The automotive differential is a differential transmission mechanism used to ensure power transfer to each driving wheel under various motion conditions, avoiding tire slippage on the ground.
When a car is turning, the outer wheel travels a longer distance than the inner wheel; when the car is driving straight on an uneven road surface, the lengths of the curves traveled by the two wheels are also unequal. Even on a very flat road surface, due to tire manufacturing size errors, different degrees of wear, different loads, or unequal inflation pressures, the actual rolling radii of each tire are not equal. If both wheels are fixed on the same rigid axle and the speeds of the two wheels are equal, the wheels will inevitably experience rolling and sliding. The sliding of the wheels on the road surface not only accelerates tire wear and increases the power consumption of the car but also may lead to deterioration of steering and braking performance.
If the driven gear of the main reducer drives both driving wheels simultaneously through a single axle, then the two wheels can only rotate at the same speed. In order to ensure that both driving wheels are in a pure rolling state, it is necessary to use two half shafts to connect the two driving wheels separately, and the driven gear of the main reducer drives the two half shafts and wheels on both sides through the differential, so that they can rotate at different angular velocities.
This type of differential, installed between the two driving wheels on the same driving axle, is called an inter-axle differential.
Similar issues exist between the various drive axles of multi-axle drive vehicles. In order to adapt to the different road conditions of each drive axle and allow each drive axle to have different input angular velocities, inter-axle differentials can be installed between each drive axle.
Differentials installed on the front drive axle (front-wheel drive vehicles) and rear drive axle (rear-wheel drive vehicles) can be respectively called front differentials and rear differentials. If installed on the intermediate driveshaft of a four-wheel drive vehicle to adjust the speed of the front and rear wheels, it is called a center differential.
Differentials can be divided into two categories: open differentials and limited-slip differentials.
The helical gear differential consists of spider gears, half-shaft gears, a spider gear shaft (cross shaft or straight pin shaft), and a differential case, among other components. The left half of the differential case 2 and the right half of the differential case 8 are bolted together. The driven gear 7 of the main reducer is fixed to the flange of the right half of the differential case 8 with bolts (or rivets). The cross-shaped spider gear shaft 9 is installed in the circular hole on the mating surface of the differential case, with a straight bevel spider gear 6 on each shaft neck, each fitted with a sliding bearing (bushing). The four spider gears mesh with a straight bevel half-shaft gear 4 on each side. The shaft neck of the half-shaft gear is supported in the corresponding hole on the left and right sides of the differential case, with its internal splines connected to the half-shaft. The spider gears, which rotate (revolve) together with the differential case, drive the half-shaft gears on both sides when the resistance on the two wheels is different. The spider gears also rotate about their own axis – self-rotation – to achieve differential drive on both sides of the wheels.
Torsen limited slip differential
The back of the spider gears and the corresponding inner surface of the differential case are both made spherical. This design increases the length of the spider gear shaft hole, which helps ensure proper meshing with the two half-shaft gears.
The Working Principle of the Differential
During the power transmission process, there is a significant axial force between the planetary gears and the half-shaft gears, to reduce wear between the gears and the differential case, flat washers 3 and spherical washers 5 are installed on the back of the half-shaft gears and planetary gears, respectively. Washers are usually made of soft steel, copper, or polyoxymethylene plastic. The lubrication of the differential is carried out together with the main reducer. In order to allow lubricating oil to enter the differential, windows are often opened on the differential case. To ensure that the lubricating oil can smoothly reach between the planetary gears and the shaft necks of the planetary gears, a flat surface is milled on the shaft necks of the planetary gears, and radial oil holes are drilled between the teeth of the planetary gears. In cars below the intermediate level, since the torque of the driving wheels is not large, two planetary gears are often used in the differential. The corresponding planetary gear shafts are straight pin shafts, and the differential case can be made into an integral shell with large window holes. Through the large window holes, the operation of disassembling and assembling the planetary gears and half-shaft gears can be performed.
Diagram of the Working Principle of the Differential
A typical differential is mainly composed of two side gears (connected to the wheels through half shafts), two planetary gears (connected to the ring gear through the carrier), and a ring gear (connected to the power input shaft).
The power transmitted by the driveshaft is transferred to the ring gear through the driving gear, which drives the planetary gear shaft to rotate together, and at the same time drives the side gears to rotate, thereby propelling the driving wheels forward.
When a vehicle is driving straight, the resistance on the left and right wheels is the same, so the planetary gears do not self-rotate. The power is transmitted to the two half-shafts, and at this point, the speeds of the left and right wheels are the same (similar to a rigid connection).
When the vehicle is turning, the resistance on the left and right wheels is different. The planetary gears rotate around the half-shafts and also self-rotate, thus absorbing the difference in resistance. This allows the wheels to rotate at different speeds, ensuring smooth cornering of the vehicle.
