Compared with normal acceleration signal

the outline signal of high frequency acceleration coming from demodulation is of more capacities for canceling noise and reveals faults nature more directly. Several typical faults in gear transmission

such as looseness or rupture of tooth in a simple gear box

fault of shaft nonalignment

complex faults about gear and bearing in a more complex gear box

are analyzed with their signals both in time and frequency region in details. The signal characteristics and diagnosis evidences about these faults are put forward

the effectiveness of he outline signal of high frequency acceleration coming from demodulation is interpreted.