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Thread: Extending a motor shaft
06-08-2010, 06:06 AM #1
Extending a motor shaft
Has anyone come up with a way of successfully extending the 5/8" dia output shaft of a 1/2 hp electric motor? I need it for a drill press that has a mechanical variable speed drive, so shaft wobble afterward will not be cool.
Right now its the standard 2" long shaft, and I need to end up with something about 5" long.
A welder buddy says that he's done it before with good results, but I thought I would check to see if anyone here has any tried and true techniques.
06-08-2010, 06:18 AM #2
Hardly 5/8" but it worked well to make a sleeve for this 1 3/8" shaft to fit this long 1 5/8" bore sheave. Sheave is held on sleeve with the visible end bolt /washer and then the whole works was taper pinned to original shaft by drilling/reaming in the bottom of one of the grooves near the motor.
Sleeve in this case is also shaft extension. A stepped key is part of the assembly and extends through the wall of the sleeve.
06-08-2010, 06:20 AM #3
If you can drill/ tap the current shaft to accept an additional one you can turn on the lathe with a male thread on one end. Then you could prep the original with a roll pin or something to prevent it from spinning off.
06-08-2010, 06:33 AM #4
Weld on a 3/4 shaft with a big V- weld all through and then turn it down to 5/8 holding it in a steady rest on the 5/8 part That way all is in alignment
Check your material is weldable
Peter from Holland
06-08-2010, 06:36 AM #5
06-08-2010, 06:39 AM #6
Thank you for all the great suggestions, gents.
Peter's idea is similar to what my buddy Bob was thinking, though Bob thought he could do it with a 5/8" extension and careful prep & setup in a Vee block (his ...).
I like Peter's idea of starting off with a larger diameter for the extension, and then machining to size afterward to ensure alignment.
06-08-2010, 07:16 AM #7
Push out the old shaft from the rotor (a decent 25 ton ton press should do it), make a new shaft, push it back in. If you don't want to do this, any electric motor rebuild shop shoud be able to do it for you.
06-08-2010, 07:23 AM #8
A welded shaft will fail at the weld from rotating bending fatigue if the pulley does not adequately straddle the weld. It is a cycles (revolutions) and load-determined life, but it will fail eventually.
Half-horse motors sell cheap on eBay, if you can find one with the long shaft.
06-08-2010, 07:32 AM #9
Good suggestion about ensuring the pulley straddles the weld !
06-08-2010, 07:57 AM #10
Everything will fail in the end
If don propperly it will hold a very long way
Almost as long as a new shaft
make sure the extension is the same material as the motorshaft
And that it is weldable
A simple check is to give the material about a 1cm weld and then see if you can file or saw right through it in the shaft If you come across a hard zone you better not weld it or at least not without a proper heat threatment and magneflux or anything afterwards
If you weld a hardenable shaft without proper precautions you can wait for it to snap If you are not experienced with it I would advice you not to
Peter from Holland
06-08-2010, 08:04 AM #11
I used to repair motors, tach generators and position transmitters for a local GE shop. Often they were very old so no replacements were available or quite expensive, worth salvaging. Frequently, the shaft was either bent or broken off, usually at the bearing. I chucked the opposite end and held the bearing in the steady rest so it ran the same way it would in service. I bored them about 80% of the OD in the bearing, which was usually larger than the shaft size, back more than a diameter, then stepped down to 60% diameter, and sometimes used a third smaller step. I made a replacement shaft with steps a light press and larger than original OD for the extension with the outside end center drilled. I filed small flats on the stepped portion, just enough for air and Loctite to escape, wet the replacement with Loctite and pressed it in with the tailstock. After the Loctite had time to cure, I backed the tailstock off and turned the shaft to size, not running on the center, giving perfect concentricity with the bearing. Done right, you couldn't tell that it had been repaired and I never had a complaint in years of repairs. A good machined finish on the bores and steps is fine because the machine marks provide some mechanical locking when filled with Loctite. Naturally, use a strong grade of Loctite.
06-08-2010, 08:37 AM #12
I think 9100 Bill's method is likely to produce better results than welding. He was surely working with large shafts. A completely new shaft is better yet, especially on a small motor.
06-08-2010, 08:46 AM #13
Electric motor shafts are generally a quite readily weldable low carbon steel. A small diameter shaft like that is going to get pretty hot, and will more or less temper itself, if it ever had a trace of hardness due to the welding.
We've built up and welded new stubs on literally hundreds if not thousands of motor (and other) shafts over many years. We always preheat any heavy section, as it is important to avoid that initial 'hard pass' which results from putting a small joiner bead on two cold pieces of iron. This is because the weld is usually done in a two sided Vee, and the first pass most often looks okay, but the second pass (when you roll it over to the other Vee) is what can crack the first pass. Then, you're welding over top of a crack when you roll it back over for the third pass and that is bad news to incorporate a crack in the middle of your weld, particularly if that crack reaches to the surface.
So the preheat is something that a good welder will take advantage of, if he is trying to produce a reliable repair. Use of a plain steel filler rod that is similar in strength to the parent material is a good method. Don't use high alloy rods, as these can produce unintentional hardening because of mixing in the weld puddle and along the edge of the Heat Affected Zone. Generally the weld quality metal is better than the parent anyway, if you can apply proper welding technique and produce 'honest welds' that are as good as they look.
BTW, pressing the motor shaft out, making a new one can be a good method......to try your patience Hint: the shaft only comes out one way. Push it backwards, it will stick, seize and score, at which time you will then have to drill it out. Installing a new shaft with a press will likely bend the new slender shaft as well, so a heat shrink is advisable. It only requires maybe a .001" interference in the rotor, so you don't have to go crazy with the heat, but you do need accuracy to ensure that the fit is an honest one, and not just tight in one or two of the laminations.
06-08-2010, 09:37 AM #14
The longest extension I did was on a 1/2 hp 3 phase motor for a centrifugal pump like a sump pump for a printed circuit etcher. Everything had to be protected from the ferric chloride etchant. I made the entire pump from PVC and sleeved the shaft with it. As I recall from 20+ years ago, the shaft was 3/4" and extended about a foot.
One thing I didn't mention, on an extension too long to turn down without support, don't center drill before pressing it in. Then carefully center drill it running in its bearing.