Gear vibration signals can be modeled as amplitude modulation and frequency modulation (AMFM) processes, and have cyclostationarity, so cyclic spectral analysis is used to extract the modulation features of gearbox vibration signals, and to detect and assess localized gear damage. The cyclic spectral density of AMFM signals is deduced, and its properties in joint cyclic frequency-frequency domain are summarized. The ratio between the sum of cyclic spectral density magnitude along frequency axis at the cyclic frequencies of modulating frequency and 0 Hz can indicate the amplitude modulation magnitude, so it can be used as an indicator to assess the health status of gearboxes. The analysis of a gearbox run-to-failure test shows that the ratio has a monotonically increasing trend with the development of the gear damage. It has potential to early detect incipient damage, to prognosticate severe damage, and to assess gearbox damage.
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