Advice on extending motor shaft
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  1. #1
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    Default Advice on extending motor shaft

    I have been restoring the variable speed option on the Autometric Model B I bought late last year (in this thread: New K&T Autometric Model B user ). A previous owner deleted it, I suspect over the cost of repair. When I received the machine it had a 2 sheave input to the gearbox and a matching 2 sheave on the motor, with what I thought was a 7/8" shaft. I replaced the 2 sheave sheave with the appropriate companion sheave for the 1922V by doing what the PO did - they'd shrunk it on after turning down the original sheave, so I pressed it off and put on a companion sheave I'd bored to be a slight interference fit. The other end looked to be a 7/8" shaft so I found a variable speed pulley matching the original from another user (Roto-Cone MS75). When I went to fit it I found that the PO had turned down the motor shaft to 20mm as well as cutting it off so it's now too short.

    So I need to either replace the motor - 220V, 204 frame 1.5HP 3 phase 1725. 204 frame is kind of uncommon. Also I'd need one with something like 5" shaft. Unlikely. I might get something in another frame but the mounting pad isn't real deep relative to where the sheave needs to go, and the mounting pad is integral with the tilting and stuff.

    Adapt the shaft - 20mm bore on one end to a 7/8" shaft on the other. No fun for the keyways - 1/4 on the remaining shaft but shallow as heck after being turned down from 1" to 20mm. Blind keyway on the adapter if I go integral which I think I should. Doable but ugh, a blind inside keyway.

    Finally, just press the shaft from the armature, measure it up and make a new shaft to match but with a 7/8" end about 5" long. Maybe too long for a variable speed pulley adjusted by belt pressure on 7/8? I could go up to 1" but that would require replacing the 7/8 MS75 with one needed a 1" shaft.

    Thoughts? Kinda leaning towards #2 since that's the least work. But if I need a 1" shaft for that length #3 would likely be the way to go.

  2. #2
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    I had a similar problem a few years back, the blind keyway was not possible with the tools I had. My answer was to use 2 set screws that went into the keyway on the motor shaft, its still working today.

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    If you disturb the existing shaft/rotor hub by pressing a new shaft in, plan on rebalancing/truing.

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    I extended the motor shaft on a KO Lee grinder as follows:
    I set the armature in the lathe, Collet in the headstock and center in the tailstock. I set the steady rest to fit the the armature at the bearing location. I took the armature out of the lathe and welded on an extension. The extension was oversize in diameter and length to allow machining stock. I put the armature back in the lathe supported by the collet and steady. I faced the end and center drilled. I bored the center so it was running true. I backed off the steady and supported the shaft on a center. Then I turned the OD to size and chamfered the end.

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    Where is the new shaft going to come from. New they are hard chrome plated. Probably doesn't matter.
    I would say that if you had to re-balance the rotor with a new shaft t would not be a hard job.

    I once turned a rotor on my lathe just to trim off the cast aluminum from a sloppy assembly. Then a
    static balance with threaded nuts as counter-weights were screwed on. This reduced the vibration level
    and now that motor is on my air compressor.

  7. #6
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    Quote Originally Posted by rke[pler View Post
    just press the shaft from the armature, measure it up and make a new shaft to match.....
    .. match.. whatever you NEED it to match.

    Do this.

    Rocket-science it was never. When is the last time you saw any volume-market electric motor with no shaft? How hard can it be if that's the way they are made to begin with?

    Leave the SECOND one you create, same day, oversized. Aside it for next go.

    It will be the fastest, cheapest and bestest approach. It is only an ignorant cylindrically turned SINGLE PIECE of steel.

    Why do MORE machine work and waste time crafting and aligning potentially fragile joinings when it is faster and better to need none at all - nor even bushings.

    "Fit it to what you got." The asided backup can be turned to fit NEW sheaves if/as/when time cometh.

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    I had to do this on my lathe, just made an extension adapter. couldn't do the blind keyway, had another local shop do it for $75. and had a set screw to tighten it on the existing shaft. it had to stick out past the casting for me to accept a 4 V belt pulley.

  9. #8
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    I have an Arboga drill press that came with a bad motor. Long shaft, proprietary frame size, dual speed, etc. Motor shop want $2500 to rewind it.

    I pressed the rotor stuff off of the shaft, cut the shaft shorter inside the "can", and put a Lovejoy coupling in the can. Then machined an adapter to mount a standard frame motor on the end of the original motors can.

    It works fine. Machine is functional. Doesn't look original, but for my purpose, that didn't matter.

    20201219_174316.jpg

    20201220_081615.jpg

    20210101_152047.jpg

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