How to balance rotating plastic drum?

[VTStang66]

I'm in the (very early) prototype stage of designing a machine with a rotating plastic drum, specifically a 50 gallon drum. It rotates about the long axis. How would you go about balancing this? I'm thinking take some careful measurements and stick a rod through it, then fine tune it with weights. I don't know if there's a better way though, or how to actually figure out where the weights go. People have been balancing wheels for a long time so I wouldn't think it would require anything too sophisticated. Who's got my slick trick?

 

[unclesyd]

Could you give us some sizes and rpm along with the material of construction?

 

[VTStang66]

Length 34", diameter 23" as is. I'm going to put some vanes on it, so total outside diameter maybe around 33" or so. Total weight probably less than 100 lbs, max RPM should be under 500, probably more like 200. At later stages that could increase, but for now I don't forsee it spinning any faster than that.

Material is whatever they make those plastic 50 gal drums out of. I'm thinking I'll run a rod through the middle and sandwich the plastic with flanges, so I need a good method of finding the center first.

 

[drawoh]

VTStang66,

What is going inside the drum?

You have to balance that too!

JHG

 

[Compositepro]

A plastic drum will likely change shape and balance under rotational stresses. Balancing can be simple or very complex, depending on how accurate you need to be. Google balancers for info.

 

[VTStang66]

Drum will be empty except for the rod running through the middle. Doesn't have to be too accurate, I just don't want it visibly wobbling or shaking the fixture it's mounted on.

 

[unclesyd]

Sounds strangely like a wind generator rotor.

 

[drawoh]

VTStang66,

The last time I watched a mechanic replace my tire, he balanced the wheels statically. This worked okay because a car wheel is a short fat cylinder. Perhaps I got lucky.

In your case, the worst scenario is that your cylinder is balanced statically, but the opposing weights are at opposite ends, creating a centrifugal moment, and a dynamic imbalance. If your drum is longer and thinner in aspect than a car wheel, you could have a problem.

Is your drum symmetric about the axis?

JHG

 

[VTStang66]

unclesyd: You're on to me! Now I have to scrap the top secret plans and work on something else

Drum symmetric? It's supposed to be, but nothing ever really is, is it?

 

[MikeHalloran]

Put the head on the drum, put a hole and flanges on that too, and run the shaft through both ends of the drum.

If the shaft has to be cantilevered to do that, so be it; space the bearings apart, and make the bearing adjacent the drum bigger than the other one. That allows you to taper the shaft in both directions, fat in the middle of course, which can make assembly easier.

If you can keep the drum fairly round, you may be able to make it balance itself once it's rotating, with a partial load of sand, balls, or water.





Mike Halloran
Pembroke Pines, FL, USA

 

[gerhardl]

Vertical or horizontal construction?

I go with Mike Halloran, but this is also about dimensioning and selecting the right type and strength of ballbearings in additioning to balancing your drum.

With this size you could perhaps use directly same machine and methode as a normal vehicle wheel-balancing machine?

 

[VTStang66]

Thanks for the responses, this is on hold for a while. I sort of put the cart ahead of the horse.