AC 3ph 3hp motor rotor shaft replacement
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  1. #1
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    Default AC 3ph 3hp motor rotor shaft replacement

    I have a 3ph 3hp that came out of an AC unit with bad vibration. I got this motor for free. The run-out on the pulley was clearly visible. I thought the bearings went bad. The front bearing spun well, but the rear ones did not. But after then measuring the bearing run-out (rear 0.0005in, shaft stationery) and the shaft run-out (rear 0.025in, rear bearing stationery) it seems to be the shaft. There is a rub line at the half inch mark from the lathe chuck, which also shows run-out (which confused me at first).

    I have taken some notes from this: Repairing induction motor rotor shaft

    What kind of metal is suitable if I made a new shaft? Would a Chinese 2hp 13x40 lathe be sufficient to turn a new one?

    Getting the rotor off. From what I read from the other thread above the rotor is shrink fit. I would have to heat up the rotor body (I read 400F, with a torch?) and push it out with a press (I have a 12 ton) or a sledge hammer. The rotor has aluminum fins. Should I make a jig or go up against the fins with a 1 inch plate?

    The shaft is about 30mm diameter at the rear, but gets wider after the rotor. The bearings are 6206 and 6205. Motor is "U.S. Electrical Motors" brand 3ph, 3hp, 1740 rpm.


    20190817_172721-2.jpg
    20190817_164201-2.jpg

  2. #2
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    The late John Stevenson posted many pix and "how to" over at the Home shop forum.

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    Before you do this, ask yourself if it is really worth doing. A 3hp 3 phase motor is not an expensive motor.

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    Slap some new bearings on it and run it as-is, or look for a newer/different motor, new shaft is more work than its worth imho.

    If you absolutely must do this, need the challenge, then yes your lathe should be adequate for this job. As for pressing shaft, get a very heavy wall tube that has an OD smaller then the fins, and ID greater than shaft, use the press, 12 ton might do it, no guarantees, good luck.

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    Re assemble the motor, hit shaft hard enough to bent it back straight, replace bearings.

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  8. #6
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    The one in the 3613 bandsaw was just worn down on the pulley fit by Bubba not paying attention

    I left the front bearing on - PUT IT IN THE STEADY REST - and turned to clean up true.

    Since that pulley was long lost, I just made one to fit

    Perfectly functional now - some labor, no expense

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    Put the rotor on Vee blocks and see if it really is bent. If it is, your 12-ton press will straighten it, and that's the only thing I would try. Making a new shaft and getting it to fit properly will take more time and cussing than stealing a different motor.

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    You might want to test this shaft between centers.

  12. #9
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    As suggested above I retested this again between centers. The run-out now measures at 0.005 at the rear, 0.005 on the rotor and 0.0045 at the front.

    I didn’t think to try to bend it back. Since the run out is through the whole shaft I suspect this might not work (unless that’s exactly why it would).

    This is of course a waste of time. But I am wondering if I could do this though. I have not made enough things on the lathe so far so I figured this might be an interesting project. It looks like the shaft size is 1.25x13, but I think I can get away with a 1.25x12 and a narrower pulley. If I do go for this and manage to get the rotor off I’ll order a new shaft from McMaster-Carr (1045 instead of 1566, since it is easier to machine).

    EDIT: As suggested I have looked up some of the John Stevenson posts on homeshopmachinist. It's interesting how he would reduce the size of the shaft, build up weld, and then cut. I'll keep reading on.


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