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Extending a motor shaft

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  • Bob Engelhardt
    replied
    Follow up: I kept it simple and made a 5/8 extension bored 1/2 to fit over the shaft. Loctite-ed for remove-ability. Cutting a key way was a problem: a 3" cutter on my horizontal mill would hit the armature winding before the key way was long enough. Finished it (badly) with a carbide burr and file. Next time I'll cut the key way before Loctite-ing

    As careful as I tried to be, the result had 0.009 TIR. Ugh! I fixed that by grinding in place:
    Click image for larger version  Name:	IMG_E1981.JPG Views:	2 Size:	1.79 MB ID:	1917999

    Of course the shaft is no longer 5/8, but the runout is now 0.0002-3. Next time I'll make it oversize and grind to size.
    Last edited by Bob Engelhardt; 12-25-2020, 11:15 AM.

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  • J Tiers
    replied
    On-center is a requirement anyway, if you expect to get a usable taper.

    If you use the compound, you should be able to use it for both ID and OD at same setting, but you obviously change tools between the two.

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  • darryl
    replied
    Way back when I did a lot of taper turning on the Unimat- mostly for fun. Tool height being exactly on center is a good reminder.

    In my case, the headstock rotated to allow this, while the tool rode only left and right, not at any compound angle. The hard part was duplicating the rotated angle to allow boring a matching taper. The Unimat had a pin that would align the headstock for normal turning, but only rough markings for other angles. Straight turning was a bit of a task, as the pin would need some play in order to insert it, and that play could have your headstock alignment off by some unknown amount, and I'd have to do some testing to see when I got it right. I ended up making a tapered cutter and a matching tapered pin to eliminate this problem.

    I've gone off topic of course- back to the original problem now. Has a decision been made as to how to do the shaft extension?

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  • old mart
    replied
    I would like to add a comment to J Tiers taper recommendation in post #17. Make absolutely certain that the tool height of both external and internal tools are exactly on the centreline, or the tapers will not match even if the compound is not moved.

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  • Bob Engelhardt
    replied
    Originally posted by Bob Engelhardt View Post
    ...
    I have some LocTite 609, 660, and 680 retaining compounds. The 609 ...[has shear psi such that] torque limit would be 150 ft-lbs (203 N-m)! I do need to test that it hasn't timed out (exceeded its shelf life).
    I did the LocTite test: a 1/2" rod in a 1/2" plate took 30 ft-lbs to break. Works out to 1800 psi shear (2300 psi spec). It has an expiration of 2006. So, 22% loss in 14 years. I can live with that.

    My shaft extension will have a 1/2" shaft in a 1" long extension - 60 ft-lbs yield. More than good enough - the motor is rated at 1.9Nm (1.4 ft-lbs) at speed. At stall it will be considerably more, probably not 40x. I hope.

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  • J Tiers
    replied
    There is a world of different tapers. Fast tapers would be little use, they would push right off from side pressure. Very shallow (locking) tapers would wedge on tightly and be hard to ever remove. But that may not be a bad thing.

    An intermediate taper, not quite self-locking, would be the best. And, yes, certainly a retaining bolt, even an MT or B&S taper wants a bolt to hold in the arbor.

    Something you do have a reamer for tends to sound good, although that would force you to match tapers, which is more fuss.

    Just do not move the compound between cutting the male taper and boring the socket. You can then choose any taper in the wide world to use.

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  • Arcane
    replied
    Originally posted by Bob Engelhardt View Post

    I had never considered its shelf life until I had a red and a blue thread lockers go bad. They were probably 5 years old and 95% unused. Really annoying to throw out so much unused product. I replaced them with much smaller sizes.
    I am shocked. 5 years? Did Loctite change their formula to shorten the shelf life?

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  • Bob Engelhardt
    replied
    Originally posted by Arcane View Post

    From what I have read Loctite has an extremely long shelf life regardless of what the company says. Don't forget, they want you to buy new stuff regularly. I have some that I got back must be close to 35 years now and it still works!
    I had never considered its shelf life until I had a red and a blue thread lockers go bad. They were probably 5 years old and 95% unused. Really annoying to throw out so much unused product. I replaced them with much smaller sizes.

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  • Paul Alciatore
    replied
    I have mused about tapered fit extensions in the past for small diameter shafts. The taper does seem to be a stronger way to do it and, as you say, it can be easily removed. But then, perhaps too easily. You would not want the constant vibration and cyclic loading from the motor to work it loose. Taper plus a screw perhaps? That is used for tooling.

    The 1/2" size here does border on small, but is probably large enough to be OK with the method I showed. A taper fit would require both a taper reamer and a matching taper on the shaft.

    When you said "press fit" a wild thought came to me. Yes, make it per my sketch with a press fit. But add a larger size, internal thread in the extension. Normally that thread would not be in use and the retaining bolt would pass through them without engaging. But if disassembly is needed, a bolt with that thread and a smaller diameter for it's first inch or so could be used to disassemble it. This disassembly tool would reach past the threads in the shaft to the bottom of that threaded hole and the threads on the section in the extension would allow it to push the extension off. It would be a somewhat long bolt/tool.

    The biggest problem I see with this is the disassembly tool would almost certainly be lost long before disassembly was needed and the poor fool looking at how to do it would be hard pressed to guess how to make a replacement. I did say it was a wild idea, didn't I?



    Originally posted by J Tiers View Post
    Paul A's solution would be even better if the joint is tapered and not parallel. Tapered joints can be made very tight but still removable. A parallel joint cannot be made both tight and easily removable, there must always be some clearance or it becomes a press fit.

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  • ed_h
    replied
    I had to do something similar to a garage door opener motor. I turned about 5/8" of the the 7/16" motor shaft down to 1/4", and threaded it. Then drilled the end of a short piece of 1/2" rod and tapped it 1/4-20. Screwed the extension on, then turned it down to 7/16 in situ. Voila--a same size shaft extension that runs true.

    Ed

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  • Arcane
    replied
    Originally posted by Bob Engelhardt View Post
    .......... I do need to test that it hasn't timed out (exceeded its shelf life).
    From what I have read Loctite has an extremely long shelf life regardless of what the company says. Don't forget, they want you to buy new stuff regularly. I have some that I got back must be close to 35 years now and it still works!

    Leave a comment:


  • Bob Engelhardt
    replied
    Originally posted by Paul Alciatore View Post
    I am not a big fan of reducing the diameter of a shaft to extend it. ...
    Yeah, I'm thinking that 3/8" is really pretty small.

    As for welding an extension on, beyond the danger of melting internal components of the motor, heaven only knows what that would do to the strength of the shaft. ...
    The welding alternative was a way to secure a sleeved extension to the shaft. It was just using a couple of plug welds. The advantage would be much less heat than silver soldering.

    ... Here is a drawing of what I mean:
    Click image for larger version  Name:	ShaftExtensionBnW.jpg Views:	123 Size:	96.1 KB ID:	1915171
    Nice drawing.

    ...An appropriate LocTite adhesive (thread locker) can be used to ensure that the joint does not slip. I would recommend a diameter of at least 3/4" for the extension, but you may be able to get away with less. ..
    I have some LocTite 609, 660, and 680 retaining compounds. The 609 is for close fitting assembly (from press fit up to .006 clearance) & has a shear strength of 2300 psi. Which means for my 1/2" diam, 1" long joint, the shear torque limit would be 150 ft-lbs (203 N-m)! I do need to test that it hasn't timed out (exceeded its shelf life).
    Last edited by Bob Engelhardt; 12-12-2020, 11:06 PM.

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  • Duffy
    replied
    Why not a suitably sized taper pin. Of course this presupposes that you have a matching taper pin reamer

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  • J Tiers
    replied
    Paul A's solution would be even better if the joint is tapered and not parallel. Tapered joints can be made very tight but still removable. A parallel joint cannot be made both tight and easily removable, there must always be some clearance or it becomes a press fit.

    Leave a comment:


  • dian
    replied
    make a light press fit addapter, pin it and thats it.

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