May 23, 2013
Non-Repeatability, a Little Movement can Cause a Lot of Headaches!
By James Pekarek
A recent Fixturlaser GO Basic Training class performed a field alignment on a 100 HP, 3 Phase, 5600 RPM electric motor coupled to a crude oil pump in a refinery. The class decided it would be permissible to use the 3600 RPM setting in GO Basic active tolerance table, which is +/- 0.5 mils/1” angular and +/- 2.0 mils offset misalignment, both in the vertical and horizontal planes.
During the alignment process, we were getting non-repeatable results in both the vertical and horizontal planes. The angular and offset coupling values would change just enough to throw us out of tolerance.
After correcting and re-measuring several times, were we getting a little frustrated. Something HAD to be moving.
It occurred to me that during the horizontal adjustment phase, the motor shaft was being “held in place” by one of the mechanics to steady it. This is simply the wrong procedure. Even on a heavy electric motor, it is difficult to prevent shaft movement by hand. On some larger 3 phase electric motors, the shaft will roll very freely in the normal direction of rotation, but there will be some resistance in the opposite direction due to “residual magnetism” in the windings and stator.
I tried to steady the shaft myself on the next alignment and had the same results. I did notice although, that no matter how steady I held the motor shaft during the horizontal adjustment, the shaft was indeed rolling slowly causing the inclinometers to change. Almost unable to perceive, it moved just 1/10th of a degree every couple of seconds. During the process of moving the motor horizontally, this movement would add up to a few degrees of rotation, enough to skew the readings and create inaccuracy. So if the shaft was rolling during the horizontal correction process it was probably “drifting” during the measuring process affecting our repeatability.
Aligning to a close alignment tolerance will be extremely difficult if you have movement in the sensors.
My solution was one that everyone should be doing. We built a “steady rest” from a 2×4 and a steel cylinder to rest the sensors on when rotated to a horizontal plane. With the sensors gently resting on the steady rest, we were able to take the measurements, make the live horizontal adjustments, and follow a bolt tightening sequence without any shaft roll.
The pump came right into alignment on the very next check. This goes to show that shaft movement (sensor movement) during an alignment can cause unexpected and unrepeatable results.