September 15, 2011
Do Vertical Pumps Need Alignment?
By Brad Case
Yes? No? Maybe? Yes!
You may have heard this in the past…”We can’t align our vertical pumps since there is no adjustment due the ‘machined fit’ between the electric motor and pump housing. Plus it has a rigid coupling!”
You may want to rethink this one.
Since we didn’t want the coupling and shaft stiffness to influence the alignment readings, the decision was made to remove the coupling bolts and rest the impeller in the pump bowl, treating the pump as a non-rotating shaft. The other option was to loosen the coupling bolts just enough for an approximate 0.050″ gap between the coupling hubs which would still allow pump shaft rotation but would have eliminated the coupling stiffness issue.
In this case the class wished to practice a non-rotating shaft alignment using the Fixturlaser thin magnetic brackets.
The sensors were mounted and alignment measurements were taken. The thin magnetic bracket with the S sensor on the pump shaft was moved to each measurement location, then the motor shaft with the M sensor was rotated to the same position before the measurement was registered. Note: When using the Vertical Alignment program in the XA, a bolt in the motor mounting circle is labeled #1. The alignment measurements are then registered with the sensors first pointed at the #1 bolt, second at 90 deg. and third 180 deg. from the #1 bolt.
For 1200 rpm, the allowable angular misalignment is +/-1.0 mil/1” with an allowable offset of +/-6.0 mils at the coupling center. The initial alignment results revealed the offset misalignment (as viewed from 90 deg. to the #1 bolt) was out of tolerance. The other coupling values were within tolerance.
The four motor hold down bolts were loosened with the XA monitoring the moves in real-time and the motor was pushed from right to left (as viewed from 90 deg. to the #1 bolt) to correct the out of tolerance offset misalignment. Yes–there was clearance in the “machined fit”. The motor hold down bolts were tightened and a fresh set of alignment measurements were taken and the results analyzed.