I am sure there is a very simple answer to this question, but with no formal mechanical engineering education, I don't know what it is.

I have a 2.5 HP, 3,450 RPM continuous duty electric motor with a 1/2" diameter (smooth) drive shaft with 3/8"-16 threads cut into the last 1/2" or so of the shaft.

I want to use this motor to power a machine I am building (an apple grinder) which uses V-belt pulleys for power transmission. I therefore need to be able to attach a V-belt pulley to this shaft. Every V-belt pulley I've ever seen uses keyed shafts (a smooth shaft with a ex. 3/16" slot cut lengthwise, into which key stock (square-profile steel) is inserted to prevent the pulley from rotating on the shaft).

How do I adapt / convert this motor's threaded shaft into a keyed one?

I would have thought there would be a simple part, with a keyway on the outside and a hole in the middle tapped with threads, which I could screw onto the motor shaft's threads... but all of my looking has not turned up such a part.

All the other parts of the machine use a 5/8" shaft size, with 3/16" key stock, so if I can convert this motor drive shaft to that, that would be fantastic.

(It goes without saying, but I'd rather not have to pay a machine shop a few hundred dollars to create this small custom part... and I thought this would be a common enough issue that there would be a standard part for this purpose.)

Any thoughts?

Many thanks.

EDIT: In the absence of a standard, easily-purchasable part for this purpose, I’m considering trying to make an adapter, by cutting a small section of 5/8” keyed shaft, then drilling a hole through the center and tapping with 3/8”-16 threads. Given that shafting is hardened steel however, I don’t think I’ll be able to do this. (I don’t have a forge available with which I could anneal a shaft, or a lathe on which I could turn my own.)

  • $\begingroup$ Sounds like you'd have to dig thru supplier websites or catalogs (eg McMaster, though its getting expensive) until you find one. Endless combinations of shaft couplings/adapters/reducers. If your situation is truly uncommon, you may have to use two adapters in series $\endgroup$
    – Pete W
    Mar 12 at 21:39
  • $\begingroup$ Thread the pulley hole or turn the thread off the shaft with a sanding jig using the motor as its own lathe but you only have one shot at that. Then forget the key and pin the shaft to the motor. That theoretically should only require vises, indicators, and a drill press. $\endgroup$
    – DKNguyen
    Mar 13 at 2:04
  • $\begingroup$ PeteW: Sadly I’ve gone through McMaster’s catalogue as thoroughly as I know how… to no avail. Some of these small parts are so costly I could buy an entirely new continuous duty motor for the cost of a part or two; I was really hoping there was just a common standard part I was missing. $\endgroup$ Mar 13 at 2:39
  • $\begingroup$ DKNguyen: I thought about buying a pulley which I could custom machine and tap 3/8”-16 threads into, but the only ones McMaster had for which that would be possible were HDPE (plastic) pulleys… which won’t be nearly strong enough for my application. Now I’m considering machining my own 5/8” hole and keyway into a steel plate and welding it into an existing pulley. I’m just astonished there isn’t a standard part for exactly this purpose. $\endgroup$ Mar 13 at 2:43
  • 1
    $\begingroup$ McMasters is not the only supplier of pulleys... $\endgroup$
    – Solar Mike
    Mar 13 at 6:04


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