1. Automatic Transmission Components and Operation
This presentation will explore:
Automatic Transmission System Layout
Torque Converter Construction and Operation
Planetary Gear Construction and Operation
Clutches, Bands and Servos

2. Input shaft (from engine)
Basic Automatic Transmission System
The basic components of an automatic transmission system are:
Torque converter
Input shaft (from engine)
Valve body
Oil pump
Torque Converter - Fluid coupling that connects the engine to the transmission.
Oil Pump - Produces hydraulic oil pressure to operate the hydraulic components.
Valve Body - Mounted inside pan, controls oil flow to pistons and servos.

3. Basic Automatic Transmission System
Clutch assemblies
Output shaft
Planetary gearsets
Piston or servo
Pistons and Servos - Operate bands and clutches, selecting ratios.
Planetary (Epicyclic) Gearsets - Provide gear ratios and reverse.
Bands and Clutches - Actuate planetary gearsets by clamping pressure.
Output Shaft - Transfers torque to the drive shaft.

4. Transmission Housing
There are four main components to the transmission housing:
Converter Housing - Attaches the transmission to the engine, houses the converter and is usually made of aluminium.
Transmission case
Converter
housing
Transmission Case -
Holds the components of the transmission system.
Extension housing
Oil (Transmission) Pan - Holds the hydraulic oil and is sealed by a gasket.
Pan
Extension Housing - Protects and houses the output shaft and is sealed by a gasket.

5. Torque Converter Principle
The torque converter couples the engine to the transmission.
Its principle of operation can be demonstrated by two electric fans.
Driving fan
Driven fan
One fan power cord is connected to the AC supply and faces the other.
The air flow created by this fan causes the second fan to rotate, even though it is not connected to the supply.
A torque converter uses oil instead of air to perform the turning motion.

6. Torque Converter Construction
The torque converter is not a serviceable item. Its main components are:
Converter Housing - Made from two parts, an outer housing and an impeller. They are precision welded together and oil filled.
Turbine
Impeller
Impeller - (Driving fan) provides power to drive turbine. It contains blades that direct oil flow.
Turbine - (Driven fan) contains blades that receive oil and redirect it, to produce a turning force.
Stator
Outer housing
Stator - Redirects oil flow from turbine to impeller, using blades.

7. Torque Converter Operation
Turbine
Stator
Impeller
Flywheel
Housing
Oil flow
The converter housing is bolted to the flywheel of the engine. The housing and the impeller therefore turn at engine speed.
As the impeller spins, oil is flung outward. A vacuum is created in the centre of the impeller, which draws in more oil.
The turbine is connected to the
input shaft of the transmission
system. It is turned by oil entering its blades. The blades force the oil toward the centre of the turbine. Oil exits the turbine in the opposite direction from impeller rotation, due to turbine design.

8. Torque Converter Operation
The stator is designed to redirect oil flow, so that it assists the impeller. It can only spin in one direction, against turbine oil flow.
Turbine
Stator
Impeller
Flywheel
Housing
Oil flow
At low engine speeds, the turbine turns at a lower speed than the impeller, producing a gain in output torque. This is ideal for moving a vehicle from stop (or during acceleration), when an engine is not operating at its full potential. This is known as torque multiplication.
As engine speed increases, torque
multiplication reduces until the speed of the turbine approaches that of the impeller (3000 RPM plus). The engine power characteristics then compensate for the loss of torque multiplication.

9. Stator One-Way Clutch Operation
Impeller
One-way clutch
(A)
(B)
Rollers jam
Rollers disengaged
The stator rotates in one direction only. It is locked by an overrunning (one-way) clutch mechanism. This method may be used on other gearsets, within the transmission system.
Operation
When the shaft tries to turn counter-clockwise, the rollers ride up on the cams and lock up (A), preventing opposite rotation.
When the shaft turns clockwise, the rollers return to their disengaged position (B) allowing the shaft to turn freely.

10. Lockup Torque Converters
At higher engine speeds, the transmission is moving at nearly the same speed as the engine. In an ideal situation, they should be travelling at the same speed, because speed difference (slippage) equals power loss.
Friction pad
Clutch piston
Friction disc
This is part of the reason why vehicles with automatic transmissions use more fuel, than those with manual versions.
Some manufacturers overcome this problem by using a lockup torque converter. A typical converter contains a clutch, friction discs and pads.
When the turbine and the impeller are up to speed, fluid is channelled to the clutch piston. Piston action pushes the friction discs / pads together, locking the turbine and impeller to turn as one. The converter may be fitted with torsion springs to dampen engine power pulses.
This system improves efficiency and prevents slippage.

11. Planetary Gears
In an automatic transmission, planetary gearsets are used to provide forward and reverse gear ratios. The gears rotate, but never move in a lateral direction.
Ring gear
Planet gear
Sun gear
Planet gears
Planet carrier
The basic planetary gear set consists of a sun gear, a ring gear and planet gears.
Sun Gear - Middle gear around which the other gears revolve.
Planet Gears - Usually two or more gears that mesh with, and revolve around, the sun gear. They are held together using a planet carrier.
Ring Gear - Surrounds and meshes with the planet gears.

12. Planetary Gears - Movement Examples
By clamping or releasing some of these revolving gears, a combination of different gear ratios may be produced on the output shaft.
Ring gear
Typical examples:
Ring gear clamped - sun gear and planet carrier both rotate in the same direction (e.g. clockwise).
Sun gear
Sun gear clamped - planet carrier and ring gear both rotate in the same direction (e.g. clockwise).
Planet carrier
Planet gear
Planet carrier clamped - sun gear rotates in one direction (e.g. clockwise), the ring gear rotates in the other direction (e.g. counter-clockwise).

13. Planetary Reduction
When the output shaft rotates slower than the input shaft, an increase in output torque is achieved. This is obtained by using gear reduction. There are two typical methods:
Sun gear clamped - Input is applied to the ring gear, the planet gears move around the sun gear, the planet carrier is the output.
Ring Gear - Input (Driven)
Planet Carrier - Output
Sun Gear - Clamped
Ring gear clamped - Input is applied to the sun gear, the planet carrier is turned by the planet gears to become the output.
Ring Gear - Clamped
Planet Carrier - Output
Sun Gear - Input (Driven)
Fast
Slow
In both cases, the direction of input and output rotation is the same.

14. Overdrive
When the output shaft rotates faster than the input shaft, an increase in vehicle speed is achieved, at the cost of reduced output torque. This is obtained by using overdrive. There are two typical methods:
Ring gear clamped - Input is applied to the planet carrier and the output is taken from the sun gear.
Ring Gear - Clamped
Planet Carrier - Input (Driven)
Sun Gear - Output
Fast
Slow
Sun gear clamped - Input is applied to the planet carrier and the ring gear is turned by the planet gears to become the output.
Ring Gear - Output
Planet Carrier - Input (Driven)
Sun Gear - Clamped

15. Reverse
Planet carrier clamped - The input is applied to the sun gear and the output is taken from the ring gear.
Ring Gear - Output
Planet Carrier - Clamped
Sun Gear - Input (Driven)
Input
Output
Planet carrier clamped - The input is applied to the ring gear and the output is taken from the sun gear.
Ring Gear - Input (Driven)
Planet Carrier - Clamped
Sun Gear - Output
In both cases, the output will turn in the opposite direction from the input.

16. Direct Drive
If two gears are locked together, the third gear is directly driven by the other two. This is known as direct drive.
In the diagram to the right, the sun gear and planet carrier are locked together.
The ring gear is forced to rotate at the same speed as the locked gears.

17. Compound Planetary Gearset
The gears found in a real transmission system are more complex than the examples just discussed. A compound planetary gearset can give a wider range of gear ratios. It has two planetary gearsets inside a single case and/or ring gear. The ring gear couples with both planet sets.
Rear sun gear
Short planet gear
Front sun gear
Long planet gear
Ring gear
Planet carrier
Some may have two sun gears, as shown to the right. Short planet gears mesh with the front sun gear and long planet gears mesh with the rear sun gear.
One of the most common types is the Simpson Compound Gearset. This type has one sun gear, two planet gearsets and one ring gear.

18. Gearset overrunning clutch
Clutches and Bands
Planetary gearsets
Gearset overrunning clutch
Clutches
Output shaft
Stator overrunning clutch
Bands
Clutches and bands are the devices that use friction to clamp or lock the required components of planetary gearsets to give the required gear ratios.

19. Clutch Construction Driving discs (friction discs) Clutch hub
Driven discs (steel plates)
Clutch drum
Clutch (Cylinder) Drum - Encloses the clutch components.
Clutch Hub - Fits inside the clutch drum. It also fits inside the diving discs. It has teeth on its outer edge that mesh with those of the driving discs.
Driving Discs - They are friction lining covered and have teeth that mesh with the clutch hub.
Driven Discs - These have outer tabs that lock into the clutch drum.

20. Clutch Construction Pressure plate Apply piston Output shaft
Clutch release spring
Clutch Apply Piston - By the use of hydraulic pressure. This clamps the driven and driving discs together. Oil seals prevent fluid leaking from the piston when the clutch is applied.
Pressure Plate - This limits the travel of the clutch discs allowing plates to be clamped together when the piston activates the clutch.
Clutch Release Spring - Returns the apply piston to its rest position when hydraulic pressure is released and the clutch is disengaged.

21. Release springs hold piston away
Clutch Operation
Piston
Release springs hold piston away
Clutch discs locked
Oil pressure low
Oil pressure high
Hub
Input shaft
Hub
Output shaft
Drum stationary
Drum rotates
(A)
(B)
With no hydraulic pressure applied, the release springs hold the driving and driven discs disengaged and free to revolve independently of each other (A). The input shaft drives the clutch hub, but not the drum or output shaft.
When hydraulic pressure is applied, the piston forces the driving discs and the driven discs together to clamp the hub to the drum so that they turn together (B) as one unit.

22. Brake Bands Adjusting screw Steel brake band body
Friction lining
Pin
Brake bands are friction devices used to lock clutch drums and ring gears in position. The friction material resists the lubrication properties of the transmission oil.
The brake band fits around a clutch drum or ring gear. It is tightened by movement of a push rod, which protrudes from the piston of a servo.
Servo body/cover
Several brake bands are commonly used in automatic transmissions.
Rod
Retaining ring
An adjusting screw is provided for initial adjustment and to compensate for lining wear.
Piston
Seals

23. Brake Band Operation
Servos are hydraulically operated. When a clutch has to be clamped, oil is routed to the servo that operates the band of the clutch. Oil pressure pushes the servo piston away from its base, moving the push rod that tightens the band, around the drum (the opposite end of the band is secured to the transmission case).
Brake
band
The band prevents one of the planetary gearset components from turning, allowing different gear ratios to be selected.
Return spring
Push rod
When the oil pressure is removed, the return spring moves the piston away to release the clamping force and allow the associated planetary gear component to rotate again.
Pressure chamber
Adjusting screw
Servo piston

24. The Hydraulics System
The hydraulics system is a complex maze of passages and tubes. It uses pressurized transmission fluid to control transmission and torque converter operations. The fluid also cools and lubricates components.
The Oil Pump provides a constant supply of pressurized fluid. It is directly connected to a flange on the torque converter housing and turns at engine speed.
Inner gear
Outer gear
Crescent
The pump contains two gears. The inner gear, which is driven at engine speed, drives the outer gear. Fluid is drawn up from the pan on one side of the crescent and forced into the hydraulic system on the other side.
From pan

25. The Hydraulics System
The Valve Body is responsible for distributing transmission fluid to control the transmission system. It is constructed with a maze of channels and passages, and houses valves that activate clutch packs and band servos.
As each driving situation occurs, the valve body directs fluid to the required valve, to ensure smooth gear shifting takes place.
The valve body contains a manual valve, which is directly connected to the gear shift handle. It covers and uncovers various passages, depending upon handle position.
The latest transmissions are controlled by computers, allowing precise gear shifting to take place. Some systems allow manual control of gear changing.

26. Bands, Clutches and Gears
Typical examples of gear ratio selection, using bands, clutches and gears.
Neutral gear (all gears rotating free):
First gear (sun gear locked using band):
High gear (two gears locked using clutch):