Land Rover Discovery: Drive Clutches

Multiplate Drive or Brake Clutch - Typical

1. Cylinder
2. Piston
3. Disk spring
4. Metal plates
5. Friction plates
6. Baffle plate
7. Pressure equalization chamber
8. Piston chamber

There are two multiplate drive clutches and two multiplate brakes used in the ZF 9HP48 automatic transmission. Each clutch or brake comprises a number of friction plates dependent on the output controlled. A typical clutch or brake consists of a number of steel outer plates and inner plates with friction material bonded to each face.

The drive clutches have both the friction plate and the metal plates rotating when the clutch is open. Multiplate brakes have either the friction plate or the metal plate rotating, with one fixed stationary.

The multiplate clutch and brake plates are held apart mechanically by a disk spring and hydraulically by ATF pressure. The ATF pressure is derived from a lubrication channel which supplies ATF to the transmission components. The ATF is passed via drillings in the input shaft into the chamber between the baffle plate and the piston. To prevent inadvertent clutch application due to pressure build up produced by centrifugal force, the fluid in the pressure equalization chamber overcomes any pressure in the piston chamber and holds the piston off the clutch plate assembly. The multiplate brakes do not require a baffle plate and pressure equalization chamber to compensate for centrifugal pressure which occurs in a rotating piston.

When clutch application is required, pressure from the ATF pump is applied to the piston chamber from the supply port. This pressure overcomes the low pressure fluid present in the pressure equalization chamber. The piston moves, against the pressure applied by the disk spring, and compresses the clutch plate assembly. When the pressure falls, the disk spring pushes the piston away from the clutch plate assembly, disengaging the clutch.

DOG CLUTCHES

Two dog clutches are used on the transmission; dog clutch 'A' connects the input shaft to sun gear S2 and ring gear R1, and dog clutch 'F' connects sun gears S3 and S4 to the centering plate mounted in the transmission casing.

Both dog clutches are similar in their operation. Each clutch is operated by ATF pressure acting on a double acting piston to move the dog clutch into and out of engagement.

Dog Clutch 'A'

1. Planetary gear set 1
2. Sensing piston
3. Dog 'A'
4. ATF pressure supply for dog clutch 'A' release
5. Input shaft
6. Piston
7. Planet carrier
8. ATF pressure supply for dog clutch 'A' engagement

Dog clutch 'A' is located at the end of the input shaft and is controlled by a double acting piston located within the input shaft.

A double acting piston is located internally in the input shaft and can move within the shaft when ATF pressure is applied to either side of the piston.

The piston is connected to the dog 'A' by a pin which moves in a slot in the input shaft.

Dog 'A' is a sleeve with internal and external splines. Dog 'A' is permanently engaged with the input shaft via the internal splines. When the piston moves dog 'A' along the input shaft to the 'closed' position, dog 'A' engages with splines on the gearset 1 and 2 planet carrier, transferring drive from the input shaft to the gear set 2. When the dog 'A' is to be disengaged to the 'open' position, ATF pressure is applied to the opposite side of the piston and dog 'A' is moved along the input shaft and is disengaged from the planet carrier. Dog 'A' is in the 'closed' position in gears 1 through 7.

The dog 'A' has two states; open and closed. The piston cannot determine if the dog 'A' has travelled its full distance into or out of engagement with the ring gear carrier or has remained in an intermediate position. The piston is fitted with a sensing piston and the ATF pressure leakage through the sensing piston can be measured by the pressure sensor in the sensor unit.

The sensing piston is hollow and moves axially within the piston. Referring to the below illustration, if ATF pressure is applied to the right side of the piston, the piston and the sensing piston are pushed to the left. During the movement of the piston, a small amount of ATF pressure is passed through the sensing piston. This leakage pressure is measured at the left side of the piston by the pressure sensor. When the piston has moved fully to the left and reached its end position, the leakage through the sensing piston is blocked. The pressure drop on the left side of the piston is sensed and the TCM can determine the dog clutch 'A' is fully disengaged with the ring gear carrier.

If the pressure on the left side of the piston does not drop within a specified shift time, the TCM can determine that the dog 'A' has stopped in an intermediate position.

The dog clutch 'A' has four possible states of operation as follows:

1. Dog 'A' open - piston at end position
2. Dog 'A' closing - piston at intermediate position
3. Dog 'A' closed - piston at end position
4. Dog 'A' opening - piston at intermediate position

1. ATF Pressure applied - dog 'A' open
2. ATF pressure applied - dog 'A' closed
3. Leakage through sensing piston for pressure sensing.

A. Dog 'A' Open - ATF pressure is applied to the piston from the left side chamber and the dog clutch 'A' is open. The ATF pressure in the right chamber is almost zero because the sensing piston is pushed to its limit of movement and leakage through the sensing piston is prevented.

B. Dog 'A' Closing - ATF pressure is applied from the right side chamber which starts the piston moving to the left into engagement with the ring gear carrier. The piston is now in the intermediate position and leakage pressure is passed through the sensing piston into the left side chamber.

Pressure in the left side chamber can be measured and will be approximately 2 bar (29 lbf in³).

C. Dog 'A' Closed - ATF pressure is applied to the piston from the right side chamber, the dog 'A' is closed and fully engaged with the ring gear carrier. The ATF pressure in the left chamber is almost zero because the sensing piston is pushed to its limit of movement and leakage through the sensing piston is prevented.

D. Dog 'A' Opening - ATF pressure is applied to the piston from the left side chamber which starts the piston moving to the right and disengaging from the ring gear carrier. The piston is now in the intermediate position and leakage pressure is passed through the sensing piston into the right side chamber. Pressure in the right side chamber can be measured and will be approximately 2 bar (29 lbf in³).

Dog Clutch 'F'

1. ATF pressure supply - open
2. Spur pinion
3. Angular contact ball bearing race
4. ATF pressure supply - close
5. Pressure sensing leakage hole
6. Sun gear 3 and 4
7. Dog 'F'
8. Bearing support housing

Dog clutch 'F' is located between the multiplate clutch 'E' and planetary gear set 4. The dog clutch is controlled by a double acting piston located within the bearing support housing.

Dog 'F' is a sleeve with internal and external splines. Dog 'F' is permanently engaged on the splines with the bearing support housing, which in turn is fixed and static within the transmission casing. When dog 'F' is moved to the closed position, it acts as a brake for sun gears 3 and 4 in planetary gear set 4.

The dog 'F' has two states; open and closed. Dog 'F' employs a more simple sensing system than dog 'A'. Dog 'F' is also the piston itself and does not use a sensing piston. The piston of dog 'F' has a leakage sensing hole which is used to detect its current position via pressure sensing.

Referring to the below illustration, if ATF pressure is applied to the right side of the piston, the piston is pushed to the left. During the movement of the piston, a small amount of ATF pressure is passed through the leakage sensing hole. This leakage pressure is measured at the left side of the piston by the pressure sensor in the sensor unit. When the piston has moved fully to the left and reached its end position, the leakage through the leakage sensing hole is blocked. The pressure drop on the left side of the piston is sensed and the TCM can determine that dog clutch 'F' is fully engaged with sun gears 3 and 4.

If the pressure on the left side of the piston does not drop within a specified shift time, the TCM can determine that the dog 'F' has stopped in an intermediate position.

The dog clutch 'F' has four possible states of operation as follows:

1. Dog 'F' open - piston at end position
2. Dog 'F' closing - piston at intermediate position
3. Dog 'F' closed - piston at end position
4. Dog 'F' opening - piston at intermediate position

1. ATF Pressure applied - dog 'F' open
2. ATF pressure applied - dog 'F' closed
3. ATF pressure applied - dog 'F' in intermediate position
4. Leakage through leakage sensing hole for pressure sensing

A. Dog 'F' Open - ATF pressure is applied to the piston from the right side chamber and the dog clutch 'F' is open. The ATF pressure in the left chamber is almost zero because the piston is pushed to its limit of movement and leakage through the leakage sensing hole is prevented.

B. Dog 'F' Closing - ATF pressure is applied from the left side chamber which starts the piston moving to the right into engagement with the sun gears 3 and 4. The piston is now in the intermediate position and leakage pressure is passed through the leakage sensing hole into the right side chamber. Pressure in the right side chamber can be measured and will be approximately 2 bar (29 lbf in³).

C. Dog 'F' Closed - ATF pressure is applied to the piston from the left side chamber, the dog 'F' is closed and fully engaged with the sun gears 3 and 4.

The ATF pressure in the right chamber is almost zero because the piston is pushed to its limit of movement and leakage through the leakage sensing hole is prevented.

D. Dog 'F' Opening - ATF pressure is applied to the piston from the right side chamber which starts the piston moving to the left and disengaging from the sun gears 3 and 4. The piston is now in the intermediate position and leakage pressure is passed through the leakage sensing hole into the left side chamber. Pressure in the left side chamber can be measured and will be approximately 2 bar (29 lbf in³).

One-Way Clutch - Torque Converter

1. Unlocked condition
2. Locked condition

1. Roller
2. Cage
3. Spring
4. Inner race

The roller clutch uses parallel rollers, located between the smooth, cylindrical inner race and the inclined cam faces of the clutch body. Springs are used to hold the rollers in position between the two contact faces.

When the clutch is rotated in a clockwise direction, the rollers become trapped between the inner race and the inclined cam faces of the clutch body, providing positive (locked) rotation of the inner race, locking the clockwise rotation of the stator. When the clutch is rotated in a clockwise direction, the rollers are moved away from the inclined cam faces and can rotate freely (unlocked) with the clutch body, this allows the torque converter stator to rotate freely when the vehicle is decelerating.

Valve BlockPlanetary Gear Trains

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