Basic knowledge of vehicle axles

Author: Driving Vision

Link: https://zhuanlan.zhihu.com/p/53631596

Source: Zhihu

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The drive axle is a mechanism located at the end of the transmission system that can change the speed and torque from the transmission and transmit them to the drive wheels. The powertrain is longitudinally mounted, and the power output from the transfer case is transmitted to the front drive axle. The drive axle is generally composed of the main reducer, differential, wheel drive device, and drive axle housing. The steering drive axle also has a constant velocity universal joint. ▲ A disconnected (independent suspension) drive axle composed of a main reducer and a constant speed drive shaft. In addition, the drive axle also bears vertical, longitudinal, and lateral forces acting on the road surface and the frame or body of the vehicle, as well as braking torque and reaction forces. The car axle (also known as the axle) is connected to the frame (or load-bearing body) through suspension, with wheels installed at both ends. The function of the axle is to bear the load of the car and maintain its normal operation on the road. A trailer axle, also known as a vehicle axle in a broad sense, can be an integral structure, like a huge barbell, with both ends supported by a suspension system to support the vehicle body. Therefore, an integral vehicle axle is usually paired with a non independent suspension; The axle can also be detachable, like two umbrellas inserted on both sides of the vehicle, and each supported by the suspension system, so the detachable axle is used in combination with independent suspension. ▲ In front wheel drive passenger cars, the gearbox is actually composed of three parts: transmission, reducer, and differential. The reducer and differential, which originally belonged to the drive axle, are integrated into the gearbox, so there is no conventional axle. ▲ A typical powertrain, where the engine+transmission+drive axle is divided into four types based on the different driving modes: steering axle, drive axle, steering drive axle, and support axle. Both the steering axle and the support axle belong to the driven axle. Most cars use front rear drive (FR), so the front axle serves as the steering axle and the rear axle serves as the driving axle; For front wheel drive (FF) cars, the front axle becomes the steering drive axle, while the rear axle serves as the supporting axle. The front axle is mostly a driven axle, also known as a steering axle, which is generally distributed at the front end of the vehicle, so it is called a front axle. It is connected to the steering system using the steering knuckles. Can transmit the steering force output by the steering gear to the wheels to achieve vehicle steering. It not only supports the spring-loaded mass at the front of the vehicle, withstands vertical loads, but also various longitudinal forces, lateral forces, and related moments. The steering axle structure of various types of cars is basically the same, mainly composed of the front axle (beam), steering knuckle, main pin, wheel hub, brake and other parts as the main components. The front axle is generally made of medium carbon steel through forging and heat treatment. Its cross-section is I-shaped or tubular, as shown in the figure. To improve the torsional strength, there are thickened parts in a fist shape near both ends of the front axle, with through holes where the main pin is inserted. The central part is bent downwards into a concave shape, with the aim of lowering the engine position and thus lowering the center of mass of the car; Expand the driver's field of view; Reduce the angle between the drive shaft and the transmission output shaft. The steering knuckle is the hinge for turning the wheels, which is a fork shaped component. The upper and lower forks have two coaxial holes for installing the main pin, and the steering knuckle journal is used to install the wheel. The two ears of the pin hole on the steering knuckle are connected to the fist shaped parts at both ends of the front axle through the main pin, allowing the front wheel to deflect a certain angle around the main pin and turn the car. In order to reduce wear, a bronze bushing is pressed into the pin hole of the steering joint, and the lubrication of the bushing is injected with lubricating grease through the oil nozzle installed on the steering joint. To ensure flexible steering, bearings are installed between the lower ear of the steering knuckle and the front axle fist shaped part. There are also adjustment shims installed between the ear and the fist shaped part of the steering knuckle to adjust the gap between them. The function of the main pin is to hinge the front axle and steering knuckle, causing the steering knuckle to swing around the main pin to achieve wheel steering. There is a groove in the middle of the main pin. During installation, use the main pin fixing bolt 5 to fit with the groove above it, and fix the main pin in the fist shaped hole of the front axle. The pin hole on the main pin and steering joint is a dynamic fit to achieve steering. The wheel hub is supported on the journal at the outer end of the steering knuckle by two tapered roller bearings. The tightness of the bearing can be adjusted by adjusting the nut (installed on the outer end of the bearing). The outer end of the wheel hub is covered with a stamped metal cover, and the inner end is equipped with an oil seal. The brake base plate and dust cover are fixed together on the steering knuckle. Front axle sectional view. In many sedans and all wheel drive off-road vehicles, the front wheels not only serve as steering axles, but also serve as driving axles. Therefore, it is called a steering drive axle. A steering drive axle has the main reducer, differential, and half axle that typical drive axles have; It also has the steering joint housing, main pin 1, and wheel hub that typical steering axles have. Compared with individual drive and steering axles, its difference is that due to the need for steering, the half shaft is divided into two sections, namely the inner half shaft (connected to the differential) and the outer half shaft (connected to the wheel hub), which are connected by an equal angular velocity universal joint. At the same time, the main pin is divided into upper and lower sections, which are respectively fixed on the spherical support of the universal joint. The steering knuckle journal is made hollow so that the outer half shaft passes through it. The connecting fork of the steering joint is a spherical steering joint housing, which not only meets the needs of steering but also adapts to the power transmission of the steering joint. The steering drive axle is widely used in all wheel drive off-road vehicles. The lightweight drive rear axle often adopts a tube insertion type, which refers to the casting of the middle axle package and the use of seamless steel pipes inserted on both sides for fixation. The advantages are simple manufacturing process, low cost, and the disadvantages are limited bearing capacity and accuracy. Now, there is a gradual transition towards heavy-duty single stage reduction bridges with only a central primary reducer. The bridge shell is made of steel plate and is mainly used for highway logistics, without overloading, requiring lightweight and high efficiency. Due to the relatively high speed, the speed ratio of the vehicle axle is generally between 3-4, matched with disc brakes, quasi hyperbolic gears, and no differential lock. In some heavy-duty situations, the heavy-duty single stage reduction drive rear axle may have a double rear axle, namely a through rear axle and rear axle, with a maximum bearing capacity of 13Tx2 and a maximum reduction gear diameter of 485mm. Currently, the commonly used 440/469/485 single rear axle output torque is generally 35000Nm+. The casting of the axle housing is to improve the bearing capacity. In addition, the casting of the axle housing can improve the transmission accuracy of the axle transmission system, eliminate deformation and other accuracy errors caused by steel plate welding parts, and is often used in harsh engineering and other applications. In order to improve the ground clearance, in addition to the central main reducer, a wheel side two-stage reduction is also added, with a maximum bearing capacity of 16Tx2 heavy-duty wheel side reduction double drive axle in the wheel side reducer. There are two types of gears, cylindrical gears and bevel gears, with cylindrical gears being the mainstream. In order to ensure ground clearance, the diameter of the main reduction gear of the double stage axle is generally not greater than 300mm. In order to obtain greater torque, the speed ratio is usually above 5. However, the strength of the main reduction driving gear decreases as the speed ratio increases. Generally, the output torque of a single rear axle can exceed 50000Nm. The double stage reduction drive front axle double stage reduction drive front axle integral rear axle is mostly used for longitudinal powertrain models, and it is often matched with plate spring suspension. The double rear axle model also has a balanced suspension. If two axles (such as the middle and rear axles of a three-axle car) are installed at both ends of the balance bar, and the middle of the balance bar is hinged to the frame, lifting one axle will cause the other to be lifted. Lower the axle. Moreover, due to the equal length of the two arms of the balance bar, the vertical load of the two axles is always equal, and there will be no individual wheel suspension situation. This type of suspension that ensures equal vertical load on the middle and rear axle wheels is called a balanced suspension. In addition to matching plate springs, the integral light drive rear axle can also be matched with spiral springs. This type of axle has a load capacity of generally less than 2 tons, and the main reducer, brake, and other components are no different from conventional axles. The cost is relatively low, and it can achieve a drive axle perspective structure of less than 1000 yuan. The worm gear structure is rare, and the drive axle perspective structure (portal axle) is rare. The differential casting of the integral axle housing can be made of ductile iron, malleable iron, or cast steel. Long seamless steel pipes are pressed into the two ends of the cast integral axle housing as half axle sleeves and fixed with pins. On heavy-duty trucks, in order to further improve the strength and stiffness of the axle housing, the rear cover and axle housing are cast into one. Casting bridge shell 1. The bearing capacity of casting bridge shell is higher than that of punching and welding bridge shell. The price of cast bridge shells in the market is lower than that of punched welded bridge shells. 3. The unit weight is large and not suitable for lightweight transportation. 4. Currently, the bridge housing is still mainstream in China, and the maintenance of parts is convenient. Stamped welded bridge shells are composed of a pair of bridge shell main components, triangular steel plates, reinforcement rings, half shaft sleeves, rear covers, and steel plate spring seats welded together along the seams between them. The semi-circular ends on both sides of the main components of the upper and lower axle shells are closely adjacent to the outer circle of the inner end of the half shaft sleeve. In addition to welding one circle along the seam, plug welding process is also required. 1. Heavy duty vehicles use thick steel plates for the welded bridge housing, but the welded bridge housing is relatively light. 2. The process requirements for steel plate welds are high and the cost is higher than that of casting bridge shells. 3. Domestic axles need to be strengthened in key stress areas to ensure a low failure rate. The earliest heavy-duty welding axle technology in China was introduced from Nissan Diesel, which sold the axle technology to Dongfeng and FAW. Subsequently, other domestic manufacturers imitated it one after another, which is a microcosm of homogeneous competition in the domestic axle industry. From the manufacturer's perspective, the main drawbacks of casting bridge shells are increased self weight and high scrap rate. An increase in self weight means a reduction in loading capacity, while a large amount of scrap means a waste of processing capacity. The impact welding bridge shell has the characteristics of high material utilization, low scrap rate, and high productivity, which is the demand for future vehicle production and development. The final reduction drive is a mechanism within the drive axle that can change torque and speed. The basic function is to increase the torque from the gearbox or universal transmission device, while reducing the speed and changing the direction of torque transmission. The main reducer consists of one or several pairs of reduction gear pairs. The power is input from the driving gear and output from the driven gear. The main reducer is the main component in the transmission system that plays a role in reducing speed and increasing torque. When the engine is longitudinally mounted, it also has the function of changing the direction of torque rotation. It relies on gears with fewer teeth and gears with more teeth to achieve deceleration, and using bevel gear transmission can change the direction of torque rotation. Placing the main reducer in the position before the power is diverted to the driving wheels helps to reduce the torque transmitted by the transmission components in front of it (such as clutch, gearbox, transmission shaft, etc.), thereby reducing the size and mass of these components. When a car is driving normally, the engine speed is usually around 2000 to 3000 r/min. If such a high speed is reduced solely by the gearbox, the transmission ratio of the gearbox gear pair needs to be large. The larger the transmission ratio of the gear pair, the larger the radius ratio of the two gears, in other words, the larger the size of the gearbox. In addition, as the rotational speed decreases, the torque inevitably increases, which increases the transmission load between the gearbox and the primary transmission mechanism behind the gearbox. Therefore, a main reducer should be installed before the differential that distributes power to the left and right drive wheels. The existence of the main reducer has two functions. The first is to change the direction of power transmission, and the second is to provide a common transmission ratio for various gears as an extension of the transmission. The output of the transmission is a torque that rotates around the longitudinal axis, while the wheels must rotate around the transverse axis of the vehicle, which requires a device to change the direction of power transmission. The reason why it is called the main reducer is that regardless of the gear position of the transmission, the gear ratio of this device is a factor of the total gear ratio. With this transmission ratio, the requirement for the deceleration capability of the transmission can be effectively reduced. The advantage of this design is that it can effectively reduce the size of the transmission and make the overall layout of the vehicle more reasonable. The characteristics of a through type main reducer are its simple structure, small mass, compact size, and the ability to make most parts of the middle and rear axles, especially the main parts such as the axle housing and half shaft, interchangeable. The differential lock allows the right wheel to roll longer than the left wheel at the same time. To balance the difference between the two tires, the left wheel needs to rotate slower and the right wheel needs to rotate faster, using different speeds to compensate for the distance difference. Simply put, the function of a differential is to allow the vehicle's tires to rotate at different speeds during turns, and to eliminate wheel slippage caused by various factors, in order to maintain consistency and smooth turns. But nothing is omnipotent, there are always its advantages and disadvantages. Due to the differential allowing the vehicle to rotate at different speeds, when encountering muddy and slippery roads, the differential may foolishly give most of the power to the slipping wheels, causing the other wheels to lose power and the entire vehicle to be unable to move. Therefore, when driving on muddy or slippery roads, if one tire of the vehicle slips, both tires need to maintain the same power output. In this case, the differential lock plays a crucial role. Simply put, the differential lock is a lock used to lock the differential. The differential lock is the locking switch of the differential. Its function is to forcibly lock the differential, causing it to lose its differential effect. The speed of two tires on the same axle or the speed of each axle of a multi axle drive vehicle reach a unified level, in order to increase the vehicle's passing force on muddy and slippery roads. Differential locks are divided into mechanical differential locks and electronic differential locks. Mechanical differential locks are commonly used on heavy-duty trucks. On heavy-duty trucks with a driving form of 6x4 or 8x4, there are generally two types of differential locks between wheels and axles. On some single wheel drive models, only the wheel differential lock is equipped. The axle differential lock locks the differential on the through axle, allowing the two drive axles to be hard connected and maintain the same speed. The differential lock between the wheels locks the differential between the wheels, causing it to lose its differential effect and keeping the left and right tires at the same speed. When a vehicle sinks on muddy and slippery roads, there are generally several situations. One is that the tires on one axle have "sunk" and continue to slip, losing drive. At this point, the differential gives most of the power to the "trapped" shaft, while the "unoccupied" shaft only has a small amount of power, and no matter how hard it is used, it cannot move. In theory, at this point, it is necessary to open the differential lock between the axles to keep the two drive axles at the same speed and work together to increase the vehicle's passing capacity. Another situation is that a tire on one axle has "fallen" and is constantly slipping. At this time, we need to open the wheel differential lock to keep both sides of the wheels at the same speed, in order to try our best to get out of the predicament. The differential lock can only be engaged when the vehicle is in a stopped state. When both differential locks need to be engaged, the axle differential lock should be engaged first, and then the wheel differential lock should be engaged. Do not turn after activating the differential lock. Because the speed of the wheels on both sides is not synchronized during turning, it will damage the main reducer and differential. After passing through muddy and slippery roads, it is necessary to promptly turn off the differential lock switch. Note that when turning off, the vehicle speed must also be zero. Input flange - front through shaft - front through shaft gear - helical gear - driving bevel gear - passive bevel gear - differential housing - cross shaft differential planetary gear - half shaft gear - half shaft - sun gear - planetary gear - planetary frame - bell hub - tire bolt - steel ring - wheel The entire transmission system, generally speaking, is a Steyr bridge. The use of wheel reducers is to further increase the driving force of the car, in order to meet or correct the matching of the driving force of the entire transmission system. The currently used wheel reducer is an additional gear transmission device that reduces speed and increases torque to meet the matching needs of the entire transmission system. The power is transmitted from the engine through the clutch, transmission, and transfer case to the main reducer of the front and rear axles, and then from the output end of the main reducer to the wheel reducer and wheels to drive the car. In this process, the working principle of the wheel reducer is to reduce the speed and torque transmitted by the main reducer, increase the torque, and then transmit it to the wheels, so as to generate a greater driving force under the reaction of ground adhesion.