automatic transmission of vehicles

7/23/2019 Automatic Transmission of vehicles

http://slidepdf.com/reader/full/automatic-transmission-of-vehicles 1/19

Automatic Transmission

Name: Mohamed Ashraf Sayed

Section :2

Types of Automatic Transmissions



Automatic transmissions can be basically divided into two types: those used

in front−engine, front−wheel drive !!" vehicles and those used in

front−engine, rear−wheel drive !#" vehicles$ Transmissions used in

front−wheel drive vehicles are designed to be more compact than

transmissions used in rear−wheel drive vehicles because they are mounted

in the engine compartment$ They are commonly referred to as a %transa&le$%

The di'erential is an integral part of the front−wheel drive transmission,

whereas the di'erential for the rear−wheel drive transmission is mounted

e&ternally$ The e&ternal di'erential is connected to the transmission by a

driveshaft$ The basic function and purpose for either front or rear drive

automatics are the same$ They share the same planetary gear train design

which is used in all Toyota automatic transmissions and the ma(ority of

automatics in production today$



The automatic transmission is composed of three major

components:

) Tor*ue converter

) +lanetary gear unit

) ydraulic control unit

!or a full understanding of the operation of the automatic transmission, it is

important to understand the basic role of these components$

The tor*ue converter provides a means of power transfer from the engine to

the input shaft of the transmission$ -t acts li.e an automatic clutch to engage

engine tor*ue to the transmission and also allows the engine to idle while

the vehicle is standing still with the transmission in gear$

The planetary gear unit provides multiple gear ratios in the forward direction

and one in reverse$ The design includes two simple planetary gear sets and a

common sun gear$ These ratios are provided by use of holding devices which

hold members of the planetary set$ These holding devices can be multiplate

clutches or bra.es, bra.e bands or one−way clutches$

The hydraulic control unit regulates hydraulic pressure and shift points based

on vehicle speed and throttle position$ -t is made up of a highly precision

housing and spool valves which are balanced between spring tension and

hydraulic pressure$ The spool valves in turn control hydraulic passages to

holding devices and regulate pressure$



TORQUE CONVERTERAutomatic transmissions use a /uid clutch .nown as a tor*ue converter to

transfer engine tor*ue from the engine to the transmission$ The tor*ue

converter operates through hydraulic force provided by automatic

transmission /uid, often called transmission oil$ The tor*ue converter

changes or multiplies the twisting motion of the engine cran.shaft and

directs it through the transmission$

The tor*ue converter automatically engages and disengages power from the

engine to the transmission in relation to engine rpm$ 0ith the engine running

at the correct idle speed, there is not enough /uid /ow for power transfer

through the tor*ue converter$ As engine speed is increased, the added /uid

/ow creates su1cient force to transmit engine power through the tor*ue

converter assembly to the transmission$



Role of the torque conerter

) Multiplies tor*ue generated by the engine$

) Serves as an automatic clutch which transmits engine tor*ue to the

transmission$

) Absorbs torsional vibration of the engine and drivetrain$

) Smoothes out engine rotation$

) rives the oil pump of the hydraulic control system$

The tor*ue converter is 3lled with automatic transmission /uid, and

transmits the engine tor*ue to the transmission$ The tor*ue converter can

either multiply the tor*ue generated by the engine or function as a /uid

coupling$

The tor*ue converter also serves as the engine /ywheel to smooth out

engine rotation as its inertia helps to maintain cran.shaft rotation between

piston power pulses$ -t tends to absorb torsion vibration from the engine and

drivetrain through the /uid medium since there is no direct mechanical

connection through the converter$ -n addition, the rear hub of the tor*ue

converter body drives the transmission oil pump, providing a volume of /uidto the hydraulic system$ The pump turns any time the engine rotates, which

is an important consideration when a vehicle is towed$ -f the vehicle is towed

with the drive wheels on the ground and the engine is not running, the a&les

drive the transmission output shaft and intermediate shaft on bearings that

receive no lubrication$ There is a great potential for damage if the vehicle is

towed for a long distance or at greater than low speeds$



Torque Conerter Component

The tor*ue converter4s three ma(or components are5 the pump impeller,

turbine runner and the stator$ The pump impeller is fre*uently referred to as

simply the impeller and the turbine runner is referred to as the turbine$

!ump "mpeller

The impeller is integrated with the tor*ue converter case, and many curved

vanes that are radially mounted inside$ A guide ring is installed on the inner

edges of the vanes to provide a path for smooth /uid /ow$ 0hen the impeller

is driven by the engine cran.shaft, the /uid in the impeller rotates with it$

0hen the impeller speed increases, centrifugal force causes the /uid to /ow

outward toward the turbine

Tur#ine



The turbine is located inside the converter case but is not connected to it$

The input shaft of the transmission is attached by splines to the turbine hub

when the converter is mounted to the transmission$ Many cupped vanes are

attached to the turbine$ The curvature of the vanes is opposite from that of

the impeller vanes$ Therefore, when the /uid is thrust from the impeller, it is

caught in the cupped vanes of the turbine and tor*ue is transferred to thetransmission input shaft, turning it in the same direction as the engine

cran.shaft$

$tator

The stator is located between the impeller and the turbine$ -t is mounted on

the stator reaction shaft which is 3&ed to the transmission case$ The vanes of

the stator catch the /uid as it leaves the turbine runner and redirects it so

that it stri.es the bac. of the vanes of the impeller, giving the impeller an

added boost or tor*ue$ The bene3t of this added tor*ue can be as great as

678 to 978$

The one−way clutch allows the stator to rotate in the same direction as the

engine cran.shaft$ owever, if the stator attempts to rotate in the opposite

direction, the one−way clutch loc.s the stator to prevent it from rotating$



Therefore, the stator is rotated or loc.ed depending on the direction from

which the /uid stri.es against the vanes$

Conerter Operation

Now that we4ve loo.ed at the parts which ma.e up the tor*ue converter, let4s

loo. at the phenomenon of /uid /ow within the tor*ue converter$ 0hen the

impeller is driven by the engine cran.shaft, the /uid in the impeller rotates inthe same direction$ 0hen the impeller speed increases, centrifugal force

causes the /uid to /ow outward from the center of the impeller and /ows

along the vane surfaces of the impeller$ As the impeller speed rises further,

the /uid is forced out away from the impeller toward the turbine$ The /uid

stri.es the vanes of the turbine causing the turbine to begin rotating in the

same direction as the impeller$

After the /uid dissipates its energy against the vanes of the turbine, it /ows

inward along the vanes of the turbine$ 0hen it reaches the interior of the

turbine, the turbine4s curved inner surface directs the /uid at the vanes of

the stator, and the cycle begins again



!%ANETAR& 'EAR$

Nearly all automatic transmissions rely on planetary gear sets to transfer

power and multiply engine tor*ue to the drive a&le$ ompound gear sets

combine two simple planetary gear sets so load can be spread over a greater

number of teeth for strength and also to obtain the largest number of gear

ratios possible in a compact area$

A simple planetary gear set consists of three parts: a sun gear, a carrier with

planetary pinions mounted to it, and an internally toothed ring gear or

annulus$ The sun(ear is located in the center of the assembly -t can be

either a spur or helical gear design$ -t meshes with the teeth of the planetary

pinion gears$ +lanetary pinion gears are small gears 3tted into a framewor.

called the planetary carrier$ The planetary carrier can be made of cast

iron, aluminum, or steel plate and is designed with a shaft for each of the



planetary pinion gears$ !or simplicity, planetary pinion gears are called

planetary pinions$"

)o* !lanetary 'ears +or,

;ach member of a planetary gearset can spin revolve" or be held at rest$

+ower transfer through a planetary gearset is only possible when one of the

members is held at rest, or if two of the members are loc.ed together$ Anyone of the three members can be used as the driving or input member$ At

the same time, another member might be .ept from rotating and thus

becomes the reaction, held, or stationary member$ The third member then

becomes the driven or output member$ epending on which member is the

driver, which is held, and which is driven, either a tor*ue increase



underdrive" or a speed increase overdrive" is produced by the planetary

gearset$ <utput direction can also be reversed through various combinations$

-or*ard .irection

0hen the ring gear or sun gear is held in a 3&ed position, and either of the

other members is an input member, the output gear rotational direction is

always the same as the input gear rotational direction$ 0hen the internal

teeth of the ring gear turns cloc.wise, the e&ternal teeth of the pinion gears

wal. around the 3&ed sun gear while rotating cloc.wise$ This causes the

carrier to rotate at a reduced speed$



The gear ratio is computed as follows: =ear ratio > Number of output gearteeth =ear ratio ?Number of input gear teeth =ear ratio @9 B9

=ear ratio > B 6:B @9 > B$6:B

0hen the carrier turns cloc.wise, the e&ternal toothed pinion gears wal.

around the e&ternal toothed sun gear while rotating cloc.wise$ The pinion

gears cause the internal toothed ring gear to accelerate to a speed greater

than the carrier speed in a cloc.wise direction$

Reerse .irection

0henever the carrier is held and either of the other gears are input

members, the output gear will rotate in the opposite direction$ 0ith the

carrier held, when the e&ternal toothed sun gear turns cloc.wise, the

e&ternal toothed pinion gears on the carrier idle in place and drive the

internal toothed ring gear in the opposite direction$



The gear ratio is computed as follows: =ear ratio @9?B9

=ear ratio > 6:B B9 >

)oldin( .eices for !lanetary 'ear $et There are three types of holding devices used in the planetary gear set$ ;ach

type has its speci3c design advantage$

The three include multiplate clutches?bra.es, bra.e bands and one−way

clutches$

) Multiplate lutch C holds two rotating planetary components



) Dra.e C holds planetary components to the housing

− bra.e band

) #oller or Sprag <ne−0ay lutch C holds planetary components in one

rotational direction

/ultiplate clutch

Transmission $eros

The sero assembly converts hydraulic pressure into amechanical force that

applies a band to hold a drum stationary$ Simple and compound servos are

used to engage bands in modern transmissions$

0ra,e 0and



The bra.e band is located around the outer circumference of the direct clutch

drum$ <ne end of this bra.e band is located to the transmission case with a

pin, while the other end contacts the bra.e piston which is operated by

hydraulic pressure$

One1+ay Clutches

A one−way clutch is a holding device which re*uires no seals or hydraulic

pressure to apply$ They are either a roller clutch or sprag clutch$ Although the

sprag clutch is most often used in Toyota automatics, we4ll mention both$

Their operation is similar in that they both rely on wedging metal between



two races$ Two one−way clutches are used in the Simpson +lanetary =ear

Set$

Vale 0ody!or e1cient transmission operation, the bands and multipleEdisc pac.s must

be released and applied at the proper time$ The ale #ody assembly is

responsible for the control and distribution of pressuriFed /uid throughout



the transmission$ This assembly is made of two or three main parts: a valve

body, separator plate, and transfer plate$ These parts are bolted as a single

unit to the transmission housing$ The valve body is machined from aluminum

or iron and has many precisely machined bores and /uid passages$

The purpose of a valve body is to sense and respond to engine and vehicleload as well as to meet the needs of the driver$ Galve bodies are normally

3tted with three di'erent types of valves: spool ales2 chec, #all ales,

and poppet ales$ The purpose of these valves is to start, to stop, or to

use movable parts to regulate and direct the /ow of /uid

throughout the transmission$

Chec, 0all Vale

The chec, #all ale is a ball that operates on a seat located on the valve

body$ The chec. ball operates by having a /uid pressure or manuallyoperated lin.age force it against the ball seat to bloc. /uid /ow3 +ressure on

the opposite side unseats the chec. ball$ hec. balls and poppet valves can

be normally open, which allows free /ow of /uid pressure, or normally

closed, which bloc.s /uid pressure /ow$ At times, the chec. ball has two

seats to chec. and direct /uid /ow from two directions, being seated and

unseated by pressures from either source



!oppet Vale

A poppet valve can be a ball or a /at disc$ -n either case, the poppet valve

bloc.s /uid

/ow$ <ften the poppet valve has a stem to guide the valve4s operation$ The

stem normally 3ts into a hole acting as a guide to the valve4s opening andclosing$ +oppet valves tend to pop open and closed, hence their name$

Normally poppet valves are held closed by a spring$

$pool Vale

The most commonly used valve in a valve body is the spool ale$ A spool

valve loo.s similar to a sewing thread spool$ The large circular parts of the

valve are called the lands$ There is a minimum of two lands per valve$ ;ach

land of the assembly is connected by a stem$ The space between the lands

and stem is called the valley$ Galleys form a /uid pressure chamber between

the spools and valve body bore$ !luid /ow can be directed into other

passages depending on the spool valve and valve body design$

automatic transmission of vehicles

Documents