Hello All,

This is in reply to the poll we did over the last 2 weeks.

Voting results as of December 13, 2006 on http://maintenanceforums.com/eve/forums/a/tpc/f/3751089011/m/8011021482

quote:

4 (10%)
0 Degrees?
0 (0%)
45 Degrees?
1 (2%)
90 Degrees?
3 (7%)
135 Degrees?
8 (20%)
180 Degrees?
7 (17%)
225 Degrees?
16 (39%)
270 Degrees?
2 (5%)
315 Degrees?
0 (0%)
360 Degrees?
0 (0%)
None of the above?

I see 16 voted for 270 degrees. The 225 and 315 votes would have been close, huh?

We placed the weight, 40 grams, at the 270 degree position. The result was an amplitude of 0.5 mils at 115 degrees, in the original position of "vertical". The horizontal amplitude was 0.4 mils and the axial was 0.45 mils.

The correction calculation called for correction weight of 46.2 grams at 277 degrees (7 degrees from the trial weight position), which split into 43.7 grams at 270 and 3.4 grams at 315 degrees. The guy running the meter was happy with this 1/2 mil, so we did not change to the calculated correction weight.

If anyone is interested in a simple explanation in how I came up with the first trial weight position on this balance job, just email me at info@alertanalytical.com. Attention: Ralph

Thanks for the great responses and the 41 who voted.

Ralph,

I was one of the 16 that voted for 270-degrees. I always try to guesstimate the unbalance location so that the trial weight provides some reduction in vibration. Plus/minus 45-degrees is usually helpful. It is very important if one is removing weight by drilling, milling or grinding.

I also try to quesstimate the amount of trial weight based on one of these methods:

1) Previous data on actual rotor
2) Previous data on another identical machine
3) Existing trim weights (some cases it is wise to remove if near heavy spot)
4) Calc based on fraction of rotor weight
5) Impact Test (refer to Art Crawford's book)
6) Weight materials available
7) Wild guess

Oh, by the way; good job Ralph!

Walt

Ralph,

Great job. Half-mil not too shabby at all.

I would have been lucky to get that if I had been on top of it!!

Gary Forsythe

quote:

Originally posted by Walt Strong:
I also try to quesstimate the amount of trial weight based on one of these methods:
...5) Impact Test (refer to Art Crawford's book)...

Can you briefly explain that?

El-Pete,

"The Simplified Handbook of Vibration Analysis Volume 1" by Art Crawford Copyright 1992 by CSI
Pages 99 - 104

Machine is shutdown
Need 2-channel analyzer with accelerometer and impact hammer
Measure Transfer Function between bearing housing and correction weight plane on rotor
Calculate vibration per unit of force and system phase lag
Measure existing vibration and phase and then calculate trial weight and location based on Transfer Function

This is a good method if you have the instrumentation for natural frequency testing and want to minimize trial runs on a critcal machine.

Walt

Walt - thanks for explaining that.

If anyone wants to look up some original research in this area, here are a couple of references.

A.~H. Church and R. Plunkett (1961)~\cite{Church:61} used a mobility
(modal) method to generate influence coefficients without trial weights.
Church and Plunkett excited the non-rotating shaft with a shaker, and
they tested this theory on a very flexible shaft, whose first three
resonances occurred at 550 cpm, 2000 cpm, and 4180 cpm, mounted in stiff
ball bearings. While quite efficient in theory this method did not
produce good results, and furthermore, one could have significant
difficulties in applying this method to an actual machine, as is pointed
out in the discussion by Josef K. Sevcik. J.~Tonnesen and J.~Lund
\cite{Tonnesen:88} used impact excitation to determine the influence
coefficients. This method has some limitations as well as sources of
error and would not be practical for field balancing.

\bibitem{Church:61}
{\sc A.~H. Church and R.~Plunkett}, Balancing flexible rotors,
\emph{Transactions of the American Society of Mechanical Engineers, Journal
of Engineering for Industry}, \textbf{83}(4) (1961), 383--389.

\bibitem{Tonnesen:88}
{\sc J.~Tonnesen and J.~W. Lund}, Impact excitation tests to determine the
influence coefficients for balancing lightly damped rotors,
\emph{Transactions of the American Society of Mechanical Engineers Journal of
Engineering for Gas Turbines and Power}, \textbf{110}(4) (1988), 600--604.

As I recall Lund and Tonnessen impacted the spinning rotor. A very flexible rotor might tend to lessen the effect of the bearings for Church and Plunkett if used in other circumstances. Note that this technique never gained wide acceptance.

This message has been edited. Last edited by: William_C._Foiles,