Abstract: Time synchronous averaging of vibration data is a fundament technique for helicopter gearbox diagnosis.Currently,this technique relies on hardware tachometer to give phase synchronous information. Empirical Mode Decomposition (EMD) is introduced to replace time synchronous averaging of gearbox vibration signal. A simple model of gearbox vibration signal is established and the characteristic vibration signals are extrated.The process of EMD is theoretical expressed under this condition, from which it is proved that different characteristic signals could be separated under a sufficient condition. The method is validated by experimental vibration data from recordings of a helicopter intermediate gearbox with fatigue pitting or cracking. The results show that EMD separates exactly the characteristic signals, which are more reliable in diagnosis of gear faults.

Key words: empirical mode decomposition, intrinsic mode function, gearbox diagnosis