Working principle of automotive parts differential

The differential of a car is a key component in the drive axle (rear wheel drive or four-wheel drive vehicles) or drive shaft (front wheel drive vehicles). Its core function is to allow the left and right drive wheels to rotate at different speeds during turns or uneven road surfaces, while transmitting the power of the engine to both wheels.

##Why do we need a differential?

1. Turning requirements: When a car turns, the inner wheels travel a shorter path while the outer wheels travel a longer path. If the two wheels are rigidly connected together through a rigid shaft, forcing them to rotate at the same speed, then when turning:

*The inner wheels will experience slippage (being dragged away), resulting in additional friction and wear.

*The outer wheels will slip (attempting to turn faster but being restricted), which also generates friction.

*This can lead to abnormal tire wear, difficult steering, increased power consumption, and even the possibility of the vehicle losing control.

2. Uneven road surface: When one side of the wheel encounters a low adhesion road surface (such as ice or mud) or bumps, it is also necessary for the wheels on both sides to have a certain speed difference to avoid unnecessary friction and power loss.

**The core function of the differential is to solve the contradiction of "coaxial different speeds". **

##Basic working principle of differential (taking the most common open bevel gear differential as an example)

The differential is mainly composed of the following core components:

1. * * Differential housing: * * A housing that is driven to rotate by the driven bevel gear (crown gear) of the main reducer. It carries other internal components of the differential and serves as an input carrier for power.

2. * * Planetary gear shaft: * * A short shaft fixed to the differential housing.

3. * * Planetary gears: * * Usually there are 2 or 4 (the most common being 2), which are loosely fitted on the planetary gear shaft and can rotate around the shaft, while also revolving around the wheel axis along with the differential housing.

4. * * Half shaft gears: * * Two, respectively connected to the left and right half shafts (drive shafts) through splines. The half shaft gear meshes with the planetary gear.

###Detailed explanation of the work process

1. * * Straight driving (equal resistance on both wheels):**

*The engine power is transmitted to the main reducer through the gearbox and transmission shaft.

*The main reducer (driving bevel gear and driven bevel gear/crown gear) increases torque and changes the direction of power transmission by 90 degrees (rear wheel drive).

*The driven bevel gear drives the differential housing to rotate.

*The differential housing drives the planetary gear shaft and the planetary gear to revolve together.

*At this point, due to the same driving resistance encountered by both driving wheels, the resistance applied to the two half shaft gears is also equal and balanced.

*Planetary gears do not rotate around their own axis during revolution, and they are wedged between two half shaft gears like wedges.

*Therefore, the two half shaft gears are forced to rotate at the same speed (i.e. the speed of the differential housing).

*The left and right drive wheels rotate at the same speed. **The power is evenly distributed to both wheels. **

2. Turning driving (inner resistance>outer resistance):**

*When turning, the driving resistance encountered by the inner wheel is greater than that of the outer wheel (because the inner wheel has a shorter trajectory and needs to "wait" for the outer wheel to travel a longer distance).

*This difference in resistance is transmitted to the half shaft gear: * * The resistance of the inner half shaft gear increases, while the resistance of the outer half shaft gear decreases * *.

*The differential housing is still revolving, driving the planetary gear shaft and planetary gears to revolve.

*Due to the unbalanced resistance experienced by the two half shaft gears, the planetary gears begin to rotate around their own axis while revolving around the axis.

*The planetary gear's self transfer "drives" the half shaft gear with lower resistance (* * outer half shaft gear * *) * * to rotate faster * *.

*At the same time, due to the principle of gear meshing, the rotation of planetary gears will also "hinder" the half shaft gear with greater resistance (* * inner half shaft gear * *) * * to rotate slower * *.

*The result is: * * outer wheel speed=differential housing speed+planetary gear rotation speed increase * *; **Inner wheel speed=differential housing speed - planetary gear rotation reduction speed * *.

*The sum of the wheel speeds on both sides is always equal to twice the differential housing speed (N left+N right=2 * N housing).

*The power will be automatically allocated to the wheels with lower resistance (easier to turn). **

3. * * One side of the wheel slips (with minimal resistance):**

*If a driving wheel completely loses its grip (such as on ice), the resistance it encounters becomes very small.

*According to the characteristic of "power distribution to the path of least resistance" in the differential, almost all power will be transmitted to this slipping wheel.

*A slipping wheel will spin at a high speed close to twice the speed of the differential case (because N slip ≈ 2 * N case - N has adhesion ≈ 2 * N case -0).

*However, wheels with adhesion receive almost no power (revs close to 0), and the vehicle cannot move forward. This is the main drawback of an open differential.

###Summarize the core characteristics of differential

** * Average torque distribution: * * When the differential is in operation (with a speed difference on both sides), it always distributes * * torque * * (rotational force) * * evenly * * to the left and right half shaft gears/wheels. No matter how large the speed difference between the two sides is, the torque obtained by the left and right wheels is always equal (ignoring internal friction losses).

** * Allowable speed difference: * * By the rotation of planetary gears, the output shafts (half shafts) on both sides can rotate at different speeds to meet the needs of turning and uneven road surfaces.

** * "Bullying the hard and fearing the soft": * * Power will preferentially flow to the wheels with less resistance (easier to turn). This is both an advantage (allowing turning) and a disadvantage (trapping the vehicle when skidding on one side).

##Types of differentials

*Open differential: As mentioned above, it is the most common and basic. Advantages: Simple structure, low cost, and lightweight steering. Disadvantage: When the vehicle slips on one side, it loses driving force.

** * Limited slip differential: * * On the basis of an open differential, friction plates, clutches, or multi plate clutches are added. When a large speed difference is detected (such as one wheel slipping), it can automatically lock or limit the speed difference, transmitting more power to the wheels with adhesion. Improved the vehicle's passability and handling stability. There are multiple implementation methods (mechanical, viscous coupling, Torsen, electronically controlled multi plate clutch, etc.).

*Locking differential: It can completely rigidly lock the left and right half axles, forcing them to rotate at the same speed, providing the strongest off-road ability. It is usually manually activated in extreme road conditions and must be unlocked when driving on paved roads.

**In summary, the differential is a sophisticated and indispensable device in the automotive transmission system. Through the unique design of the planetary gear set, it cleverly solves the basic contradiction of the need for different speeds of the driving wheels on both sides during turning, ensuring the smoothness, stability, and tire life of the vehicle during driving. **Understanding its working principle is crucial for comprehending how a car operates.