According to structural form, drive axles can be divided into three major categories

1. Central Single-Stage Reduction Drive Axle
This is the simplest type of drive axle structure, the basic form of the drive axle, and it dominates in heavy-duty trucks. Generally, when the main transmission ratio is less than 6, a central single-stage reduction drive axle should be used as much as possible. The central single-stage reducer tends to use double-helical hypoid gears, the active pinion uses a saddle-type support, and a differential lock device is available as an option.
2. Central Two-Stage Reduction Drive Axle
In the domestic market, there are mainly two types of central two-stage drive axles: one type is the rear axle design of heavy-duty vehicles, such as Eaton series products. Space is reserved in advance in the single-stage reducer; when it is required to increase traction and speed ratio, a cylindrical planetary gear reduction mechanism can be installed, changing the original central single-stage into a central two-stage drive axle. This modification has a high degree of "three-ization" (i.e., serialization, generalization, and standardization), and the axle housing and main reducer can be used interchangeably, and the bevel gear diameter remains unchanged. Another type, such as Rockwell series products, when it is necessary to increase traction and speed ratio, the first-stage hypoid gear needs to be modified before installing a second-stage cylindrical spur gear or helical gear to become the required central two-stage drive axle. In this case, the axle housing can be used interchangeably, but the main reducer cannot be used interchangeably, and the bevel gear has two Specifications. Since the above central two-stage reduction axles are derived as a model from the central single-stage axle as a series product when the speed ratio exceeds a certain value or the total traction mass is large, it is difficult for them to be transformed into front drive axles, and their use is subject to certain limitations. Therefore, comprehensively speaking, the two-stage reduction axle is generally not developed as a basic type of drive axle, but exists as a drive axle derived from a particular consideration.
3. Central Single-Stage, Wheel-Side Reduction Drive Axle
Wheel-side reduction drive axles are widely used in off-road vehicles and military vehicles in oil fields, construction sites, and mines. Currently, wheel-side reduction axles can be divided into two categories: one is the conical planetary gear type wheel-side reduction axle; the other is the cylindrical planetary gear type wheel-side reduction drive axle. The conical planetary gear type wheel-side reduction axle is composed of a wheel-side reducer with conical planetary gear transmission, and the wheel-side reduction ratio is a fixed value of 2. It is generally combined with a central single-stage axle to form a series. In this series, the central single-stage axle still has independence and can be used independently. If it is necessary to increase the output torque of the axle to increase traction or speed ratio, a conical planetary gear reducer can be added to both ends of the axle without changing the central main reducer, thus turning it into a two-stage axle. The difference between this type of axle and the central two-stage reduction axle is that it reduces the torque transmitted by the half-shaft and directly increases the increased torque to the wheel-side reducer at both ends of the axle, and its degree of "three-ization" is higher. However, because the wheel-side reduction ratio of this type of axle is a fixed value of 2, the Size of the central main reducer is still relatively large. It is generally used in highway and off-highway military vehicles. For cylindrical planetary gear type wheel-side reduction axles, single-row, gear ring fixed cylindrical planetary gear reduction axles generally have a reduction ratio between 3 and 4.2. Due to the large wheel-side reduction ratio, the speed ratio of the central main reducer is generally less than 3, so the large bevel gear can have a smaller diameter to meet the requirements of heavy-duty trucks for ground clearance. This type of axle is larger and more expensive than a single-stage reducer, and the wheel valley contains gear transmission, which will generate a large amount of heat during long-term highway driving, causing overheating. Therefore, as a drive axle for highway vehicles, it is not as good as a central single-stage reduction axle.