The mechanical fault comprehensive simulation test bench produced by a company is used to carry out the fault feature extraction test of gearbox under variable speed. The test-bed is mainly composed of motor, tachometer, acceleration sensor, two-stage parallel gearbox and magnetic powder brake. The speed is changed by matching motor control software. The main parameters of the test are as follows: the gear box input shaft driving gear (G1) has 41 teeth; the intermediate shaft driven gear (G2) has 79 teeth, the driving gear (G3) has 36 teeth; the output shaft driven gear (G4) has 90 teeth. The acceleration sensor is arranged at the output end of the intermediate shaft, and the tachometer is arranged at the output end of the motor. The specific position is shown in Figure 1.

During the test, the sampling frequency is set at 20 480 Hz, and the motor speed is set at 1 200 R / min ~ 1 800 R / min. the vibration data of any interval under variable speed condition are collected. The feature extraction tests of normal gear, broken gear and uniform wear gear are carried out respectively. The fault gear is the driving gear (G3) of the gearbox intermediate shaft, and the gearbox input shaft is used as the reference shaft.

Fig. 2 shows the acceleration time-frequency diagram of the gearbox under normal state. It can be observed from the diagram that with the increase of time, the rotation frequency and meshing frequency of each shaft and gear of the gearbox change, in which the speed order and meshing order are obvious; Fig. 3 and Fig. 4 are the time-frequency diagrams of the gearbox under the fault state. It can be seen that there are many order signals in the diagram, and the higher order signals are obvious, which indicates that the gearbox has a fault, but the fault state and position cannot be known.

Fig. 5 ~ Fig. 7 are the corresponding frequency spectrums of the normal gearbox and the fault gearbox. There is obvious spectrum smearing phenomenon in the graphs. As shown in Fig. 5, the frequency variation range corresponding to the meshing order ratio is circled with dotted lines. The traditional spectrum analysis method can not effectively extract the state information, so it is difficult to carry out fault diagnosis.