Okay guys - let me try to state this in real world terms. First of all, if you have to mud in a rotor prior to grinding, then you must not have a lot of confidence in your balancing equipment. Let us take your scenario where the rotor is out (single plane), say 25 grams at the desired correction radius. You would place 25 grams of temporary weight at the desired place using the "add" mode of the instrument. Now you will mark a spot opposite the temporary weight and grind a little to see what happens. Remember, grinding is an "art", not a science. You would also have removed the temporary weight. Now you respin the part in the "remove" mode and see what has happened. What if the machine is working properly and the next answer is 20 grams at a slightly different angle (indicating that you are not a Van Gogh with a grinder!)? Given your premise that you cannot overcorrect, do you then change the instrument back to "add" mode and re-mud again? Probably not, right? Then what is the difference between correction run #1 and correction run #2? All I am telling you guys is, that in the real world, unless we are charging by the hour, NOBODY muds in a part prior to grinding unless they suspect a problem. And, yes, I have seen rotors that look like swiss cheese because the balancing machine operator continues to try to correct after the second attempt when the first attempt did not work properly. If it did not work the first time, what makes him think it will work the 15th time? Either the calibration needs to be re-done on soft suspension or the constants need to be re-verified on a hard suspension in this case - but it is rare.

Arne's point is much more important - not only must eccentricity be taken into effect on a mandrel, but fit-up on the pump shaft must be considered. A lot of our effort, given the trend toward tighter tolerances, is wasted when we have a part with a 0.001" or so fit-up slop on its shaft. Do the numbers - multiply the possible centerline offset by the grams of the part weight and compare to, say, 4W/N - are you balancing a heavy spot or creating one by correcting for eccentricity?

Earl,
I respect your knowledge. I don't believe I ever thanked you for the article on the importance of ecentricity and balancing that you sent me on another thread. Thanks, I appreciated your input as I have many people on this forum.
Not long ago we had some couplings come into our shop just like several I had balanced in the past. I was swamped with work as was the other guy who had been balancing them and there was another guy who knew how to balance in the stand (IRD 260 balancer). I went over it with him to make sure he understood what the customer wanted and he assured me he could handle it. I went to my office and was working on reports and about an hour later he asked me to look at the couplings to see what he was doing wrong. He had drilled three holes on each end of the coupling and they weren't close to the right spot. After checking his setup, he had it blocked wrong on the ends causing it to wobble slightly which resulted in erronous readings. His confidence in balancing led him to believe he could do what the machine told him. We corrected the setup and I asked him to verify on the first run after calibration to insure everything was correct and to drill one small hole to start with to make sure he was not taking off to much metal. Once you understand how much and where, then proceed to balance with confidence. My advice from now on will be confirm before drilling or grinding unless you are confident enough you would not be asking my advice anyway. And just to clarify this, I believe confirming just the first correction is sufficient. Unfortunately the calibraton run does not do this. Regardless of how knowledgeable we are, bad data in will always result in bad data out.

Ronnie

Hi Folks,

Sorry if I am beating this Pump Balancing Thread to death but I must make this point.

When we have selected the balancing plane that fits us the most for unbalance corrections does not mean that this is where the unbalance in the rotor resides... agreed ?

So it is normal that we may be chasing our tail when taking material off right at the beginning without first establishing the effect of adding temporarily material at 180 degrees from it and noting its effect.

Supposing now...just supposing for a moment that farther on the hub, say 4-5 inches away from the back face of that fan, there is a void or a small eccentricity perhaps in the casting and this is what causes the unbalance, therefore if we must use the back face for correction because of no other possibility... are we sure that we are not introducing a couple unbalance ?
And subsequently finding a new sets of readings after corrections have been applied ?

So, it is not surprising that balancing the odd impeller becomes a nightmare.

Markoleo