# A Digital Twin-Based State Monitoring Method of Gear Test Bench
## 1 Introduction  
With excellent transmission characteristics (e.g., wide transmission power, high transmission efficiency, long service life, smooth movement, and compact structure), the gear is widely applied to various types of mechanical transmission systems of national industries (e.g., automobile, aviation, shipping, energy, metallurgy, and electrical power). The dynamic characteristics of the gear directly affect the equipment working performance to which it belongs. With the progress of science and technology and the continuous development of the manufacturing industry, higher requirements are also put forward for the gear performance. In the whole process of gear product design, production, manufacturing, and operation and maintenance, various performance tests of gears are indispensable. Generally, gears are used in different transmission systems in the form of reducers or gearboxes. During the gear meshing, there may have various defects (e.g., gear tooth shape errors, tooth surface contact deformation, insufficient lubrication, gear and bearing installation errors, and insufficient lubrication) in the gearbox, which will cause some faults (e.g., pitting, fatigue peeling, wear, plastic flow, and gluing) of the gear tooth surfaces and the root crack, which may even lead to the gear fatigue and the broken teeth. Aiming at this problem, many scholars have conducted in-depth research on the design [1; 2; 3; 4; 5; 6], running monitoring [7; 8; 9], defects [10; 11; 12; 13], and wear [14; 15; 16; 17; 18] of the gearboxes and achieved good results, which provide useful theoretical and technical references for the design and manufacture of gears and gearboxes. Thus, in order to improve the reliability of gear transmission, on the basis of ensuring the design machining precision and assembly quality of gears, it is necessary to test the performance of gear transmission devices in the gear test bench to ensure the normal safe operation of gearboxes. The gear test bench provides the basic data for the gear design and machining based on a series of experiments, such as strength lifetime, performance degradation measuring, noise and vibration testing, tooth contact analysis, etc. Compared with theoretical analysis methods, data-based experimental methods are more accurate and flexible, and suitable for accuracy control under variable working conditions [19; 20]. In order to obtain the exact experimental data, the safe and reliable operation of gear testing has always attained widespread attention. In the running process of the gear test bench, many problems will occur, such as process data collection, running state monitoring, and performance prediction, which have a great influence on the reliability of experimental results [21]. It is particularly urgent to combine new information technologies to monitor the running state of the gear test bench and predict its timely and effective running performance. In recent years, digital twin has become increasingly relevant with the development of the Fourth Industrial Revolution and the transition to intelligent manufacturing [22], which has become essential to modern industrial developments and production paradigms [23]. The concept of digital twin was first proposed by Professor [PERSON] in the early 21 st century [24]. Digital twin refers to virtual and real mapping by constructing the mapping relationship between the digital virtual entity and physical entity. It maps the physical entity in the physical space to the digital space and has functions such as data mapping, analysis and decision-making, and control execution [25]. It uses data to simulate the behavior of physical entities in the real environment, so as to increase or expand the ability of physical entities through virtual real interaction feedback, data fusion analysis, decision iteration optimization, and other means [26].