Bearing
Outer Race Damage
Case History Number 3
A
request came into AVT's site office to carry out a vibration survey on P9917 (VIL) pump on
one of the refineries production units due to a suspected fault with the electric motor.
This machine did not form part of the routinely monitored items and therefore no
historical vibration data was available. Once the speed and bearing types were identified
random points could be set-up in the data collector for the 5 locations on the motor,
these being the motor non drive end and drive end bearing locations in the vertical,
horizontal and axial directions.
A frequency bandwidth of 2000 Hz was selected, as this was regarded high enough to capture
any possible bearing defects.
Overall and spectral vibration data was collected from all the databased points and then
uploaded to the Vibration database held on the AVT office computer. Examination of the
vibration spectrum indicated frequency components that would be attributable to bearing
damage, with the highest overall reading recorded at the motor NDE bearing in the
horizontal direction, however, the spectrum indicating the most informative signature was
recorded in the axial direction.
Evident on this signature is a peak at 155 Hz, along with harmonic activity thereafter,
see Figure 1.
Figure 1 - Vibration spectrum
indicating bearing outer race damage.
This 155 Hz peak was
attributed to a defect on the outer race of the motor NDE bearing,
(ref SKF 6314-C3) and was determined by the following calculation :-
| N = Motor speed divided by 60 |
= |
49.4
(Hz) |
| n = Number of balls or rollers |
= |
8 |
| d = Ball/roller diameter (mm) |
= |
24
mm |
| D = Pith diameter (mm) |
= |
110
mm |
| ß = Contact angle of ball to race |
= |
0 |
0.5 x
((49.4 x 8) x (1- (24/110))) x COS ß = 154.5 Hz
The vibration spectrum in Figure 2 was
taken from the same test point on the motor after bearing renewal.
Figure 2 - Vibration spectrum taken after bearing
replacement.
On examination of the motor
NDE bearing, an area of metal loss, (flaking), of approximately 6mm diameter was evident
on the outer race of the bearing see Figure 3.
Figure 3 - Photograph indicating bearing damage.
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