What are the basic knowledge of drive shafts?

1. Introduction to drive shaft assembly (combined with specific assembly diagram)
Transmission shaft, English PROPELLER (DRIVING) SHAFT. Power is transmitted between two axes with different axes and even between two axes whose relative position changes continuously during the working process.
The transmission shaft can be classified into different categories according to its important component, the universal joint. If the universal joint has obvious elasticity in the direction of twisting, it can be divided into a rigid universal joint drive shaft and a flexible universal joint drive shaft. The former relies on the hinged connection of parts to transmit power, while the latter relies on elastic parts to transmit power and has a buffering and vibration damping effect. Rigid universal joints can be divided into non-constant velocity universal joints (such as cross-axis universal joints), quasi-constant velocity universal joints (such as double-joint universal joints, three-pin universal joints) and constant velocity universal joints. Universal joints (such as ball and cage universal joints, ball and fork universal joints). Constant speed and unequal speed refer to whether the rotation angular rates of the driven shaft and the driven shaft are equal when the driven shaft rotates with the driving shaft. Of course, the average speeds of the driving shaft and the driven shaft are equal.
​ A universal joint in which the angular velocities of the main and driven shafts remain equal when the angle between the two shafts changes is called a constant velocity universal joint or a constant angular velocity universal joint. They are mainly used in wheel transmission devices such as steering drive axles and disconnected drive axles, and are mainly used for power transmission in cars. When the car is rear-wheel drive, a cross-shaft universal joint drive shaft is often used. For some high-end cars, constant velocity ball joints are also used; when the car is front-wheel drive, a constant velocity universal joint is often used – etc. The speed universal joint is also a kind of transmission shaft, but it has a different name.
In cars with front-engine rear-wheel drive (or all-wheel drive), due to suspension deformation during the movement of the car, there is often relative movement between the input shaft of the main reducer of the drive shaft and the output shaft of the transmission (or transfer case). In addition, in order to effectively avoid certain mechanisms or devices (which cannot achieve linear transmission), there must be a device to achieve normal transmission of power, so universal joint transmission appeared. Universal joint transmission must have the following characteristics: a. Ensure that power can be reliably transmitted when the relative positions of the two connected axes change within the expected range; b. Ensure that the two connected axes can operate evenly. The additional load, vibration and noise caused by the angle of the universal joint should be within the allowable range; c. The transmission efficiency should be high, the service life should be long, the structure should be simple, easy to manufacture and easy to maintain. For automobiles, since the output shaft of a cross-shaft universal joint rotates at unequal speed relative to the input shaft (with a certain angle), double universal joints (or multiple universal joints) must be used for transmission. And arrange the two universal joint forks connected to the transmission shaft on the same plane, and make the angles between the two universal joints equal. This is very important. The included angle of the universal joint should be minimized during design.
​The imbalance of the drive shaft assembly is the main cause of bending vibration of the drive train. The vibration and noise caused by it are obvious. In addition, the axial movement of the universal joint cross shaft, the gap in the sliding spline of the transmission shaft, the centering accuracy of the connection between both ends of the transmission shaft assembly, the elastic deformation of the transmission shaft during high-speed rotation and the balance of spot welding on the transmission shaft Temporal thermal factors can change the unbalance of the drive shaft assembly. Reducing the unbalance of the drive shaft is extremely important for cars, especially high-speed cars such as cars, otherwise it will cause many related failures or abnormal damage.
The cross-shaft rigid universal joint drive shaft is the most widely used in automobile transmission systems and has the longest history. The drive shaft usually refers to a cross-shaft rigid universal joint drive shaft. The cross-shaft rigid universal joint is mainly used to transmit angle changes. It generally consists of a flange fork, a cross-shaft needle roller bearing assembly, a universal joint fork or a sliding fork, an intermediate connecting fork or a spline shaft fork, and a needle roller bearing. Composed of axial fixing parts, etc. The flange fork is usually connected to the gearbox, drive axle or other drive shaft. The flange fork is a fork-shaped part with a flange. It is generally made of forgings of medium carbon steel or medium carbon alloy steel. There are also sand castings of ductile iron and precision castings of medium carbon steel or medium carbon high-quality alloy steel. The flange fork generally has a flat flange, and some also have an end trapezoidal tooth flange. The cross shaft needle roller bearing assembly generally includes four needle roller bearings, a cross shaft, and a grease nipple. Needle roller bearings generally consist of several needle rollers, a bearing bowl, and a multi-edge rubber oil seal (some with a skeleton). In some needle roller bearings, there is also a circular gasket with an oil groove, either nylon or copper or other materials. It is mainly used to reduce the axial clearance of the universal joint and improve the dynamic balance quality of the drive shaft. . The universal joint fork is a fork-shaped part, generally made of forgings of medium carbon steel or medium carbon alloy steel, and also precision castings of medium carbon steel. The axial fixing parts of needle roller bearings are generally elastic retaining rings (internal and external cassettes) for holes (or shafts), or bearing pressure plates, locking plates, bolts, etc.
Another important component of the transmission shaft is the sliding spline pair, which is composed of inner and outer splines (rectangular, involute) and is used to transmit changes in length.
The universal joint swing angle of the drive shaft and the maximum expansion and contraction of the sliding spline pair are determined based on the runout check of the drive shaft during vehicle layout.
The transmission shaft tube is generally made of electric welded steel tube rolled from cnc drilling carbon fiber. For large-sized shaft tubes, cold-drawn seamless tubes are also used. The outer diameter and wall thickness (or inner diameter) of the shaft tube are determined based on the maximum working torque and maximum working speed transmitted by the drive shaft. The hollow shaft tube has a smaller mass and can transmit greater torque, and has a higher critical speed than a solid shaft with the same outer diameter.
Under normal circumstances, the center distance between two universal joints is not greater than 1.5m. When the distance is short, it usually consists of two universal joints and a sliding spline pair, with no shaft tube in the middle. When the distance is far and the length of the transmission shaft exceeds 1.5m, it is often divided into two or three, using three or four universal joints, and the last one has a sliding spline pair, and the rest have a structural type with intermediate supports. .
A typical intermediate support generally consists of a cylindrical ball bearing, a bearing seat, two oil seals, a rubber pad, and a grease nipple. The intermediate drive shaft is connected to the frame beam by an intermediate support bracket, and the axis center line of the intermediate drive shaft is required to be perpendicular to the intermediate support plane. It is generally required that the intermediate support of the transmission shaft is arranged at the first-order critical speed node of the transmission shaft system.
During use, regular maintenance is generally required. In places with grease nipples such as universal joints, sliding spline pairs, intermediate supports, etc., the specified grease must be added regularly according to relevant regulations. There are also maintenance-free drive shafts with reserved grease in the universal joint and intermediate support bearings, so there is no need to refill the grease regularly.
2. Introduction to universal joints
The universal joint is a key component on the automobile drive shaft. On front-engine, rear-wheel drive vehicles, the universal joint drive shaft is installed between the transmission output shaft and the drive axle main reducer input shaft; while in front-engine, front-wheel drive vehicles, the drive shaft is omitted and the universal joint is installed Between the front axle half-shaft and the wheels, which are responsible for both driving and steering.
A car is a moving object. In a rear-drive car, the engine, clutch and transmission are installed on the frame as a whole, and the drive axle is connected to the frame through elastic suspension. There is a distance between the two and needs to be connected. When the car is running, the uneven road surface will cause jumps, load changes or differences in the installation positions of the two assemblies, which will cause the angle and distance between the transmission output shaft and the drive axle main reducer input shaft to change. Therefore, a “variable strain” should be used. ” device to solve this problem, so there is a universal joint. ​
Application of universal joint
In addition, this universal joint is needed as a “joint” between the off-road vehicle transmission and the transfer case, and between the front-drive steerable transaxle and the axle shaft. The structure and function of the universal joint is a bit like the joints on the limbs of the human body, which allows the angle between the connected parts to change. But it is different from the activity form of limb joints. It only allows the angle to change within a certain range.
Universal joints include cross-shaft rigid universal joints, quasi-constant velocity universal joints (double coupling type and three-pin type), constant velocity universal joints (ball fork type and ball cage type), and dynamic universal joints . At present, the most widely used type of rear-drive automobile is the cross-axis universal joint.
A single universal joint cannot make the instantaneous angular velocity of the output shaft and the input shaft equal, which can easily cause vibration, aggravate damage to the machine parts, and produce a lot of noise. Therefore, the universal joint transmission form of rear-drive cnc machining automotive adopts double universal joints, that is, there is a universal joint at each end of the transmission shaft. Its function is to make the angles at both ends of the transmission shaft equal and ensure that the output shaft and the input shaft The instantaneous angular velocity is always equal.
In order to meet the angular changes caused by power transmission, steering and the up and down jump produced when the car is running, universal joints are also commonly used to connect the drive axle, half shaft and wheel axle of a front-drive car. Due to the limitation of axial size and the requirement of relatively large deflection angle, ordinary universal joints are unable to do the job, so a variety of constant velocity universal joints are widely used. On ordinary front-drive cars, each half-shaft uses two constant velocity universal joints. The universal joint close to the transaxle is the inner joint of the half-shaft, and the one close to the axle is the outer joint of the half-shaft. Among various constant velocity universal joints, the common one is the ball cage universal joint, which uses six steel balls to transmit force. At any intersection angle between the driving shaft and the driven shaft, the steel balls are located at the intersection of the two circles. , that is, it is located on the bisector of the intersection angle of the two shafts, thus ensuring equal angular speed transmission of the main and driven shafts.