Application of pressure at the inner section of the
diaphragm will cause the outer rim to move away from
the flywheel and draw the pressure plate away from the
clutch disc, disengaging the clutch.
The clutch disc, also called friction lining, consists
of a splined hub and a round metal plate covered with
friction material (lining). The splines in the center of
the clutch disc mesh with the splines on the input shaft
of the manual transmission. This makes the input shaft
and disc turn together. However, the disc is free to slide
back and forth on the shaft.
Clutch disc torsion springs, also termed damping
springs, absorb some of the vibration and shock
produced by clutch engagement. They are small coil
springs located between the clutch disc splined hub
and the friction disc assembly. When the clutch is
engaged, the pressure plate jams the stationary disc
against the spinning flywheel. The torsion springs
compress and soften, as the disc first begins to turn
with the flywheel.
Clutch disc facing springs, also called the
cushioning springs, are flat metal springs located
under the friction lining of the disc. These springs have
a slight wave or curve, allowing the lining to flex
inward slightly during initial engagement. This also
allows for smooth engagement.
The clutch disc friction material, also called disc
lining or facing, is made of heat-resistant asbestos,
cotton fibers, and copper wires woven or molded
together. Grooves are cut into the friction material to
aid cooling and release of the clutch disc. Rivets are
used to bond the friction material to both sides of the
metal body of the disc.
The flywheel is the mounting surface for the
clutch. The pressure plate bolts to the flywheel face.
The clutch disc is clamped and held against the
flywheel by the spring action of the pressure plate. The
face of the flywheel is precision machined to a smooth
surface. The face of the flywheel that touches the
clutch disc is made of iron. Even if the flywheel were
aluminum, the face is iron because it wears well and
dissipates heat better.
The pilot bearing or bushing is pressed into the end
of the crankshaft to support the end of the transmission
input shaft. The pilot bearing is a solid bronze bushing,
but it also may be a roller or ball bearing.
The end of the transmission input shaft has a small
journal machined on its end. This journal slides inside
the pilot bearing. The pilot bearing prevents the
transmission shaft and clutch disc from wobbling up
and down when the clutch is released. It also assists the
input shaft center the disc on the flywheel.
When the operator presses the clutch pedal, the
clutch release mechanism pulls or pushes on the clutch
release lever or fork (fig. 4-8). The fork moves the
release bearing into the center of the pressure plate,
causing the pressure plate to pull away from the clutch
disc releasing the disc from the flywheel. The engine
crankshaft can then turn without turning the clutch disc
and transmission input shaft.
When the operator releases the clutch pedal, spring
pressure inside the pressure plate pushes forward on
the clutch disc (fig. 4-8). This action locks the
Figure 4-8.Clutch operation.