variable speed Bearing fault diagnosis 1 Characteristic-frequency 1 Envelope spectra 1 Envelope spectrum analyse 1 Failure analysis 1 Fault characteristic frequency band 1 Fault characteristics 1 Fault detection 1 Order tracking 1 Roller bearings 1 Spectra analysis 1 Spectrum analysis 1 Variable speed 1 Vibration 1 Vibration analysis 1 article 1 bearing fault diagnosis 1 biophysics 1 calculation 1 collective motion 1 controlled study 1 correlation coefficient 1 diagnosis 1 diagnosis time 1 diagnostic accuracy 1 diagnostic test accuracy study 1 envelope spectrum analysis 1 fault characteristic frequency band 1 frequency analysis 1