A differential is a device, usually but notnecessarily employing gears, capable oftransmitting torque and rotation throughthree shafts, almost always used in one oftwo ways. In one way, it receives one inputand provides two outputs; this is found inmost automobiles. In the other way, itcombines two inputs to create an outputthat is the sum, difference, or average, ofthe inputs.
A cutaway view of a car final drive unit whichcontains the differential
In automobiles and other wheeled vehicles, thedifferential allows each of the driving roadwheelsto rotate at different speeds, while for mostvehicles supplying equal torque to each of them. In automotive applications, the differentialhousing is sometimes colloquially called a"pumpkin" as the differential housing typicallyresembles a “ pumpkin”
Purpose: The differential is designed to drive a pair of wheels with equalforce, whilst allowing them to rotate at different speeds. In vehicles without a differential, such as karts, both drivingwheels are forced to rotate at the same speed, usually on acommon axle driven by a simple chain-drive mechanism. When cornering, the inner wheel needs to travel a shorterdistance than the outer wheel, so with no differential, the resultis the inner wheel spinning and/or the outer wheel dragging, andthis results in difficult and unpredictable handling, damage totires and roads, and strain on (or possible failure of) the entiredrivetrain.
History:•1050 BC-771 BC: The Book of Song claimed the South Pointing Chariot, whichuses a differential gear, was invented during the Western Zhou Dynasty in China.•200-100 BC - The antikythera Mechanism: The device is remarkable for the levelof miniaturization and for the complexity of its parts, which is comparable to thatof 18th century clocks. It has over 30 gears, although Michael Wright (see below)has suggested as many as 72 gears, with teeth formed through equilateraltriangles, In Greece•227 - 239 AD - Despite doubts from fellow ministers at court, Ma Jun from theKingdom of Wei in China invents the first historically verifiableSouth Pointing Chariot, which provided cardinal direction as a non-magnetic,mechanized compass.•658, 666 AD - two Chinese Buddhist monks and engineers create South PointingChariots for Emperor Tenji of Japan.•1027, 1107 AD - Documented Chinese reproductions of the South PointingChariot by Yan Su and then Wu Deren, which described in detail the mechanicalfunctions and gear ratios of the device much more so than earlier Chineserecords.•1720 - Joseph Williamson uses a differential gear in a clock.•1810 - Rudolph Ackermann of Germany invents a four-wheel steering system forcarriages, which some later writers mistakenly report as a differential.
•1827 - modern automotive differential patented by watchmakerOnésiphore Pecqueur (1792-1852) of theConservatoire des Arts et Métiers in France for use on a steam cart.•1832 - Richard Roberts of England patents gear of compensation, adifferential for road locomotives.•1876 - James Starley of Coventry invents chain-drive differential for useon bicycles; invention later used on automobiles by Karl Benz.•1897 - first use of differential on an Australian steam car byDavid Shearer.•1913 - Packard introduces the spiral-gear differential, which cuts gearnoise.•1926 - Packard introduces the hypoid differential, which enables thepropeller shaft and its hump in the interior of the car to be lowered.•1958 - Vernon Gleasman patents the Torsen Dual-Drive Differential, atype of limited slip differential that relies solely on the action of gearinginstead of a combination of clutches and gears.
Input torque is applied to the ring gear (blue), which turns the entire carrier (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, theplanet gear (green) rotatesabout the left side gear, inturn applying extra rotationto the right side gear (yellow)
Functional description: A cutaway drawing of a cars rear axle , showing the crown wheel and pinion of the final drive, and the smaller differential gears
Torque is supplied from the engine, via the transmission, to a drive shaftwhich runs to the final drive unit and contains the differential. A spiral bevel pinion gear takes its drive from the end of the propeller shaft,and is encased within the housing of the final drive unit. This meshes with the large spiral bevel ring gear, known as the crown wheel. The crown wheel gear is attached to the differential carrier or cage, whichcontains the sun and planet wheels or gears, which are a cluster of fouropposed bevel gears in perpendicular plane. The two sun wheel gears are aligned on the same axis as the crown wheelgear, and drive the axle half shafts connected to the vehicles driven wheels.The other two planet gears are aligned on a perpendicular axis which changesorientation with the ring gears rotation As the differential carrier rotates, the changing axis orientation of theplanetgears imparts the motion of the ring gear to the motion of the sun gears bypushing on them rather than turning against them but because the planet gearsare not restricted from turning against each other, within that motion, the sungears can counter-rotate relative to the ring gear and to each other under thesame force
The rotation of the crown wheel gear is always the average of the rotations ofthe side sun gears. This is why, if the driven roadwheels are lifted clear of theground with the engine off, and the drive shaft is held (say leaving the transmissionin gear, preventing the ring gear from turning inside the differential), manuallyrotating one driven roadwheel causes the opposite roadwheel to rotate in theopposite direction by the same amount. When the vehicle is traveling in a straight line, there will be no differentialmovement of the planetary system of gears other than the minute movementsnecessary to compensate for slight differences in wheel diameter, undulations in theroad (which make for a longer or shorter wheel path), etc.
Loss of traction: One undesirable side effect of a conventional differential is that it can reduceoverall torque - the rotational force which propels the vehicle. The amount of torque required to propel the vehicle at any given momentdepends on the load at that instant The torque applied to each driving roadwheel is a result of the engine andtransmission applying a twisting force against the resistance of the traction atthat road wheel. Unless the load is exceptionally high, the engine and transmission can usuallysupply as much torque as necessary, so the limiting factor is usually the tractionunder each wheel It is therefore convenient to define traction as the amount of torque that canbe generated between the tire and the road surface, before the wheel starts toslip. A proposed way to distribute the power to the wheels, is to use the conceptof gearless differential, of which a review has been reported by Provatidis ,but the various configurations seem to correspond either to the "sliding pinsand cams" type, such as the ZF B-70 available for early VWs, or are a variation ofthe ball differential.
Traction-aiding devices: ARB, Air Locking Differential
One solution is the limited slip differential (LSD), the most well-known of which is theclutch-type LSD. With this differential, the side gears are coupled to the carrier via a multi-discclutch which allows extra torque to be sent to the wheel with high resistance than availableat the other driven roadwheel when the limit of friction is reached at that other wheel. Belowthe limit of friction more torque goes to the slower (inside) wheel. If there is no load on onewheel then no torque goes to the other so the LSD provides no torque except for springloading, but some extra effect can be obtained by partially applying the vehiclesparking brake when one roadwheel is spinning, as this can provide some resistance there toincrease the overall torque, and allow the other driven roadwheel to move the vehicle. Thisonly works where the handbrake acts on the driven wheels, as in the traditionalrear-wheel drive layout. Naturally, the handbrake should be released as soon as the vehicle ismoving again.
A high-friction Automatic Torque Biasing (ATB) differential, such as the Torsendifferential, where the friction is between the gear teeth rather than at addedclutches. This applies more torque to the driven roadwheel with highest resistance(grip or traction) than is available at the other driven roadwheel when the limit offriction is reached at that other wheel. When tested with the wheels off the ground,if one wheel is rotated with the differential case held, the other wheel will stillrotate in the opposite direction as for an open differential but there with be somefrictional losses and the torque will be distributed at other than 50/50. Althoughmarketed as being "torque-sensing", it functions the same as a limited slip
A very high-friction differential, such as the ZF "sliding pins andcams" type, so that there is locking from very high internal friction.When tested with the wheels off the ground with torque applied toone wheel it will lock, but it is still possible for the differential actionto occur in use, albeit with considerable frictional losses, and with theroad loads at each wheel in opposite directions rather than the same.
An additional function of the conventional electronictraction control systems usually use the anti-lock braking system (ABS)roadwheel speed sensors to detect a spinning roadwheel, and applythe brake to that wheel. This progressively raises the reaction torqueat that roadwheel, and the differential compensates by transmittingmore torque through the other roadwheel - the one with bettertraction.
In a four-wheel drive vehicle,a viscous coupling unit canreplace a centre differentialentirely, or be used to limit slipin a conventional opendifferential. It works on theprinciple of allowing the twooutput shafts to counter-rotaterelative to each other, by way ofa system of slotted plates thatoperate within a viscous fluid,often silicone. The fluid allowsslow relative movements of theshafts, such as those caused bycornering, but will strongly resisthigh-speed movements, such asthose caused by a single wheelspinning. This system is similar toa limited slip differential.
Epicyclic differential Epicyclic gearing is used here to apportion torque asymmetrically. The input shaft is the green hollow one, the yellow is the low torque output, and the pink is the high torque output. The force applied in the yellow and the pink gears is the same, but since the arm of the pink one is 2x-3x bigger the torque will be 2x-3x higher An epicyclic differential uses epicyclic gearing to split and apportion torqueasymmetrically between the front and rear axles. An epicyclic differential is at theheart of the Toyota Prius automotive drive train, where it interconnects the engine,motor-generators, and the drive wheels (which have a second differential forsplitting torque as usual). It has the advantage of being relatively compact along thelength of its axis (that is, the sun gear shaft). The yellow shaft carries the sun gear which is almost hidden. The blue gears arecalled planet gears and the pink gear is the ring gear or annulus
Spur-gear differential This is another type of differential that was used in some early automobiles, more recently the Oldsmobile Toronado , A spur-gear differential has two equal-sized spur gears, one foreach half-shaft, with a space between them. Instead of the Bevel gear,also known as a miter gear, assembly (the "spider") at the centre ofthe differential, there is a rotating carrier on the same axis as the twoshafts. Torque from a prime mover or transmission, such as the driveshaft of a car, rotates this carrier.
A relatively newActive differentials: technology is the electronically-controlled active differential. An electronic control unit (ECU) uses inputs from multiple sensors, including yaw rate, steering input angle, and lateral acceleration - and adjusts the distribution of torque to compensate for undesirable handling behaviours like understeer. Fully integrated active differentials are used on the Ferrari F430, and on the rear wheels in the Acura RL. A version is also being offered on the latest Audi S4.
The second constraint of the differential is passive – it isactuated by the friction kinematics chain through theground. The difference in torque on the roadwheels andtires (caused by turns or bumpy ground) drives the seconddegree of freedom, (overcoming the torque of inner friction)to equalize the driving torque on the tires. The sensitivity ofthe differential depends on the inner friction through thesecond degree of freedom. All of the differentials (so called“active” and “passive”) use clutches and brakes forrestricting the second degree of freedom, so all suffer fromthe same disadvantage – decreased sensitivity to adynamically changing environment. The sensitivity of theECU controlled differential is also limited by the time delaycaused by sensors and the response time of the actuators.
Live axle: Live axle A live axle, sometimes called a solid axle, is a type of beam axlesuspension system that uses the driveshafts that transmit power to the wheels toconnect the wheels laterally so that they move together as a unit. A live axle consists of a central differential in a single housing that also contains thedriveshafts that connect the differential to the driven wheels. The differential is connected tothe engine via a swinging drive shaft and a universal joint. The complete assembly maytypically be suspended with leaf springs, coil springs or air bags.
In small trucks solid front axles have generally beenreplaced by independent front suspension.Some live axles use trailing arms, semi-trailing arms,Panhard rod, or Watts linkage to control the verticaland lateral movements of the axle. Others, particularlyolder vehicles, use Hotchkiss drive, in which the leafsprings provide axle location as well as suspension.
Advantages and disadvantages Static Bump ReboundSolid axle suspension characteristics: Camber change on bumps, noneon rebound, large unsprung weight
Advantages:Advantages of the live axle are relative simplicity, lowermanufacturing costs, lighter overall vehicle weight, andthe fact that the axle and suspension systems take uplittle interior volume. Because the axle assembly is afairly simple and rigid arrangement, it can easily bemade strong and robust, which is an advantage forvehicles with substantial power or that are intended foruse in rugged environments or off-road usage. A furtheradvantage of a live/beam axle in off-road use is thatground clearance under the axle remains constant, evenif one wheel rises over a bump and the other doesnt.
Disadvantages:The principal disadvantage is the negative effect on ride qualityand handling. The wheels cannot move independently inresponse to bumps. Although the overall mass of the totalsuspension is low, the mass of the differential and driveshaftsare part of the vehicles unsprung weight, so the greaterunsprung mass transmits larger forces to the body of the vehicleand its occupants. Conversely, in an independent rear suspensionsystem the differential is rigidly attached to the vehicle. Thelower unsprung mass of the suspension results in a greaterability to absorb imperfections in the road. In passenger carapplications, often now fitted withmulti-link independent suspension, the useful ability to changetoe and camber independently left to right under cornering loadsis not given with a live axle.
Applications:Until the 1980s the live axle was the most common rearsuspension system on rear-wheel drive cars in theUnited States. It remains common on trucks, buses andother heavy vehicles, owing to its greater potentialrobustness and relatively low maintenance requirements,but most passenger cars have now adopted independentrear suspension instead.Examples of some passenger-vehicle types that haveemployed a live axle with various suspension elements are:Leaf springs — the early Land-Rover and Jeep models;Coil springs — the Range Rover 1 & 2, Rover SD1, Volvo 240;Panhard rods - the Ford Mustang;
Leaf springs — the early Land-Rover and Jeep models;Coil springs — theRange Rover 1 & 2,Rover SD1, Volvo240; Panhard rods - the Ford Mustang