Revisiting Pump-to-Motor Alignment: Why 0.002-in and Not 0.020-in?

Written by Lev Nelik, P.E. APICS

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From the chart they developed, motor power is found to increase by 1 percent (not 4 percent stated by Mr. Bloch) between the 0.001-in and 0.020-in alignment. Such data is of interest, and if one percent savings in energy is multiplied on many millions of installed pumps, the overall energy savings for the entire continent is, of course, impressive. However, one can now understand a reluctance to accept a motor power decrease from, say, 6.80-kW to 6.80 x 0.99 = 6.73-kW as significant, for most plants with installed pump population of less than millions. From this data, one may draw a conclusion that is less optimistic and, in fact, opposite to Mr. Bloch's comments with regard to practical significance on energy, when alignment changes from 0.001-in to 0.020-in. In light of these facts, we can understand (and not condemn too quickly) any manager's reason to save money on a more reasonable alignment effort.

Having studied his references, I must unfortunately point out to Mr. Bloch that the subject of our discussion is the relationship between alignment and pump life - not the energy aspect (although even this topic appears to be within 1 percent effect). The Prüftechnik white paper focuses on the energy aspect - not equipment reliability, nor does it cover anything relating misalignment to vibrations. Thus, I must unfortunately dismiss this as an irrelevant source on this specific subject (although it is a very good study otherwise), and ask Mr. Bloch to find another reference with data that relates to the subject of the discussion.

Furthermore, I could not find any tested filed data on quoted page 303 of Mr. Bloch's book. Perhaps I studied the wrong edition of the book in the library. However, in either case, his statement about the anticipated 50 percent decrease in life with vibration does not address the issue. I originally stated that the vibrations did not change with alignment value, thus we are not discussing effect of vibration on life, but misalignment on life.

It would be interesting, however, to see test data proving the alleged decrease in pump life when vibration increases from 0.1-in/sec to 0.3-in/sec. As a matter of reference, I am aware of similar paper published by NASA that states a similar effect. Unfortunately, after obtaining the paper, I did not find actual test data - the conclusions are based strictly on theoretical expectations.

Regarding Mr. Bloch's third reference, Mr. Berry did an excellent job describing the fundamentals of bearing life equation, vibration forces, unbalance forces, misalignment forces, V-belt tension forces, looseness forces, and so forth. He uses good illustrations to explain the mechanism of these forces, including images of a deflected shaft under misalignment similar to the one I presented in my discussion (Checking In, April 2007, p. 106). Mr. Berry's paper is excellent material for a self-study course for any mechanic and engineer involved with rotating machinery. Again, however, there is no factual test data in it, and as such, it cannot be relied upon as a relevant reference for our subject in hand.

If any of our readers have factual data relating misalignment to pump life, please send it to us. But please, no preaching - just the facts.

Until facts are presented there is, unfortunately, no data to illustrate that a 0.002-in alignment is better than 0.020-in with regard to pump life. I am sure there are additional facts I am unaware of that may present a different conclusion. If so, I will be the first one to applaud.

I conclude by noting that the Prüftecknik white paper recommends "to align machines to within an offset tolerance of 0.005-in." If so, most millwrights can actually achieve this without the expense of sophisticated alignment machinery.

¹ Thanks to Ludeca, Inc. (Doral, FL) for providing the white paper for this research.

Dr. Nelik (aka "Dr. Pump") is president of Pumping Machinery, LLC, an Atlanta-based firm specializing in pump consulting, training, equipment troubleshooting, and pump repairs. Dr. Nelik has 30 years experience in pumps and pumping equipment. He has published over fifty documents on pump operations, the engineering aspects of centrifugal and positive displacement pumps, and maintenance methods to improve reliability, increase energy savings, and optimize pump-to-system operations. With questions, comments, or to attend his Pump School, he can be contacted at http://www.pumpingmachinery.com/.

Here are the facts I believe we know (not that we think we know):

Gross misalignments cause reliability problems and reduced equipment life
It is not good to not align machinery at all
There are no arguments against making alignment as best as possible
There is little proof of what constitutes as minimum required alignment value
Much theory exists, but very little field test data exist to confirm it
There is an intuitive feel that high energy (or hot temperature) machines may have better justification for precession alignment than the majority of pumps (which are mainly low and medium energy machines), but field test data relating its life to alignment values is still scarce
No factual data on correlations between amount of misalignment and life
No factual data on correlation between misalignment and vibrations
No factual data on correlation between vibrations and life, (perhaps) non-linear
Everyone wants to save money
Everyone wants to sell something
Laser manufacturers, dial indicator manufacturers, and straight edge manufacturers all have a point