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Are Solid, Precision Shafts, Naturally Balanced?

evansste

evansste

Electrical
Jul 25, 2015
4
I'm working on a project that requires me to spin a 2-inch diameter shaft, with a length that is less than 14 inches long, at a rate of 16,000 RPM. When attempting to spin the shaft, at this speed, I've noticed extreme shaking and vibration. However, up until now, all of my shafts have been hollow. This may easily explain why the shafts, themselves, are unbalanced; hence the extreme vibration.

I've been able to confirm that my hollow shaft is unbalanced. It was designed to work inside of an air bushing. So, I've placed it inside of an air bushing, horizontally, gave it a little spin, and have noticed that it always settles in the same orientation. This clearly shows that the shaft has a heavy side.

So, here's my question. If I buy a solid shaft, will it not do this, by default? Will I be able to spin it, and notice that it won't have a heavy side?

I recognize that shafts, for air bushings, naturally have strict tolerances. So, if the material is completely homogeneous, it would make sense that all such solid shafts, would be naturally balanced. However, is this true in actuality? I'd hate to buy one, only to find that I still have the problem of the shaft being unbalanced.

It's important that the solid shaft be able to pass the horizontal, static balance test. This is because, if it does, I'd like to use it to static balance a disk, that I have.

Is anyone able to confirm that solid shafts, for air bushings, are balanced? If anyone is, then I'd love to hear from you.

If material type is a factor, I'll add that I'd like to buy a solid, aluminum, shaft for my air bushing. However, if it turns out that the shaft will be unbalanced, then I won't be able to use it, for this purpose, and that money will have been wasted. For this reason, it'd be great to hear from anyone who has tried something like this, or from those who have experience, in this area.

Thanks for your time. All input will be greatly appreciated.
 
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What is the runout of the bore in your hollow shaft? What is it's straightness? How uniform is the wall thickness? Have you attempted to static balance it? (It seems you should be able to do so). Have you sent it to a balancing shop to have it balanced on a calibrated balance machine?

I don't have experience at 16,000rpm but at lower speeds a fully machined, dead straight solid shaft does not require balancing. So I can't say if it will meet your requirements but it certainly has fewer variables than a hollow shaft.
 
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Thanks, so much, for your input.

The hollow shaft was designed to work inside of an air bushing. However, it wasn't, necessarily, designed for rotation. Thomson Linear, the company that made it, is known for making linear shafts. These hollow shafts are listed as being hollow, in order to reduce the weight of the shaft, and that the hole can be used to route fluid, cabling, etc. So, I don't believe they ever considered any real degree of rotational precision, when drilling a hole in them. All of the tight tolerances are with regard to their ability to act well as a guide for air bushings. So, for these reasons, I shouldn't be surprised that it shakes and vibrates.

To the naked eye, the hole appears to be centered, allowing for a uniform wall thickness. The hole goes through the entire length of the shaft. I've attempted to static balance it, by clamping an adjusted weight to one end. However, during the process, I realized that it'd make a whole lot more sense to use a solid shaft; if solid shafts are naturally balanced.

I haven't taken it to a shop that does balancing.

If it turns out that solid shafts are naturally balanced, I figured that, getting a solid shaft would be more feasible. As you've said, there are fewer variables, and it has a much better chance of not having issues with being rotationally balanced.

Thanks, again, for your insight.
 
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Drilling a hole to be very straight takes extra work. Generally you can't, and then you have to grind the OD a concentric as possible to that hole and balance the part anyway to remove the residual error from the not-perfectly-straight hole.

I agree that ultra precision ground solid shafting will be easier to work with and possibly have enough inherent balance for your application.
 
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There will always be some microscopic deflection between the bearings. Are there sufficient bearings to support the shaft?
 
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Granted our shafts aren't spinning as fast as yours, but our only factor in "balancing" them is straightness. I don't have any drawings to confirm this now, but a 10ft bar has to be within a few thou. Admittedly our tolerances are probly a little tighter than needed.

So as long as you can verify a high degree of straightness, yes it will work.
 
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OP
I have extensive experience building dynamic balance gear shafts. for high torque and rpm apllication..

on the drawing must require dynamic balancing to inch oz. .010 0r better.

manufacturing will be expensive for a hollow shaft, reason is shaft requires preping for manufacturing , and depending on the hardness, ( tensile and met lab requirements)
a solid shaft must be cinterless grind, then gun drilled, then the hole has to be used as the datums both ends. and the od true to the hole with in .005 or less.
if the shaft is farmed out just add dynamic balance requirement and it's good.

there will be high percent of scap with parts not conforming.
a solid shaft would be much simple and more economical to manufacture.
with less loss of parts.
 
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OP
I should add that shaft alignment is critical.
and support. bearings have to be high precision.
 
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The bearing tolerance may have a lot to do with how it's driven. Turbo machines are quite sloppy from a tolerance perspective. Their plain bushings have clearance on both ID and OD. A 3/4" shaft in a turbocharger may have 0.005" of radial run out while a typical 2 inch journal bearing would be 0.002".
 
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Yes part of the issue with out of balance shafts supported with bearing if out of balancing will
Cause damage to the bearings. Do to vibration and out of balance. Causing shorten cycle life.

Good Case of that is quality control of the Japanese cars. Massive precise bearings, gears and shafts. And other components.

The fall of the the Renault I had foolishly purchased. I had a bearing failure in the manual transmissions. Which is usually unheard of.
I disassembled it and the ball bearings fell out of the split case. The bearing casing was the most
Poor design with thin sheet metal casing.
 
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