The final drive is generally used to change the direction of transmission, reduce the rotational speed, increase the torque, and ensure that the vehicle has sufficient driving force and an appropriate speed. There are many types of final drives, including single-stage, double-stage, two-speed, and wheel-side reducers.
Single-stage Final Drive: A device that achieves speed reduction through a pair of reduction gears is called a single-stage reducer. It has a simple structure and is lightweight. It is widely used in light and medium-duty trucks such as the Dongfeng BQ1090.
Double-stage Final Drive: For some heavy-duty trucks that require a larger reduction ratio, if a single-stage final drive is used, the diameter of the driven gear will have to increase, which will affect the ground clearance of the drive axle. Therefore, double-stage reduction is adopted, which is usually called a double-stage reducer. A double-stage reducer has two sets of reduction gears to achieve two-stage speed reduction and torque increase. To improve the meshing smoothness and strength of the bevel gear pair, the first-stage reduction gear pair is a spiral bevel gear, and the second-stage gear pair is a helical cylindrical gear. When the driving bevel gear rotates, it drives the driven bevel gear to rotate, thus completing the first-stage reduction. The driving cylindrical gear of the second stage rotates coaxially with the driven bevel gear and drives the driven cylindrical gear to rotate for the second-stage reduction. Since the driven cylindrical gear is installed on the differential housing, when the driven cylindrical gear rotates, the wheels are driven to rotate through the differential and the half shafts.
The differential is used to connect the left and right half shafts, enabling the wheels on both sides to rotate at different angular velocities while transmitting torque, ensuring the normal rolling of the wheels. In some multi-axle drive vehicles, a differential is also installed in the transfer case or between the axles of a through-drive system, which is called an inter-axle differential. Its function is to create a differential effect between the front and rear drive wheels when the vehicle is turning or driving on uneven roads. Currently, domestic cars and most other vehicles basically use the symmetrical bevel gear ordinary differential. The symmetrical bevel gear differential consists of planetary gears, half-shaft gears, a planetary gear shaft (either a cross shaft or a single straight shaft), and a differential housing. At present, most vehicles use a planetary gear differential. An ordinary bevel gear differential consists of two or four conical planetary gears, a planetary gear shaft, two conical half-shaft gears, and left and right differential housings.
The half shaft is a solid shaft that transmits the torque from the differential to the wheels, driving the wheels to rotate and propelling the vehicle forward. Due to the different installation structures of the hubs, the force conditions of the half shafts are also different. Therefore, half shafts are divided into three types: full-floating, semi-floating, and three-quarter floating.
Full-floating Half Shaft: Generally, large and medium-sized vehicles adopt the full-floating structure. The inner end of the half shaft is connected to the half-shaft gear of the differential by splines, and the outer end is forged with a flange and connected to the hub by bolts. The hub is supported on the half-shaft sleeve by two widely spaced tapered roller bearings. The half-shaft sleeve is press-fitted into the rear axle housing to form the drive axle housing. With this support form, the half shaft has no direct connection with the axle housing, so the half shaft only bears the driving torque and no bending moment. This type of half shaft is called a “full-floating” half shaft. The so-called “floating” means that the half shaft does not bear bending loads. For the full-floating half shaft, the outer end flange is integrated with the shaft. However, in some trucks, the flange is made as a separate part and is sleeved on the outer end of the half shaft by splines. Thus, both ends of the half shaft have splines and can be used interchangeably.
Semi-floating Half Shaft: The inner end of the semi-floating half shaft is the same as that of the full-floating one and does not bear bending and torsion. Its outer end is directly supported on the inner side of the half-shaft housing through a bearing. This support method will cause the outer end of the half shaft to bear a bending moment. Therefore, in addition to transmitting torque, this type of half shaft also partially bears the bending moment, so it is called a semi-floating half shaft. This structure type is mainly used in passenger cars.
Three-quarter Floating Half Shaft: The three-quarter floating half shaft bears a bending moment to an extent between that of the semi-floating and full-floating half shafts. This type of half shaft is not widely used currently and is only applied in some individual small sedans, such as the Warszawa M20.
Axle Housing
Integral Axle Housing: The integral axle housing is widely used because of its good strength and stiffness performance, which facilitates the installation, adjustment, and maintenance of the final drive. Depending on the manufacturing method, it can be divided into integral casting type, middle-section casting with pressed-in steel pipe type, and steel plate stamping and welding type.
Segmented Drive Axle Housing: The segmented axle housing is generally divided into two sections, which are connected into one body by bolts. The segmented axle housing is relatively easy to cast and machine.
In the automotive industry, the design and functionality of drive axles have been continuously evolving. With the pursuit of higher vehicle performance, lighter weight, and greater durability, new materials and manufacturing techniques are being introduced. For the final drive, advanced gear materials with higher strength and wear resistance are being developed. These materials can withstand heavier loads and harsher operating conditions, reducing the frequency of component replacements and maintenance. In the differential, electronic differentials are emerging. Unlike traditional mechanical differentials, electronic differentials can precisely control the torque distribution between wheels based on various sensors’ data, such as wheel speed, vehicle speed, and steering angle. This improves vehicle handling, especially in challenging driving scenarios like high-speed cornering or off-road driving.
Regarding half shafts, new composite materials are being tested. These materials aim to further reduce the weight of half shafts while maintaining or even enhancing their strength. Lighter half shafts contribute to overall vehicle weight reduction, which in turn improves fuel efficiency. For axle housings, the use of high-strength steel alloys and optimized manufacturing processes, like precision casting and advanced welding techniques, are enhancing the structural integrity of the housing. This not only makes the drive axle more reliable but also allows for more compact designs, saving space within the vehicle chassis.
In addition, with the rise of electric vehicles, the concept of drive axles is also undergoing changes. Electric drive axles integrate electric motors directly into the axle assembly, eliminating the need for some traditional mechanical components. This new design simplifies the power transmission chain, improves energy conversion efficiency, and enables more precise control of vehicle dynamics. Research is also focused on developing smart drive axles that can communicate with other vehicle systems, such as the battery management system and the autonomous driving control unit, to optimize power delivery and vehicle performance in real-time.
