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How to make motor shaft longer?

G

Gennady

Well-Known Member
Hello gang.
I am working on the horizontal power feed gearbox for the BP mill. Motor is gone unfortunately, and I am trying to fit something I have in hand. It is treadmill motor that looks suitable. There is one problem, though - business end of the shaft is too short. What common wisdom will suggest to make it longer?
Original motor has a keyed shaft for the worm gear and tightens with the nut, while treadmill motor has a left hand thread

IMG_1354.jpeg
IMG_1353.jpeg
 
Gennady said:
Hello gang.
I am working on the horizontal power feed gearbox for the BP mill. Motor is gone unfortunately, and I am trying to fit something I have in hand. It is treadmill motor that looks suitable. There is one problem, though - business end of the shaft is too short. What common wisdom will suggest to make it longer?
Original motor has a keyed shaft for the worm gear and tightens with the nut, while treadmill motor has a left hand thread

View attachment 50455View attachment 50456
Click to expand...

I had a similar issue needing to change the diameter of the shaft to match the timing pulley on my lathe. I also needed a tiny bit of extension due to modified motor mount as well. I made an extender like this:

Screenshot 2024-08-06 164402.webp




Screenshot 2024-08-06 164424.webp
 
Do you have a welder? There are Youtube videos of people doing this by dialing in the armature in the lathe (steady rest), welding on an extension (keyed to the existing shaft, perhaps with an axial light press or slip fit), V-ing the two pieces, then turning the new piece true to the old shaft. I have a couple of motors I will be doing this to when other projects are done and I actually have a need for those motors.

There was a gentleman on the HomeShopMachinist forum (John Stevenson, died a few years back) who I believe did a lot of this type of work professionally. Not from him, but some discussion of difference approaches here:

 
Horizontal power feed. That tells me that the motor will turn both ways, and that the connection needs to be slip critical. That probably eliminates using the existing threads or any kind of press fit connection. You really need a key way of some kind. Does the shaft only need to get longer or does the diameter need to change too?
 
mbond said:
That tells me that the motor will turn both ways,
Click to expand...
Original gear box has a mechanical reverse conical gear set, so technically I don't need reverse on a motor. is there any issues with cross pin? - it will secure extension axially and rotation-wise at he same time
 
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The only thing I would look at is if there is any extra play involved. Even a small amount will throw off the dial on your feed. And since pins tend to be weak (versus keys) this may be fine on day 1, but get worse over time. Since the motor will turn only one way, this is a much smaller issue since the 'drift' won't 'wiggle' back and forth as the motor reverses. it will just accumulate as the pin bends over time - probably any single cut will be totally fine
 
mbond said:
Horizontal power feed. That tells me that the motor will turn both ways, and that the connection needs to be slip critical. That probably eliminates using the existing threads or any kind of press fit connection. You really need a key way of some kind. Does the shaft only need to get longer or does the diameter need to change too?
Click to expand...
Good point but loctite should solve that problem, no?
 
If the motor turns in only one direction, then self-tightening threads + lock tight or a pin or some combination ought to work great (mill power feeds should not run in high vibration environments).

If the motor has to turn in both directions, the threads can't be self-tightening because one of the ways the power of the motor will work against the thread. I guess it depends on how much resistance the feed will have in that direction whether something like lock tight would be enough. That depends on gear ratios, motor torque, load applied on a cut etc. etc. etc. and is hard to estimate. Eventually, it won't be, but it depends on you duty cycle.

A typical key has large flat surfaces exactly for the purpose of transferring the forces in both directions. Other shapes work too - square, hex, triangular - and the parts can be disassembled and reassembled much more easily.

If the OP has to increase the diameter of the shaft as well as its length, he could machine a keyway (or flats) into the existing shaft and use that to force a positive connection. Probably that's not any more work than finding a left tap of the right size and using it. But if it is just longer - or worse smaller and longer - that get to be quite a bit more work quickly.

since the OP says that it only has to turn in one direction, I would machine parallel flats, make an extension of the correct size and then insert a roll pin. The right tap / die should work equally well
 
DPittman said:
You could make a shaft extension that has an internal thread to mate with the treadmill motor shaft.
Click to expand...
I've seen this done on a 10hp motor. He machined a stub with a male thread and drill and tapped the end of the motor shaft. Then he coated it with solder and screwed the two together. He checked runout and it was 5 thous so acceptable to him. This was a 3 phase industrial application and I was an envious kid watching him use the lathe.
 
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