Motor shaft options, choices, decisions decisions...

better-lathe-than-never

better-lathe-than-never

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I'm starting a new project building a belt grinder. I want to re-use an existing 230V motor that I have laying around. I took some measurements of the shaft:

OD = .850 (just under 7/8'')
Keyway = .250''

My options for the drive wheel of the grinder are either 7/8'' bore or 5/8'' bore, both with 3/16'' key slots - I'm trying to figure out what's more economical and easier to accomplish (with my skill set). I have these choices as I see it:

1) Buy a drive wheel with 7/8'' bore and make a shim for the undersized shaft, then cut a 1/4'' slot through it; re-cut the keyway
CON: may be difficult for me to make such a precision bushing: (.875-.850) / 2) = .0125'' wall thickness);
will need to broach the key slot to 1/4''

2) Buy a wheel with 5/8'' bore and turn it to the dimension I need (.850'') for the correct fit
CON: will need a broach to make a 1/4'' key way slot

3) buy a new motor... :(

Maybe there are other options that I'm not seeing, could use some help.
 
Another option would be to machine the motor shaft to 5/8ths and key to 3/16" which is standard.
 
You can get stepped key stock 1/4-3/16, So that would simplify it a bit.
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I would probably go with door #2. Mike
 
You could also wrap a piece of .012 shim stock around the shaft to fit the .875" bore.
Also, you can get .750" bore pulleys so no need to go from .625" If you go that route.
I'd recommend springing for cast iron pulleys as they run truer than the zinc ones, much less vibration.
 
The first question I have is what did that motor come from? 0.850” diameter shaft is very unusual and doesn’t calculate out to a metric size motor either. Did something spin on the shaft and wear it down? If so, you will want to clean that up so the pulley you put on there will actually fit properly. How confident are you that this motor will last for a reasonable amount of time? If you are not that confident, then I would recommend doing what it takes to modify the least amount of parts for the replacement motor when it fails. Since 7/8” and 5/8” are the most common replacements, I would make your parts compatible with those sizes.

The shim stock solution could very well be the best way to do it. When I was working at a wastewater plant, we had a fluid clutch with a worn bearing housing that was causing vibration in a thermal dryer drive system. A replacement clutch was about 3 months delivery. A Coke can was the right thickness for a shim, so we shimmed the bearing bore and it lasted several months of 24/7 operation until the new clutch arrived.
 
Ischgl99 said:
1) The first question I have is what did that motor come from? 0.850” diameter shaft is very unusual and doesn’t calculate out to a metric size motor either. Did something spin on the shaft and wear it down?
2) I would recommend doing what it takes to modify the least amount of parts for the replacement motor when it fails. Since 7/8” and 5/8” are the most common replacements, I would make your parts compatible with those sizes.
Click to expand...
1) The motor came from a milling machine with a cracked casting. I parted it out and have the motor left over - it runs well, quiet and smooth. There was a 4.5in aluminum pulley on it - and it was tight... no play.
2) That's good advice either way

I plan on pursuing the shim solution - will measure the Coke can - would be a hoot if that worked. Thx -
 
better-lathe-than-never said:
1) The motor came from a milling machine with a cracked casting. I parted it out and have the motor left over - it runs well, quiet and smooth. There was a 4.5in aluminum pulley on it - and it was tight... no play.
2) That's good advice either way

I plan on pursuing the shim solution - will measure the Coke can - would be a hoot if that worked. Thx -
Click to expand...
Is it an old milling machine? Wondering if maybe from pre NEMA standards days, or maybe it was just a custom motor that they decided to make the way they wanted and who cares about standards. That’s my milling machine by the way, my motor is 100% custom and it will be a lot of work to replace it, hopefully something for the next owner of my machine and not in my lifetime.

I was surprised myself the Coke can worked, but I had the vibration data to prove it. The housing of the clutch was aluminum, so going with an aluminum shim sounded right, and I had plenty of “shim stock” in my office :)
 
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