Repairing induction motor rotor shaft

This motor was on my lathe as a replacement I did from single to 3phase. I removed the rotor to cut the key shaft down to the original pulley of the lathe. It was 7/8", and I reduced it to 5/8". Do to shock and cantilever of pulley position on shaft, this failed. The shaft snapped off.
So the task is to press out the broken motor shaft from the rotor steel laminations, and make a new one.

I assumed the shaft was pressed in. So I made a press jig to protect the aluminum cast fins, which are part of the squirrel cage electrical shorting bars (I think that's what they are called). The shaft will not budge on a 20 ton press. Motor size is 1HP, and special inverter duty winding insulation. In fact the cast iron bucking plates that came with the press, failed catastrophically (the plate), and allowed the rotor and pressing jig to explosively shoot into the sheet rock wall.
Is there a reason this can not be pressed out?

Also is there a recommended steel for the shaft? It was very soft to machine (Rc 5 or so), so appears to have zero heat treat or hardened alloy qualities. I don't believe that the material is some sort of eddy current blocking metal. The shaft can only be pressed out one way, as one end has a raised section for the bearing that is larger diameter then the shaft section that goes through the laminations.







I'm thinking the only way to remove this is to cut it off, and drill it out, then bore to clean it up, and make a shaft that will press fit properly.
It just seems this should have budged from the laminant stackup.

I haven't found a motor I can't press the shaft out with a sledge hammer and my anvil.

Try heating the rotor up to 400F and try it again.

Given this is only 1 hp, if you support the rotor by the aluminum shorting end ring ( the fins are for cooling, electrically negligible) 20 tons should be enough to bend the rotor like a stack of Bellville washers.

---edit---:
I believe the reason you can't press it out, is because you can't press the shaft out the same way it went in. the rotor plates are probably belleville shaped just slightly and you have to bend them back the other way to get the shaft out.

You have to heat the rotor FAST. Big rosebud is the ticket. Same going back in. Shrink it on and you don't have much time. Heat the rotor ON THE PRESS and then drop the shaft in it. Use no more than about .002 interference, as the grip is so long it doesn't take much to hold fast. You do this on the press in case it happens to grab, in which case you get on it with the press, hoping to get it home before it bites. If it grabs partway dwon, don't keep trying to press or you'll bend it. Let it cool completely, then heat it again and try to press it home. We don't even try to press out anything over about 2" in diam at work, and we have a 60ton Dake air over hydraulic press. Big ones we try to avoid doing at all, but if no choice, we bore the old shaft out of the rotor. By the time you get to the hassle of trying to get a big shaft bored out, made and then put back in the rotor, it is usually cheaper to buy a whole new motor with the corresponding warranty.

I'd bore the old shaft and insert a stub. I'd make the stub from a pre heat treated material. If you like finish the diameter after installation for easy concentricity.

IIRC the aluminum rotor bars and end rings (the things with the fins) are
molded in place.

John Stevenson has detailed repairing several motors by welding on a stub
and remachining.

You've got to watch heating up laminations, you'll mess up the coating.

Coating is pretty pointless on a rotor. 90% of ours have no coating or have it stripped off, everything from fractional to 600+hp. I am back at the pump shop. Repairing rotors and shafts is 98% of my work. Damned things are pretty much indestructible. Stub shaft probably will not hold if that one failed.

Really I think it's new/used motor time, over all I believe you will be much happier in the end.
Cast Iron back up plates scare the hell out of me, cut some out of good old steel for a 20 ton press get your self some 1" plate. Plate that thick should prevent deflection when heavy pressing.

Post #4 with precision or Kaput.......

I've heard it said that the laminations can have punching burrs that will dig into the shaft, sort of like a ratchet and lock it tight, but when the rotor is flipped over it will press out easier.

Thanks for the replies. I wanted to fix this motor, but from the replies, and the effort to make a new shaft, as I had already damaged the end with the small protruding shaft for the fan (and had cut it off after my first attempt to press destroyed it), as this is a TEFC motor. Looks like scrapping a low hour motor is the lowest entropy path.
From the posts about shrink fits, they probably put the shaft in the motor as part of the pre heat to cast the aluminum shorting bars. There is no way they would have pressed the shaft in with the forces I'm against.

I did replace the bucking plates with 1 inch hot rolled, as if this shot out at me, it would have broken my femur.

Thanks again.

Mike C. said:

Coating is pretty pointless on a rotor. 90% of ours have no coating or have it stripped off, everything from fractional to 600+hp. I am back at the pump shop. Repairing rotors and shafts is 98% of my work. Damned things are pretty much indestructible. Stub shaft probably will not hold if that one failed.

Click to expand...

I'm referring to the coating applied to the punching steel before they
were stamped out, it cuts down on hysteresis.

That's why a rebuilt (if overheated in the burnout oven) will not be exactly
the same as new.

Turn the shaft and shrink fit a new stub on it, there's probably some room in the motor for sure. Then turn it to desired dimmensions, will work just fine and is by far the most easiest way of fixing this.

Marko

As above, you already state one end was over bore size, ergo you know how it was originally inserted. Machine that down and that will push out with ease going that way. Yep the stamped edge of the laminations very much act as barbs. Over heating em will majority impact there electrical properties, don't quench them either, theres a lot more going on in a rotor than a typical person realises.

Next time bore the pulley out, theres bloody good reasons so many of the diffrent motor standards have such similar shaft sizes. Equally if you do ever have to turn one, you have to have some radi at every step and even then it pays not to be a moron when it comes to belt tension. If pulleys are in good order and the belts not F'ed you don't need to get stupid tension wise to transmit a few Hp with typical V belts!!! If belts are slipping its normally better to step back and ask why before going tighter if there not noticeably lose to start with.

adama said:

As above, you already state one end was over bore size, ergo you know how it was originally inserted. Machine that down and that will push out with ease going that way. Yep the stamped edge of the laminations very much act as barbs. Over heating em will majority impact there electrical properties, don't quench them either, theres a lot more going on in a rotor than a typical person realises.

Next time bore the pulley out, theres bloody good reasons so many of the diffrent motor standards have such similar shaft sizes. Equally if you do ever have to turn one, you have to have some radi at every step and even then it pays not to be a moron when it comes to belt tension. If pulleys are in good order and the belts not F'ed you don't need to get stupid tension wise to transmit a few Hp with typical V belts!!! If belts are slipping its normally better to step back and ask why before going tighter if there not noticeably lose to start with.

Click to expand...

I would have bored the pulley out, but it was a 2 step pulley, and the minor diameter would have split the pulley at the small V. And I have that belt set for the lower speed operation.
If I had been "think'n" I would have bored the broken end out, and press in a new shaft. But in my furver to press the shaft from the rotor laminations, that shaft is now fubar. I think I had $250 in it, so lesson learned, that probably will never be realized again.
I don't ever tension the shit out of a belt. In this case it was the weight of the motor providing all the tension, which was nice the few times I've had a crash that the belt slipped. Still screws the carbide tool up.

"That's why a rebuilt (if overheated in the burnout oven) will not be exactly
the same as new. "

Rotors aren't burnt out. Stators are, but a standard three phase rotor has nothing to burn out. Again,we have done probably several thousand shaft swaps on a certain brand of pump that were were making new shafts for, in house. Never had a problem with it. Have swapped shafts in rotors from fractional (most recently the 1/12hp fan motor on my home heat pump compressor unit) to a couple hundred hp.