Differential (mechanical Device) - Wikipedia

Purpose

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During cornering, the outer wheels of a vehicle must travel farther than the inner wheels (since they are on a larger radius). This is easily accommodated when the wheels are not connected, however it becomes more difficult for the drive wheels, since both wheels are connected to the engine (usually via a transmission). Some vehicles (for example go-karts and trams) use axles without a differential, thus relying on wheel slip when cornering. However, for improved cornering abilities, many vehicles use a differential, which allows the two wheels to rotate at different speeds.

The purpose of a differential is to transfer the engine's power to the wheels while still allowing the wheels to rotate at different speeds when required. An illustration of the operating principle for a ring-and-pinion differential is shown below.

•   Differential operation while driving in a straight line:Input torque is applied to the ring gear (purple), which rotates the carrier (purple) at the same speed. When the resistance from both wheels is the same, the planet gear (green) doesn't rotate on its axis (although the gear and its pin are orbiting due to being attached to the carrier). This causes the sun gears (red and yellow) to rotate at the same speed, resulting in the car's wheels also rotating at the same speed.
•   Differential operation while turning left:Input torque is applied to the ring gear (purple), which rotates the carrier (purple) at the same speed. The left sun gear (red) provides more resistance than the right sun gear (yellow), which causes the planet gear (green) to rotate anti-clockwise. This produces slower rotation in the left sun gear and faster rotation in the right sun gear, resulting in the car's right wheel turning faster (and thus travelling farther) than the left wheel.

Ring-and-pinion design

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A relatively simple design of differential is used in rear-wheel drive vehicles, whereby a ring gear is driven by a pinion gear connected to the transmission. The functions of this design are to change the axis of rotation by 90 degrees (from the propshaft to the half-shafts) and provide a reduction in the gear ratio.

The components of the ring-and-pinion differential shown in the schematic diagram on the right are: 1. Output shafts (axles) 2. Drive gear 3. Output gears 4. Planetary gears 5. Carrier 6. Input gear 7. Input shaft (driveshaft)

Epicyclic design

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An epicyclic differential uses epicyclic gearing to send certain proportions of torque to the front axle and the rear axle in an all-wheel drive vehicle.[citation needed] An advantage of the epicyclic design is its relatively compact width (when viewed along the axis of its input shaft).[citation needed]

Spur-gear design

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A spur-gear differential has equal-sized spur gears at each end, each of which is connected to an output shaft.[8] The input torque (i.e. from the engine or transmission) is applied to the differential via the rotating carrier.[8] Pinion pairs are located within the carrier and rotate freely on pins supported by the carrier. The pinion pairs only mesh for the part of their length between the two spur gears, and rotate in opposite directions. The remaining length of a given pinion meshes with the nearer spur gear on its axle. Each pinion connects the associated spur gear to the other spur gear (via the other pinion). As the carrier is rotated (by the input torque), the relationship between the speeds of the input (i.e. the carrier) and that of the output shafts is the same as other types of open differentials.

Uses of spur-gear differentials include the Oldsmobile Toronado American front-wheel drive car.[8][further explanation needed]

Locking differentials

edit Main article: Locking differential

Locking differentials have the ability to overcome the chief limitation of a standard open differential by essentially "locking" both wheels on an axle together as if on a common shaft. This forces both wheels to turn in unison, regardless of the traction (or lack thereof) available to either wheel individually. When this function is not required, the differential can be "unlocked" to function as a regular open differential.

Locking differentials are mostly used on off-road vehicles, to overcome low-grip and variable grip surfaces.

Limited-slip differentials

edit Main article: Limited-slip differential

An undesirable side-effect of a regular ("open") differential is that it can send most of the power to the wheel with the lesser traction (grip).[9][10] In situation when one wheel has reduced grip (e.g. due to cornering forces or a low-grip surface under one wheel), an open differential can cause wheelspin in the tyre with less grip, while the tyre with more grip receives very little power to propel the vehicle forward.[11]

In order to avoid this situation, various designs of limited-slip differentials are used to limit the difference in power sent to each of the wheels.

Torque vectoring

edit Main article: Torque vectoring

Torque vectoring is a technology employed in automobile differentials that has the ability to vary the torque to each half-shaft with an electronic system; or in rail vehicles which achieve the same using individually motored wheels. In the case of automobiles, it is used to augment the stability or cornering ability of the vehicle.