16802_Automatic Transmission System

7/28/2019 16802_Automatic Transmission System

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Devices Torque converters

Planetary gearing

Electronic control unit

Automatic transmission brake bands

Multi-disc clutches

Hydraulic controlling


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Torque converter In an automatic transmission, transmission is done by

both the transmission control system, automaticallyselecting the gearing according to load and speed, andby the torque converter.


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Torque converter The torque converter is mounted on the engine in the

same place as a manual clutch, and does the same job,transmitting engine torque to the input shaft of thetransmission. It can also multiply torque according todriving conditions.


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Planetary gears Most automatic transmissions use epicyclic or planetary

gears. They are constantly in mesh with each other. A basic planetary gearset has a sun gear, which meshes with

planet gears, also called planet pinions. The planet pinions, in sets of three or more, rotate onbearings, on hardened steel pins, on a planet carrier, whichspaces the pinions equally around the sun gear. It alsolocates them so they can mesh with an internally toothedring gear.

This means the planet pinions are always in mesh with thesun gear and the ring gear.

In operation, their motion is described as either Walking"or "Idling".


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Planetary gears Walking means that if either the sun gear, or the ring

gear, is held stationary, the alternative driving memberrotates the planet gears on their pins. This turns theplanet carrier in the same direction as the drivingmember.

Planet gears always turn in the same direction on their

pins as planet carrier rotation, while they walk arounda stationary sun gear. They always turn in the oppositedirection on their pins while walking inside astationary ring gear.


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"Idling" refers to the rotation of the planet gears ontheir pins whenever the planet carrier is stationary.Torque is transmitted from the sun gear to the ringgear, or from the ring gear to the sun gear, via theplanet gears and the stationary carrier.


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Neutral gear The ring gear is not held. When the input shaft rotates

the sun gear, the planet gears idle on their stationarycarrier pins, and turn the ring gear in the oppositedirection to engine rotation.

This free rotation of the gears provides neutral. Nodrive is transmitted to the output.


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1st

and 2nd

gears For low gear, a brake band, anchored to the

transmission case, is placed around the ring gear.Applying this holds the ring gear stationary.

Now when the sun gear rotates, the planet gears can nolonger idle. They must walk around the inside of thestationary ring gear.

The planet carrier must move with them in the samedirection as engine rotation, but at a slower speed. Andwith an increase in torque.


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Top gears A direct-drive or top-gear condition with a 1 to 1 ratio is

obtained by locking together any 2 members of the gearset.

A multi-plate clutch can be used for this purpose.

The outer drum and outer plates are attached to the ringgear. The inner plates and inner drum are attached to theplanet carrier.

When fluid under pressure is directed onto the clutchpiston, the clutch plates are locked together. This locks theplanet carrier to the ring gear.

Now when the sun gear is rotated, the planet gears canneither idle nor walk. The whole gearset turns as one unitto give a direct drive.


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Reverse gear For reverse, the ring gear is attached to the output

shaft, and the planet carrier is held stationary by abrake band.

Rotating the sun gear causes the planet gears to idle ontheir stationary pins. This turns the ring gear, and itsoutput shaft, in the opposite direction to enginerotation.


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Automatic transmission brake

bandsAll bands are externally contracting types and have a

thin layer of plain or grooved friction material bondedonto a spring steel or cast steel backing.

One end of the band rests against a stop or strut in thetransmission case. The other end accommodates apush rod or linkage from a hydraulically operatedservo, which contracts the band onto the drum


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Multiple-disc clutches Multiple-disc clutches can be used to hold members to

the case instead of using bands but in mosttransmissions, they couple planetary gear membersand shafts together.

A servo-operated band can only hold a memberstationary, but multiple-disc clutches can hold or driveindividual members.


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Electronic control In an electronically controlled transmission, the speed

of the vehicle and the throttle opening are sensed bythe vehicle speed sensor, and the throttle positionsensor. This information is sent as electrical signals toan ECU.

Signals to hydraulically-controlled transmissionsystem


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Hydraulically-controlled

transmission The hydraulic system comprises a crescent type gear

pump, a centrifugally operated hydraulic governor anda control valve body assembly which regulates thepressure and directs fluid to the appropriatetransmission components.

The governor is driven by the transmission outputshaft and provides a varying hydraulic pressureaccording to vehicle speed.


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Other components Levers

Valves

Sensors Transmitters

Flow lines

Program

Switch board


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Continuously variable

transmissionA continuously variable transmission has no fixed gear

ratio changes. Changes occur in a smooth steplessprogression to suit speed and load conditions.

electronically controlled magnetic clutch.


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transmissionA small gap between the driving and driven members

is filled with magnetic powder. A coil on the drivingmember receives electrical current via two slip rings

and brushes. Current through the coil generates a magnetic force

that links up the magnetic powder in a chain fashion.Torque is transmitted from the driving member to the

driven member. Clutch current is controlled by a transmission

computer reacting to input data about vehicleoperating conditions.


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transmissionAt idle speeds with the vehicle stationary, current flow

is small, which allows slippage between engine andtransmission.

As the accelerator is depressed, the computer deliversmore current to the coils. The powder locks the enginerigidly to the transmission input shaft.

With the engine off, or the selector in Park or Neutral,reverse direct current sent to the coil de-magnetizesthe clutch powder and releases the clutch completely.

Documents

16802_Automatic Transmission System