chapter 18 bennet

8/10/2019 Chapter 18 Bennet

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Chapter 18

Automatic Transmissions


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Objectives (1 of 2)

Identify the components of a simple planetary

gearset.

Explain the operating principles of a

planetary gear train.

Define a compound planetary gearset and

explain how it operates.

Describe a multiple-disc clutch and explain itsrole in the operation of an automatic

transmission.


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Objectives (2 of 2)

Outline torque path powerflow through a

typical four- and five-speed automatic

transmission.

Describe the hydraulic circuits and flowsused to control automatic transmission

operation.

List the two types of hydraulic retarders usedon Allison automatic transmissions and

explain their differences.


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Planetary Gear Set (1 of 3)

A simple planetary gear setcan provide: Overdrive

Reverse

Forward reduction

Neutral Direct drive ratios

There are three maincomponents in a simpleplanetary gearset. A sun gear

A carrier with multipleplanetary pinion gearsmounted to it

An internally toothed ring gearor annulus


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The sun gear is located in

the center of the

assembly.

Planetary pinion gears

are small gears fitted into

a planetary carrier.

The planetary carrier can

be made of cast iron,

aluminum, or steel plate.

Planetary pinions rotate

on needle bearings.


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The carrier and its pinions are considered

one unitthe mid-size gear member.

The ring gear is the largest component of a

simple planetary gearset.

The ring gear is located the greatest distance

from the axis of the sun gear and is therefore

capable of exerting the most leverage.


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How Planetary Gears Work (1 of 4)

Power transmission is possible only when one ofthe three members is held stationary, or if two of themembers are locked together.

Any one of the three memberssun gear, piniongear carrier, or ring gearcan be used as thedriving or input member.

At the same time, another member might be keptfrom rotating and thus becomes the held or

stationary member. The third member then becomes the driven or

output member.


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Depending on which member is the driver,

which is held, and which is driven, either a

torque increase or a speed increase is

produced by the planetary gearset. Output direction can also be reversed

through various combinations.


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Laws of simple planetary gear operation

See Table 18-1 on page 531 of the textbook.


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When an external-to-external gear tooth setis in mesh, there will bea change in the

direction of rotation atthe output.

When an external geartooth is in mesh with an

internal gear, theoutput rotation for bothgears will be the same.


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Maximum Forward Reduction

With the ring gear stationaryand the sun gear turningclockwise, the external sungear teeth rotate the planetarypinions counterclockwise.

Each planetary pinion pushes

against its shaft, moving theplanetary carrier clockwise.

The small sun gear mustrotate several times to turn thelarger planetary carrierthrough one complete

revolution. This combination represents

the most gear reduction thatcan be achieved in oneplanetary gearset.


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Minimum Forward Reduction

The ring gear drives theplanetary pinions clockwiseand walks around thestationary sun gear.

The planetary pinions drive

the planetary carrier in thesame direction as the ringgearforward.

More than one turn of theinput is needed for onecomplete revolution of the

output. But because a large gear

is driving a small gear, theamount of reduction is notas great.


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Maximum Overdrive

The ring gear is stationaryand the planetary carrierrotates clockwise.

The planetary pinion shaftspush against planetarypinions.

The pinions are forced towalk around the inside ofthe ring gear, driving thesun gear clockwise.

The carrier is rotating lessthan one turn inputcompared to one turnoutput.


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Slow Overdrive

In this combination, the sungear is stationary and thecarrier rotates clockwise.

As the carrier rotates, thepinion shafts push against

the pinions and they areforced to walk around theheld sun gear.

This drives the ring gearfaster.

The carrier turning less than

one turn causes the pinionsto drive the ring gear onecomplete revolution in thesame direction as theplanetary carrier.


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Slow Reverse

Here, the sun gear is drivingthe ring gear while theplanetary carrier is held.

The planetary pinions, rotatecounterclockwise on their

shafts. While the sun gear is

driving, the planetarypinions are used as idlergears to drive the ring gearcounterclockwise.

Because the driving sungear is small and the drivenring gear is large, the resultis slow reverse.


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Fast Reverse

For fast reverse, the carrieris still held as in slowreverse, but the sun gearand ring gear reverse roles,with the ring gear now beingthe driving member and thesun gear driven.

In this combination, theinput ring gear uses theplanetary pinions as idlers todrive the output sun gear in

reverse. The sun gear rotates in

reverse to the input ringgear.


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Direct Drive

Both the ring gear and the

sun gear are input

members.

Opposing forces lock the

planetary pinions againstrotation so that the entire

planetary gearset rotates as

one complete unit.

For direct drive, both input

members must rotate at thesame speed.


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Neutral

When no member is

held stationary or

locked, there will be

input into the gearset,

but no output.

The result is a neutral

condition.


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Summary of Simple

Planetary Gearset Operation

When the planetary carrier is the input, the gearset produces anoverdrive. Speed increases and torque decreases.

When the planetary carrier is the driven (output) member, thegearset produces a forward reduction. Speed decreases andtorque increases.

When the planetary carrier is held, the gearset will produce areverse. To determine if the speed produced is fast or slow,remember the rules regarding large and small gears.

A large gear driving a small gear increases speed and reducestorque of the driven gear.

A small gear driving a large gear decreases speed andincreases torque of the driven gear.


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Planetary Gear Controls

Multiple-disc clutches serve as both braking andpower transfer devices.

A multiple disc clutch uses a series of circularfriction discs to transmit torque or apply braking

force. The discs have internal teeth that are sized and

shaped to mesh with splines on the clutch assemblyhub.

This hub is connected to a planetary geartraincomponent so that gearset members receive thedesired braking or transfer force when the clutch isapplied or released.


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Multiple Disc Clutch Design

The drum holds all other clutch components: the cylinder, hub,piston, piston return springs, seals, pressure plate, clutch pack(including friction plates), and snap rings.

The cylinder in a multiple-disc clutch is relatively shallow. Thecylinder bore acts as a guide for piston travel.

The piston is made of cast aluminum or steel with a seal ringgroove around the outside diameter.

A seal ring seats in the groove. This rubber seal retains fluidpressure required to stroke the piston and engage the clutchpack.

The piston return springs overcome the residual fluid pressure inthe cylinder and move the piston to the disengaged positionwhen clutch action (holding or transfer) is no longer needed.


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Multiple Disc Clutch Cooling

Operating temperatures can reach 1,100F, soclutch disc cooling is an important designconsideration in transmission manufacturing.

Friction disc surfaces are grooved to improve

cooling. Grooving provides the following advantages:

It allows the disc to store fluid to lubricate, cool, andquiet clutch engagement.

Grooves in the friction disc surface provide channels

through which the fluid can escape and drain quickly. When a clutch is disengaging, any fluid on the disc

and plate surfaces can create a suction effect, makingthe two surfaces more difficult to separate.


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Compound Planetary Gearsets(1 of 3)

Multiple planetary gearsets are coupled to produce

the required gear ratios and direction.

The four-speed heavy-duty transmissions have

three simple gearsets that we call front, center, andrear.

Five-speed transmissions can add a low planetary

gearset to the three-gear configuration for a total of

four gearsets. Input torque is directed to one of the planetary sun

gears through the use of clutches.


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Compound Planetary Gearsets (2 of 3)

A four-speed, three-gearset transmission uses onlyfive clutches, excluding the torque converter lockupclutch.

A five-speed, four-gearset transmission uses six

clutches. They function as follows: Forward clutch locks converter turbine to main input

drive shaft.

First clutch anchors rear planetary ring gear.

Second clutch anchors front planetary carrier.

Third clutch anchors sun gear shaft. Fourth clutch locks main input drive shaft to center

sun gear shaft.

Low clutch (five speeds only) anchors low planetarycarrier.


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Compound Planetary Gearsets (3 of 3)

Because the simple planetary gearsets areinterconnected by common shafts and drums, theoutput of one planetary gearset can be used as theinput of another planetary gearset.

Therefore, one gearset can take engine inputtorque, modify speed, torque, and direction, andpass this onto a second gearset.

The second gearset then further modifies speed,torque, and direction before directing the power flow

to the output shaft and vehicle drivetrain. With two simple planetaries working together,

torque input is compounded.


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Laws of Simple

Planetary Gear Operation



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Planetary Gearset Combinations



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Oil Pump

An oil pump is used to circulate oil for: Lubrication

Cooling

Hydraulic clutch application

The pump is located ahead of the gearing andclutches, and it will not be driven when the vehicleis towed or pushed.

Whenever the vehicle must be towed or pushedmore than a short distance (one-half mile

maximum), the driveline must be disconnected orthe driving wheels raised. Failure to do this will result in premature wear and

damage to internal components.


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Four-speed Clutch Application

See Table 18-4 on page 543 of textbook.


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Five-speed Clutch Application

See Table 18-5 on page 546 of textbook.


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Converter Cooler Circuit

Converter-in oil flows to the torque converter. Oil must flowthrough the converter continuously to keep it filled and to removeheat generated by the converter. The converter pressureregulator valve regulates converter-in pressure by exhaustingexcessive oil to sump.

Converter-out oil, exiting the torque converter, flows to anexternal cooler (a vehicle OEM-supplied component). Ram air orengine coolant is used to remove heat from the transmission oilat the cooler heat exchanger.

Lubrication oil is directed from there through the transmission to

those components requiring continuous lubrication and cooling.


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Modulator Pressure Circuit

Modulator pressure is used to modify shift points in response tothrottle position.

Modulator pressure is highest when the engine is running at idleand decreases in proportion to accelerator pedal travel.

The modulator valve assists the governor pressure in moving

any of the shift signal valves during an upshift. It can also delay a downshift. A decrease in modulator pressure

will cause a downshift if governor pressure is not sufficient tooppose the shift valve spring force.

The trimmer boost and trimmer regulator valve are controlled by

modulator pressure.


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Trimmer Regulator Valve

The trimmer regulator valve reduces main pressure

to a regulated pressure. The regulated pressure is

raised or lowered by changes in modulator

pressure.

This varies the clutch apply pressure pattern of the

trimmer valves.

A higher modulator pressure at zero accelerator

pedal travel will reduce trimmer regulator pressure. A lower modulator pressure (open throttle) results in

higher regulator pressure and a higher initial clutch

apply pressure.


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Lockup Circuit

Lockup clutch engagement and release are controlled bythe modulated lockup valve and the lockup relay valve.

The purpose of the modulated lockup valve is to prolonglockup clutch engagement while vehicle speed

decreases (closed throttle). This feature provides engine-braking action at speeds

lower than the normal lockup disengagement point.

In its downward position, the modulated lockup valvedirects main pressure to the top of the lockup valve.

Main pressure pushes the lockup valve downward. In itsdownward position, the valve directs main pressure tothe lockup clutch. This engages the clutch.


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Priority Valve

Gives the control system priority

The priority valve ensures that the control system

upstream from the valve will have sufficient pressure

during shifts to perform its automatic functions.

Without the priority valve, the filling of the clutch

might require a greater volume of oil (momentarily)

than the pump could supply and still maintain

pressure for the necessary control functions.

In many models, first clutch pressure can bypass

the priority valve when the transmission is in

reverse.


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Inhibitor Valve

Inhibits downshifts In five-speed transmissions, a 21 inhibitor

valve prevents a downshift from second gearto first gear when road speed is too high.

Prevents overspeeding

It will also protect the engine fromoverspeeding during downgrade operation in

first gear by making a 1

2 upshift if roadspeed exceeds that which is safe for first gearoperation.

Cl h Ci i


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Clutch Circuits

There are five separate clutches in four-speed transmissions andsix clutches in five-speed models.

The first, second, third, and fourth clutches are each connectedto a relay valve and to a trimmer valve.

The forward clutch is connected directly to the selector valve and

does not connect to a trimmer valve. Four-speed transmissions will automatically shift from first to

second, second to third, and third to fourth.

Five-speed models will automatically shift from second to third,third to fourth, and fourth to fifth. These shifts occur as a result of

governor pressure, and (if the throttle is not fully open) modulatorpressure.

A t ti U hift


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Automatic Upshifts

A combination of governor pressure and modulator pressure, orgovernor pressure alone, upshifts the transmission to second orthird gear, respectively.

At closed or part throttle, modulator pressure exists and willassist governor pressure. When the throttle is closed, shiftsoccur at lower wheel speeds.

At full throttle, there is no modulator pressure, meaning thatupshifts occur at higher road speeds.

The greater the vehicle speed, the greater the governorpressure. When governor pressure is sufficient, the first upshift will occur.

With a further increase in governor pressure, the second upshiftwill occur, etc.

In any automatic upshift, the shift signal valve acts first. Thisdirects a shift pressure to the relay valve. The relay valve shifts,exhausting the applied clutch and applying a clutch for a highergear.

A t ti D hift


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Automatic Downshifts

Automatic downshifts, like upshifts, are controlledby governor and modulator pressures.

Downshifts occur in sequence as governor pressureand/or modulator pressure decrease.

Low modulator pressure (full accelerator pedaltravel) will hasten the downshift; high modulatorpressure (zero accelerator pedal travel) will delaydownshifts.

In any automatic downshift, the shift signal valve

acts first. This exhausts the shift signal holding therelay valve downward. The relay valve then movesupward, exhausting the applied clutch and applyingthe clutch for the new lower gear.

D hift d R I hibiti


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Downshift and Reverse Inhibiting

The system is designed, to prevent downshifts attoo rapid a rate or to prevent a shift to reverse whilemoving forward.

The progressive downshift occurs because thepressure and the valve sectional areas arecalibrated to only shift the signal valves downwardagainst governor pressure when governor pressuredecreases to a safe downshift speed. So if speed is too great, governor pressure is

sufficient to hold the shift signal valve upward.

As governor pressure decreases, all shift signalvalves move downward in sequence.

H d li R t d


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Hydraulic Retarders

Hydraulic retarders are one of several types ofauxiliary braking systems used on heavy-dutytrucks.

Some retarder systems are designed as an integralcomponent of the automatic transmission.

They provide some vehicle braking on severedowngrades and help maximize service brake lifeby providing braking action at the driveline.

Integral retarders are usually available in two

automatic transmission configurations: The input retarder

The output retarder

I t R t d


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Input Retarders

The input retarder is located between the torque converterhousing and the main housing. It is designed primarily forhighway applications.

The unit uses a paddle wheel design with a vaned rotor mountedbetween stator vanes in the retarder housing.

Retardation or brake mode occurs when transmission oil isdirected into the retarder housing.

The oil causes resistance to the rotation of a vaned rotor.

Retarding capacity can be enhanced by downshifting.

Variable control is achieved by moving a hand lever or activating

a separate foot pedal. Heat is dissipated by circulating the oil through a high-capacity

transmission oil cooler.

Output Retarders


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Output Retarders

Most output retarders are mounted on the rear of thetransmission without adding additional length to it.

The output retarder uses a two-stage principle.

The first stage consists of a rotor/stator hydraulic design. Thesecond stage can be a powerful, oil-cooled, friction clutch pack.

On activation of the output retarder, quick initial response isprovided by a momentary application of the friction clutch packwhile the hydraulic section is being charged with oil.

The hydraulic section provides better retardation at higherspeeds. As the vehicle slows, the second stage friction clutch

pack phases in, providing the low-speed retardation powercapable of slowing the vehicle to a virtual stop.

Summary (1 f 6)


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Summary (1 of 6)

Automatic transmissions may be hydromechanicalor electronic. All automatic transmissions upshiftand downshift with no direct assistance from thedriver.

Factors such as road speed, throttle position, andgoverned engine speed control shifting betweengears.

Automatic transmissions use compoundedplanetary gearsets.

The simple planetary gearset consists of three maincomponents: a sun gear, a carrier with planetarypinions mounted to it, and an internally toothed ringgear or annulus.

Summary (2 f 6)


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Summary (2 of 6)

Advantages of the planetary gearset are as follows: Constantly meshed gears

Gear torque loads divided equally

Compact and versatile

Additional ratio and direction variations can be addedby compounding

In a planetary gearset, any one of the three mainmemberssun gear, pinion gear carrier, or ringgearcan be used as the driving or input member.

Depending on which member of the planetarygearset is the driver, which is held, and which isdriven, torque or speed increases are produced.

Summary (3 f 6)


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Summary (3 of 6)

To produce an output, one of the planetary gearmembers must be held stationary. In heavy-duty truck transmissions this is achieved

with multiple-disc clutches that can serve as bothbraking and power transfer devices.

Compound planetary combinations are severalplanetary gearsets coupled to produce the requiredgear ratios and direction.

Four-speed, heavy-duty transmissions have threesimple planetary gearsetsfront, center, and rear.

Five-speed transmissions add an additional lowplanetary gearset for a total of four gearsets.

Summary (4 f 6)


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Summary (4 of 6)

In all automatic transmissions, power flowsfrom the torque converter through the

transmission planetary gearing and out to the

output shaft. Transmission oil is drawn from the sump

through a filter by the input-driven oil pump.

Oil pressurized by the pump flows into thebore of the main pressure regulator valve.

Summary (5 of 6)


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Summary (5 of 6)

The converter/cooler/lubrication circuit originates atthe main pressure regulator valve.

The selector valve/forward regulator circuit is

manually shifted to select the operating range

desired.

Main pressure is directed to the governor valve.

The speed of the transmission output shaft controls

the position of the governor valve, which determinesthe amount of pressure in the governor circuit.

Summary (6 of 6)


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Summary (6 of 6)

The modulator actuator varies modulator pressureas the accelerator pedal moves.

Lockup clutch engagement and release arecontrolled by the modulated lockup valve and the

lockup relay valve. There are five separate clutches in four-speed

transmissions and six clutches in five-speedmodels, each clutch having its own circuit.

Hydraulic retarders are one of several types ofauxiliary braking systems used on heavy-dutytrucks; they are applied by the driver as needed.

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