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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
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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$
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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$
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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$
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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$
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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
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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$
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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
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!%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
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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
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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$
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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$
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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
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) 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
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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
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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
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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
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!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$
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