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God Bless America 09/11/2001
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CASE HISTORY #29 (1750 HP Motor Bearing) -Bearing Cage Defect Frequencies – The cage frequency, or fundamental train frequency, is the rotating speed of the ball or roller cage assembly. This frequency is not encountered very often, but it can occur when some defect affects the rotation of the train. Excessive clearance in an antifriction bearing can cause the generation of a frequency at the FTF and/or modulations of the FTF at rotating speed and its harmonics. Outer race defects and roller defects can also be modulated with the FTF fundamental frequency. This letter will discuss a pure case of cage defects, with harmonics of cage fundamental frequency. The motor this case history had a NU226E bearing on the out by end. We always recommend using all available tools to evaluate a rotating unit. I used enveloped acceleration and velocity in this report since the problems can be seen clearly in these spectrum. Below is the monthly trend waterfall graph, which exhibits a large increase in the cage frequency to almsot 3 Gs env. We normally have our alarm set at 1 Gs env. of energy.
The next spectrum is in velocity and in the horizontal direction. This spectrum displays overlaid cage frequencies. It is evident that the 2X, 4X and 6X of the cage frequency are dominant.
The spectrum below shows the alarm envelope from our baseline spectrum. I have found this to be a fabulous tool in finding bearing degradation.
The motor was immediately scheduled to be checked. When the bearing was removed the rollers fell out, and it was found that the cage was destroyed. Below is a picture of the bearing with only parts of the cage available.
There was not excessive degradation of the rollers, outer race, or inner race. It was also found that the outer race was starting to turn in the housing. This is an example of vibration analysis finding a problem and giving management time to correct the problem before serious damage was done to the rotor shaft or experiencing downtime and production loss from the bearing locking up. In the next newsletter I will illustrate a different case study of vibration analysis, which detects a problem and verification of the problem. If you have any questions please contact us. |
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