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.
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.
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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