Rotating mass

[banshee1]

1st time posting here and I'm not sure if this is where I should post my question, so feel free to move it if it's not.

For my project (an inertia dyno) I need to spin a pair of rollers at roughly 4lbs each at roughly 4500 rpm's. The rollers are Ø3" and will be made from mild steel which will be welded onto a Ø5/8 or Ø3/4" shaft. My plan is to have both rollers welded onto the same shaft, but I may have to use 2 shorter shafts and couple them together. I plan to have the diameters of the rollers turned once they are welded to the shaft so everything is on the same centerline to help eliminate vibration. I'm going to have a bearing on each side of each roller.

Obviously I don't want anybody getting hurt as this will be used as an educational tool so I want to make sure this is safe. Does anybody see any problems with what I'm trying to do?

Instead of 2 separate rollers I can make it one roller about Ø2" in diameter about 13" long to get the same moment of inertia, but my thinking is because the roller will now have to spin at 6500 rpm's I'll have problems with the center of the shaft trying to bend and causing more problems. I would be concerned if I had a good way to add a bearing and support in the center, but a bearing with a 2" ID is quite costly.

Any ideas, or directions anybody can give would be greatly appreciated.

 

[EdStainless]

Large shafts, short bearing spans, and good dynamic balancing should help.
I would go with 3" and the slower speed.
Will these be solid or hollow?
It may be very difficult to balance hollow ones, most commercial seamless tube has serious concentricity issues. If you go hollow make sure that you use welded tube.

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P.E. Metallurgy, Plymouth Tube

 

[DeleteriousPhases]

any drawings you can share?

 

[metalman8357]

Definitely send these out for dynamic balancing as EdStainless suggested. I made a similar educational setup recently and sent my fabricated shafts out for balancing for only $30/piece.

 

[Tmoose]

solid rollers with less than 0.001" OD runout would have pretty good "inherent" balance.

For the relationship see Figure 1-A here -

http://www.irdbalancing.com/downloads/TechPaper1BalQualityReqmts.pdf

As Ed said Hollow rollers often/usually suffer from wall thickness variation. Machining the OD true does not fix that.
If the ID runout is more than a few thousandths then balancing can quickly become a nightmare.
Pre-balancing the rough machined hollow cores allows adding weights to the inside. So hopefully the remaining unbalance can be corrected externally.

 

[banshee1]

Sorry, for not getting back to you folks sooner. Was away on vacation.

I don't have any prints, it's just all in my head right now. Haven't committed anything to paper yet.

The rollers will be solid. I thought about using tube, but I know that stuff is very inconsistent and I don't want to have to get it all welded up and have to turn the OD and then sent out to be balanced if I can help it.

My full time job is as a machinist/ CNC programmer. I'm doing this for my local hobby shop who was asked by a teacher from the local high school if this was something that could be done. Because the school doesn't have a lot of money for this, they obviously can't afford to pay a machine shop to make the whole thing they went to the hobby shop thinking it was something that could be purchased. I've made a few things for the hobby shop in the past (good friends with the owners and I spend a lot of time there with my son) so they asked me if it's something I could build which I'm happy to do even though the school isn't near where I live or one my kids will go to. I've just requested that I be present when they use it and that I own it but they are more than welcome to call me to use it. This way I can inspect it as it's used and make sure it's used correctly.

Thanks again for the info/links guys/gals.