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Encyclopedia > Differential (mechanics)
 Input torque is applied to the ring gear, which turns the entire carrier (all blue), providing torque to both side gears (red and yellow), which in turn may drive the left and right wheels. If the resistance at both wheels is equal, the pinion gear (green) does not rotate, and both wheels turn at the same rate. If the left side gear (red) encounters resistance, the pinion gear (green) rotates about the left side gear, in turn applying extra rotation to the right side gear (yellow).

In an automobile and other four-wheeled vehicles, a differential is a device, usually consisting of gears, for allowing each of the driving wheels to rotate at different speeds, while supplying equal torque to each of them. Illustration of a differential gear, made by Wapcaplet in Blender and finished in the GIMP. File links The following pages link to this file: Differential (mechanics) Categories: GFDL images | Images created with Blender ... Illustration of a differential gear, made by Wapcaplet in Blender and finished in the GIMP. File links The following pages link to this file: Differential (mechanics) Categories: GFDL images | Images created with Blender ... Karl Benzs Velo model (1894) - entered into the first automobile race An automobile (or motor car) is a wheeled passenger vehicle that carries its own motor. ... Spur gears found on a piece of farm equipment. ... A driving wheel on a steam locomotive. ... Torque applied via an adjustable end wrench Relationship between force, torque, and momentum vectors in a rotating system In physics, torque can informally be thought of as rotational force or angular force which causes a change in rotational motion. ...

## Contents

A vehicle's wheels rotate at different speeds, especially when turning corners. The differential is designed to drive a pair of wheels with equal force, while allowing them to rotate at different speeds. In vehicles without a differential, such as karts, both driving wheels are forced to rotate at the same speed, usually on a common axle driven by a simple chain-drive mechanism. When cornering, the inner wheel travels a shorter distance than the outer wheel, resulting in the inner wheel spinning and/or the outer wheel dragging. This results in difficult and unpredictable handling, damage to tires and roads and strain on the entire drive train. Go Kart racing, Bairnsdale Kart Club Rental karts on an outdoor track Kart racing or Karting is a variant of open-wheeler motor sport with simple, small four-wheeled vehicles called karts, go-karts, or gearbox/shifter karts depending on the design. ... An axle is a central shaft for a rotating wheel or gear. ...

## Functional description

The differential on the rear axle of a car

The following description of a differential applies to a "traditional" rear-wheel-drive car or truck: Power is supplied from the engine, via the gearbox, to a driveshaft (UK term: propeller shaft), which runs to the rear axle. A pinion gear at the end of the propeller shaft is encased within the differential itself, and it engages with the large ring gear (British term: crownwheel), shown in the diagrams. The ring gear is attached to a carrier, which holds a set of small planetary gears. The three planetary gears are set up in such a way that the two outer gears (the side gears), can rotate in opposite directions relative to each other. The pair of side gears drive the axle shafts to each of the wheels. The entire carrier rotates in the same direction as the ring gear, but within that motion, the side gears can counter-rotate relative to each other. Image File history File links Download high resolution version (1000x712, 85 KB) Summary diagram of a differential between drive shaft and the rear wheels, my own drawing (used digital art program to increase contrast and remove parts not relevant) Licensing File links The following pages on the English Wikipedia link... Image File history File links Download high resolution version (1000x712, 85 KB) Summary diagram of a differential between drive shaft and the rear wheels, my own drawing (used digital art program to increase contrast and remove parts not relevant) Licensing File links The following pages on the English Wikipedia link... An axle is a central shaft for a rotating wheel or gear. ... A gearbox is an assembly of gears allowing the rotational speed of an input shaft to be changed to a different speed. ... Cardan driveshaft with universal joints A driveshaft or driving shaft or Cardan shaft is a mechanical device for transferring power from the engine or motor to the point where useful work is applied. ... A propeller shaft connects a propeller to an engine. ... An axle is a central shaft for a rotating wheel or gear. ... Epicyclic gearing is used here to increase output speed. ... An axle is a central shaft for a rotating wheel or gear. ...

Thus, for example, if the car is making a turn to the right, the main ring gear may make 10 full revolutions, and during that time, the left wheel will speed up because it has further to travel, and the right wheel will slow down correspondingly, as it has less distance to travel. The side gears will turn in opposite directions relative to each other by, say, 2 full turns each (4 full turns with regard to each other), resulting in the left wheel making 12 revolutions, and the right wheel making 8 revolutions.

When the vehicle is travelling in a straight line, there will be almost no movement of the planetary system of gears, other than the minute movements necessary to compensate for slight differences in wheel diameter, undulations in the road (which make for a longer or shorter wheel path), etc.

## Loss of traction

One undesirable side effect of a differential is that it can reduce overall torque - the rotational force which propels the vehicle. The amount of torque required to propel the vehicle at any given moment depends on the load at that instant - how heavy the vehicle is, how much drag and friction there is, the gradient of the road, the vehicle's momentum and so on. For the purpose of this article, we will refer to this amount of torque as the "threshold torque". Torque applied via an adjustable end wrench Relationship between force, torque, and momentum vectors in a rotating system In physics, torque can informally be thought of as rotational force or angular force which causes a change in rotational motion. ...

The torque on each wheel is a result of the engine and transmission applying torsion, a twisting force, against the resistance of the traction at that wheel. Unless the load is exceptionally high, the engine and transmission can usually supply as much torque as necessary, so the limiting factor is usually the traction under each wheel. It is therefore convenient to define traction as the amount of torque that can be generated between the tire and the ground before the wheel starts to slip. If the total traction under all the driven wheels exceeds the threshold torque, the vehicle will be driven forward; if not, then one or more wheels will simply spin. Look up torsion in Wiktionary, the free dictionary. ... Traction is applied mechanical force used to achieve motion. ...

To illustrate how a differential can limit overall torque, imagine a simple rear-wheel-drive vehicle, with one rear wheel on asphalt with good grip, and the other on a patch of slippery ice. With the load, gradient, etc., the vehicle requires, say, 2000 Nm of torque to move forward (i.e. the threshold torque). Let us further assume that the traction on the ice equates to 400 Nm, and the asphalt to 3000 Nm. Newton metre is the unit of moment in the SI system. ...

If the two wheels were driven without a differential, each wheel would push against the ground as hard as possible. The wheel on ice would quickly reach the limit of traction (400 Nm), but would be unable to spin because the other wheel has good traction. The traction of the asphalt plus the small extra traction from the ice exceeds the threshold requirement, so the vehicle will be propelled forward.

With a differential, however, as soon as the "ice wheel" reaches 400 Nm, it will start to spin. The planetary gears inside the differential carrier will start to rotate because the "asphalt wheel" encounters greater resistance. Instead of driving the asphalt wheel with more force, the differential will allow the ice wheel to spin faster, and the asphalt wheel to remain stationary, compensating for extra speed of the spinning ice wheel. The torque on both wheels will be the same - limited to the lesser traction of 400 Nm each. Since 800 Nm is less than the required threshold of 2000 Nm, the vehicle will not be able to move.

Note that an observer will simply see one stationary wheel and one spinning wheel. It will not be obvious that both wheels are generating the same torque (i.e. both wheels are in fact pushing equally, despite the difference in rotational speed). This has led to a widely held misconception that a vehicle with a differential is really only "one-wheel-drive". In fact, a normal differential always provides equal torque to both driven wheels (unless it is a locking, torque-biasing, or limited slip type).

There are various devices for getting more traction from vehicles with differentials. One solution is the limited slip differential (LSD), the most well-known of which is the clutch-type LSD. With this differential, the side gears are coupled to the carrier via a stack of clutch plates which limits the speed difference between the two wheels. In automotive applications, a limited slip differential (LSD) is a modified or derived type of differential gear arrangement that allows for some difference in rotational velocity of the output shafts, but does not allow the difference in speed to increase beyond a preset amount. ...

Another solution is the locking differential, which employs a mechanism for allowing the planetary gears to be locked relative to each other, causing both wheels to turn at the same speed regardless of which has more traction; this is equivalent to removing the differential entirely. A locking differential or locker is a modified type of automotive differential. ...

Electronic traction control systems usually use the ABS system to detect a spinning wheel and apply the brake to it. This progressively raises the reaction torque at that wheel, and the differential compensates by transmitting more torque through the other wheel - the one with better traction. Traction control systems, on current production vehicles, are typically (but not necessarily) electro-hydraulic systems designed to prevent loss of traction (and therefore the control of the vehicle) when excessive throttle or steering is applied by the driver. ... An anti-lock braking system (ABS) is a system on motor vehicles which prevents the wheels from locking while braking. ... A brake is a device for slowing or stopping the motion of a machine or vehicle, and to keep it from starting to move again. ...

A viscous coupling unit replaces the differential entirely. It works on the principle of allowing the two output shafts to counter-rotate relative to each other within a viscous fluid. The fluid allows slow relative movements of the shafts, such as those caused by cornering, but will strongly resist high-speed movements, such as those caused by a single wheel spinning. Viscous coupling is an application used in all-wheel drive systems. ...

A four-wheel-drive vehicle will have at least two differentials (one for each pair of wheels) and possibly a center differential to apportion power between the front and rear axles. Vehicles without a center differential should not be driven on dry, paved roads in four wheel drive mode, as small differences in rotational speed between the front and rear wheels cause a torque to be applied across the transmission. This phenomenon is known as "wind-up" and can cause damage to the transmission. On loose surfaces these differences are absorbed by the slippage on the road surface. The Jeep Wrangler is a 4WD vehicle with a transfer case to select low range or high range 4WD. The Lamborghini MurciÃ©lago is a 4WD/AWD that powers the front via a VCU if the rear slips. ... Image:Gearbox. ...

## Non-automotive applications

Reconstructed 19th century carding mill differential

A differential gear train can also be used to give the difference between two input axles. Mills often used such gears to apply torque in the required axis. Image File history File linksMetadata Download high-resolution version (1200x800, 973 KB) Ox powered treadwheel, New Salem, Illinois, 2006. ... Image File history File linksMetadata Download high-resolution version (1200x800, 973 KB) Ox powered treadwheel, New Salem, Illinois, 2006. ...

The oldest known example of a differential, in the Antikythera mechanism, used such a train to produce the difference between two inputs, one input related to the position of the sun on the zodiac, and the other input related to the position of the moon on the zodiac. The output of the differential gave a quantity related to the moon's phase. In the first half of the twentieth century, mechanical analog computers, called differential analyzers, were constructed that used differential gear trains to perform addition and subtraction. The Antikythera mechanism (main fragment) The Antikythera mechanism (Greek: O Î¼Î·Ï‡Î±Î½Î¹ÏƒÎ¼ÏŒÏ‚ Ï„Ï‰Î½ Î‘Î½Ï„Î¹ÎºÏ…Î¸Î®ÏÏ‰Î½ transliterated as O mÄ“chanismÃ³s tÅn AntikythÄ“rÅn) is an ancient mechanical analog computer (as opposed to most computers today which are digital computers) designed to calculate astronomical positions. ... For other uses, see Sun (disambiguation). ... It has been suggested that this article be split into multiple articles. ... Adjective lunar Bulk silicate composition (estimated wt%) SiO2 44. ... It has been suggested that New moon be merged into this article or section. ... A page from the Bombardiers Information File (BIF) that describes the components and controls of the Norden bombsight. ... The differential analyser was a mechanical analog computer invented by Vannevar Bush in 1927. ... 3 + 2 with apples, a popular choice in textbooks Addition is the basic operation of arithmetic. ... 5 - 2 = 3 Subtraction is one of the four basic arithmetic operations; it is essentially the opposite of addition. ...

## Active differentials

A relatively new technology is the electronically-controlled active differential. A computer uses inputs from multiple sensors, including yaw rate, steering angle, and lateral acceleration and adjusts the distribution of torque to compensate for undesirable handling behaviors like understeer. Active differentials used to play a large role in the World Rally Championship, but in the 2006 season the FIA has limited the use of active differentials only to those drivers who have not competed in the World Rally Championship in the last five years. Yaw or Yam is the name for the Levantine god of chaos and the power of the untamed sea as found in texts from the ancient city of Ugarit. ... Understeer is a term for a car handling condition during cornering in which the circular path of the vehicles motion is of a markedly greater diameter than the circle indicated by the direction its wheels are pointed. ... Stephane Sarrazin driving a Subaru Impreza WRC on the Monte Carlo Rally Carlos Sainz driving a Toyota Corolla WRC on the Monte Carlo Rally The World Rally Championship (WRC) is a rallying series organised by the FIA, culminating with a champion driver and manufacturer. ... The FÃ©dÃ©ration Internationale de lAutomobile, commonly referred to as the FIA, is a non-profit association established in 1904 to represent the interest of motoring organisations and motor car users. ... Stephane Sarrazin driving a Subaru Impreza WRC on the Monte Carlo Rally Carlos Sainz driving a Toyota Corolla WRC on the Monte Carlo Rally The World Rally Championship (WRC) is a rallying series organised by the FIA, culminating with a champion driver and manufacturer. ...

The first use of this technology on a production automobile was in the 1989 Nissan Skyline GT-R R32 Model. The lockup of the rear differential and center coupling were controlled as part of the ATTESA-ETS system. Fully integrated active differentials are used on the 2005 MR Ferrari F430 and on rear wheels in the Acura RL. This does not cite its references or sources. ... 2005 (MMV) was a common year starting on Saturday of the Gregorian calendar. ... It has been suggested that this article or section be merged into Mid-engine design . ... The Ferrari F430 is a high-performance sports car produced by the Italian automaker Ferrari to succeed the Ferrari 360. ... It has been suggested that this article or section be merged into Honda Legend. ...

The second constraint of the differential is passive – it is actuated by the friction kinematics chain through the ground. The difference in torque on the tires (caused by turns or bumpy ground) drives the second degree of freedom, (overcoming the torque of inner friction) to equalise the driving torque on the tires. The sensitivity of the differential depends on the inner friction through the second degree of freedom. All of the differentials (so called “active” and “passive”) use clutches and brakes for restricting the second degree of freedom, so all suffer from the same disadvantage – decreased sensitivity to a dynamically changing environment. The sensitivity of the computer controlled differential is also limited by the time delay caused by sensors and the response time of the actuators. In mechanical engineering, aeronautical engineering and robotics, degrees of freedom (DOF) describes flexibility of motion. ... In mechanical engineering, aeronautical engineering and robotics, degrees of freedom (DOF) describes flexibility of motion. ... In mechanical engineering, aeronautical engineering and robotics, degrees of freedom (DOF) describes flexibility of motion. ...

## History

There are many claims to the invention of the differential gear, but it is likely that it was known, at least in some places, in ancient times. Here are some of the milestones in the history of this device.

South Pointing Chariot (replica) Supposedly invented sometime around 2600BC in China by the Yellow Emperor Huang Di, the South Pointing Chariot (Zhi Nan Ju æŒ‡å—è»Š) is widely regarded as the most complex geared mechanism of the ancient Chinese civilization. ... Yellow Emperor The Yellow Emperor (&#40644;&#24093; Hu ng D ) is a Chinese mythical character, a culture hero said in legend to be the ancestor of all Chinese people. ... (2nd millennium BC - 1st millennium BC - 1st millennium) The 1st century BC started on January 1, 100 BC and ended on December 31, 1 BC. An alternative name for this century is the last century BC. The AD/BC notation does not use a year zero. ... The Antikythera mechanism (main fragment) The Antikythera mechanism (Greek: O Î¼Î·Ï‡Î±Î½Î¹ÏƒÎ¼ÏŒÏ‚ Ï„Ï‰Î½ Î‘Î½Ï„Î¹ÎºÏ…Î¸Î®ÏÏ‰Î½ transliterated as O mÄ“chanismÃ³s tÅn AntikythÄ“rÅn) is an ancient mechanical analog computer (as opposed to most computers today which are digital computers) designed to calculate astronomical positions. ... 1810 was a common year starting on Monday (see link for calendar). ... Rudolph Ackermann (April 20, 1764 - March 30, 1834) was an Anglo-German inventor and publisher. ... Naval Battle of Navarino by Carneray 1827 was a common year starting on Monday (see link for calendar). ... 1792 was a leap year starting on Sunday (see link for calendar). ... 1852 was a leap year starting on Thursday (see link for calendar). ... The Conservatoire National des Arts et MÃ©tiers (CNAM) is a higher education establishment operated by the French government dedicated to providing education and conducting research for the promotion of science and industry. ... 1832 was a leap year starting on Sunday (see link for calendar). ... Richard Roberts Richard Roberts (22 April 1789 â€“ 11 March 1864) was a British engineer whose development of high-precision machine tools contributed to the birth of production engineering and mass production. ... 1876 (MDCCCLXXVI) was a leap year starting on Saturday. ... James Starley was an English inventor. ... The Precinct in Coventry city centre. ... This article is becoming very long. ... Karl Benz Karl Friedrich Benz (November 26, 1844 â€“ April 4, 1929) was a German engine designer and automobile engineer, generally regarded as the inventor of the gasoline-powered automobile. ... 1897 (MDCCCXCVII) was a common year starting on Friday (see link for calendar). ... 1913 (MCMXIII) was a common year starting on Wednesday. ... Packard red hexagon symbol made its debut in 1905, with the color red added in 1913 Packard was a United States based brand of luxury automobile built by the Packard Motors Company of Detroit,Michigan, and later by the Studebaker-Packard Corporation of South Bend, Indiana. ... Year 1926 (MCMXXVI) was a common year starting on Friday (link will display the full calendar). ... Packard red hexagon symbol made its debut in 1905, with the color red added in 1913 Packard was a United States based brand of luxury automobile built by the Packard Motors Company of Detroit,Michigan, and later by the Studebaker-Packard Corporation of South Bend, Indiana. ... A hypoid is the name given to a type of helical (spiral) gear. ...

Results from FactBites:

 Differential (mechanics) - WOI Encyclopedia Italia (1919 words) In vehicles without a differential, such as karts, both driving wheels are forced to rotate at the same speed, usually on a common axle driven by a simple chain-drive mechanism. Vehicles without a center differential should not be driven on dry, paved roads in all wheel drive mode, as small differences in rotational speed between the front and rear of the vehicle cause a torque to be applied across the transmission. The oldest known example of a differential, in the Antikythera mechanism, used such a train to produce the difference between two inputs, one input related to the position of the sun on the zodiac, and the other input related to the position of the moon on the zodiac.
 BBTF's Bullpen Mechanics (2647 words) Not only does it have good movement and speed differential from his fastball, but what he does best is that he "sells" his changeup. After making some changes to his mechanics in the offseason and pitching well in Spring Training, he was released by the Brewers. What I do want to point out is that his mechanics have changed for the better from last year to this year and he's throwing harder because of it.
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