INFORMATION SHEET

PROGRAM: NCAM CODE: TCAM 101

SUB-MODULE: AUTOMOTIVE TECHNOLOGY AND PREPARED BY: OKIDI THOMAS BECKET
MAINTENANCE I
CONTENT: MANUAL & AUTOMATIC TRANSMISSION DATE OF EXECUTION:

MANUAL TRANSMISSION SYSTEM

Manual transmissions is also referred to as stick shift transmission or just „stick', 'straight drive', or standard
transmission because you need to use the transmission stick every time you change the gears. To perform the gear
shift, the transmission system must first be disengaged from the engine. After the target gear is selected, the
transmission and engine are engaged with each other again to perform the power transmission. Manual transmissions
are characterized by gear ratios that are selectable by locking selected gear pairs to the output shaft inside the
transmission.

Components of manual transmission

The main components of manual transmission are:

 Clutch
 Gear box
 U- joint
 Shafts
 Differential gear box

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Clutch:
Clutch is a device which is used in the transmission system of automobile to engage and disengage the engine to the
transmission or gear box. It is located between the transmission and the engine.
When the clutch is engaged, the power flows from the engine to the rear wheels in a rear- wheel-drive transmission
and the vehicle moves.
When the clutch is disengaged, the power is not transmitted from the engine to the rear wheels and vehicle stops even
if engine is running.
It works on the principle of friction. When two friction surfaces are brought in contact with each other, they are united
due to the friction between them. If one is revolved the other will also revolve.

The friction depends upon the surface area contact. The friction surfaces are so designed that the driven member
initially slips on driving member when initially pressure is applied. As pressure increases the driven member is
brought gradually to speed the driving member.

The three main parts of clutch are:

 Driving member
 Driven member
 Operating member
The driving member consists of a flywheel mounted on the engine crank shaft. The flywheel is bolted to cover which
carries a pressure plate or driving disc, pressure springs and releasing levers. Thus the entire assembly of flywheel and
cover rotates all the times. The clutch housing and the cover provided with openings dissipate the heat generated by
friction during the clutch operation.

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The driven member consists of a disc or plate called clutch plate. It is free to slide length wise on the splines of the
clutch shaft. It carries friction materials on both of its surfaces when it is gripped between the flywheel and the
pressure plate; it rotates the clutch shaft through splines.
The operating members consists of a foot pedal, linkage, release or throw-out bearing, release levers and springs
necessary to ensure the proper operation of the clutch.
Now the driving member in an automobile is flywheel mounted on crank shaft, the driven member is the clutch plate
mounted on transmission or gear box input shaft. Friction surfaces or clutch plate is placed between two members.

Types of Friction Materials:

The friction materials of the clutch plate are generally of 3 types:
 Mill Board Type
 Molded type
 Woven type

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  Mill Board type friction materials mainly include asbestos material with different types of impregnates.
 Molded type friction materials are made from a matrix of asbestos fiber and starch or any other suitable
binding materials. They are then heated to a certain temperature for moulding in dies under pressure. They are
also made into sheets by rolling, pressing and backs till they are extremely hard and dense. Metallic wires are
used sometimes to increase wear properties.
 Woven types facing materials are made by impregnating a cloth with certain binders or by weaving threads of
copper or brass wires covered with long fiber asbestos and cotton. The woven sheets treated with binding
solution are baked and rolled.

Properties Of Good Clutching:

 Good Wearing Properties
 High Resistance to heat
 High coefficient of friction
 Good Binders in it

OPERATION OF CLUTCH
When the clutch pedal is pressed through pedal movement, the clutch release bearing presses on the clutch release
lever plate which being connected to clutch release levers, forces these levers forward. This causes the pressure plate
to compress pressure springs, thus allowing it to move away from the clutch driven plate. This action releases the
pressure on the driven plate and flywheel, the flywheel is now free to turn independently, without turning the
transmission.
When the clutch pedal is released, reverse action takes place i.e. the driven plate is again forced against the flywheel
by the pressure plate- because of the force exerted by pressure springs. The pressure plate will keep on pressing the
facings of driven plate until friction created becomes equal to the resistance of the vehicle. Any further increase in
pressure will cause the clutch plate and the transmission shaft to turn along with flywheel, thus achieving vehicle
movement.

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 SINGLE CLUTCH PLATE
It is the most common type of clutch plate used in motor vehicles. Basically it consists of only one clutch plate,
mounted on the splines of the clutch shaft. The flywheel is mounted on engine crankshaft and rotates with it. The
pressure plate is bolted to the flywheel through clutch springs, and is free to slide on the clutch shaft when the clutch
pedal is operated. When the clutch is engaged the clutch plate is gripped between the flywheel and pressure plate. The
friction linings are on both sides of the clutch plate. Due to the friction between the flywheel, clutch plate and the
pressure plate, the clutch plate revolves. As the clutch plate revolves, the clutch shaft also revolves. Clutch shaft is
connected to the transmission gear box. Thus the engine power is transmitted to the crankshaft and then to the clutch
shaft.
When the clutch pedal is pressed, the pressure plate moves back against the force of the springs, and the clutch plate
becomes free between the flywheel and the pressure plate. Thus the flywheel remains rotating as long as the engine is
running and the clutch shaft speed reduces slowly and finally it stops rotating. As soon as the clutch pedal is pressed,
the clutch is said to be disengaged, otherwise it remains engaged due to the spring forces.

Clutch in disengaged and engaged position

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 MULTI-PLATE CLUTCH
Multi-plate clutch consists of a number of clutch plates instead of only one clutch plate as in case of single plate
clutch. As The number of clutch plates are increased, the friction surfaces also increases. The increased number of
friction surfaces obliviously increases the capacity of the clutch to transmit torque.
The plates are alternately fitted to engine and gear box shaft. They are firmly pressed by strong coil springs and
assembled in a drum. Each of the alternate plate slides on the grooves on the flywheel and the other slides on splines
on the pressure plate. Thus, each alternate plate has inner and outer splines.
The multi-plate clutch works in the same way as a single plate clutch by operating the clutch pedal. The multi-plate
clutches are used in heavy commercial vehicles, racing cars and motor cycles for transmitting high torque. The multi-
plate clutch may be dry or wet. When the clutch is operated in an oil bath, it is called a wet clutch. When the clutch is
operated dry it is called dry clutch. The wet clutch is used in conjunction with or part of the automatic transmission.

CONE CLUTCH
Cone clutch consists of friction surfaces in the form of cone. The engine shaft consists of female cone. The male cone
is mounted on the splined clutch shaft. It has friction surfaces on the conical portion. The male cone can slide on the
clutch shaft. Hence, when the clutch is engaged the friction surfaces of the male cone are in contact with that of the
female cone due to force of the spring. When the clutch pedal is pressed, the male cone slides against the spring force
and the clutch is disengaged.

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The only advantage of the cone clutch is that the normal force acting on the friction surfaces is greater than the
axial force, as compare to the single plate clutch in which the normal force acting on the friction surfaces is
equal to the axial force.
The disadvantage in cone clutch is that if the angle of the cone is made smaller than 200 the male cone tends to
bind in the female cone and it becomes difficult to disengage the clutch.
Cone clutches are generally now only used in low peripheral speed applications although they were once common in
automobiles and other combustion engine transmissions. They are usually now confined to very special transmissions
in racing, rallying, or in extreme off-road vehicles, although they are common in power boats. Small cone clutches are
used in synchronizer mechanisms in manual transmissions.

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 DOG & SPLINE CLUTCH

This type of clutch is used to lock two shafts together or to lock a gear to shaft. It consists of a sleeve having two sets
of internal splines. It slides on a splined shaft with smallest diameter splines. The bigger diameter splines match with
the external dog clutch teeth on driving shaft. When the sleeve is made to slide on the splined shaft, its teeth match
with the dog clutch teeth of the driving shaft. Thus the sleeve turns the splined shaft with the driving shaft.
The clutch is said to be engaged. To disengage the clutch, the sleeve is moved back on the splined shaft to have no
contact with the driving shaft. This type of clutch has no tendency to slip. The driven shaft revolves exactly at the
same speed of the driving shaft, as soon as the clutch is engaged. This is also known as positive clutch.

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 CENTRIFUGAL CLUTCH

The centrifugal clutch uses centrifugal forces, instead of spring force for keeping it in engaged position. Also, it does
not require clutch pedal for operating the clutch. The clutch is operated automatically depending on engine speed. The
vehicle can be stopped in gear without stalling the engine. Similarly the gear can be started in any gear by pressing the
accelerator pedal.
A centrifugal clutch works through centrifugal force. The input of the clutch is connected to the engine crankshaft
while the output drives gear box shaft, chain, or belt. As engine R.P.M. increases, weighted arms in the clutch swing
outward and force the clutch to engage. The most common types have friction pads or shoes radially mounted that
engage the inside of the rim of housing.
On the center shaft there are an assorted amount of extension springs, which connect to a clutch shoe. When the center
shaft spins fast enough, the springs extend causing the clutch shoes to engage the friction face. It can be compared to a
drum brake in reverse. The weighted arms force these disks together and engage the clutch.
When the engine reaches a certain RPM, the clutch activates, working almost like a continuously variable
transmission. As the load increases the R.P.M. drops thereby disengaging the clutch and letting the rpm rise again and
reengaging the clutch. If tuned properly, the clutch will tend to keep the engine at or near the torque peak of the
engine.

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These results in a fair bit of waste heat, but over a broad range of speeds it is much more useful then a direct drive in
many applications. Weaker spring/heavier shoes will cause the clutch to engage at a lower R.P.M. while a stronger
spring/lighter shoes will cause the clutch to engage at a higher R.P.M.

GEARBOX
A gearbox is a mechanical method of transferring energy from one device to another and is used to increase torque
while reducing speed. Torque is the power generated through the bending or twisting of a solid material. This term is
often used interchangeably with transmission. Located at the junction point of a power shaft, the gearbox is often used
to create a right angle change in direction, as is seen in a rotary mower or a helicopter. Each unit is made with a
specific purpose in mind, and the gear ratio used is designed to provide the level of force required. This ratio is fixed
and cannot be changed once the box is constructed. The only possible modification after the fact is an adjustment that
allows the shaft speed to increase, along with a corresponding reduction in torque. In a situation where multiple
speeds are needed, a transmission with multiple gears can be used to increase torque while slowing down the output
speed. This design is commonly found in automobile transmissions. The same principle can be used to create an
overdrive gear that increases output speed while decreasing torque.

Principle Of Gearing
Consider a simple 4-gear train. It consists of a driving gear a on input shaft and a driven gear d on the output shaft. In
between the two gears there are two intermediate gears b, c. Each of these gears are mounted on separate shaft. We
notice that:
Gear a drives gear b

Gear b drives gear c

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Gear c drives gear d

Therefore, the overall all speed ratios are,

Types of Gear Boxes

The following types of gear box are used in automobiles:
 Sliding Mesh
 Constant Mesh
 Synchromesh.

SLIDING MESH GEAR BOX

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It is the simplest gear box. The above figure shows 3-speed gear box in neutral position. 3 gears are connected to the
lay shaft/counter shaft. A reverse idler gear is mounted on another shaft and always remains connected to the reverse
gear of countershaft. This “H” shift pattern enables the driver to select three different gear ratios and a reverse gear.

Gears in Neutral
When the engine is running and clutch is engaged the clutch (input) shaft gear drives the countershaft gear. The
countershaft rotates opposite in direction of the input shaft. In neutral position only the input shaft gear is connected to
the countershaft gear. Other gears are free and hence the transmission main (output) shaft is not turning. The vehicle is
stationary.

First or low shaft gear

By operating the gear shift lever the larger gear (number 5) on the main shaft is moved along the shaft to mesh with
gear number 4 of the counter shaft. The main shaft turns in the same direction as that of the clutch shaft. Since the
smaller countershaft shaft gear is engaged with larger main shaft gear, a gear reduction of approximately 3:1 is
obtained.

Second speed gear

By operating the gear shift lever, the sixth gear on the main shaft is moved along the shaft to mesh with the third gear
of the counter shaft. The main shaft turns in same direction as clutch shaft. A gear reduction of approximately 2:1 is
obtained.

High-Speed Gear
By operating the gear shift lever the first and six gears of the input and main shafts are locked at their dog teeth. The
main shaft turns along with the clutch shaft and a gear ratio of approximately 1:1 is obtained.

Reverse gear
By operating the gear shift lever, the last gear present on the main shaft is engaged with the reverse idler gear. The
reverse idler gear is always in mesh with the counters haft gear. Interposing the idler gear between the counter-shaft
reverse gear and main shaft gear, the main shaft turns in the direction opposite to the clutch shaft. This reverses the
rotation of the wheels so that the wheel backs.

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Constant Mesh Gear Box
In this type of gear box, all gears of the main shaft are in constant mesh with the corresponding gears of the
countershaft (Lay shaft). Two dog clutches are provided on the main shaft- one between the clutch gear and the
second gear, and the other between the first gear and reverse gear. The main shaft is splined and all the gears are free
on it. Dog clutch can slide on the shaft and rotates with it. All the gears on the countershaft are rigidly fixed with it.
When the left hand dog clutch is made to slide to the left by means of the gear shift lever, it meshes with the clutch
gear and the top speed gear is obtained. When the left hand dog clutch meshes with the second gear, the second speed
gear is obtained. Similarly by sliding the right hand dog clutch to the left and right, the first speed gear and reverse
gear are obtained respectively. In this gear box because all the gears are in constant mesh they are safe from being
damaged and an unpleasant grinding sound does not occur while engaging and disengaging them.

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Synchromesh Gear Box

It is that gear box in which sliding synchronizing units are provided in place of sliding dog clutches as in case of
constant mesh gear box. With the help of synchronizing unit, the speed of both the driving and driven shafts is
synchronized before they are clutched together through train of gears. The arrangement of power flow for the various
gears remains the same as in constant mesh gear box. The synchronizer is made of frictional materials. When the
collar tries to mesh with the gear, the synchronizer will touch the gear first and use friction force to drive the gear to
spin at the same speed as the collar. This will ensure that the collar is meshed into the gear very smoothly without
grinding. Synchromesh gear devices work on the principle that two gears to be engaged are first bought into frictional
contact which equalizes their speed after which they are engaged readily and smoothly.

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 U- JOINT
A universal joint, U-joint, Cardan joint, Hardy-Spicer joint, or Hooke's joint is a linkage that transmits rotation
between two non-parallel shafts whose axes are coplanar but not coinciding., and is commonly used in shafts that
transmit rotary motion. It is used in automobiles where it is used to transmit power from the gear box of the engine to
the rear axle. The driving shaft rotates at a uniform angular speed, whereas the driven shaft rotates at a continuously
varying angular speed.
A complete revolution of either shaft will cause the other to rotate through a complete revolution at the same time.
Each shaft has fork at its end. The four ends of the two fork are connected by a Centre piece, the arms of which rest in
bearings, provided in fork ends. The Centre piece can be of any shape of a cross, square or sphere having four pins or
arms. The four arms are at right angle to each other.
When the two shafts are at an angle other than 180° (straight), the driven shaft does not rotate with constant angular
speed in relation to the drive shaft; the more the angle goes toward 90° the jerkier the movement gets (clearly, when
the angle β = 90° the shafts would even lock). However, the overall average speed of the driven shaft remains the
same as that of driving shaft, and so speed ratio of the driven to the driving shaft on average is 1:1 over multiple
rotations.
The angular speed ω2 of the driven shaft, as a function of the angular speed of the driving
shaft ω1 and the angle of the driving shaft φ1, is found using: ω2 = ω1 cosα / (1-sin2α.cos2θ)
For a given and set angle between the two shafts it can be seen that there is a cyclical variation in the input to output
velocity ratio. Maximum values occur when sin θ = 1, i.e. when θ = 900 and 2700. The denominator is greatest when
θ = 0or 1800 and this condition gives the minimum ratio of the velocities.

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Slip joint in the propeller shaft

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Hook joint in the propeller shaft

The Drive Shaft

The drive shaft, or propeller shaft, connects the transmission output shaft to the differential pinion shaft. Since all
roads are not perfectly smooth, and the transmission is fixed, the drive shaft has to be flexible to absorb the shock of
bumps in the road. Universal, or "U-joints" allow the drive shaft to flex (and stop it from breaking) when the drive
angle changes.
Drive shafts are usually hollow in order to weigh less, but of a large diameter so that they are strong. High quality
steel, and sometimes aluminum are used in the manufacture of the drive shaft. The shaft must be quite straight and
balanced to avoid vibrating. Since it usually turns at engine speeds, a lot of damage can be caused if the shaft is
unbalanced, or bent. Damage can also be caused if the U-joints are worn out.
There are two types of drive shafts, the Hotchkiss drive and the Torque Tube Drive. The Hotchkiss drive is made up
of a drive shaft connected to the transmission output shaft and the differential pinion gear shaft. U-joints are used in
the front and rear. The Hotchkiss drive transfers the torque of the output shaft to the differential.

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No wheel drive thrust is sent to the drive shaft. Sometimes this drive comes in two pieces to reduce vibration and
make it easier to install (in this case, three U-joints are needed).The two-piece types need ball bearings in a dustproof
housing as center support for the shafts. Rubber is added into this arrangement for noise and vibration reduction.
The torque tube drive shaft is used if the drive shaft has to carry the wheel drive thrust. It is a hollow steel tube that
extends from the transmission to the rear axle housing. One end is fastened to the axle housing by bolts. The
transmission end is fastened with a torque ball. The drive shaft fits into the torque tube. A U-joint is located in the
torque ball, and the axle housing end is splined to the pinion gear shaft. Drive thrust is sent through the torque tube to
the torque ball, to transmission, to engine and finally, to the frame through the engine mounts. That is, the car is
pushed forward by the torque tube pressing on the engine.

Differential Unit
Differentials are a variety of gearbox, almost always used in one of two ways. In one of these, it receives one input
and provides two outputs; this is found in every automobile. In automobile and other wheeled vehicles, the differential
allows each of the driving wheels to rotate at different speeds, while supplying equal torque to each of them. In the
other, less commonly encountered, it combines two inputs to create an output that is the sum (or difference) of the
inputs. In automotive applications, the differential and its housing are sometimes collectively called a "pumpkin"
(because the housing resembles a pumpkin).
Purpose:-
The differential gear box has following functions:
 Avoid skidding of the rear wheels on a road turning.
 Reduces the speed of inner wheels and increases the speed of outer wheels, while drawing a curve.
 Keeps equal speeds of all the wheels while moving on a straight road.
 Eliminates a single rigid rear axle, and provides a coupling between two rear axles.

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The following description of a differential applies to a "traditional" rear- or front-wheel-drive car or truck:
Power is supplied from the engine, via the transmission or gearbox, to a drive shaft termed as propeller shaft, which
runs to the differential. A spiral bevel pinion gear at the end of the propeller shaft is encased within the differential
itself, and it meshes with the large spiral bevel ring gear termed as crown wheel. The ring and pinion may mesh in
hypoid orientation.
The ring gear is attached to a carrier, which holds what is sometimes called a spider, a cluster of four

The following description of a differential applies to a "traditional" rear- or front-wheel-drive car or truck:
Power is supplied from the engine, via the transmission or gearbox, to a drive shaft termed as propeller shaft, which
runs to the differential. A spiral bevel pinion gear at the end of the propeller shaft is encased within the differential
itself, and it meshes with the large spiral bevel ring gear termed as crown wheel. The ring and pinion may mesh in
hypoid orientation.
The ring gear is attached to a carrier, which holds what is sometimes called a spider, a cluster of four bevel gears in a
rectangle, so each bevel gear meshes with two neighbors and rotates counter to the third that it faces and does not
mesh with. Two of these spider gears are aligned on the same axis as the ring gear and drive the half shafts connected
to the vehicle's driven wheels.

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These are called the side gears. The other two spider gears are aligned on a perpendicular axis which changes
orientation with the ring gear's rotation. These two gears are just called pinion gears, not to be confused with the main
pinion gear. (Other spider designs employ different numbers of pinion gears depending on durability requirements.)
As the carrier rotates, the changing axis orientation of the pinion gears imparts the motion of the ring gear to the
motion of the side gears by pushing on them rather than turning against them (that is, the same teeth stay in contact),
but because the spider gears are not restricted from turning against each other, within that motion the side 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 ring gear 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 side gears 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 ring gear is always the average of the rotations of the side gears. This is why if the wheels are
lifted off the ground with the engine off, and the drive shaft is held (preventing the ring gear from turning inside the
differential), manually rotating one wheel causes the other to rotate in the opposite direction by the same amount.
When the vehicle is traveling 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.

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 AUTOMATIC TRANSMISSION
An automatic transmission (commonly "AT" or "Auto") is an automobile gearbox that can change gear ratios
automatically as the vehicle moves, freeing the driver from having to shift gears manually.

Automatic Transmission Modes

In order to select the mode, the driver would have to move a gear shift lever located on the steering column or on the
floor next to him/her. In order to select gears/modes the driver must push a button in (called the shift lock button) or
pull the handle (only on column mounted shifters) out. In some vehicles position selector buttons for each mode on
the cockpit instead, freeing up space on the central console. Vehicles conforming to U.S. Government standards must
have the modes ordered P- R-N-D-L (left to right, top to bottom, or clockwise). Prior to this, quadrant-selected
automatic transmissions often utilized a P-N-D-L-R layout, or similar. Such a pattern led to a number of deaths and
injuries owing to un-intentional gear miss-selection, as well the danger of having a selector (when worn) jump into
Reverse from Low gear during engine braking maneuvers.
Automatic Transmissions have various modes depending on the model and make of the transmission. Some of the
common modes are:

Park Mode (P)

This selection mechanically locks the transmission, restricting the car from moving in any direction. A parking pawl
prevents the transmission—and therefore the vehicle—from moving, although the vehicle's non-drive wheels may still
spin freely. For this reason, it is recommended to use the hand brake (or parking brake) because this actually locks the
(in most cases, rear) wheels and prevents them from moving. This also increases the life of the transmission and the
park pin mechanism, because parking on an incline with the transmission in park without the parking brake engaged
will cause undue stress on the parking pin. An efficiently-adjusted hand brake should also prevent the car from
moving if a worn selector accidentally drops into reverse gear during early morning fast-idle engine warm ups.

Reverse (R)
This puts the car into the reverse gear, giving the ability for the car to drive backwards. In order for the driver to select
reverse they must come to a complete stop, push the shift lock button in (or pull the shift lever forward in the case of a
column shifter) and select reverse. Not coming to a complete stop can cause severe damage to the transmission.

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Many modern automatic gearboxes have a safety mechanism in place, which does to some extent prevent (but doesn't
completely avoid) inadvertently putting the car in reverse when the vehicle is moving.
This mechanism usually consists of a solenoid-controlled physical barrier on either side of the Reverse position,
which is electronically engaged by a switch on the brake pedal. Therefore, the brake pedal needs to be depressed in
order to allow the selection of reverse. Some electronic transmissions prevent or delay engagement of reverse gear
altogether while the car is moving.

Neutral/No gear (N)

This disconnects the transmission from the wheels so the car can move freely under its own weight. This is the only
other selection in which the car can be started.

Drive (D)
This allows the car to move forward and accelerate through its range of gears. The number of gears a transmission has
depends on the model, but they can commonly range from 3, 4 (the most common), 5, 6 (found in VW/Audi Direct
Shift Gearbox), 7 (found in Mercedes 7G gearboxes, BMW M5 and VW/Audi Direct Shift Gearbox) and 8 in the
newer models of Lexus cars. Some cars when put into D will automatically lock the doors or turn on the Daytime
Running Lamps.

Overdrive ([D], Od, Or A Boxed D)

This mode is used in some transmissions to allow early Computer Controlled Transmissions to engage the Automatic
Overdrive. In these transmissions, Drive (D) locks the Automatic Overdrive off, but is identical otherwise. OD
(Overdrive) in these cars is engaged under steady speeds or low acceleration at approximately 35-45 mph (approx. 72
km/h). Under hard acceleration or below 35-45 mph, the transmission will automatically downshift. Vehicles with this
option should be driven in this mode unless circumstances require a lower gear.

Second (2 or S)
This mode limits the transmission to the first two gears, or more commonly locks the transmission in second gear.
This can be used to drive in adverse conditions such as snow and ice, as well as climbing or going down hills in the
winter time. Some vehicles will automatically up-shift out of second gear in this mode if a certain rpm range is
reached, to prevent engine damage.

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First (1 or L)
This mode locks the transmission in first gear only. It will not accelerate through any gear range. This, like second,
can be used during the winter season, or for towing.
As well as the above modes there are also other modes, dependent on the manufacturer and model. Some examples
include:
D5:- In Hondas and Acuras equipped with 5-speed automatic transmissions, this mode is used commonly for highway
use (as stated in the manual), and uses all five forward gears.
D4:- This mode is also found in Honda and Acura 4 or 5-speed automatics and only uses the first 4 gears. According
to the manual, it is used for "stop and go traffic", such as city driving.
D3:- This mode is found in Honda and Acura 4-speed automatics and only uses the first 3 gears. According to the
manual, it is used for stop & go traffic, such as city driving. This mode is also found in Honda and Acura 5-speed
automatics.
This is the manual selection of gears for automatics, such as Porsche's Tiptronic. This feature can also be found in
Chrysler and General Motors products such as the Dodge Magnum and Pontiac G6. The driver can shift up and down
at will, by toggling the shift lever (console mounted) like a semi-automatic transmission. This mode may be engaged
either through a selector/position or by actually changing gear (e.g. tipping the gear-down paddles mounted near the
driver's fingers on the steering wheel).
The predominant form of automatic transmission is hydraulically operated, using a fluid coupling/ torque converter
and a set of planetary gear-sets to provide a range of torque multiplication.

Parts And Operation

A hydraulic automatic transmission consists of the following parts:
 Torque Converter/Fluid Coupling
 Planetary Gear Set
 Clutch packs & Bands
 Valve Body
 Hydraulic or Lubricating Oil

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 Cut section model of a torque converter

Torque Converter/Fluid Coupling: -Unlike a manual transmission system, automatic transmission does not use a
clutch to disconnect power from the engine temporarily when shifting gears. Instead, a device called a torque
converter was invented to prevent power from being temporarily disconnected from the engine and also to pre-vent
the vehicle from stalling when the transmission is in gear.
A fluid coupling/torque converter consists of a sealed chamber containing two toroidal- shaped, vaned components,
the pump and turbine, immersed in fluid (usually oil). The pump or driving torus (the latter a General Motors
automotive term) is rotated by the prime mover, which is typically an internal combustion engine or electric motor.
The pump's motion imparts a relatively complex centripetal motion to the fluid. Simplified, this is a centrifugal force
that throws the oil outwards against the coupling's housing, whose shape forces the flow in the direction of the turbine
or driven torus (the latter also a General Motors term).

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Here, Corolis force reaction transfers the angular fluid momentum outward and across, applying torque to the turbine,
thus causing it to rotate in the same direction as the pump. The fluid leaving the center of the turbine returns to the
pump, where the cycle endlessly repeats. The pump typically is connected to the flywheel of the engine—in fact, the
coupling's enclosure may be part of the flywheel proper, and thus is turned by the engine's crankshaft. The turbine is
connected to the input shaft of the transmission. As engine speed increases while the transmission is in gear, torque is
transferred from the engine to the input shaft by the motion of the fluid, propelling the vehicle. In this regard, the
behavior of the fluid coupling strongly resembles that of a mechanical clutch driving a manual transmission.
A torque converter differs from a fluid coupling in that it provides a variable amount of torque multiplication at low
engine speeds, increasing "breakaway" acceleration. This is accomplished with a third member in the "coupling
assembly" known as the stator, and by altering the shapes of the vanes inside the coupling in such a way as to curve
the fluid's path into the stator. The stator captures the kinetic energy of the transmission fluid in effect using the left-
over force of it to enhance torque multiplication.
Tiptronic transmission is a special type of automatic transmission with a computer controlled automatic shift. The
driver can switch the transmission to manual mode, which lets her shift the gear at her wish sequentially up (+) or
down (-) without disengaging the clutch. This works just like a manual transmission; however, it still uses a torque
converter to transfer power from the engine. Unfortunately, this is less efficient than a manual transmission.

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Planetary Gear-Set
The automatic system for current automobiles uses a planetary gear set instead of the traditional manual transmission
gear set. The planetary gear set contains four parts: sun gear, planet gears, planet carrier, and ring gear. Based on this
planetary set design, sun gear, planet carrier, and ring gear spin centrifugally. By locking one of them, the planetary
set can generate three different gear ratios, including one reverse gear, without engaging and disengaging the gear set.
The gear set is actuated by hydraulic servos controlled by the valve body, providing two or more gear ratios.

Clutch Packs And Band

A clutch pack consists of alternating disks that fit inside a clutch drum. Half of the disks are steel and have splines that
fit into groves on the inside of the drum.
`The other half have a friction material bonded to their surface and have splines on the inside edge that fit groves on
the outer surface of the adjoining hub. There is a piston inside the drum that is activated by oil pressure at the
appropriate time to squeeze the clutch pack together so that the two components become locked and turn as one.

Structure of the actuator piston, lever link and band system

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 Planetary gear set

A band is a steel strap with friction material bonded to the inside surface. One end of the band is anchored against the
transmission case while the other end is connected to a servo. At the appropriate time hydraulic oil is sent to the servo
under pressure to tighten the band around the drum to stop the drum from turning.
The bands come into play for manually selected gears, such as low range or reverse, and operate on the planetary
drum's circumference. Bands are not applied when drive/overdrive range is selected, the torque being transmitted by
the sprag clutches instead.
The sun gear is connected to a drum, which can be locked by a band. The ring gear is directly connected to the input
shaft, which transfers power from the engine. The planet carrier is connected to the output shaft, which transfers
power into the wheels.
Based on this design, when in neutral, both band and clutch sets are released. Turning the ring gear can only drive
planet gears but not the planet carrier, which stays static if the car is not moving. The planet gears drive the sun gear
to spin freely.

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In this situation, the input shaft is not able to transfer power to the output shaft. When shifting to 1st gear, the band
locks the sun gear by locking the drum. The ring gear drives the planet carrier to spin. In this situation, the ring gear
(input shaft) spins faster than the planet carrier (output shaft).
To shift to higher gear, the band is released and the clutch is engaged to force the sun gear and planet carrier (output
shaft) to spin at the same speed. The input shaft will also spin at the same speed as the output shaft, which makes the
car run faster than in 1st gear. Using a compound planetary gear set generates more gear ratios with a special gear
ratio, over-drive gear whose gear ratio is small than 1.
This will make the gear shift smoother. Both the band and clutch piston are pressurized by the hydraulic system. The
part connecting the band or clutches to the hydraulic system is called the shift valve, while the one connecting the
hydraulic system to the output shaft is called the governor.
The governor is a centrifugal sensor with a spring loaded valve. The faster the governor spins, the more the valve
opens. The more the valve opens, the more the fluid goes through and the higher the pressure applied on the shift
valve. Therefore, each band and clutch can be pushed to lock the gear based on a specific spin speed detected by the
governor from the output shaft. To make the hydraulic system work efficiently, a complex maze of passages was
designed to replace a large number of tubes. For modern cars, an electronic con-trolled (computer controlled) solenoid
pack is used to detect throttle position, vehicle speed, engine speed, engine load, brake pedal position, etc., and to
automatically choose the best gear for a moving vehicle.
Principally, a type of device known as a sprag or roller clutch is used for routine upshifts/downshifts. Operating much
as a ratchet, it transmits torque only in one direction, freewheeling or "overrunning" in the other. The advantage of
this type of clutch is that it eliminates the sensitivity of timing a simultaneous clutch release/apply on two planetaries,
simply "taking up" the drivetrain load when actuated, and releasing automatically when the next gear's sprag clutch
assumes the torque transfer.

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Valve Body
Hydraulic control center that receives pressurized fluid from a main pump operated by the fluid coupling/torque
converter. The pressure coming from this pump is regulated and used to run a network of spring-loaded valves, check
balls and servo pistons.
The valves use the pump pressure and the pressure from a centrifugal governor on the output side (as well as
hydraulic signals from the range selector valves and the throttle valve or modulator) to control which ratio is selected
on the gearset; as the car and engine change speed, the difference between the pressures changes, causing different
sets of valves to open and close.

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Each of the many valves in the valve body has a specific purpose and is named for that function. For example the 2-3
shift valves activate the 2nd gear to 3rd gear up-shift or the 3-2 shift timing valve which determines when a downshift
should occur.
The hydraulic pressure controlled by these valves drives the various clutch and brake band actuators, thereby
controlling the operation of the planetary gearset to select the optimum gear ratio for the current operating conditions.
However, in many modern automatic transmissions, the valves are controlled by electro-mechanical servos which are
controlled by the Engine Management System or a separate transmission controller.
The most important valve and the one that you have direct control over is the manual valve. The manual valve is
directly connected to the gear shift handle and covers and uncovers various passages depending on what position the
gear shift is placed in. When you place the gear shift in Drive, for instance, the manual valve directs fluid to the clutch
pack(s) that activates 1st gear.
It also sets up to monitor vehicle speed and throttle position so that it can determine the optimal time and the force for
the 1 - 2 shifts. On computer controlled transmissions, you will also have electrical solenoids that are mounted in the
valve body to direct fluid to the appropriate clutch packs or bands under computer control to more precisely control
shift points.
Hydraulic & Lubricating Oil: - A component called Automatic Transmission Fluid (ATF) which is part of the
transmission mechanism provides lubrication, corrosion prevention, and a hydraulic medium to convey mechanical
power.
Primarily it is made of refined petroleum and processed to provide properties that promote smooth power transmission
and increase service life. ATF is one of the parts of the automatic transmission that needs routine service as the
vehicle ages.

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