Reliability Engineering Snapshot TM

Illustrated Case Studies in the Maintenance Engineering World of Failure Analysis, Predictive Maintenance, and Non Destructive Evaluation



Vibration Case No. 22 - 800 HP Motor Loose Fit Ball Bearing and Shaft with Fretting Damage

Distinct yet subtle changes in vibration data always signals a problem ... no matter how much you don't want to believe it. Motor Enbells & Bearing Internal Seal Plate
800 HP Motor Disassembled ViewTo the left is a disassembled 800 HP induction motor. The motor bearing endbells are shown above. This motor was in service for a little over two years. It wasn't doing anything to alarm anyone, excepting that the vibration data on the outboard motor bearing showed otherwise. The vibration was very subtle. The trend data showed a gradual rise until the last month when it sky rocketed. The motor was taken out of service, but not without a great deal of resistance from some very concerned people.

Upon inspection of the entire unit, it was discovered that the outboard motor bearing had become loose on the shaft. The looseness allowed the rotor to slip and gouge out material. The bearing seat on the outboard motor endbell (above, upper right) required repair. It had been wallowed out by the outer race banging loosely in the endbell (use cursor to see close up, the darkened area is fretting damage). The stator did not require any repair, the rotor never contacted the stator.


Inboard Motor Brng Journal Area View Outbrd Motor Brng End of Rotor Shaft


Rotor Inboard Bearing Surface is in good condition. The seat is located to the right of the threads and is at the same elevation as the threads.


Rotor Outboard Bearing Surface is damaged. The inner race wore into the bearing journal surface. The seat is located to the left of the threads in the photo to the right. ( Use the cursor to zoom in on two damaged areas. The bearing seat is to the left of the threads in both close up photos).

Fretting Damage on Loose Outer Race on Outboard Motor Bearing
Vibration Trend PlotThe loose outer race of the outboard motor bearing is shown above. The dark area in the middle is fretting damage caused by the constant pounding of the outer race against the endbell bearing seat.

The last set of vibration data (lower right) was distinctly different than the baseline data (lower left). There was impacting occurring at a magnitude of 10 times the normal amplitude, as evidenced by the waveform (bottom, lower right). The vibration frequency spectrum changed around the stator slot frequency (72X). Instead of visible frequency bands in this neighborhood with a very low noise floor, the noise floor almost washed out the stator slot frequencies. The amplitude scales are not equal as shown here in order to emphasize the low noise floor around the stator slot frequencies for a well aligned rotor/stator assembly, in the spectrum (top signature, lower left).

Good Signature and Waveform Vibration Signature for a Bad Outbrd Brng.

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