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DIFFERENTIALS AND TRANSFER BOX INFORMATION:
A differential is a device, usually but not necessarily employing gears, capable
of transmitting torque and rotation through three shafts, almost always used in
one of two ways: in one way, it receives one input and provides two
outputs--this is found in most automobiles--and in the other way, it combines
two inputs to create an output that is the sum, difference, or average, of the
inputs.
In automobiles and other wheeled vehicles, the differential allows each of the
driving road wheels to rotate at different speeds, while for most vehicles
supplying equal torque to each of them.
Purpose
A vehicle's wheels rotate at different speeds, mainly when turning corners. The
differential is designed to drive a pair of wheels with equal torque 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 needs to travel a shorter distance than the outer
wheel, so with no differential, the result is the inner wheel spinning and/or
the outer wheel dragging, and this results in difficult and unpredictable
handling, damage to tires and roads, and strain on (or possible failure of) the
entire drivetrain.
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. Some historical
milestones of the differential include:
* 1050 BC–771 BC: The Book of Song (which itself was written between 502 and 557
A.D.) makes the assertion that the South Pointing Chariot, which uses a
differential gear, was invented during the Western Zhou Dynasty in China.
* 227 – 239 AD – Despite doubts from fellow ministers at court, Ma Jun from the
Kingdom of Wei in China invents the first historically verifiable South Pointing
Chariot, which provided cardinal direction as a non-magnetic, mechanized
compass.
* 658, 666 AD – two Chinese Buddhist monks and engineers create South Pointing
Chariots for Emperor Tenji of Japan.
* 1027, 1107 AD – Documented Chinese reproductions of the South Pointing Chariot
by Yan Su and then Wu Deren, which described in detail the mechanical functions
and gear ratios of the device much more so than earlier Chinese records.
* 1720 – Joseph Williamson uses a differential gear in a clock.
* 1810 – Rudolph Ackermann of Germany invents a four-wheel steering system for
carriages, which some later writers mistakenly report as a differential.
* 1827 – modern automotive differential patented by watchmaker Onésiphore
Pecqueur (1792–1852) of the Conservatoire des Arts et Métiers in France for use
on a steam cart. (Sources: Britannica Online and [1])
* 1832 – Richard Roberts of England patents 'gear of compensation', a
differential for road locomotives.
* 1876 – James Starley of Coventry invents chain-drive differential for use on
bicycles; invention later used on automobiles by Karl Benz.
* 1897 – first use of differential on an Australian steam car by David Shearer.
* 1913 – Packard introduces the spiral-gear differential, which cuts gear noise.
* 1926 – Packard introduces the hypoid differential, which enables the propeller
shaft and its hump in the interior of the car to be lowered.
* 1958 – Vernon Gleasman patents the Torsen dual-drive differential, a type of
limited slip differential that relies solely on the action of gearing instead of
a combination of clutches and gears.
Note: The Antikythera mechanism (150 BC – 100 BC), discovered on an ancient
shipwreck near the Greek island of Antikythera, was once suggested to have
employed a differential gear. This has since been disproved.
Functional description
A cutaway drawing of a car's rear axle, showing the crown wheel and pinion of
the final drive, and the smaller differential gears
The following description of a differential applies to a "traditional"
rear-wheel-drive car or truck with an "open" or limited slip differential:
Torque is supplied from the engine, via the transmission, to a drive shaft
(British term: 'propeller shaft', commonly and informally abbreviated to
'prop-shaft'), which runs to the final drive unit that 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 and pinion may mesh in hypoid orientation, not shown. The crown
wheel gear is attached to the differential carrier or cage, which contains the
'sun' and 'planet' wheels or gears, which are a cluster of four opposed bevel
gears in perpendicular plane, so each bevel gear meshes with two neighbours, and
rotates counter to the third, that it faces and does not mesh with. The two sun
wheel gears are aligned on the same axis as the crown wheel gear, and drive the
axle half shafts connected to the vehicle's driven wheels. The other two planet
gears are aligned on a perpendicular axis which changes orientation with the
ring gear's rotation. In the two figures shown above, only one planet gear
(green) is illustrated, however, most automotive applications contain two
opposing planet gears. Other differential designs employ different numbers of
planet gears, depending on durability requirements. As the differential carrier
rotates, the changing axis orientation of the planet gears imparts the motion of
the ring gear to the motion of the sun gears by pushing on them rather than
turning against them (that is, the same teeth stay in the same mesh or contact
position), but because the planet gears are not restricted from turning against
each other, within that motion, the sun gears can counter-rotate relative to the
ring gear and to each other under the same force (in which case the same teeth
do not stay in contact).
Thus, for example, if the car is making a turn to the right, the main crown
wheel may make 10 full rotations. During that time, the left wheel will make
more rotations because it has further to travel, and the right wheel will make
fewer rotations as it has less distance to travel. The sun gears (which drive
the axle half-shafts) will rotate in opposite directions relative to the ring
gear by, say, 2 full turns each (4 full turns relative to each other), resulting
in the left wheel making 12 rotations, and the right wheel making 8 rotations.
The rotation of the crown wheel gear is always the average of the rotations of
the side sun gears. This is why, if the driven roadwheels are lifted clear of
the ground with the engine off, and the drive shaft is held (say leaving the
transmission 'in gear', preventing the ring gear from turning inside the
differential), manually rotating one driven road wheel causes the opposite
road wheel to rotate in the opposite direction by the same amount.
When the vehicle is travelling in a straight line, there will be no differential
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 conventional 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".
The torque applied to each driving road wheel is a result of the engine and
transmission applying a twisting force against the resistance of the traction at
that road 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 road surface, 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.
To illustrate how a differential can limit overall torque, imagine a simple
rear-wheel drive vehicle, with one rear road wheel on asphalt with good grip, and
the other on a patch of slippery ice. With the load, gradient, etc., the vehicle
requires, say, 2,000 Newton metres (1,480 ft·lbf) 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 N·m (300 ft·lbf), and the asphalt to 3,000
N·m (2,210 ft·lbf).
If the two roadwheels were driven without a differential, each roadwheel would
be supplied with an equal amount of torque, and would push against the road
surface as hard as possible. The roadwheel on ice would quickly reach the limit
of traction (400 Nm), but would be unable to spin because the other roadwheel
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 ~300 Nm. 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 still symmetrically split the total amount of
available torque equally. ~300 Nm is sufficient to make the ice wheel to spin,
but the equal amount of ~300 Nm is not enough to turn the asphalt wheel. Since
the asphalt wheel remains stationary, the spinning ice wheel will rotate twice
as fast as before. As the actual torque on both roadwheels is the same – the
amount is determined by the lesser traction of the ice wheel. So both wheels
will get 300 Nm each. Since 600 Nm is less than the required threshold torque of
2000 Nm, the vehicle will not be able to utilise the output from the engine, and
will not move.
An observer will simply see one stationary roadwheel on one side of the vehicle,
and one spinning roadwheel on the opposite side. 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 allows the transmission
of equal torque to both driven roadwheels; unless it is a specific type of
differential, such as locking, torque-biasing, or limited slip type.
A proposed way to distribute the power to the wheels, is to use the concept of
gearless differential, of which a review has been reported by Provatidis [1],
but the various configurations seem to correspond either to the "sliding pins
and cams" type, such as the ZF B-70 available for early VWs, or are a variation
of the ball differential.
[edit] Traction-aiding devices
This section may contain original research. Please improve it by verifying the
claims made and adding references. Statements consisting only of original
research may be removed. More details may be available on the talk page. (July
2009)
ARB, Air Locking Differential
There are various devices for getting more usable 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 multi-disc clutch which allows extra torque to be sent to
the wheel with high resistance than available at the other driven roadwheel when
the limit of friction is reached at that other wheel. Below the limit of
friction more torque goes to the slower (inside) wheel. If there is no load on
one wheel then no torque goes to the other so the LSD provides no torque except
for spring loading, but some extra effect can be obtained by partially applying
the vehicle's parking brake when one roadwheel is spinning, as this can provide
some resistance there to increase the overall torque, and allow the other driven
roadwheel to move the vehicle. This only works where the handbrake acts on the
driven wheels, as in the traditional rear-wheel drive layout. Naturally, the
handbrake should be released as soon as the vehicle is moving again.
* A locking differential, such as ones using differential gears in normal use
but using air or electrically controlled mechanical system, which when locked
allow no difference in speed between the two wheels on the axle. They employ 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 effectively bypassing the differential gears
entirely. Other locking systems may not even use differential gears but instead
drive one wheel or both depending on torque value and direction.
* A high-friction 'Automatic Torque Biasing' (ATB) differential, such as the
Torsen differential, where the friction is between the gear teeth rather than at
added clutches. 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 of friction 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 still rotate in the opposite direction as for an open
differential but there with be some frictional losses and the torque will be
distributed at other than 50/50. Although marketed as being "torque-sensing", it
functions the same as a limited slip differential.
* A very high-friction differential, such as the ZF "sliding pins and cams"
type, so that there is locking from very high internal friction. When tested
with the wheels off the ground with torque applied to one wheel it will lock,
but it is still possible for the differential action to occur in use, albeit
with considerable frictional losses, and with the road loads at each wheel in
opposite directions rather than the same (acting with a "locking and releasing"
action rather than a distributed torque).
* An additional function of the conventional electronic traction control systems
usually use the anti-lock braking system (ABS) roadwheel speed sensors to detect
a spinning roadwheel, and apply the brake to that wheel. This progressively
raises the reaction torque at that roadwheel, and the differential compensates
by transmitting more torque through the other roadwheel – the one with better
traction. In Volkswagen Group vehicles, this specific function is called
'Electronic Differential Lock' (EDL).
* In a four-wheel drive vehicle, a viscous coupling unit can replace a centre
differential entirely, or be used to limit slip in a conventional 'open'
differential. It works on the principle of allowing the two output shafts to
counter-rotate relative to each other, by way of a system of slotted plates that
operate within a viscous fluid, often silicone. 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.
This system is similar to a limited slip differential.
A four-wheel drive (4WD) vehicle will have at least two differentials (one in
each axle for each pair of driven roadwheels), and possibly a centre
differential to apportion torque between the front and rear axles. In some cases
(e.g. Lancia Delta Integrale, Porsche 964 Carrera 4 of 1989 [2]) the centre
differential is an epicyclic differential (see below) to divide the torque
asymmetrically, but at a fixed rate between the front and rear axle. Other
methods utilise an 'Automatic Torque Biasing' (ATB) centre differential, such as
a Torsen – which is what Audi use in their quattro cars (with longitudinal
engines).
4WD vehicles without a centre 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 considerable damage to the
transmission or drive train. On loose surfaces these differences are absorbed by
the tire slippage on the road surface.
A transfer case may also incorporate a centre differential, allowing the drive
shafts to spin at different speeds. This permits the four-wheel drive vehicle to
drive on paved surfaces without experiencing "wind-up".
[edit] 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 2×–3× as big, the torque will be
2×–3× as high.
An epicyclic differential uses epicyclic gearing to split and apportion torque
asymmetrically between the front and rear axles. An epicyclic differential is at
the heart of the Toyota Prius automotive drive train, where it interconnects the
engine, motor-generators, and the drive wheels (which have a second differential
for splitting torque as usual). It has the advantage of being relatively compact
along the length of its axis (that is, the sun gear shaft).
Epicyclic gears are also called planetary gears because the axes of the planet
gears revolve around the common axis of the sun and ring gears that they mesh
with and roll between. In the image, the yellow shaft carries the sun gear which
is almost hidden. The blue gears are called planet gears and the pink gear is
the ring gear or annulus.
[edit] Spur-gear differential
This is another type of differential that was used in some early automobiles,
more recently the Oldsmobile Toronado, as well as other non-automotive
applications. It consists of spur gears only.
A spur-gear differential has two equal-sized spur gears, one for each
half-shaft, with a space between them. Instead of the Bevel gear, also known as
a miter gear, assembly (the "spider") at the centre of the differential, there
is a rotating carrier on the same axis as the two shafts. Torque from a prime
mover or transmission, such as the drive shaft of a car, rotates this carrier.
Mounted in this carrier are one or more pairs of identical pinions, generally
longer than their diameters, and typically smaller than the spur gears on the
individual half-shafts. Each pinion pair rotates freely on pins supported by the
carrier. Furthermore, the pinions pairs are displaced axially, such that they
mesh only 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. Therefore, each pinion couples that spur gear to
the other pinion, and in turn, the other spur gear, so that when the drive shaft
rotates the carrier, its relationship to the gears for the individual wheel
axles is the same as that in a miter-gear differential.
[edit] Non-automotive applications
A differential gear train can also be used to allow a difference between two
input axles. Mills often used such gears to apply torque in the required axis.
It's also used in fine mechanical watches with a hand to show the amount of
reserve power in the mainspring.
The oldest known example of a differential was once thought to be in the
Antikythera mechanism. It was supposed to have 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. It has now been proven that the assumption of the existence of a
differential gearing arrangement was incorrect.[2]
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 U.S. Navy Mk.1 gun fire control computer
used about 160 differentials of the miter gear type.
Differentials, usually flat but also spherical, are used in wristwatches to
allow the power reserve to be indicated. Power from the mainspring is split via
the differential to the time indications and the power reserve indicator.
Differentials are also used in watchmaking to link two separate regulating
systems with the aim of averaging out errors. Greubel Forsey use a spherical
differential to link two double tourbillon systems in their Quadruple
Differential Tourbillon.
Active differentials
A relatively new 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. 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.
Fully integrated active differentials are used on the Ferrari F430, Mitsubishi
Lancer Evolution, and on the rear wheels in the Acura RL. A version manufactured
by ZF is also being offered on the latest Audi S4 and Audi A4[3].
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
roadwheels and 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 ECU
controlled differential is also limited by the time delay caused by sensors and
the response time of the actuators.
TRANSFER CASE
A transfer case is a part of a four wheel
drive system found in four wheel drive and all wheel drive vehicles. The
transfer case is connected to the transmission and also to the front and rear
axles by means of drive shafts. It is also referred to as a "transfer gearcase",
"transfer gearbox","transfer box" or "jockey box" (Australia).
* The transfer case receives power from the transmission and sends it to both
the front and rear axles. This can be done with a set of gears, but the majority
of transfer cases manufactured today are chain driven.[1] On some vehicles, such
as four wheel drive trucks or vehicles intended for off-road use, this feature
is controlled by the driver. The driver can put the transfer case into either
"two wheel drive" or "four wheel drive" mode. This is sometimes accomplished by
means of a shifter, similar to that in a manual transmission. On some vehicles
this may be electronically operated by a switch instead. Some vehicles, such as
all wheel drive sports cars, have transfer cases that are not selectable. Such a
transfer case is permanently "locked" into all-wheel-drive mode.
* The transfer case may contain one or more sets of low range gears. Low range
gears are engaged with a shifter or electronic switch. On many transfer cases,
this shifter is the same as the one that selects 2WD or 4WD operation. Low range
gears slow down the vehicle and increase the torque available at the axles.
Low-range gears are used during slow-speed or extreme off road manoeuvres, such
as Rockcrawling, or when pulling a heavy load. This feature is often absent on
all wheel drive sports cars. Some very large vehicles, such as heavy equipment
or military trucks, have more than one low-range gear.
Types of transfer cases
Those used on "part-time four-wheel drive" system off-road vehicles such as
trucks, truggies, rock-crawling vehicles, and some military vehicles generally
allow the driver to select 2WD versus 4WD as well as high versus low gear
ranges. Those used in sports cars are usually "transparent" to the driver; there
is no shifter or select lever.
There are two different types of "internal workings" found in most transfer
cases. Gear-driven transfer cases can use sets of gears to drive either the
front or both the front and rear driveshaft. These are generally strong, heavy
units that are used in large trucks, but there are currently several gear drive
cases in production for passenger cars.
Chain-driven transfer cases use a chain to drive most often only one axle, but
can drive both axles. Chain-driven transfer cases are quieter and lighter than
gear-driven ones. They are used in vehicles such as compact trucks, full size
trucks, Jeeps and SUVs. Some off-road driving enthusiasts modify their vehicles
to use gear-driven transfer cases, accepting the additional weight and noise to
gain the extra strength they generally provide.
Transfer cases are also classified as either "independent" or "married".
"Married" transfer cases are bolted directly to the transmission. Sometimes a
"married" transfer case is an integral part of the transmission and the two
components share the same housing, as is commonly found on recent Subaru
products and some other four-wheel drive cars. An "independent" transfer case is
completely separate from the transmission; it is bolted to the transmission
output shaft and a short driveshaft travels from the transfer case to the front
and rear differentials. Married transfer cases are by far the most common found
in cars. Most pickups and SUVs use an independent transfer case. In fact, this
is so common that the term "married" is rarely used. Independent transfer cases
are used on very long wheelbase vehicles, such as commercial trucks or military
trucks.[2]
Manual transfer cases have a selector lever on the driver's side floor
transmission hump and may also have either two sealed automatic front axle
locking hubs or two manual front axle hub selectors of "LOCK" and "UNLOCK" or
"FREE". To engage the four wheel drive system the vehicle must be moving at a
slow speed less than 30 miles per hour, this is only for the four wheel drive
high setting. To engage the four wheel drive low setting the vehicle must be
stopped and the transmission must be shifted to neutral, then the four wheel
drive low can be selected.
Electronic Shift On-the-Fly (ESOF) transfer cases have a dash-mounted selector
switch or buttons with front sealed automatic locking axle hubs. Some models
also have what is called selec trac which has a slider switch on the center
console. This was usually found in the Jeep Cherokee vehicles. Unlike the manual
transfer case, this system has a transfer case motor and vacuum operated front
hubs. To engage or disengage, the vehicle can be completely stopped or moving at
any speed under 55 mph.
Most mud trucks are set up with a transmission and dual transfer cases because
of the high relocation after a massive body lift is installed. This saves money
on custom drive shafts for high pinion angles. Most of the time, monster trucks
and mud trucks are full time four wheel drive and aren't set up with a
transmission at all, just an independent three speed transfer case. These trucks
are set up to be run at one constant speed and only need to have 1 to 2 gears
plus reverse.
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