Un differential It is an essential mechanical component for any car, except many electrical. Thanks to her outer wheel that moves a vehicle can rotate faster than the inside during a curve. An essential difference, since the first has to travel more distance than the second during this maneuver.
Another way to describe its function is this: the motor only rotates at a certain speed at any one time, but it must be able to move two wheels at different speeds. The differential adds a few more gears to the transmission of a car for this to be possible.
The cars only have a differential for the drive wheels. That is: in front for front-wheel drive cars, behind in rear-wheel drive cars, and in both places for all-wheel drive cars. Although the latter can also have a central differential, which distributes the turning force between the front and rear axles.
Parts of a differential
Before explaining how a differential works, we are going to list what its fundamental parts. But always with a key idea in mind: the differential receives the rotation of a single axle and has to transmit that rotation to each wheel, which must also be able to rotate at different speeds.
- Corona: it is the gear in charge of transforming the axis of rotation that comes from the gearbox into one compatible with the wheels.
- Satellites: these are the pinions welded to the structure of the crown, which rotate freely but will transmit the rotation of the crown to the bearings. The fact that they rotate freely is the key to the operation of the differential, because it is what absorbs the difference in rotation between the two bearings.
- Planetariums: they are the pinions that receive the movement of the satellites so that the bearings rotate. They are the end of the chain of gears that make up the differential.
How does a differential work?
Now that we know what the parts of a differential are, we are ready to know how it works. We are going to describe the mission of each of its gears well so that it is very clear.
Function 1 – Change the axis of rotation
The differential receives the movement of the pinion that is at the end of the transmission shaft, but this rotates about a longitudinal axis regarding the car. The crown transforms that rotation into one compatible with the rotation of the wheels, which is therefore on a transverse axle to the car.
Function 2 – Allow the wheels to turn at different speeds
If we simply connected the planetary pinions to the crown, we would already have a turning transmission to the wheels. It would be very simple because, as you can see in the image, they already have a compatible twist. However, with this we would inevitably have two wheels turning at the same speed. The satellite sprockets are the intermediaries to prevent this from happening.
If the car is going straight, the satellites do not rotate on themselves. They act as a block that transmits the rotation to both wheels equally. But, when one has to turn faster than the other in a curve, the pinions turn on themselves so that this can be so. In the following video you can see how this happens clearly:
The secondary effect of the differential
When one of the wheels loses grip due to a low grip area or because it lifts off the ground, all the rotation generated by the motor goes to that wheel. In this way, the wheel that does grip, and which is therefore the useful one, does not receive any movement and remains stationary.
Precisely to avoid this problem there are self-locking differentials. Some mechanical devices that can fulfill the task described so far without this happy side effect. These are components that most cars have with a somewhat sporty focus.
Types of self-locking differentials
Electronic self-locking differential
Actually this device is just a imitation of a limited-slip differential. Automotive electronics use the sensors of the ABS and the ESP to detect if one of the wheels spins too much due to losing grip. At that moment, use the brake to stop it so that the other can receive the turning force normally.
Although its operation is fast, thanks to the evolution that the electronics of today's cars have undergone, it does not fulfill its function as well as the real limited-slip differentials. Furthermore, it is not the best solution for a car with sporty aspirations, as it causes a increased wear and heating of the brakes by resorting to them more times than normal.
Torsen type self-locking differential
The Torsen type limited slip differential eliminates the problem of differentials in a simple and elegant way. It works thanks to a helical spur gear (endless screw) can move a cogwheel, but the cogwheel cannot move the worm.
The parts of a Torsen type differential are: the same crown type gear that a normal differential has, some sprockets with diagonal teeth to fit the helical spur gears, some sprockets with horizontal teeth and finally the helical spur gears. In the following video you can see how it works clearly:
limited slip differential
This differential works thanks to a set of clutches arranged in a row that control traction. When detects that a wheel loses grip, they get together so that they don't keep doing it. The mechanics usually work through a set of gears that reacts when exceeding a certain speed difference. Within this type of limited slip differentials there is also the Ferguson-type or viscous-coupled. This works thanks to an oil with silicones that, when heated by friction, becomes denser and begins to transmit movement to the wheel that remains stationary.
Haldex or controlled slip differentials
This type of limited slip differential also works thanks to a multi-disc clutch system. However, instead of operating with a simple set of gears, it does so thanks to electronics and a hydraulic system. Thanks to this, it is able to control the amount of torque that reaches each wheel according to how much the discs press against each other. They can cut all transmission of the rotation, give it completely or distribute the torque in the proportion that is necessary.
Pictures – kurt, fasteddy760, Satish Krishnamurthy