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James.

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Reply with quote  #1 
Hi all,

I have carried out many on site balancing, and I have a question about workshop balancing.

In the repair shop they have some very old Schenk balancing machines, soft bearings I think. In these machines they only run about 700cpm, how do they calculate the residual unbalance for the fan when it actually operates at 3000cpm? Is it a ratio down?

Thanks in advance.
Vibe-Rater

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Reply with quote  #2 
Hi James, hope all is well with you and now formidable family.

Think of it like this.  If a rotor is imbalanced by say 30 grams (at particular radius).  Then it will be 30 grams at 700 rpm and 3000 rpm, and any speed in between or above because the rotor mass does not change.  And subsequently that point where we assume all imbalance is concentrated is the same for all speeds and the amount is the same.  The forces change with speed, not the residual imbalance mass.

So what you do is perform the calculations to figure out the max permissible imbalance mass AT THE OPERATING / RUNNING SPEED.  and that is your target at 700 rpm or other lower speed you would balance at.  

If you achieve that your 2130/40 or other instrument will give you values recommended for further trim and if those fall below your max allowable then you are done.  If not you apply those trims.

Took me a while to get my head around this but now understand better (but do not consider myself master Yoda status by any means)

I always collect initial physical vibration measurements before beginning to balance and then again after. So to make sure I have actually improved things. Luckily in my situation I have opportunity to carry out pretty much as many runs as I need - on soft balance stand. Unlimited runs is not always the case. I have a spreadsheet I could send you but find that developing your own will force you to understand the entire procedure better and the calculations involved to arrive at the correct allowable residual for an ISO G grade. The IRD balance chart from all those years ago was also a very important part of learning.  I now always compare my results to the chart as a double check. and triple check with physical vibration measurements.

If you want the spreadsheet send me an email off forum. rgds


James.

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Posts: 113
Reply with quote  #3 
Hi John,

Many thanks for the reply. I think I got it when you wrote "The forces change with speed, not the residual imbalance mass".

A bit strange to get my head around as the vibration due to mass unbalance increases with speed, but I guess if you target the residual imbalance for the operational speed it will be good at that speed.

Thanks again
James
John from PA

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Posts: 963
Reply with quote  #4 
The key to understanding this is realizing that unbalance is caused by a difference in the mass center and rotating center.  That distance, so long as we balance below the 1st flexural mode, is constant.  Mass center takes into account weight distribution.  If I have a perfectly round shaft, and add a keyway, the mass center is shifted due to the removal of material.  The rotating center doesn't change.  Speed is not a factor.
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