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Encyclopedia > Differential (mechanical device)
 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 planet gear (green) does not rotate, and both wheels turn at the same rate. If the left side gear (red) encounters resistance, the planet 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, that allows 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 (vÃ©lo means bicycle in French) model (1894) - entered into the first automobile race 2005 MINI Cooper S. An automobile (also motor car or simply car) is a wheeled passenger vehicle that carries its own motor. ... Spur gears found on a piece of farm equipment A gear is a wheel with teeth around its circumference, the purpose of the teeth being to mesh with similar teeth on another mechanical device -- possibly another gear wheel -- so that force can be transmitted between the two devices in a... The force bearing on the axle has an eccentricity e with the point of contact to the rolling surface and exerts a moment about the contact point. ... Torque applied via an adjustable end wrench Relationship between force, torque, and momentum vectors in a rotating system In physics, torque (or often called a moment) can informally be thought of as rotational force or angular force which causes a change in rotational motion. ...

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, and possible failure of the entire drive train. World Champion Davide Fore demonstrate 2007 KF1 Zanardi chassis and KF1 at the Paul Ricard in Feb. ... An axle is a central shaft for a rotating wheel or gear. ...

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.

Functional description

The differential on the rear axle of a car

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 (or often called a moment) 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 non-spinning traction on the ice equates to 400 Nm, and the asphalt to 3000 Nm. Newton metre is the unit of moment (torque) 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, and then develop less traction~300Nm. 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 300 Nm each. Since 600 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. ... In mechanics, a transmission or gearbox is the system of gears and/or the hydraulic system (called variously hydrodynamic, fluid or automatic transmission) that transmits mechanical power from a prime moverâ€”such as an engine or electric motorâ€”to a typically rotary output device at a lower angular momentum but...

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 Sun (Latin: Sol) is the star at the center of the Solar System. ... It has been suggested that this article be split into multiple articles. ... Apparent magnitude: up to -12. ... Lunar phase refers to the appearance of the illuminated portion of the Moon as seen by an observer, usually on Earth. ... 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 = 5 with apples, a popular choice in textbooks[1] Addition is the mathematical operation of combining or adding two numbers to obtain an equal simple amount or total. ... 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. The word yaw can refer to: Yaw, the name for the Levantine god of chaos, rivers, the sea, and tempests; Yaw, an aeronautical and nautical term which indicates how far a craft is pointing away from its direction of travel due to rotation about its vertical axis. ... 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. ... 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. ... The World Rally Championship (WRC) is a rallying series organised by the FIA, culminating with a champion driver and manufacturer. ...

Fully integrated active differentials are used on the 2005 MR Ferrari F430 and on rear wheels in the Acura RL. 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 . ... This article or section does not adequately cite its references or sources. ... The Acura RL is the top-of-the-line sedan in the range of Hondas North American luxury brand Acura. ...

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. ...

Spur gears found on a piece of farm equipment. ... The cycloidal gear profile is used in mechanical clocks. ... Epicyclic gearing is used here to increase output speed. ... Worm and worm gear A worm gear, or worm wheel, is a type of gear that engages with a worm to greatly reduce rotational speed, or to allow higher torque to be transmitted. ... Non-circular gear example A non-circular gear (NCG) is a special gear design with special characteristics and purpose. ... The involute gear profile is the most commonly used system for gearing today. ... This article does not cite its references or sources. ... A hypoid is the name given to a type of helical (spiral) gear. ... Illustration of a Gear train with a pinion shown. ... Rack and pinion animation A rack and pinion is a pair of gears which convert rotational motion into linear motion. ... Hub gears or internal-gear hubs are a type of gear system used on bicycles. ... On a bicycle, the cassette or the freewheel is the set of rear sprockets that attaches to a hub on the rear wheel. ... Download high resolution version (500x680, 39 KB)Description: Photograph of spur gears on a piece of farm equipment. ... 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. ... A gear coupling is a mechanical device for transmitting torque between two shafts that are not coaxial. ... A gear train is a set or system of gears arranged to transfer rotational torque from one part of a mechanical sytem to another. ... There are very few or no other articles that link to this one. ... A hobbing machine is a special form of milling machine that mills gears. ... The Antikythera mechanism (main fragment). ... Photo showing two ball screws. ... Jackscrew ( animation) A jackscrew mechanism uses a long threaded rod used to position a matching nut at any position along its length. ... 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. ... The Falkirk Wheel The Falkirk Wheel, named after the nearby town of Falkirk in central Scotland, is a rotating boat lift connecting the Forth and Clyde Canal with the Union Canal. ... A leadscrew is a threaded shaft that is used in machines to create linear motion. ... Bicycle gearing is concerned with the connection between the pedals and the rear wheel of a bicycle, and with the effect this connection has on how easy or hard it is to turn the pedals. ...

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