September 23, 2014
Is Your Coupling Relaxed?
By James Pekarek
During the OJT portion of a Fixturlaser NXA Pro training class, at a power plant, the students asked me to allow them to perform the precision laser shaft alignment of a 75 HP electric motor to pump without interrupting them or offering any advice. I obliged and let them go on their way.
The first go around they seemed very confident and went about the alignment seemingly without issue. They removed all the old shims, cleaned under the feet, corrected soft foot, took alignment readings, made the exact recommended corrections and re-measured all in proper order. The results were less than perfect.
The horizontal results showed 1.8 mil of offset misalignment and 0.5 mil/inch of angular misalignment at the coupling, this is well within tolerance for 1800 rpm, however the vertical results were a different story. They showed 6.7 mil of offset misalignment and 0.9 mil/inch of angular misalignment at the coupling, which was out of tolerance. Another shim adjustment and a new set of alignment measurements verified they had the motor and pump within 1800 RPM tolerances.
I asked them to start over from the beginning and perform the alignment again in its entirety. They received the same results almost exactly, the horizontal was fine and vertical was out requiring a second shimming sequence. We had lots of time so I asked them to repeat the entire alignment 2 more times. Each time they got almost the same results.
They felt they alignment should have been completed in one Verti-Zontal Compound Move so they asked me to assist and help with the alignment. I knew all along exactly what the issue was. We started from the beginning again and removed all of the shims. We then performed the rough alignment with a straight edge, this is the step they left out each time. When the rough alignment step is skipped and the two machines are significantly misaligned, it creates strain in the flexible coupling element. The two machine shafts are going to be pulled towards each other taking up any clearances in the bearings, which can be significant depending upon the bearing type and class, which in turn influences the shaft’s rotational centerlines.
Performing a rough alignment during your pre-alignment steps will relax the coupling element and allow the shaft bearings to sit in their proper positions while laser alignment measurements are being taken. This will typically allow the alignment to be performed in one Verti-Zontal Compound Move saving you a considerable amount of time. (Not roughing in will also affect dial indicator alignment measurements).
For more information on how bearings clearances can impact shaft alignment measurements go to: http://www.vibradev.wpengine.com/video-library/shaft-alignment-concepts/