Reliability Engineering Snapshot TM

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




Vibration Case Study No. 72 - Diagnosing Split Roller Bearing Problems

Split Bearing DesignSplit bearings are tricky to diagnose when interpreting the acceleration spectrum. For diagnostic purposes, they should not be put into the same class as a normal ball bearing.

The bearing at the left is a split bearing. When using acceleration for diagnostic purposes there could be some misleading information. The information is correct, but under normal interpretation rules one would believe that there was a problem when in fact there wasn't a problem.

Any technician would say that since most ball and roller bearing designs are predominantly rolling contact and not sliding contact, between the ball and raceway, any broadband vibration would be a good indication of metal to metal contact. This would be construed as a problem.

End View of Split Bearing Inner RacewayHowever, looking at the cross section of a typical split bearing one immediately sees that there is a lot of sliding contact happening between the bottom half of the roller sidewall, and the side of the locking collar (see red circle, picture left). In addition, there is a lot of sliding contact happening between the roller cage and locking collar. As long as there is plenty of lubricant there isn't a problem. Regardless as to the amount of lubricant, the well lubricated sliding motion is still picked up as broadband frequencies in the spectrum (picture lower left). There's also a fair amount of impacting.
Vibration Spectrum & Waveform of New Split Bearing As tricky as this bearing is to monitor, when it goes bad it generates normal bearing fault frequencies. The nonsynchronous bearing frequencies will pop up out of the mountain of broadband frequencies.

There is another weakness behind this bearing and it is showcased in the article "Machine Design - Case Study No. 70: Pros and Cons of Using Split Bearings." In addition, this bearing tends to run hot and the reason is showcased in the article "Lubrication - Case Study No. 69: Eliminating Pressure Lubricated Systems and Living with High Heat."


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