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Differentials are used on all modern cars and trucks, including AWD and 4WD vehicles. Differentials serve the purpose of enabling the wheels of a vehicle to spin at speeds conducive to turning and changing directions. This usually requires that the wheels on the outside of a turn must spin at a faster speed and travel a further distance to keep up with the inner wheels and to accomplish the turn successfully. The differential is a device consisting of grinding meshes that enable this. It also provides torque or power from the engine to the wheels.
The invention of differentials is difficult to pinpoint, as many make claims for it. Its history extends back to ancient times and it has evolved through the use of modern technology. Some sources indicate that differentials go as far back as 2634 B.C., when legend has it that the South Pointing Chariot exercised differential gears. The South Pointing Chariot, an ancient vehicle traveling on two wheels, was invented by Yellow Emperor Huang Di from China. The chariot was said to utilize select wheel size and gear ratios to ensure that the figure placed on the top of the chariot would continue to point in one continuous direction.
The oldest proof of a gear mechanism device is present in the Atikythera mechanism, which is traced as far back as the first century B.C. The device, discovered in 1900, consisted of a bronze and wooden frame. The only explanation that historians have been able to devise for its purpose is as a planetary/heavenly tracking device. Research confirms that this was the likely purpose for the earliest proven differential gear.
Differential gears became mobile on the first four-wheel steering system invented by a German man named Rudolph Ackerman in 1810. The device was created for a carriage system. In 1827, watchmaker, Onesiphore Pecqueur, from France, developed and patented the differential for use on a steam-powered car. Within five years, it was developed for road locomotives by an Englishman, Richard Roberts. As well, David Sherer, an Australian, was responsible for inventing the first steam car to use differential gears in 1897.
 Features/How it Works
Differentials have several purposes: they aim to provide power from the engine to the wheels; they reduce the gears of the vehicle so that the rotational speed slows down before the wheels come to a complete stop; and they allow the wheels to move freely at different speeds, hence the name. The differential essentially divides the force of the engine—the torque—in two ways so that the wheels can move in different speeds.
This is necessary because without this feature, turning and changing directions become difficult to achieve. With differentials, regardless of the method, vehicles can easily accomplish this. Wheels located on the inner section of the turn have less distance and slower speed to travel then the outer wheels. The differential remedies this by splitting the torque. Similarly, the front wheels move fast and have more distance to travel then the rear wheels. The differential also makes up for this. Cars that consist of a set of wheels that drive and a set that don’t, however, do not suffer this problem. In this case, the wheels are not conjoined, so either the rear or the front wheels are capable of rotating independently. In the circumstance where there is no differential, the wheels are locked together and unable to produce simple, smooth turns, often relying on the slippage of a wheel to achieve this function. In order for a slip to occur, a great deal of traction is required between the car and the road. The weight of the force would have to be substantial to be transmitted from the axles to the wheels and would likely cause strain to the axles.
Both 4WD and AWD vehicles can have differentials, although they are usually designed in a certain way to meet the needs of both. AWD, for example, requires differentials between each set of the wheels used for driving and also one between the front and the back wheels due to the fact that the front wheels travel a different distance.
In 4WD, the wheels all turn together simultaneously as they are connected by one differential. The obvious drawback of this type is that because the wheels are traveling at the same speed, regardless of the difference in distance between them, it becomes harder to turn in a direction, especially on concrete. In this case, the 4WD would have to be disengaged.
 Open Differential
The open differential supplies the same amount of torque to each of the wheels. Traction can determine the amount of torque that is supplied to the wheels. The engine can limit the amount of traction needed on hard, dry surfaces that do not require as much. In slippery, wet, or unstable surfaces, torque is applied so that traction can be achieved without risking slippage. In circumstances where one wheel has steady traction and another is at risk for slipping, the differential divides the torque between both wheels, reducing the torque for stable wheel to that of the slipping one. This has the disadvantage of reducing revolutions per minute (rpm) to zero.
Another disadvantage of the open differential is if one of the wheels leave the ground it will reduce the torque to zero, which means the wheel on the ground will have zero torque as well.
 Limited Slip Differential
With limited slip differential, also known as positraction, the wheels are not at risk of being reduced to zero torque when one wheel leaves the ground. While it is similar to the open differential in many respects, it is made distinct with a spring pack, a function that grinds the gears against clutches and keeps the wheels turning at similar speeds. The clutches act to ensure that the speed of the wheels remain the same in the odd circumstance that they are not. In situations that cause a wheel to leave the ground or encounter slippage, the limited differential will provide torque for the wheel that is not experiencing this. The vehicle may not move as smoothly or quickly as it normally would, but motion would still occur.
 Viscous Coupling
Used in AWD vehicles, viscous couples connect the rear wheels to the front and in the event that one set stops working, torque is transferred to the other set. Viscous coupling is beneficial in that it actively controls the amount of equal torque that is received by both sets of wheels. However, it is not as effective in the process of turning, as it does not transmit torque when one of the wheels are experiencing a slip. The coupling consists of plates filled with black liquid and is connected to the output shaft. The plates spin and act as a control in situations when a set of wheels attempts to spin faster than the other. Torque is also transferred to the slower wheels to get both sets up to equal speed.
In the event of turning or changing, viscous coupling has little effect, leaving the transmitted torque do most of the work in pushing the wheels at separate distances.
 Locking Differential
Similar to open differential, locking differential uses electric, pneumatic, or a hydraulic mechanism to lock the differentials. When initiated by a switch, the differentials will lock and the wheels will move the same speed. Locking differential is usually used on off-road vehicles, like heavy construction equipment.
 Torsen Differential
Designed by Torsen and used primarily for AWD vehicles, this type of differential utilizes the functions of an open differential in the sense of dividing the torque between each wheel. When traction is lost in a wheel, the Torsen differential acts to produce a torque bias ratio that can provide for up to as much as five times the torque, depending on the design. Similar to the viscous coupling, it transfers torque to the front or rear wheels as needed but goes even further in that it predicts when this is needed. One disadvantage to this is that it, like other differentials, if a wheel leaves the ground, all wheels share the torque of that one, which is zero.