This picture shows the vibration signals of a damaged bearing. From the time waveform you can see periodical impact signals. These periodical impacts can excite the natural frequencies which are usually higher than 1k Hz (see the above spectrum plot.)
By conducting an envelope filter scanning, we can see that the bearing outter race frequency can be seen clearly by setting the envelope filter at 3.25kHz or 12.5kHz.
Thie plot shows the Envelope sepctrum of this bearing with Envelope filter center set at 3 kHz. The outer race frequency of this bearing is observed very clearly.
Envelope spectrum has been widely used to find out the bearing fault frequencies at thier early stage of damage. However, if one does not set the envelope filter appropriately for his measurements, the envelope spectrum may fail to show the fault frequencies clearly. Benstone Instruments use a special approach called "Envelope filter scanning" to reduce the uncertainty of envelope spectrum measurements . As you conduct this approach, the analyzer will measure a sets of envelope spectrum with different envelope filters ranging from 1kHz to 20kHz, and then show them on a 3D waterfall plot. From this 3D plot you can find out the best filter band for conducting an envelope spectrum for a bearing. If there is no any fault frequency found on the 3D plot, one can be very sure that this bearing is still in good conditon.