Transcript
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Chapter 17
Torque Converters
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Objectives (1 of 2)
Explain the function of the torque converter in
a vehicle equipped with an automatic
transmission.
Explain how the torque converter is coupled
between the crankshaft and the transmission.
Identify the three main elements of a torque
converter and describe their roles. Define torque multiplication and explain how
it is generated in the torque converter.
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Objectives (2 of 2)
Define both rotary and vortex fluid flow and
explain how each affects torque converter
operation.
Describe the overrunning clutch, lockupclutch, and variable pitch stators.
Outline torque converter service and
maintenance checks. Remove, disassemble, inspect, and
reassemble torque converter components.
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Shop Talk
Torque converters can be confused with fluidcouplings because both use similar operatingprinciples.
The most fundamental difference is thattorque converters use curved blades, whilefluid couplings and fluid flywheels usestraight pitch blades.
Torque converters also use stators and havethe ability to multiply torque, neither of whichis characteristic of fluid couplings.
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Torque Converters
Automatic transmissions use a torque converter to
couple the engine to the transmission.
The torque converter:
Transmits the twisting force or torque delivered to itby the engine crankshaft
Multiplies engine torque when additional power is
needed
The amount of torque transferred from the engine tothe transmission by the torque converter is directly
related to engine rpm.
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Torque Converter Construction
Truck torque converters can be:
Serviceable
Many torque converters are welded together.
A normal service shop cannot disassemblethem for servicing and repair.
Non serviceable
The only types of torque converters (T/Cs)that are readily serviceable are the types that
are bolted together.
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Flex Plate
The flex plate carries thestarter motor ring gear.
The combined mass of thetorque converter and flexdisc acts like a flywheel tosmooth out the powerpulses produced by theengine.
The flex plate also allows fora slight alignment tolerance
between the engine andtorque converter assembly.
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Components
The torque converter
has three main
components.
Impeller Turbine
Stator
Optional lockup clutch
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Exterior
The exterior of the torqueconverter shell is shapedlike two bowls facing eachother.
They are either welded or
bolted together.
A pilot shaft supports theweight of the torqueconverter at the front.
At the rear of the torqueconverter shell is the pumpdrive hub with notches orflats which are used to drivethe transmission pump.
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Curved blades rotate as a unit with the shell at enginespeed. It starts the transmission oil circulating within thetorque converter shell.
The impeller is positioned with its back to thetransmission housing; the turbine is positioned with itsback to the engine.
The hub of the turbine is splined so that it can drive theturbine (transmission input) shaft. The turbine shafttransfers engine torque to the transmission gearing.
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The turbine blades are
designed to have a
greater curve than the
impeller blades.
This helps reduce oil
turbulence between the
turbine and impeller
bladesturbulence thatwould slow impeller
speed and reduce the
converters efficiency.
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A fundamental law of
hydraulics states, The more
the moving stream of fluid is
diverted (changed), the
greater the force it places onthe curved reaction surface.
As oil in the torque
converter moves around the
turbine blades, it pushes
against the blades andtransmits additional force.
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Principles of Operation
As the pump impeller rotates,centrifugal force throws the oiloutward and upward.
The faster the impeller rotates,the higher the centrifugal
force. Fluid under pressure is
continuously delivered throughthe converter hub.
It is important to note that the
oil pump delivering the fluid isdriven by the engine.
A seal or combination of sealsprevents fluid from being lostfrom the system.
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Oil leaving the turbine is
directed to an external oilcooler and then to the
transmission oil sump.
At idle there is insufficient
centrifugal force within the
torque converter to move thetruck.
As impeller speed increases
the centrifugal force of the oil
directed against the turbineblades becomes great enough
to move the vehicle.
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Shop Talk
There can be a mechanical connection
between the impeller and turbine by the use
of a lockup clutch.
A lockup clutch eliminates slippage betweenthe impeller and turbine at certain speeds.
This helps to reduce the heat generated in the
fluid and improves fuel mileage.
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Types of Oil Flow (1 of 2)
The two types of oil flow thatoccur within the T/C are: Rotary
Rotary flow describesthe centrifugal force
applied to the fluid asthe converter rotateson its axis.
Vortex
Vortex flow is thecircular flow that occurs
as the oil is forced fromthe impeller to theturbine and then backto the impeller.
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Types of Oil Flow (2 of 2)
If a toy pinwheel were heldat arms length and swungin a large circle, airmovement at the outer circlewould produce rotary flow,
while the small circles cut bythe pinwheels propellervanes would produce vortexflow.
The point when the speed of
the turbine approaches thespeed of the impeller isreferred to as the couplingpoint.
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Split Guide Rings
Fast moving oil exits theimpeller blades, striking theturbine blades withconsiderable force.
It then has a tendency to be
thrust back toward the centerof both impeller and turbine.
To control this fluid thrust andthe turbulence that results, asplit guide ring is located inboth the impeller and turbinesections of the T/C.
The guide ring suppressesturbulence, allowing moreefficient operation.
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Stator (1 of 3)
The stator is the key totorque multiplication. Itredirects the oil leaving theturbine back to the impeller.
The stator then redirects the
fluid flow so that the oilreenters the impeller,moving in the samedirection as the impeller.
The kinetic energy
remaining in the oil nowhelps rotate the impellerwith more force, multiplyingtorque.
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Stator (2 of 3)
The roller clutch is designedwith an inner race, rollers,accordion (apply) springs,and outer race.
Around the inside diameter
of the outer race are severalcam-shaped pockets.
The rollers and accordionsprings are located inthese pockets.
As the vehicle begins tomove, the stator stays in itsstationary or locked position.
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Stator (3 of 3)
Locking mode takes place when the outer race attemptsto rotate counterclockwise.
The accordion springs force the rollers down the rampsof the cam pockets.
As vehicle road speed increases, turbine speedapproaches impeller speed.
Oil exiting the turbine vanes now strikes the back face ofthe stator, causing the stator to rotate in the samedirection as the turbine and impeller, unlocking theclutch.
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Variable Pitch Stator
Each of a series of
movable stator vanes
has a crank rod fitted
into a circular groove in
the hydraulic piston.
The movement of a
hydraulic piston varies
the angle of the statorvanes.
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Lockup Clutches
A lockup torque converter eliminates the 10 percent
slip that takes place at the coupling phase of
operation.
The engagement of a clutch has the advantage ofimproving vehicle fuel economy, lowering overall
engine emissions, and reducing torque converter
operating heat and engine speed.
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Caution
The torque converter is free to move forward
when the transmission is disconnected from
the engine.
To ensure that the torque converter does notseparate from the transmission while the
transmission is being removed from the
vehicle, install a retaining strap to hold theT/C in position.
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Lockup Clutch Back Plate
The back plate should beflat to within 0.006 inch.
Inspect the football key slotfor evidence of wear frommovement of the lockup
back plate and football key. Excessive wear or
elongation of the footballkey slot will requirecomponent replacement.
Inspect the back plate forcracks, from the key slot tothe ID of plate.
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Turbine Assembly
Inspect the turbine
assembly for cracked or
broken vanes and signs of
overheating.
If the turbine assembly istaken apart, inspect the rivet
holes for signs of wear or
elongation.
The turbine assembly hub
should be checked forstripped, twisted, or broken
splines.
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Lockup Clutch Piston
Inspect the seal ring groove.
Check the lockup pinholes.
Inspect the friction surface
for wear, scoring, scratches,
signs of overheating, andflatness.
It should be flat to within
0.003 inch TIR.
Inspect the seal surface.
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Inspecting the Stator Assembly
Inspect the stator assembly.
Inspect the rivets for cracksor loose fit.
Check the stator cam rollerpockets.
Inspect the stator thrustbearing race surface.
Check the stator freewheelroller surface, thrust bearingsurface, and roller bearingsurface.
Inspect the stator freewheelrollers.
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Pump/Impeller
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Pump (Impeller) Hub (1 of 2)
Check the seal ring grooveson the pump hub forcracked edges, nicks, burrs,and sharp edges.
Inspect the front sealsurface for scoring,scratches, nicks, andgrooves. No rework of any
irregularities noted isallowed on the sealsurface.
The use of a crocus clothor light honing on thissurface could promoteleakage past the front seal.
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Pump (Impeller) Hub (2 of 2)
There should be no signs ofcracks, scoring, metaltransfer, or heat damage onthe pump drive flange.
Inspect the snap ring groovefor burrs, cracks, and nicks.
Check the bearing racesurface.
Inspect the roller bearingbore.
Inspect the gasket surface.
Inspect the pump hub forpulled, stripped, or crossedthreads.
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Summary (1 of 2)
Automatic truck (and passenger car) transmissions
use a type of fluid coupling known as a torque
converter to transfer engine torque from the engine
to the transmission.
A flex plate, sometimes called a flex disc, is used to
connect the torque converter to the crankshaft.
Transmission oil is used as the medium to transfer
energy from the engine-driven impeller to theturbine, which in turn drives the transmission.
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Summary (2 of 2)
Two types of oil flow take place inside the
torque converter.
Rotary flow and vortex flow
A converter lockup clutch enables amechanical coupling of the engine and
transmission.
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