CH40 automatic transmissions and transaxles STUDENT VERSION

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Chapter 40

Automatic Transmissions and Transaxles

Automatic Transmissions and Transaxles

• Selects gear ratios according to engine

speed, power train load, vehicle speed, and other operating factors

• Most have four speeds with an overdrive fourth gear

• Many have five, six, and up to eight speeds

Torque Converter

• A fluid clutch or coupling that transfers

torque from the engine to the transmission

• Automatically engages and disengages

power from the engine in relation to RPM

• At idle, there is not enough fluid flow for power transfer

• As RPM increases, fluid flow is sufficient

to transmit engine power

Torque Converter Internal Parts

Torque Converter Components

• Impeller or Pump Assembly

– Receives power from the engine.

Types of Oil Flow

• Coupling point is when turbine speed

approaches impeller speed

Overrunning Clutch

• Keeps the stator from

rotating when driven

in one direction

• The stator is locked

when the there is a

difference between the

turbine and impeller

speeds

Lockup Torque Converter

• Eliminates the 10% slip between the

impeller and turbine

• Improves fuel economy

• Reduces operational temperature

• Most torque converter clutches (TCC) use electronically controlled hydrualic pistons

Planetary Gearset

Planetary Gear Operation

• When the planetary carrier is the input, an overdrive condition is produced

• When the planetary carrier is the output, an underdrive condition is produced

• When the planet carrier is held, reverse gear

is produced

Types of Compound Gearsets

• Simpson

– Has two separate planetary gearsets with a common sun gear.

• Ravigneaux

– Has two separate sun gears, two separate

planetary gears, and a common ring gear.

Simpson Gearset

Ravigneaux Gearset

Other Transmission Designs

• Nonplanetary Gearset Type

– Uses constant mesh gears similar to a manual transmission.

• Continuously Variable Type (CVT)

– Has no fixed forward speeds.

– Uses belts and pulleys to provide variable

ratios.

CVT Transmission

Lepelletier System

• Used in late model 6-, 7-, and 8-speed transmissions

• Combines a simple gear set with a

Ravigneaux gear set

• Many are more compact than most four- and five-speed automatics

Apply Devices

• Bands

– Are typically used as a holding device.

• Servos

– Are used to apply the band.

• Multiple Friction Disc Assemblies

– Are used to hold or apply.

Typical Simple Servo

Multiple-Disc Clutches

• A series of friction discs and steel plates

used to transmit torque or apply braking

force

• Internal teeth mesh with clutch hub

• External teeth mesh with internal splines in transmission housing or clutch drum

Bearings, Bushings, and Thrust Washers

• Bearings are used to reduce wear and

friction:

– Most either slide or roll against the surface

• Bushings are pressed into place:

– Support rotating parts, guide parts, and control fluid flow

• Thrust washers may act as a bearing and a spacer to control endplay

Typical Bearing Locations

Bushing Locations

Snap Rings

• Both internal and external types are used

• Internal snap rings hold servo and clutch assemblies together

• May also be used to adjust clearances

Gaskets and Seals

Seal Classifications

• Static

– Is used between two

parts that do not move.

• Dynamic

– Is used between two

parts that move with

Final Drive Assemblies

• Transmit transaxle output to the differential section

• Four common configurations:

– Helical gear

– Planetary gear

– Hypoid gear

– Chain drive

Hydraulic System

• The fluid operates, lubricates, and cools the transmission

• The pump is the source of all fluid flow:

– Pressure-regulating valves change fluid

pressure to control shift quality and shift points – Flow-directing valves send to fluid to the

correct apply device for gear changes

• Check ball valves

• Poppet valves

• Spool valves

Shift Quality

• Shift feel is controlled by the pressure at

which each hydraulic member is applied or released

• Timing between band and clutch application

to prevent engine flare-up or clutch and

band slippage

Hydraulic Circuits

• The hydraulic circuit for each gear selector position is explained in the flow charts

• While examining flow charts, note gear

range, torque converter mode, engaged

planetary controls, approximate vehicle

speed, and throttle position