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Practical Solutions to Machinery and Maintenance Vibration Problems

Chapter 8, Vibration in Bearings

Section 6, Use of Velocity Spectra to Determine Bearing Condition

One of the keys to the successful analysis of distressed bearings is pattern recognition. Since there are many different configurations, types and sizes of rolling element bearings, it is very difficult to accurately determine one specific frequency and amplitude that will be generated by a bearing defect. Therefore, it is necessary to observe the familiar "patterns" developed by about 80 percent of distressed bearings rather than their absolute amplitudes and frequencies.

Stage One:
Fig. 2 represents the velocity spectrum for the first stage of a bearing defect. An increase in IBF units has occurred. Note that there is no change on the vibration velocity spectrum. At this stage, it should be noted that there is no discernible change in noise or temperature and no visual indication that the bearing is distressed.

Stage Two:
Fig. 3 represents the velocity spectrum for the second stage of failure. IBF continues to increase. The first indication of a problem on the velocity spectrum is what is often referred to as the bearing component's natural frequency. This low amplitude vibration usually occurs somewhere between 30 Kcpm and 120 Kcpm depending on the bearing. These frequencies are independent of operating speed and non-synchronous. As the problem continues to deteriorate, 1 x rpm sidebands begin to appear. As deterioration continues, the other component natural frequencies appear, again developing 1 x rpm sidebands until the familiar "haystack shape" has developed. The difference frequencies between sidebands usually indicate the rpm of the defective bearing.


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