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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 8, Vibration in Bearings

Section 2, High Frequency/Ultrasonic Measurements

One of the most difficult Bearing problems to identify is incipient failure (failure that is just about to happen). Vibration velocity readings help us to identify bearing defects in the latter stages of bearing life. However, in order to get the earliest warning of impending trouble, it is necessary to monitor the ultrasonic vibration levels. Most instruments now have the capability to measure very high frequency vibration in the 5 kHz - 50 kHz range, (300 Kcpm - 3000 Kcpm). The terminology for the particular unit of measurement varies from manufacturer to manufacturer, such as "spike energy," shock pulse," "bearing defect energy," "high frequency detection," etc., etc. However, they all measure pretty much the same thing. The instrument manufacturers will be able to furnish a detailed definition and description of their specific units of measurement. For the purposes of this text, general terms will be used.

These high frequency measurements sense the low energy, repetitive, metal-to-metal impacts that occur in the earliest stages of an incipient bearing failure. The rate, amplitude and frequency are all combined to give a single "numerical" output. To maintain impartiality, this text will identify all these units with the generic term IBF (Incipient Bearing Failure) number.

The problem with all these units is that it is almost impossible to accurately determine bearing condition based on a single measurement. Measurements are very sensitive to a host of external influences such as differences in installation, load, lubrication, pickup mounting, location and number of interfaces. Other problems, such as cavitation, gear mesh and steam leaks, also have significant effects in IBF numbers (i.e. it is possible to install two identical, brand new bearings in two identical machines and get two different IBF readings).

The use of IBF readings excels when trended over a period of time. If used correctly, IBF units will provide the earliest warning system for impending failure. In this manner, change becomes more significant than absolute values. If this technique is to be successful, accurate repeatable data has to be acquired.

Of all the vibration parameters measured, IBF units are the most sensitive to transducer location and mounting. Small changes in location and/or mounting techniques can produce large changes in IBF amplitudes, making it very difficult to identify true changes in bearing condition.

Fig. 1 shows a typical IBF trend. Notice that the IBF number actually drops either before or as the bearing fails. If a single IBF reading of, say, 0.5 is obtained, will the bearing last another 5 years, 5 months or 5 days? This is not known unless enough plots have been taken to show a trend. It is not recommended that a diagnosis be made based on IBF units alone. Instead, always verify the diagnosis with other vibration units such as velocity. Remember, trending IBF units provides only an early warning system.


 

 

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