Abstract: This article introduces the structural characteristics and performance of three types of transmissions currently used in domestic loaders, objectively analyzes the problems of each type of transmission, and predicts the requirements and development of transmissions, which is believed to be helpful to the development of domestic loader transmissions. Keywords: Transmission, Current Status, Requirements, R&D, Prediction, Domestic Loader Transmission Status Domestic loaders mainly use the following three types of transmissions. The dominant transmission is: a dual-turbo 4-element torque converter + 2-forward 1-reverse planetary transmission (referred to as transmission A for convenience). High-end loaders mainly use transmissions from the German company ZF (imported or produced by a joint venture in Liuzhou): a single-turbo 3-element torque converter + 4-forward 3-reverse fixed-shaft transmission (referred to as transmission B for convenience). Some loader manufacturers or some products use domestically produced transmissions: a single-turbo 3-element torque converter + 4-forward 4-reverse, 4-forward 2-reverse, or 3-forward 3-reverse fixed-shaft transmission (referred to as transmission C for convenience). Transmission A features an overrunning clutch that automatically integrates the power output of turbines I and II in the torque converter, enabling automatic switching between heavy and light load conditions. This type of transmission effectively has 4 forward and 2 reverse gears. Therefore, using a dual-turbo 4-element torque converter reduces the number of gears and simplifies the transmission structure, especially the shifting mechanism, requiring only a single shift lever for all shifting and reversing operations. Figure 1 shows a simplified transmission diagram of this type of transmission. The dual-turbo 4-element torque converter has a high torque ratio (stall torque ratio K0 can reach over 4) and a wide high-efficiency range (with dual high-efficiency zones). Figure 2 shows the torque ratio K and efficiency curves of the dual-turbo 4-element torque converter. Transmission A features an overrunning clutch that automatically integrates the power output of turbines I and II in the torque converter, enabling automatic switching between heavy and light load conditions. This type of transmission effectively has 4 forward and 2 reverse gears. Therefore, using a dual-turbo 4-element torque converter reduces the number of gears and simplifies the transmission structure, especially the shifting mechanism, requiring only a single shift lever for all shifting and reversing operations. Figure 1 shows a simplified transmission diagram of this type of transmission. The dual-turbo 4-element torque converter has a high torque ratio (stall torque ratio K0 can reach over 4) and a wide high-efficiency range (with dual high-efficiency zones). Figure 2 shows the torque ratio K and efficiency curves of the dual-turbo 4-element torque converter. Transmission A features an overrunning clutch that automatically integrates the power output of turbines I and II in the torque converter, enabling automatic switching between heavy and light load conditions. This type of transmission effectively has 4 forward and 2 reverse gears. Therefore, using a dual-turbo 4-element torque converter reduces the number of gears and simplifies the transmission structure, especially simplifying the shifting mechanism. All shifting and reversing operations can be achieved with just one shift lever. Figure 1 shows a simplified transmission diagram of this type of transmission. The dual-turbo 4-element torque converter has a large torque ratio (stall torque ratio K0 can reach over 4) and a wide high-efficiency range (with dual high-efficiency zones). Figure 2 shows the torque ratio K and efficiency curves of the dual-turbo 4-element torque converter. After years of development, the design and manufacturing process of the dual-turbo 4-element torque converter are well-mastered by manufacturers, with virtually no major fault points, although minor faults are more common. The 2-forward, 1-reverse planetary transmission has a simple structure and is easy to maintain. Loader maintenance personnel can be trained quickly and perform on-site repairs without specialized tools. Spare parts are readily available, allowing for rapid machine repair without delaying project progress. This transmission uses a mechanical-hydraulic shifting system, directly manipulating the shift control valve mechanically for gear changes. The overrunning clutch automatically switches between heavy and light load conditions, requiring only four positions: 2nd gear, 1st gear, neutral, and reverse. Therefore, a single shift lever is sufficient for both gear shifting and reversing. However, domestically produced 2-forward, 1-reverse planetary transmissions are often poorly manufactured. Some gears, after heat treatment, do not undergo surface strengthening and grinding processes. The processing equipment is relatively outdated and inferior, the materials are relatively poor, the tooling and cutting tools are subpar, the heat treatment process is outdated, and poor logistics control leads to parts being damaged, dirty, and roughly assembled. These issues result in numerous potential failure points and a high frequency of malfunctions, severely damaging the reputation of domestically produced loaders. The main faults of a 2-forward, 1-reverse planetary transmission include: damage to the overrunning clutch (causing the torque converter to be unable to output under heavy load conditions), broken teeth on the sun gear or planetary gears, axial movement of the reverse planetary shaft cutting off the retaining washer, axial movement of the first gear planetary shaft cutting off the retaining disc or bolts, bearing damage, wear or shaving of the friction plate in the first and reverse gears, oil leakage or damage to the seals, damage to the transmission pump, jamming or damage to the transmission control valve, blockage or damage to the oil filter, oil pipe rupture, spring failure or breakage, etc. [align=center] Figure 1: Transmission diagram of transmission A[/align] [align=center] Figure 2: Torque ratio K and efficiency η curves of the dual-turbo four-element torque converter[/align] Transmission B is a relatively reliable transmission. Figure 3 shows the transmission diagram of the ZF4WG200 transmission. This torque converter is characterized by its stamped and welded integrated core, which has a compact structure. However, there is no retaining ring on the guide wheel seat to hold the guide wheel in place, so the torque converter core may fall out when the transmission is flipped. Compared to dual-turbo torque converters, single-turbo torque converters have a smaller torque ratio (the stall torque ratio K0 is generally less than 4, such as the K0 value of 2.55 for ZFWG200) and a narrower high-efficiency range (only a single high-efficiency range). Figure 4 shows the torque ratio K and efficiency η curves of the ZF4WG200 transmission. This transmission is characterized by the fact that, except for the input and output shafts, all other intermediate shafts are fixed and do not rotate; the transmission pump uses a high-efficiency internal gear pump and is installed inside the housing; the materials are specially customized, and new heat treatment processes such as pressure quenching are used; the gears undergo grinding and tooth surface strengthening treatments; key components are all imported; the processing equipment, tooling, and cutting tools are relatively advanced (e.g., the housing is machined using an imported machining center); the production environment is clean; and advanced processes and tooling are used in assembly. The shift control adopts an electronic-hydraulic power shift, requiring only one electronic shift lever to achieve both direction and gear shifting operations; and a forced shift KD key is provided at the end of the shift lever to achieve a shift from 2nd to 1st gear. Figure 5 shows its electronic shift lever. Pushing the lever allows for reversing (and can lock in neutral for safety), while turning it completes the gear shifting operation. Since the shift lever and the shift control valve are connected by wires, installation is very convenient, and an adjustable steering gear can be easily used for personalized driving control. Because it uses a computer-controlled shifting device, in addition to 4-forward and 3-reverse shifts, it can easily achieve 6-forward and 3-reverse shifts, expanding the transmission's application range. The shift control valve has a smooth engagement valve, ensuring smooth and less impactful gear shifting. However, the biggest obstacles to the widespread adoption of this transmission are: 1) High price. Due to its compact structure, it requires high-quality materials and heat treatment processes; due to high precision requirements, it needs advanced equipment and manufacturing processes, limiting production capacity. 2) The availability of spare parts is difficult to compare with transmission A, and the price is high, requiring a considerable number of imported parts. 3) Maintenance requires specialized tools and facilities, unlike transmission A which can be repaired locally, increasing maintenance costs and potentially affecting project progress. 4) Maintenance and repair are much more stringent than with transmission A. Transmission C is a type of transmission used by a limited number of manufacturers and in a limited number of products. Domestic research, investment, and output in single-turbo 3-element torque converters are far less than in dual-turbo 4-element torque converters. For single-turbo 3-element torque converters, the best domestic product is the single-turbo 3-element torque converter core produced by Shaanxi Aerospace Power High-Tech Co., Ltd. for the ZP 4WG200 transmission. It was certified by ZF Germany and specifically supplied to Liuzhou ZP Company. Due to significant gaps between domestic and foreign countries in materials, manufacturing equipment and processes, heat treatment technology and processes, assembly technology and processes, logistics management, production environment, and personnel quality, transmission C lags far behind transmission B (in terms of technology and quality). It is not as convenient to assemble and disassemble as transmission A, especially in terms of spare parts availability. However, compared to transmission A, transmission C has fewer potential failure points. The most typical failure is oil leakage causing excessively low clutch oil pressure, leading to excessive wear of the friction plates and even clutch seizure. Low clutch oil pressure can be caused by casting defects such as blemishes or looseness on the oil filler cap, leading to damage to the oil seal ring, or damage to the oil seal ring on the piston. Prolonged low oil pressure operation can cause significant damage to power shift transmissions and should be avoided as much as possible. Figure 6 shows the Shantui 50 type transmission, which consists of a single-turbo 3-element torque converter and a 4-forward, 4-reverse fixed-shaft transmission. This transmission is a 4-degree-of-freedom transmission, requiring three clutches for each gear, thus necessitating three shift levers: one reversing lever (for F and R clutches), one shift lever (for F1, 3 and F2, 4 clutches), and one high/low gear lever (for the HL sleeve). Therefore, shifting is very cumbersome, which is the biggest drawback of this transmission. Figure 7 shows the PZl33 transmission manufactured by Zibo Kaitai. It consists of a single-turbo 3-element torque converter and a 3-forward, 3-reverse fixed-shaft transmission. This is a 3-DOF transmission, requiring three clutches for each gear, thus necessitating two shift levers: one for reversing (for clutches F and R) and one for shifting (for clutches L1, L2, and L3). This transmission can be configured on Type 15, Type 18, and Type 30 loaders. A typical fault is broken wave springs (this problem can be better solved by using coil springs as piston return springs). Transmission C generally uses a 3-DOF transmission (rarely more than 3 DOF). Each gear requires two clutches. When using a mechanical-hydraulic power transmission, two shift levers are needed: one for reversing and one for shifting. To simplify the structure, flexible shaft operation is generally used. However, flexible shafts have a large free travel, making precise stroke control difficult. Therefore, the control lever is difficult to limit with a slot, resulting in less clear and accurate gear shifting compared to a spatially articulated transmission mechanism (such as transmission A), and potentially leading to accidental gear engagement. The numerous transmission control levers not only increase the difficulty of spatial arrangement but also the difficulty of driver operation, increasing the driver's workload. Therefore, an electro-hydraulic transmission is the best choice. 2. Requirements for Transmissions in Domestic Loaders 2.1 High Reliability Due to the high failure rate of domestically produced transmissions, this is the biggest concern in the market. Reducing transmission failure points and improving transmission reliability are the most important and urgent tasks. 2.2 Price Advantage For Chinese-made products, price is a key competitive factor, both domestically and internationally. Moreover, the proportion of private purchases is increasing, so products without a price advantage have no future in today's fiercely competitive and low-profit loader industry. Therefore, transmission A will continue to play a major role for a considerable period, but manufacturers need to strive to improve product quality and reduce the failure rate. 2.3 Operation should be simple and quick. One gear shift lever should be used whenever possible, with a maximum of two, to facilitate space arrangement, reduce driver workload, and improve work efficiency. 2.4 Maintenance should be convenient, and spare parts should be readily available. Maintenance personnel should be able to quickly master maintenance techniques after training. Maintenance should be possible on-site or at a nearby repair shop, requiring minimal or no specialized tools, and spare parts should be readily available. In short, transmission failures should be repaired as quickly as possible to avoid impacting project progress. 3. Development Forecast of Domestic Loader Transmissions Transmission A has been used for many years, but reliability has always been a major challenge. Solving transmission reliability is the primary goal of transmission development. Electronic transmission control (electro-hydraulic power shift and automatic shift) has always been the direction of international transmission development. Therefore, developing a transmission suitable for China's national conditions should be our priority. 3.1 In planetary transmissions like transmission C (transmission A), the planetary gear carrier is cantilevered, the internal gears float radially (without bearing positioning), the gear teeth are relatively wide, the reverse planetary gear carrier and the first gear internal gear float axially (limited only by friction plates), and the planetary gears are prone to axial movement. These structures generate abnormal axial forces that can cause transmission failures. Furthermore, manufacturing errors can also cause failures in components such as the overrunning clutch. Since all gear assemblies are mounted on a single shaft, errors can affect other parts, and axial forces can be transmitted to them. In contrast, in a fixed-shaft transmission, the parts on each shaft are precisely positioned, and errors do not accumulate on other shafts, resulting in fewer failures compared to planetary transmissions. From a reliability perspective, fixed-shaft transmissions are more ideal, and they are now widely used in loaders abroad. With multi-gear fixed-shaft transmissions, there is no longer a need for a dual-turbo 4-element torque converter to increase gears, and therefore no need for an overrunning clutch to integrate the power output of the two turbos. Instead, a relatively simple single-turbo 3-element torque converter can be used. 3.2 To enhance product quality, the transmission may employ electro-hydraulic or automatic shifting, using a single or dual shift lever. A ZF-like electronic control lever (Figure 5) can be used for reversing and shifting. To reduce costs, when using a mechanical-hydraulic shifting transmission, two shift levers can be used: one for reversing and the other for shifting; or a lever similar to the CAT 966E can be used for both reversing and shifting (the action is similar to the ZF electronic lever), but the mechanism is more complex and requires less force to operate the shift control valve. 3.3 High-end products using ZF-like products: We also need to equip our products with high-quality transmissions similar to ZF to meet the requirements of special users and applications. Given our current capabilities, we cannot yet fully domestically produce such products; therefore, imported transmissions can also be used to meet the high requirements of our complete machine.