I have a small 6 lb rotor on a soft susp. pedestal and although the bearing journals are very smooth another frequency is dominant(10x) even when inducing a significant unbalance. Right now the rotor is riding on acetal vee blocks. I think that there is mircoscopic fluting(less than a few tenths)from machining. I think I can faintly see the light changing on the journal surface as I spin it by hand. I have changed belts, idlers and I'm pretty sure that it is not cogging from the motor. Would a pair of acetal half sleeves help reduce this by increasing contact area? I am concerned that the half sleeves will affect plane separation by reducing freedom of motion.

I can easily filter these out, but it's really bugging me to find the source of the errant frequencies as they just shold not be so prevalent.

This message has been edited. Last edited by: Andy Paul,

Well, the half sleeve bearings made a huge difference. The contact area of the v block is so small it picks up even the most minute roughness in the bearing journal. The parasitic mass of the suspension system is below 100 grams.

Hi Andy, how is the (self-described) lunatic balance guy making it? I just visited your web site and I must say I am very impressed. Looks like you incorporated most of the earlier feedback you got from folks here. I will give you a call in a few days. I am just setting up a balancing shop and may well want to use your stuff. Stay at it!

Oh, are you using any kind of lubricant on the V-blocks? I am not familiar with acetal... does it require lubrication? The 10x may be coming from friction; could be a "rub" which will often produce harmonics. Are you seeing only a 10x harmonic? Could this be "aliasing". I am not familiar with the details of aliasing but I understand most DSP analyzers use "anti-aliasing" whatever that is.

Hi Rusty!
Yes, I'm still driving myself crazy building all of this stuff. It's starting to become too much for a one man show. The acetal can run dry, but I have machined oil grooves which will hold and recirculate a light oil. I don't think it's rubbing, but what I think seems to be a contributing factor is Vee blocks are shaped like a tuning fork, so any....and I mean the slightest imperfection in the bearing journal is amplified. These small balancing cradles have a horrible signal/noise ratio, it seems everything but unbalance wants to dominate. I'm pretty sure it's not aliasing as I've verified it on my scope and also on the scopes fft function. Attached is a photo of my newest one which looks much better than it works at the moment.

This message has been edited. Last edited by: Andy Paul,

Your balancing stand looks like it is just sitting on the floor. If it is, then you might consider rigidly supporting it. There might be either a base looseness effect or a natural frequency that is impeding your balancing efforts.

Walt

quote:

Originally posted by Walt Strong:
Your balancing stand looks like it is just sitting on the floor. If it is, then you might consider rigidly supporting it. There might be either a base looseness effect or a natural frequency that is impeding your balancing efforts.

Walt

Walt, that was just a photo op! When balancing it is bolted down to a 1500 lb milling machine.
Thanks for noticing though. Image resized for better viewing!

Have you checked for resonacne at that 10x frequency? Maybe vary the speed or do a bump test?

Nice looking rig... is all that machined aluminum expensive? The V-blocks look a little tall and a little thin to me. I would keep them as short as possible, and 2x to 3x thicker to make them stiffer. The aluminum brackets could also be thicker. You want every thing really, really stiff to prevent resonances.

How are you allowing the piece to "vibrate" so that it can be measured? Is this a 'soft' or 'hard' bearing machine?

The half sleeve bearings are twice as thick, and the thin alum brackets have been replaced by thicker steel in attempt to stiffen up the whole system. It is soft suspension, and there is about 3/8 movement on a precision linear bearing. I was originally trying to keep everything thin to accomodate narrow journals and good clearance of any disk type rotors close to the journal. Most of the alum was stuff I had sitting on the shelf, probably $100 worth of alum if I had to guess.

Pete, I have tried up and down the speed range, from just over resonance to 3000 RPM.

This message has been edited. Last edited by: Andy Paul,

Andy, sounds like you're on the right track. Your v-blocks or half-brgs can be wider than the journal surface, can't they? I don't see where it would matter if the bearing surface extends out past the end of the journal surface. Might 'look' a little suspect to a casual observer, but don't think it would make a functional difference.

Oh, can I come browse through your scrap bin?