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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 8, Vibration in Bearings

Section 9, Estimating Remaining Bearing Life

The writer views all of the symptoms only as warnings. It is analogous to measuring just how
much thickness of automobile tire rubber is left above the cords. Assuming a "reading" that indicates that the tire's cords are about to be exposed, the driver might ask if the car could be driven for three hours at the maximum speed without experiencing a blowout. The answer can only be that there is just so much thickness of rubber left, that the condition can cause a blowout within the first five minutes but may not result in a blowout for several weeks or even several months. Obviously, the rubber thickness measurement is only a warning of what can probably happen. It may even predict what will more likely happen, but it is not a sure indication of what will happen. The same is true for bearings showing more and more symptoms via the spectrum, as failure approaches.

For bearings with symptoms of imminent failure, it is suggested that the analyst indicate to the operator: "From the best vibration and IBF knowledge we have today (which is incomplete), the symptoms indicate the machine should be shut down as quickly as possible. However, if you have reasons to want to take your chances and continue running, there is always a possibility that this particular failing bearing may run much longer than symptoms indicate. But at this level, the risk is yours. I can only report that I recommend changing the bearing." For bearings that are deteriorating but not as close to imminent failure, a rough estimate may be given, but in the same spirit as the analogy of the tire.

Operating under the same operating conditions as before, it is reasonably possible to predict how long a bearing will last if the symptoms and data are compared with that bearing's previous symptoms of failure. Most of the time, however, this experience is not available, so it is not recommended that specific bearing life predictions be made. Many factors other than bearing conditions, such as speed, also affect decisions as to whether a bearing should be changed or allowed to run. Try to avoid specific time predictions, since errors in time prediction often lead to a loss of credibility even though the defective bearing has been correctly identified.

 

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