components is called a clutch pack. A typical clutch pack
is shown in Figure 1-21.
Bands wrap around drums (cylindrical transmission
parts) to hold them stationary. The band is tightened
against the drum by a hydraulic piston called a servo,
which is operated by hydraulic pressure. Figure 1-22
shows a band, servo, and related linkage.
Another type of holding member is the one-way
clutch, Figure 1-23. A one-way clutch is a mechanical
device that allows the central hub to turn in one direction,
but causes it to lock up when it tries to turn in the opposite
direction. One-way clutches are always used in combina-
tion with other holding members to hold or drive parts of
the planetary gearsets.
Case and Housings
The case is the main support for the other transmis-
sion or transaxle parts. It also contains passages to deliver
pressurized fluid between various parts of the transmission.
Modern cases are made of aluminum that is cast into the
proper shape. After casting, the case is machined where
necessary to form mating surfaces for the other compo-
nents. Oil passages are then drilled in the case where nec-
essary. Bushings may be pressed into the case at wear
points. Figure 1-24 shows two common transmission and
transaxle cases. On most modern vehicles, the housings
around the torque converter and the output (or tail) shaft
are cast as an integral part of the case. When used, sepa-
rate bell housings (housing around the torque converter)
and tailshaft housings are bolted to the case.
Oil Pans
All transmissions or transaxles contain one or more
oil pans. The main purpose of the oil pan is as a storage
place for extra transmission fluid. Airflow over the oil pan
helps remove heat from the fluid. Some oil pans contain a
Chapter 1 Introduction to Automatic Transmissions and Transaxles 21
Planet gears
and carrier
Sun gear
Internal gear
Figure 1-19. This figure illustrates the three main components of a simple planetary gearset: the sun gear, planet gears and
planet carrier, and the internal, or ring, gear. (General Motors)
Figure 1-20. Simple planetary gears can be combined into mul-
tiple, or compound, gearsets to obtain many different gears.
This photograph shows a gearset containing two sun gears,
two planet carriers, and a ring gear. Many other combinations
are possible.