1. Classification of Transmission Devices in the Mechanical Industry The mechanical industry transmission sector belongs to the supporting parts industry of engineering machinery. The transmission devices in the mechanical industry can be roughly divided into three categories: (1) Gear transmission devices are mainly used in the transmissions, drive axle main drives and wheel-side reducers of earthmoving machinery, engineering cranes, compaction machinery and internal combustion forklifts, etc., where the usage requirements are not high and the main unit cost is low. Gear transmissions can be divided into two categories according to their structural form: fixed shaft type and planetary type. Drive axles can be divided into rigid, steering and through drive axles according to their function. (2) Hydraulic transmission devices are mainly used in earthmoving machinery and internal combustion forklifts, etc., where the usage and main unit performance requirements are high. Among them, hydraulic torque converters and power shift transmissions are continuously variable transmission elements in chassis power transmission, which can make the main unit have good automatic adaptability and operation performance. (3) Hydrostatic transmission devices are mainly used in hydraulic excavators and bulldozers, loaders, road machinery, road rollers and internal combustion forklifts, etc., where the main unit performance requirements are high, as well as hydrostatic transmission, steering systems and working devices, which can make the main unit have good continuously variable transmission and operation performance. 2. Development trend of gear transmission devices for construction machinery in China (1) The technical level of mechanical gear transmission devices in China is relatively low, and there is a lot of room for development. We should vigorously promote the optimization design method, improve the materials and processes of key parts such as gears, shafts, and housings, further improve service life, reduce weight, and reduce volume; adopt advanced shifting elements and shifting methods, strive to reduce the labor intensity of drivers, shorten the power interruption time, and improve the operating efficiency of construction machinery. The advanced design and components of transmissions that have been introduced in China should be vigorously promoted and adopted. (2) Due to the advantages of hydraulic torque converters and power shift transmissions, earthmoving and transport machinery has certain automatic adaptability, easy and smooth shifting, and good acceleration performance, and the production cost is relatively moderate, it has been accepted by most domestic manufacturers. However, since many large and medium-sized tire-type construction machinery in China still use hydraulic torque converters with high torque ratios and two-speed transmissions, and adopt manual shifting methods, the operating efficiency and economic efficiency are significantly lower. The advanced technologies already introduced in China, such as the single-stage hydraulic torque converters of CAT D6D and D7G from the United States and the WG series power shift transmissions from ZF from Germany, can effectively improve the efficiency and economy of engineering machinery operation and reduce system heat generation. They should be promoted and applied in domestic engineering machinery, and further adopt electro-hydraulic control and electronic control systems to replace manual control systems in order to improve working conditions and increase work efficiency. At present, a key issue in improving the level of domestic hydraulic mechanical transmission devices is that we must accelerate the pace of mergers and acquisitions, introduce advanced technologies, and build a world-class competitive professional system in my country as soon as possible. Only in this way can we realize the specialization, serialization and generalization of hydraulic mechanical products, and make this series of products have a strong vitality. (3) Promoting the application of self-locking anti-slip differentials and wet brakes in tire-type engineering machinery drive axles is one of the ways to improve the technical level of my country's engineering machinery drive axle products. Self-locking anti-slip differential can automatically realize the unequal distribution of torque between the left and right wheels to make full use of the vehicle's traction force, and can also significantly improve the vehicle's off-road performance and economy; wet brakes have high durability and reliability, and their service life is more than 1.5 times longer than that of dry caliper disc brakes, and they have large braking capacity and good braking performance. They have been widely used in large and medium-sized tire-type engineering machinery abroad, and should also be vigorously promoted. (4) In the rear axle of tracked engineering machinery, single-power flow steering clutches and brakes are generally used, so that the vehicle has only one turning radius of R=B when turning left and right, and the other turning radii are achieved by the slippage of friction elements, which not only causes serious power loss, but also reduces the service life of friction elements. In order to improve steering performance and reduce power loss, my country should accelerate the research and development of a power differential steering mechanism similar to the American CAT D8N tracked bulldozer. Most of the engine power is input to the central main drive through the transmission system, and another part of the power is transmitted to the gear ring of the first planetary gear set through the hydraulic pump-motor. When designing a power differential steering mechanism, the following should be ensured: ① When the motor speed is not zero, the left and right output speeds are equal to ensure the bulldozer has good straight-line driving performance. ② When the speed of the central main drive is zero, the steering hydraulic motor makes the left and right output speeds equal but opposite in direction. At this time, the bulldozer achieves in-situ steering around its center, i.e., R=B/2. ③ Because the hydraulic motor can continuously change speed and rotate, it achieves stepless and precise control of the left and right output speed difference, allowing the bulldozer's turning radius to be continuously adjusted, and the steering is smooth, improving steering performance and achieving zero power loss. 3. Application of Hydrostatic Transmission Devices Hydrostatic transmission has advantages such as continuously variable speed, wide speed range, constant torque or constant power speed regulation, and easy implementation of electronic control, making it a promising application in engineering machinery. However, it is rarely used as the main transmission in earthmoving and transport machinery and lifting machinery. The main problem is that the quality of domestic hydraulic components is poor, while the price of foreign hydraulic components is too high, resulting in excessively high main transmission costs. Since the 1990s, China has introduced hydrostatic forklifts from Linde and loaders from Liebherr, but their market share in China is very small. Based on the actual situation of the domestic construction machinery market, this paper puts forward the following exploratory opinions on the promotion and application of hydrostatic transmission in China: (1) Hydrostatic transmission forklifts have been widely used in developed countries. As some warehouses, docks and factories in China have higher requirements for the mobility performance (especially low-speed performance) and noise of forklifts, these departments are becoming users of hydrostatic forklifts in China. Domestic forklift and hydraulic component manufacturers should see the good prospects of hydrostatic forklifts and jointly research and develop hydrostatic systems for forklifts that are suitable for my country's national conditions, and provide products with advanced performance, reliable operation and reasonable price. They can also adopt the method of joint development with international hydrostatic component manufacturing companies to accelerate the development speed. (2) Small and medium-sized multi-functional construction machinery has been widely used in developed countries because it has multiple functions such as excavation, loading, forklift and lifting. With the development of my country's economic construction, especially urban construction, small and medium-sized multi-functional engineering machinery will also be promoted and applied in my country, and they will undoubtedly first adopt hydrostatic transmission as their main transmission device. Domestic engineering machinery enterprises should see the development prospects of small and medium-sized multi-functional engineering machinery and work with domestic and foreign hydrostatic component manufacturers to carry out research and development on them in order to promote the development of small and medium-sized multi-functional engineering machinery in my country. (3) In large earthmoving and hoisting machinery in China, due to the high technical difficulty and high price of the supporting hydrostatic and electronic control components, it is difficult for domestic users to accept them. Therefore, in my country, it is not appropriate to focus the research and development of hydrostatic transmission on supporting large earthmoving and hoisting machinery for the time being, but rather on the above two types of engineering machinery.