| Vibration vs. Coupling Alignment
We rebuilt a small, 25HP, air blower for a WWTP, the customer coupled it up with a straight edge alignment, and the vibration levels were reportedly high. Our check of the vibration levels showed the motor was vibrating radially in the .31"/sec - .36"/sec range, with axial levels at .43 and .56"/sec! The horizontal and vertical blower vibration levels were not quite as high in the .11"/sec - .17"/sec range. High axial vibrations generally point to shaft misalignment.
A check of the shaft alignment with our Optalign Shaft Alignment System confirmed the alignment was not within tolerance, but it was only .010" out vertically, and .020" horizontally. Usually with a flexible coupling, this "minor" misalignment would not have this much effect on the vibration levels. However, this was a gear type coupling. They are not near as forgiving.
We also corrected for a soft foot of .010". Our final cold alignment measurements were in tolerance at the .001" - .003" range, with just a small amount of offset for thermal growth. There were no other installation problems: all motor & blower hold-down bolts were tightened adequately. Baseplate vibration pads were positioned correctly. All blower piping was supported properly with flexible piping sleeves to isolate it from the building piping. The installation was in accordance with manufacturer's recommendations.
After the shaft alignment, all of the vibration amplitudes decreased by at least half. The axial levels decreased even more to less than half of the radial! Final vibration levels were well within tolerance for 3600 rpm operation. Correcting for soft foot was probably the key to reducing the vibration levels. It is just as important as alignment of the shafts.
Lesson Learned: Excessive vibration in rotating equipment is a release of wasted energy. Motors have to work harder, and thereby draw more amps, due to the vibration. Shaft alignment decreases vibration, and thereby reduces the operating cost of motor driven equipment. Make accurate shaft alignment your first step to "going green" and saving energy!
|
