Intuition is wonderful, but sometimes deceiving. For example, I have always intuitively felt that "the better the alignment, the longer the pump life." But intuitively preaching about the horrors of bad alignment from an air-conditioned office is one thing; actually chasing a 0.001-in alignment issue under a 100-deg summer sun is something else entirely.
In other words, intuition does not address the real question: how much longer? So let's talk facts: what we know, not what we think we know.
I received a lot of positive feedback from my column "Pump-to-Motor Alignment: Why 0.002-in and Not 0.020-in?" (Pumping Prescriptions, February 2007), which covered the theory of forces acting on bearings due to misalignment, with calculations presented showing L10 life decreasing from 2.3 years to 1.1 years when the rotor offset increased from 0.002-in to 0.020-in for a typical end suction pump.
Practicing engineers know that any theory can be quickly substantiated or disproved through field data. Because I had no actual field data, I was skeptical of my own theory. So I asked our readers to present their own field data, if available. I also personally performed vibration measurements of the same pump, initially misaligned to 0.020-in and then to 0.002-in. I found no change in vibrations: 0.08-in/sec in each case. I was a bit puzzled.
The feedback from our readers was intense. Interestingly, they are all in search of the same thing: quantifiable (tested, not analytical) answers.
Marvin Williams of PCS Nitrogen Trinidad Ltd. stated that after moving to laser alignment, the MTBF significantly improved on their 1,600 pieces of equipment (Checking In, April 2007). However, no reference to the alignment method used prior to purchasing the laser alignment tool implied that the real attention to aligning to any value came with the purchase of the laser tool, perhaps as a side benefit, but not as the main reason. The question remained that if the technicians' attention was brought to aligning with a straight edge versus no alignment at all (or by eye at best), perhaps the same improvements would have resulted.
Rick Kesig of Littleford Day, Inc. questioned the L10 life based on entire rotor mass offset to 0.020-in vs. 0.002-in (Checking In, April 2007). He believed my calculations would be valid for the case of imbalance, but not misalignment. Internal forces are very different between the imbalance and misalignment, thus the life of the bearing would not be significantly impacted. He felt this also would explain the unchanging vibrations in my test.
Rick's refreshing comment forced me to dig deeper and review in greater detail the nature of the deflections, forces, and behavior of a rotor, as well as a coupling, during misalignment. Figures 1 and 2 in that issue's discussion illustrated that his argument holds true for the case of a "weak" coupling (coupling life is reduced), but in the case of a rigid coupling, pump bearings are indeed affected due to rotor distortion. I quantified my answer with numerics.
George Gates, a millwright, pointed to additional concerns with a seal, and not only bearings (Checking In, April 2007). This too would require a rotor to deflect due to misalignment in order for seal faces to accede their normally acceptable limit of 0.002-in. Again, a 0.020-in misalignment between shafts does not necessarily equate to such at seal faces, depending on the coupling design and stiffness.
Eugene Edens of United States Lime & Minerals pointed out the "pleasures" of standing in a hot Texas sun trying to align to 0.002-in, and his research indicated no difference on pump life between 0.002-in and 0.020-in - but a big difference in the time and effort to achieve the 0.002-in (Checking In, May 2007).
Uri Sela, a P.E., expressed concerns that some narrow-minded manager might be tempted to cut his manpower costs if a 0.020-in target was allowed instead of a 0.002-in (Checking In, May 2007). He agreed that true data is required to proof or disproof the issue technically, and such data is missing. On the other hand, it would be premature to accuse a manager of being narrow-minded in his desire to cut cost: it is always easy to preach perfection under someone else's money.
Phil Mix of Invista provided input on hot oil pumps alignment, where bearings were repeatedly failing shortly after the pumps were restored to service after repairs. Upon investigation, he found that hot alignment was specified in the procedure, but due to safety issues, the decision was made to do cold alignment only.
Engineers then revitalized the hot alignment specification, insisting in its importance. However, agreeing with the validity of the safety concerns, they decided to calculate thermal deflections of the unit and set cold alignment below these calculated values without actually doing hot alignment itself. Doing this, bearing failure rates went down dramatically.
Interestingly enough, the initial cold alignment was still done by a simple method and believed to have reasonable accuracy with proper attention. Thus, it was apparently not the method of alignment per se, but doing the aligning, versus not aligning at all, that made a difference.
Heinz Bloch offered three references that he claimed contained the factual proof of effects and relationship between misalignment and pumps life (Checking In, May 2007). Hoping to finally have actual field data that showed the relationship between misalignment and equipment life, I thoroughly researched each of his three references.
In his first reference the data is brief and does not describe the details, but it is nevertheless a very good practical research work, and Prüeftechnik GmbH (Ismaning, Germany) should be complimented on what I feel is the only known publication on the subject with facts, and not hearsay.
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.
Thanks to Ludeca, Inc. (Doral, FL) for providing the white paper for this research.
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
Pumps & Systems, July 2007