following are helpful tips for remembering the basics of simple planetary
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.
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.
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.
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.
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.
band's steel strap is designed with slots or holes to release fluid trapped
between the drum and the applying band.
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
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.
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.
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.
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.
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.
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.
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
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
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
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.
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
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
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.
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.
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.
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.
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
help with this cooling and provide other performance advantages, friction disc
surfaces are normally grooved.
Planetary Control Terminology
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.
Reverse and high clutch
Front kickdown band
Low and reverse rear band
Low and reverse band or clutch
Low and reverse band or clutch
Low roller clutch
PLANETARY GEAR TRAIN
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
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.
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.
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.
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.