# A Digital Twin-Based State Monitoring Method of Gear Test Bench
## 1 Introduction
The real-time feedback of gear transmission stability was effectively solved. [PERSON] et al. [43] presented a hybrid approach framework driven by digital twin technology based on meta-action theory, to realize real-time monitoring and digital simulation of the mechanical transmission wear and performance degradation. This method can also apply to other mechanical transmission systems with high-performance requirements. [PERSON] et al. [44] established a digital twin multi-body dynamic model of a planetary gearbox for detecting the combined fault of the sun gear missing teeth and the planetary gear missing teeth or pitting under different working conditions. Furthermore, in terms of the machining accuracy improvement of gear products, [PERSON] et al. [45] presented the digital twin model construction of a face-milled hypoid pin-ion. The machine settings and the cutter parameters for the hypoid gear are obtained by minimizing the topographical error between digital twin and the sample gear using optimization software. Considering the relationship mapping model of the cutting parameters and the cutting force and temperature, [PERSON] et al. [46] provided a multi-physical modeling method driven by mechanism and data to establish a thermomechanical digital twin model of the gear skiving process. [PERSON] et al. [47] proposed a new digital twin model considering low-risk transmission performances of the spiral bevel gear, which is introduced into tooth flank grinding. An improved tooth contact analysis and loaded tooth contact analysis are performed to build a bridge between low-risk performances and hypoid generator parameters based on this model. [PERSON] et al. [48] proposed a digital twin model for the loaded contact pattern-based grinding of spiral bevel gears, which proposed different optimization strategies to realize the precise grinding of the tooth surface. In order to improve the efficiency and accuracy of measurement uncertainty evaluation, [PERSON] et al. [49] established a measurement uncertainty model of gear profile on the gear measurement center by using digital twin technology to effectively identify various errors in the measurement software. The above research work shows that digital twin has shown good application effect and prospects in the prediction and evaluation of gear performance, as well as the improvement of gear machining accuracy, which also provides the theoretical and technical references for the study of this paper. As we all know, the spiral bevel gear is a complex transmission with a complex gear surface. Its transmission performance is determined by many factors (e.g., the shape of the tooth surface, the position and shape of the tooth surface contact areas, the meshing trace, and the transmission errors). This experiment is a key way to test the transmission performance, which provides a basis for the dynamic characteristics, structural optimization design, and fault diagnosis of spiral bevel gear transmission under the actual running state [50; 51]. In this paper, the widely used closed power flow gear test bench is applied to test the performance (e.g., strength, noise, and vibration) of spiral bevel gears. The normal operation of the test bench directly determines the various indicators' (e.g., contact area position, contact ellipse direction, tooth surface deviations, geometric transmission errors, load transmission errors, and fatigue life) testing of the spiral bevel gear, which will have a great impact on the evaluation of spiral bevel gear manufacturing performance. However,a variety of problems will easily happened in the operation equipment (e.g., transmission shaft failure, motor failure, loading device failure, lubrication failure, and gear installation offset) during the working process of the test bench. The monitoring data acquisition is more difficult, as well as the real-time running state monitoring.