PRESSURE PLATE.The pressure plates are
thick clutch plates that are placed on either end of the
stack. Their purpose is to distribute the application
pressure equally on the surfaces of the clutch discs and
CLUTCH PISTON.The clutch piston uses
hydraulic pressure to apply the clutch. Hydraulic
pressure is supplied to the clutch piston through the
center of the rotating member.
CLUTCH PISTON SEALS.The clutch piston
seals serve to prevent the leakage of hydraulic pressure
around the inner and outer circumferences of the clutch
ensure rapid release of the clutch when hydraulic
pressure to the clutch piston is released. The clutch
springs may be in the form of several coil springs
equally spaced around the piston or one large coil
spring that fits in the center of the clutch drum. Some
models use a diaphragm-type clutch spring.
The operation of the multiple-disc clutch is as
follows (fig. 4-32):
RELEASEDWhen the clutch is released, there
is no hydraulic pressure on the clutch piston and the
clutch discs and plates are free to rotate within each
other. The result is that the clutch hub rotates freely and
does not drive the clutch drum.
APPLIEDWhen the clutch is applied, hydraulic
pressure is applied to the clutch piston that, in turn,
applies pressure to the clutch discs and plates, causing
them to lock together. The result is that the clutch hub
drives the clutch drum through the clutch.
An overrunning clutch is used in automatic
transmissions to lock a planetary gearset to the
transmission case so that it can act as a reactionary
member. The overrunning clutch for the planetary
gears is similar to the one in a torque converter stator or
an electric starting motor drive gear. A planetary
gearset overrunning clutch consists of an inner race,
set of springs, rollers, and an outer race.
Operation of the overrunning clutch is very simple
to understand. When driven in one direction, rollers
lock between ramps on the inner and outer race,
allowing both races to turn. This action can be used to
stop movement of the planetary member, for example.
When turned in the other direction, rollers walk off the
ramps, and the races are free to turn independently.
Figure 4-32.Multiple-disc clutch operation.