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Pumps & Systems, July 2007

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.¹