Bearing defects in general
Small defects in the bearing begin to “ring” – they excite the natural frequencies of the bearing components, which are mainly in the range of 500 Hz to 2 kHz. These resonances can also be resonances of the supporting parts of the bearing. At the end of the 2nd stage, sidebands appear around the resonance peak. This stage can be detected using demodulated envelope spectra of high frequencies.
Frequencies of bearing defects and their harmonic multiples appear. As wear increases, more harmonic multiples of the bearing fault frequencies appear and the number of sidebands that are around the above harmonic multiples as well as around the bearing fault frequencies increases. This stage can be determined from the spectra of the vibration rates.
Towards the end of the service life, the amplitude of the speed component is even affected. This increases and along with it a number of harmonic multiples of the speed. Discrete frequencies of bearing defects as well as natural frequencies of the bearing components begin to disappear from the spectrum as a result of the increased clearance in the bearing and are replaced by random broadband high-frequency “threshold noise”.
Bearing failure frequencies can be calculated on the basis of bearing dimensions and equipment speeds. Most manufacturers have databases or calculation applications where failure frequencies can be found more easily.