cabinets to show the other motors what happens if they don't behave.

Arvid: Yes, I'll post some pic's and post a new msg. when done.

[Al_The_Man]

Soon after I posted that I realized that your motor is a brushless, the one I have to re-cement was a brush type so the ferrites are on the stator, sorry.
Al