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8/10/2019 Chapter 17 Bennet

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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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