Abstract: Flexible thin-walled elliptical bearing (FTEB) is the key component of harmonic reducer, and the elliptical characteristics of the inner and outer rings make the fault feature frequency of FTEB show periodic fluctuation even when the bearing rotates at a fixed speed, which is completely different from that of ordinary rolling bearing. The elliptical characteristics of FTEB can also cause background periodic shocks, which can greatly cover up the fault periodic shocks. The Morlet wavelet is used to extract fault periodic shocks from the mixed shock signal of FTEB based on the waveform similarity between the Morlet wavelet and the fault periodic shocks. The fault periodic shock extraction mechanism of the continuous Morlet wavelet transform is analyzed, and it is theoretically proved that the Morlet wavelet can obtain the best extraction effect for the fault shocks similar to the wavelet waveform, while the background shocks caused by the ellipse will be greatly suppressed. The instantaneous frequency of the fault shocks is extracted by the Hilbert-Huang transform. For the FTEBs with inner ring fault and outer ring fault, the fluctuation characteristics of the fault frequency are accurately extracted from their vibration signals, respectively, and the results show that the fault frequency of the FTEB does have periodic fluctuations even when the bearing rotates at a constant speed, and the fluctuation frequency is twice the rotation frequency of camshaft. As a comparison, the instantaneous frequency of the fault shock of ordinary rolling bearing is analyzed, and the results show that there is no fluctuation. The periodic fluctuation of the fault frequency is the unique property of the FTEB.

Key words: flexible thin-walled elliptical bearing, fault feature frequency, frequency fluctuation, instantaneous frequency