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Motor anti-friction or sleeve bearings

The real facts which determine the usability of anti-friction or sleeve bearings: low coast over a long period; cheaper periodic maintenance and/or reduced noise. Another important reason for the application of sleeve bearing is the rotor dynamic design. Large machines with medium or high speed (>1800rpm) generally have flexible rotors (the machine operates above the first rotor critical speed). This kind of machines requires damping in the bearing to pass by this frequency or frequencies reducing the vibration levels in this resonance situations.

However the flexibility of the rotor can often be more severe in a very slow speed motor where the rotor diameter is very large and there is a tremendous whip. We were once called in to inspect a 1500-hp, 2300-volt synchronous motor, 12 pole. The customer had borrowed a spare motor from a sister plant and once in production they had a sever vibration problem. We determined that the vibration was electrical and the motor was removed form service and brought to our facility. Before we dismantled the motor we ran it up in our test bed. at 600-volts the motor ran beautifully. As we approached 1200-volts the vibration began and at around 1800-volts the vibration was so bad that we aborted the tests. When the motor was dismantled it was evident that this motor was a Babbitt bearing motor that had been converted to Anti-friction bearing.
This in itself should not be an issue if it is done properly.
The end-covers had been converted to accommodate the new anti-friction bearings.
The new bearings were large enough to accommodate the load.
The problem was that the original Babbitt bearing was 8 inches long.
The new bearing was 2.5 inches wide.
Once again, this is not a problem if the new anti-friction bearing is positioned properly. Instead of positioning the bearing at the inner portion of the journals the bearing had been positioned at the outer portion of the journal. This created approximately 11 more inches of free shaft. We re-assembled the motor without installing the baffles so that the stator and rotor cores were visible and once again energized the motor. When the magnetic forces within the motor came in to play the rotor was flexing so much that you could see the air gap opening and closing. The more voltage that was applied the closer the rotor got to the stator core and the intensity of the vibration increased.
There is a critical relationship between the rotor diameter, the speed of the rotor and shaft diameter.

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