In this research, we discuss about the peculiar vibration behavior of a helical gear pair that appears at non-integer order of the meshing frequency. It is supposed that the vibration is generated because of the undulation on tooth surfaces having different cycles of transmission error from tooth mesh cycle. But, it is difficult to detect the undulation with direct measurement of tooth shapes because its amplitude is very small and the length of measured tooth profile is relatively short for spectrum analysis.
At first, characteristics of tooth surface undulation are discussed with spectra of measured gear vibration. It makes clear that there are integer order surface undulation components of gear rotation. Next, possibility of the spectrum analysis of measured tooth surface deviation is also discussed using Maximum Entropy Method.