# A Digital Twin-Based State Monitoring Method of Gear Test Bench
## 5 Test Verification  
In this paper, a closed-power flow fatigue life test bench of spiral level bears, which is mainly composed of a motor, constant velocity closed box, magnetic power brake, accompanying test box, main test bevel gearbox, torque meter, torque indicator, etc., is designed and assembled. The simulation effectiveness of the proposed state monitoring system is illustrated and verified by physical quantities such as vibration, noise, and temperature in the running process of the gear test bench. The material of the gearbox is ductile iron, which has the characteristics of better support stiffness. In the test, a pair of spiral bevel gears is used as the experimental object. The magnetic powder brake, main test bevel gearbox, coupling, and motor compose a closed transmission loop. According to the design standard and working range of the gear test bench, the brake torque loaded on the gear is set at 499 N.m, and the driving speed of the pinion is set at 720 r/min. The vibration signal is collected by an M + P vibration tester and the noise is collected by a decibel tester, while the temperature is measured and recorded by an infrared thermometer. The geometric parameters of the testing spiral bevel gears are shown in Table 1, while the test bench of spiral bevel gear fatigue life is shown in Figure 10. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Items** & **Pinion** & **Gear** \\ \hline Number of teeth & 16 & 27 \\ Module (mm) & 4.25 & 4.25 \\ Mean spiral angle (\({}^{\circ}\)) & 35 & 35 \\ Normal pressure angle (\({}^{\circ}\)) & 20 & 20 \\ Shaft angle (\({}^{\circ}\)) & 90 & 90 \\ Face width (mm) & 17 & 17 \\ Hand of spiral (\({}^{\circ}\)) & Left & Right \\ Outer cone distance (mm) & 66.693 & 66.693 \\ Pitch angle (\({}^{\circ}\)) & 30.6507 & 59.493 \\ Face angle (\({}^{\circ}\)) & 35.3501 & 62.2168 \\ Root angle (\({}^{\circ}\)) & 27.7832 & 54.6499 \\ Addendum (mm) & 4.682 & 2.542 \\ Dedendum (mm) & 3.342 & 5.482 \\ Whole tooth height (mm) & 8.024 & 8.024 \\ Bottom (mm) & 0.8 & 0.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Geometric parameters of the testing spiral bevel gears. Figure 10: Test bench of spiral bevel gear fatigue life. The test process is as follows: firstly, the gear meshing running time is half an hour under one-fourth, one-half, three-fourths, and full of the brake torque, respectively. The start time of the experiment is then recorded. During the test operation periods with different brake torques, the sensors fixed in the gearbox can collect the signals, such as the temperature, noise, vibration. The data collection process is continuous until the gear tooth is broken. During the test, factors such as the tooth surface wear, extrusion, and insufficient lubrication in the gearbox will cause the rise of the gearbox temperature. The defects (e.g., tooth defect, tooth shape errors, gear tooth offset load, and insufficient bearing preload) in the gearbox can cause the edge contact on the tooth surface and the gear circumferential vibration. At the same time, under the action of alternating stress, the noise of gear mesh will be increased and the crack of the tooth will be gradually expanded, resulting in the final fracture of the tooth and the stop of the gear test bench. Under the normal running state, the temperature and noise changes during the operation of the gear test bench equipment are shown in Figures 11 and 12, respectively. More specifically, a temperature comparison between the gear test bench equipment operation and the state monitoring system simulation is shown in Figure 11, where Curves B and C represent the temperature change of the gear test bench and the state monitoring system, respectively. It is clear that the temperature of the gearbox gradually increases from the beginning of the test. When the temperature of the box reaches about 55 \({}^{\circ}\)C, the crack of the broken tooth begins to occur. After that, the temperature of the gearbox rises sharply.