Motor Anxiety
February 4th, 2009This is Why Engineers Shoot up Their Offices
Once again, I am finding selecting tools harder than using them.
I want to make a belt grinder. I read up and found out that the biggest motor I could conceivably need would be 2 horsepower, so that’s what I’m trying to buy. I want 3-phase, so I can use a VFD. I thought a 1750-RPM motor would be good, because it would run cooler and last longer. And I figured I’d get something like 2500 fpm on it, which would be sufficient.
BUT WAIT!
Turns out speeds of up to 5000 fpm can be useful. That rules out a 1750-RPM motor. I’d need an 11″ drive wheel. I think. Hope I didn’t make any careless errors. Now I’m looking for 3450-RPM motors, and good ones aren’t that easy to find.
It gets worse. There is almost no hope that I will be able to find bearings that will work with a shaft bigger than 5/8″. And a lot of 2 HP motors come with bigger shafts. If you can’t have mounted bearings on your shaft, you might as well kill yourself.
So I have to put all these factors together. Now that I think about it, don’t I need a super-long shaft? Man, that didn’t even occur to me. It will have to go through two mounted bearings plus a thick drive wheel hub. Is there such a thing as a shaft extender for an electric motor?
Excuse me while I breathe into a paper bag.
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I guess my brain was malfunctioning again. Apparently there is no reason why I can’t put a drive wheel directly on the motor shaft and stick the motor next to the frame. Other people don’t do this, because they have elaborate pulley systems to get around the lack of a VFD.
Whew.
February 4th, 2009 at 9:19 PM
“It gets worse. There is almost no hope that I will be able to find bearings that will work with a shaft bigger than 5/8?.”
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Try McMaster-Carr:
http://www.mcmaster.com/
or Grainger:
http://www.grainger.com/
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Both carry bronze (up to 1-3/4″) and roller bearings (up to 3″).
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You can also find precision ground shafts and pillow-bearings.
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You could also use a V-belt pulley in a 2:1 ratio to get your belt speed up to 5000rpm. It’s a little more complicated than direct-drive, but it gives you more motor options.
February 4th, 2009 at 9:23 PM
When I checked McMaster-Carr, they had a very limited selection of mounted bearings that would run at high RPMs. That’s all I knew to check.
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Are you saying you can put a new shaft on a motor? I thought the shaft was an integral part of the thing, impossible to replace.
February 4th, 2009 at 10:11 PM
“When I checked McMaster-Carr, they had a very limited selection of mounted bearings that would run at high RPMs. That’s all I knew to check.”
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Sorry. I was looking at unmounted roller and needle bearings. You would need to fabricate a mount (a simple flat piece of sheet metal bolted over the bearing which is pressed into a hole in the wood could work); use a flanged or retaining ring roller bearing such as 2782T89 at McMaster-Carr and insert it in some hardwood (it’s what I did for my kid’s pedal plane 😉 Yes, I know your RPM is higher…I checked and 2782T89 can handle 5000 RPM (depending on load). Other bearing sizes are available.
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“Are you saying you can put a new shaft on a motor? I thought the shaft was an integral part of the thing, impossible to replace.”
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Actually, I was thinking that you would need another shaft/drum at the other end of the belt. For the motor, you might need a shaft extension, or you may also want to consider a spider coupling to a slave shaft if the motor’s bearing is not rated for side load. Of course, a motor rated for side load would be a much simplier setup.
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A quick warning; at 3450RPM (motor speed), you’ll need to make sure the assembly is balanced or the whole thing could fly apart.
February 5th, 2009 at 4:45 PM
BEtter recheck whether you might need 5,000 rpm or 5,000 fpm belt speed. They are different and you have used both interchangeably.
February 5th, 2009 at 4:47 PM
Show me where I referred to 5000 RPM. I can’t find it.
February 5th, 2009 at 4:51 PM
Steve is off the hook.
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I said 5000 rpm above (when I meant 5000fpm). So blame it on me.
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Still, at 5000 fpm, the rpm won’t be much different (3450 rpm as Steve says). After you factor in load at speed, you’ll probably end up with a bearing that can handle 5000 rpm anyway.