Page 119 - Rotating Machinery Pratical Solutions to Unbalance and Misalignment

Misalignment of Machine Shafts

Force from
Magnetic Force Driven Equipment
➔
➔

Motor Rotor Operating
Out of Its Magnetic Center

Figure 7-13. Rotor Operating off its Magnetic Center

speed. Using a piece of chalk or a felt-tip marker, mark the shaft
where it protrudes from the bearing housing. Shut off the motor
and observe it as it coasts to a stop. Note the movement (if any)
of the mark on the shaft. If the mark moves either in or out, the
magnetic center and the gravitational center are not the same.
The bearings or the assembly of the motor should be in-
spected. Unless specifically designed for operation other than
level, the motor should be placed in service in a nearly level po-
sition. If it is far off of level, the motor should be leveled and
corrections made to the position of the driven equipment so that
proper alignment can be achieved without moving the motor from
a near-level position.
Care should be exercised to assure that the re-assembly of the
coupling between the motor and the driven equipment leaves the
motor shaft in its magnetic center position. Couplings with float
should be assembled so that the motor shaft has its mark in the
center of the movement.
After the equipment is properly assembled and aligned, re-
start the equipment an observe the position of the motor’s marked
shaft.

SUMMARY

In general, the best indicator of misalignment is the high
axial vibration. As a guide, anytime the axial vibration is at least
70% of the radial vibration (either horizontal or vertical), misalign-