The main vibration type gearing in modern aeroengine transmission is the travelling wave vibration which is divided into two kinds: the forward travelling wave vibration and the backward travelling wave vibration. If the frequency of vibration exciting force, which comes from the meshing of a set of helical gears, is the same as the frequency of travelling wave resonance, the dangerous resonant vibration will take place. However both of forward and backward travelling wave resonance can not be excited as easily. The results of an aeroengine transmission test indicate that forward travelling wave resonant vibrations are more easily excited to the driven helical gear and more dangerous.

The work done by the component force, which is generated because of the angle of travelling wave vibration, on the driven helical gear, work done on the forward travelling wave vibration is positive and the backward travelling wave vibration is negative. In other words, for backward travelling wave vibration the induced force acts as a damping force, but for forward travelling wave vibration the induced force acts as a self–exciting force. Therefore it is more dangerous to driven helical gear when forward travelling wave vibration appears.

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