Planetary Gear Control

Planetary Gear Control

The following are helpful tips for remembering the basics of simple planetary gearset operation.

- When the planetary carrier is the drive (input) member, the gearset produces an overdrive condition. Speed increases, torque decreases.

- When the planetary carrier is the driven (output) member, the gearset produces a forward direction. Speed decreases, torque increases.

- When the planetary carrier is held, the gearset produces a reverse.

Certain parts of the planetary gear train must be held while others must be driven to provide the needed torque multiplication and direction for vehicle operation. Planetary gear controls is the general term used to describe transmission bands, servos, and clutches.

Transmission Bands

A band is a braking assembly positioned around a stationary or rotating drum. The band brings a drum to a stop by wrapping itself around the drum and holding it. The band is hydraulically applied by a servo assembly. Connected to the drum is a member of the planetary gear train. The purpose of a band is to control the planetary gear train by holding the drum and connecting planetary gear member stationary. Bands provide excellent holding characteristics and require a minimum amount of space within the transmission housing.

When a band closes around a rotating drum, a wedging action takes place to stop the drum from rotating. The wedging action is known as self-energizing action. It is explained later in this chapter.

There are two types of bands used in automatic transmissions: single wrap and double wrap. A thick steel single wrap band is used to hold gear train components driven by high output engines. Self-energizing action is low because of the rigidity of the band's design. Thinner steel bands are not able to provide a high degree of holding power, but because of the flexibility of design, self-energizing action is stronger and provides more apply force.

Band lugs are either spot welded or cast as a part of the band assembly. The purpose of the lugs is to connect the band with the servo through the actuating (apply) linkage or the band anchor (reaction) at the opposite end.

The band's steel strap is designed with slots or holes to release fluid trapped between the drum and the applying band.

A typical band is designed to be larger in diameter than the drum it surrounds. This design promotes self-disengagement of the band from the drum when servo apply force is decreased to less than servo release spring tension. A friction material resembling automobile brake lining is bonded to the inside diameter of the band.

The double wrap band is a circular external contracting band normally designed with two or three segments (parts). As the band closes, the segments align themselves around the drum and provide a cushion.

The steel body of the double wrap band may be thin or thick steel strapping material. Modern automatic transmissions use thin single or double wrap bands for increased efficiency. Double wrap bands made with heavy thick steel strapping are required for high output engines.

Transmission Servos

The servo assembly converts hydraulic pressure into mechanical pressure that applies the band around a drum, holding it stationary. Simple and compound servos are used to engage bands in modern transmissions.

Simple Servo In a simple servo, the servo piston fits into the servo cylinder and is held in the released position by a coil spring. The piston is encircled by a seal ring made of rubber, which keeps fluid pressure confined to the apply side of the servo piston.

The piston pushrod is drilled through the center, which permits fluid pressure to be directed to the apply side of the servo piston. The piston pushrod locates in the band apply strut, which is indexed with the band apply lug. At the opposite end of the band is the anchor strut and adjustment screw. They receive the engagement force of a band.

To apply a band, fluid pressure is directed down the servo pushrod to the apply side of the servo piston. The servo piston strokes through the servo cylinder, compresses the servo coil spring, and develops servo apply force. As the servo piston and pushrod stroke the servo cylinder, they force the apply lever and strut against the band lug. The band tightens around the rotating drum. The rotating drum comes to a stop and is held stationary by the band.

When servo apply force is released, the servo coil spring forces the servo piston to stroke back up the servo cylinder to the released position. With the servo apply force removed, the band springs free and permits drum rotation.

Compound Servos A compound servo has a cylinder that is cast as part of the transmission housing. The servo piston located nearest the front of the transmission uses cast-iron seal rings capable of withstanding the heat generated by the torque converter and engine.

When the compound servo is applied, fluid pressure flows through the hollow piston pushrod to the apply side of the servo piston. The piston moves inward, compressing the servo coil spring and forcing the pushrod to move one end of the band toward the adjusting screw and anchor. The band tightens around the rotating drum and brings it to a stop. If the drum was stationary, the band would apply, holding the drum tight and unable to revolve. The apply of the compound servo piston is much like the simple servo, but there the similarity ends.

Fluid pressure is applied to the release side of the servo piston when the band is to be released. To release the compound servo, fluid pressure is admitted to the release side of the servo piston. Line pressure on either side of the servo piston balances out. The force of the servo spring causes the release of the servo piston.

Transmission Clutches

In contrast to a band, which can only hold a planetary gear member, transmission clutches, either overrunning or multiple-disc, are capable of both holding and driving members.

Overrunning Clutches When applied to transmission operation, the overrunning clutch is broadened to a holder or driver. Both sprag and roller overrrunning clutches can be applied to either the holding or driving application.

For example, when the turbine shaft and inner race rotate, the accordion apply strings force the rollers down their ramps and engage both the inner and outer races. Power flow passes from the inner race through the roller to the outer race, driving the low reverse sun gear and direct clutch splines at turbine shaft speed.

Should the turbine shaft decelerate and rotate slower than the driven race, the overrunning clutch rollers in an overrun mode disengage from the driving race. The low reverse sun gear and direct clutch are disengaged from the turbine shaft.

Multiple-Disc Clutches A multiple-disc clutch uses a series of hollow friction 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 assembly hub. In turn, this hub is connected to a planetary gear train component so gearset members receive the desired braking or transfer force when the clutch is applied or released.

DESIGN Multiple-disc clutches have a large drum-shaped housing that can be either a separate casing or part of the existing transmission housing. This drum housing 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 very shallow when compared to an engine cylinder. The hub of the cylinder acts as a guide for piston level.

The piston is made of cast aluminum or a steel stamping with a seal ring groove around the outside diameter. A seal ring seats in the groove. This rubber seal retains fluid pressure used to stroke the piston engaging the clutch pack. The piston return springs overcome the reduced fluid pressure in the clutch and move the piston to the disengaged position when clutch action (holding or transfer) is no longer needed.

The clutch pack consists of normal clutch plates, friction discs, and one very thick plate known as the pressure plate. The pressure plate has tabs around the outside diameter to mate with the channels in the clutch drum. It is held in place by a large snap ring. The stroking piston forces the engaging clutch pack against the fixed pressure plate. Because the pressure plate cannot move or deflect, it provides the reaction to the engaging clutch pack.

Clutch plates must be perfectly flat, and although the surface of the plate might appear smooth, it is specifically machined to promote a coefficient of friction to help transmit engine torque.

The friction discs are sandwiched between the clutch plates and pressure plate. Friction discs are designed with a steel core plate center with friction material bonded to either side. Asbestos was once the universal friction material used, but because it is hazardous to human health, cellular paper fibers, graphites, and ceramics are now being used as friction materials.

COOLING During clutch engagement, friction takes place and develops heat between the clutch plates and friction discs. The transmission fluid absorbed by the paper-based friction material transfers heat from the disc to the plates. The plates then transfer the heat to the drum housing where it can be cooled by the surrounding transmission fluid. Operating temperatures can reach 1100F, so clutch disc cooling is an important design consideration in transmission manufacturing.

To help with this cooling and provide other performance advantages, friction disc surfaces are normally grooved.

Planetary Control Terminology

Table below is a crossover chart listing the names the different manufacturers call the same planetary gear control. On some four-speed transaxles, the terminology changes because the gear train arrangement changes.

Chrysler refers to the planetary controls by their location in the transaxle or transmission housing. To help eliminate confusion, this test follows Chrysler's logical approach. Chrysler calls the clutch engaged for all forward speeds the rear clutch. The band at the front of the transaxle is the front kickdown band engaged in second gear, which is also known as the passing gear. The front clutch in the transaxle is engaged in reverse and high or third gear. The band located at the rear of the transaxle housing is known as the low and reverse band. It is engaged when the gear selector lever is placed in reverse or manually shifted to first or low gear. The overrunning clutch engages only low gear or, if the vehicle speed is low enough, on a three to one downshift for rapid torque multiplication.



General Motors

Front clutch

Reverse and high clutch

Direct clutch

Rear clutch

Forward clutch

Forward clutch

Front kickdown band

Intermediate band

Intermediate band

Low and reverse rear band

Low and reverse band or clutch

Low and reverse band or clutch

Overrunning clutch

One-way clutch

Low roller clutch


Automatic transmissions use several simple planetary gearsets tied together to generate the required gear ratios and direction needed for optimum performance. When several simple gearsets work as a unit, they are known as a gear train. The most common gear train used in automatic transmissions is the Simpson gear train.

The Simpson gear train consists of two simple planetary gearsets that share a common sun gear. This common sun gear ties the two gearsets together and allows the output of one gearset to become the input of the other.

In a Simpson gear train, both the forward ring gear and the common sun gear can be connected to the transmission input shaft using twp separate multiple-disc clutches. Although the sun gear rotates around the output shaft, it is not connected to the output shaft. The sun gear and the rear carrier can be held. The sun gear is held stationary by either a band or a multiple-disc clutch, which locks the sun gear to the transmission housing.

In manually selected low gear, the rear carrier is held by either a band or multiple-disc clutch. In both cases, there is also an overrunning clutch that prevents backward rotation of the carrier while allowing forward rotation. When the drive position is selected, the overrunning clutch only holds while the transmission is in low gear.

The forward carrier and the rear ring gear are splined to the transmission output shaft. To make this connection, the output shaft extends through the hollow sun gear to reach the forward carrier.

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