Page 123 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Advanced Machine Alignment

foreign material, and to assure they are all set to the same and
correct pitch.
Next, the stationary machine should be checked to assure it
is near level. This is more critical in certain types of equipment,
but should be considered for all rotating machinery. This will as-
sure that the adjustable piece of equipment starts out on a level
plane, and should allow for a minimum of shims being required
to bring the two pieces of equipment into alignment.

ELECTRIC MOTORS

In the case of electric motors, being out of level can result in
the rotor surging along its axis. Since some electric motors do not
have thrust bearings, the resulting pounding must be absorbed in
the coupling or the thrust bearing of the driven equipment. This
surging results from the gravitational center of the rotor being
different than the magnetic center. The rotor continually hunts for
the position it wants to run in. This manifests itself in high axial
vibrations. The phase and frequency of these vibrations may or
may not prove to be synchronous.
This condition is not difficult to determine. Use chalk or a felt
marker and draw a line where the motor’s shaft exits the bearing
housing. Disconnect and secure the coupling, and briefly run the
motor. Allow the motor to coast to a stop undisturbed. Inspect the
mark. It should be in the same location as when the motor was
running. If it has moved either in or out, there may be a problem
with the gravitational center. In the case where the motor was
moved to align it with the stationary equipment, the stationary
equipment must be re-leveled.

SHIMS

To assure a precision alignment, the feet or hold-down bolt
pads of the equipment must be clean and free from rust, dirt, and
oil. In addition, it is wise to remove all old shims and inspect