Wheel / Pulley Balancing

I'm to the point of turning the drive wheel for the belt grinder I'm building for my son. It'll be 6061 Al, 6" dia. x 2" wide, running on a 3/4" keyed shaft, and turn up to 3800 rpm. I figure I'll put a setscrew on the key and another at 90 deg., which will require drilling from the circumference. I can drill holes opposite to help balance.

Any suggestions on how to balance this for a hobbyist? I'm not buying a balancer, but could build something. Or just spin it up and see how it does? Thanks.

Pete Keillor

========================================================= [Ed]

Static balance is easy. Turn a shaft that just fits the bore. Rest it on two knife edges (I like razor blades, if the device is light, but that's overkill here). It will rotate until the heavy side is down.

At 3800 rpm, with a lathe-turned wheel, you probably aren't going to have dynamic balance (wobble) problems. Those are more difficult with home-built gadgets.

If you haven't already cut the shaft to length you could turn the other end to 1/4" and spin the shaft and pulley with a fast drill, a variable speed one to accelerate it slowly and carefully. If the shaft is long enough the pulley will rotate smoothly around the combined center of gravity of it and its end of the shaft. You could carefully touch a laundry marker to the shaft to mark the light, high side and feel the amount of vibration.

-jsw

Good idea. Since the shaft is cut, I can adapt your idea by chucking in a 3/4" 5C collet in my Hardinge mill, set it up for high speed, and use the VFD to bring it up slowly. Should be able to get a sense of an out of balance condition. More rigid, but with enough overhang, it should work.

I also thought of not drilling the opposing holes completely, and deepening as indicated.

Thanks.

Pete

The lathe is NOT a good idea. I couldn't find a clear, short article on Critical Speed to explain what happens. A loosely restrained hand drill will let the spinning shaft and pulley find their own center of rotation while the shaft and pulley may whip around dangerously in the lathe.

Or mill, I posted fast to stop you. Don't spin it with anything until I find a better explanation.

If you haven't already cut the shaft to length you could turn the other end to 1/4" and spin the shaft and pulley with a fast drill, a variable speed one to accelerate it slowly and carefully. If the shaft is long enough the pulley will rotate smoothly around the combined center of gravity of it and its end of the shaft. You could carefully touch a laundry marker to the shaft to mark the light, high side and feel the amount of vibration.

-jsw

========================================================== [Ed]

One-plane (static) balancing is generally considered plenty for this type of job:

(page 4)

That's just a knife-blade job, as I described. Make sure the knife blades are DEAD level. The back sides of two hacksaw blades should be plenty sensitive. Just stroke them with a stone or some wet-dry to make sure they're smooth.

On Wednesday, August 6, 2014 2:36:15 PM UTC, Pete Keillor wrote: \

I would probably just spin it up and see how it does. But this has come up before in this group.

Take a look at " Orthoganol balancing - twinwheel grinders " back in 09. It is a really good thread started by Jim Pentagrid.

Dan

My home made belt grinder has a DC motor with a max speed of 6000 rpm. With the pulleys that I made, the 6" x 2" contact wheel can run at 4000 rpm. This 10% over the threshold for 2 plane balancing in Ed's reference. It was balanced with the knife-blade method and does not show any imbalance.

I gotta admit that I hardly ever run at max speed, which is a belt speed of 6000 fpm.

Bob

Yeah, go with that. For a quick check maybe you could roll the shaft on two parallels on the mill table, shimmed as level as you care to.

The only difficult part of making a static balancer is drilling an accurately centered conical recess that doesn't have a small flat in the center. A split-point drill might work.

-jsw

Hmmm ... if you make the screw enough bigger (and perhaps an Allen head cap screw (if you have hollow ends and hubs within that), you could compensate for the mass missing from the drum surface. Then no need to drill an extra hole opposite. Maybe add a nut for locking and extra mass, too.

Enjoy, DoN.

The pulley is aluminum and presumably the set screws will be some kind of steel alloy so they are three times the density of the aluminum, which should make up for the "missing" volume from the threads and hex key socket. They might even be too heavy. Considering how close they are to the shaft, I bet the balance will be pretty much dead on with just the two plain set screws. If you set up the knife edges it would be interesting to check the pulley before you drill and tap the set screw holes, just to make sure it is dead on, and then you could see if the set screws change anything.

----- Regards, Carl Ijames

Hmmm ... if you make the screw enough bigger (and perhaps an Allen head cap screw (if you have hollow ends and hubs within that), you could compensate for the mass missing from the drum surface. Then no need to drill an extra hole opposite. Maybe add a nut for locking and extra mass, too.

Enjoy, DoN.

This one will be very similar speeds.

Thanks to everybody for the suggestions. I'm out of town and didn't have access for a while. It's the wife's business junket, but spouses were invited, so who in their right minds wouldn't get out of Texas in August to Park City, Utah. Got in some very pleasant hiking.

I now plan to use the knife edge idea originally mentioned by Ed, then depending maybe add extra set screws if needed or drill opposite. Probably just spin it first, but the disc would be near the mill spindle on the 3/4" shaft which is pretty short, 10" back through the collet. No worries about whipping. If it seems smooth enough, that'll be it. The idler and contact wheels should not be a problem because they run on their own bearings and are uniform.

Pete Keillor

Does an unbalanced flycutter or boring head make your mill vibrate?

-jsw