*4.1. Experiment Description*

The experimental test rig is illustrated in Figure 6a,b. The motor was controlled by an inverter and connected to the input shaft of the gearbox to transmit power by a coupling. An electromagnetic torque load was coupled with the output shaft of the gearbox through a V-belt. The electromagnetic torque load was controlled by a torque controller (TDK-Lambda, GEN 100-15-IS510; TDK-Lambda, Wuxi, China), which can adjust the torque of the load manually. Two accelerometers were mounted on the gearbox to collect signals, and the signals collected were transmitted to a laptop using the data acquisition card. Detailed information on the data acquisition system is provided in Table 1.


**Figure 6.** (**a**) Experimental test rig and (**b**) the structure of the gearbox.

The structure of the gearbox is displayed in Figure 6b. It consists of four gears, six bearings and three shafts. Shaft 1 was the input shaft, and gear Z1, with a module of 2.25 mm, a pressure angle of 20, a helical angle of 20, and 30 teeth, was installed on it. Shaft 1 transmitted the power to shaft 2 by a pair of gears (Z1 and Z2) in mesh. The output shaft (shaft 3) was driven by another helical gear Z4, with 80 teeth, which was meshed with the gear Z3. The helical gears Z2 and Z3 installed on shaft 2 both have 45 teeth and other parameters of them are the same to Z1. We installed one of the faulty components: bearing 1, bearing house 1, and gears Z1, Z2, Z3, Z4 every time on the gearbox to experiment. Table 2 shows all the condition patterns of the gearbox.


**Table 2.** Condition patterns of the gearbox.
