How Clutches Work
The clutch on your car or truck can be compared to an on-off switch. When the clutch is disengaged
(clutch pedal down), the release mechanism actuates the fork. The fork contacts the release bearing,
moving it against the clutch diaphragm spring fingers. This action allows the clutch pressure plate to lift
away from the flywheel, opening a very small space between the disc, flywheel and pressure plate.
When the disc moves away from the flywheel, power flow from the engine to the transmission is
interrupted. The engine crankshaft and flywheel are rotating at a higher speed than the disc and
transmission input shaft that are coasting. When the clutch is engaged (clutch pedal up), the disc slips
briefly to provide smooth engagement and, once again, the clutch clamps the disc against the flywheel.
This causes the input shaft to turn, transmitting engine power to the transmission.
The disc is a critical component in providing long service life for the clutch system. It provides smooth
engagement and dampens engine vibrations. It is mounted to the input shaft between the flywheel and
the clutch. It can slide forward and backward on the shaft, but cannot rotate without rotating the shaft.
Important component parts include the hub-flange and the torsion springs. The hub-flange is located
between a cover plate and a retainer plate. The hub is splined to fit the input shaft. Torsion springs in
the damper assembly smooth engagement and dampens vibrations. Some discs include idle-stage
dampers, either small springs around the hub, or friction washers inside the disc. As pulsations from
the engine reach the disc, the springs compress and expand to cushion or dampen vibrations and
eliminate gear rattle.
Friction material is riveted to numerous metal components called marcels, or cushion segments.
Waves in marcels soften engagement. Some heavy-duty discs do not contain marcels. They are made
with cerametallic friction material, characterized by abrupt engagement and some chatter.
Pressure Plate or Clutch Cover
The clutch cover clamps the disc against the flywheel during engagement. During disengagement it
releases pressure on the disc, creating a gap large enough for the disc to move away from the
flywheel and enable the driver to shift gears.
A typical diaphragm spring clutch consists of a pressure plate, a diaphragm spring, a pivot ring, drive
straps and a cover. When the release bearing contacts the tips of the diaphragm spring fingers, it
moves them toward the flywheel. The outside diameter of the diaphragm spring pivots on the pivot ring
inside the cover. This action lifts the pressure plate off the flywheel through the drive straps that
connect the cover to the pressure plate.
Lever style clutches produce clamp load by pressure from coil springs. As the friction material of the
disc wears, the springs expand, reducing their clamping force. At the same time, pedal effort remains
high. As a result of these disadvantages, passenger cars and light trucks are now almost exclusively
equipped with diaphragm spring clutches.
Diaphragm spring clutches maintain higher clamp load than lever style clutches throughout the service
life of the clutch. As disc friction material wears, clamp load increases during the first half of clutch life
before decreasing gradually to its original level. Diaphragm spring clutches require less pedal effort
the further the pedal is actuated, reducing stress on release system components.
When replacing a coil spring clutch with a diaphragm spring, always remove over-center or
release-assist springs. Designed to reduce the higher pedal effort associated with coil spring
clutches, over-center springs may overcompensate when a diaphragm spring clutch is installed. This
can result in a very soft pedal and, in some cases, a pedal that will go to the floor and stay there.
Bolted to the end of the crankshaft, the flywheel provides the mounting surface for the clutch. During
engagement, the disc is clamped against the flywheel by the pressure plate. In addition to its other
functions, the flywheel acts as a heat sink, dissipating heat and moving it away from the clutch
pressure plate and disc friction material. The flywheel must provide a smooth, flat surface in order for
the clutch to operate properly.
The dual-mass flywheel is designed to absorb engine vibrations before they are transmitted to the
driveline where they pan create gear rattle. This is achieved by splitting the conventional flywheel into
two sections: a primary section that bolts to the crankshaft, and a secondary section, onto which the
clutch is bolted. The primary section of the flywheel contains springs to isolate engine vibrations and a
torque limiter to prevent engine torque spikes from exceeding engine and transmission component
strength. When torque spikes occur, the torque limiter allows the primary section of the flywheel to turn
independently of the secondary section, saving the driveline and transmission from damage.
The release bearing is attached to the fork and slides on a bearing retainer that is attached to the front
of the transmission. The movement of the fork causes the release bearing to slide across the bearing
retainer and press against the tips of the diaphragm spring fingers. Ball bearings in the release
bearing enable it to turn while applying pressure to the fingers. In order for the clutch to function
properly, the bearing retainer must be exactly parallel to the input shaft and provide a smooth surface
for the release bearing.
Angular-contact bearings, found in hydraulic release systems and self-adjusting cable systems, are in
constant contact with the diaphragm spring fingers. Self-centering bearings are designed to
compensate for slight misalignment between the engine and transmission. It is normal for these
bearings to be "off center" until they contact the diaphragm spring fingers.
Some vehicle designs utilize a concentric slave cylinder. It eliminates the need for a number of release
system components, including the release fork, pivot ball and bearing retainer. Its location inside the
bell housing makes it difficult to troubleshoot. To avoid added labor costs later, we recommend
replacing the concentric slave cylinder when the clutch is replaced.
On many vehicles, a pilot bearing or bushing is located in the end of the crankshaft. The pilot bearing
supports the end of the transmission input shaft and centers the disc on the flywheel. Types of pilots
include conventional ball bearings, needle bearings and sintered bronze bushings. A small and
relatively inexpensive component, the pilot bearing or bushing should always be replaced during clutch
installation. The variety of problems caused by a worn or defective pilot bearing or bushing are not
worth the risk of having to remove the bell housing and transmission to replace this component later.
Driving Habits and Clutch Wear
Constant engagement and disengagement of the clutch will wear away disc friction material. The rate,
at which wear occurs, however, depends largely on the driving habits of the operator and vehicle
Driving behaviors and conditions that decrease clutch service life include riding the clutch pedal, high
RPM engagement, excessive slipping, harsh downshifting, lugging the engine, excessive vehicle
loading, and engine, transmission and suspension modifications.
Slipping the clutch during engagement, creates excessive heat, damages the clutch and flywheel
contact surfaces and accelerates disc friction material wear. Riding the clutch reduces the clamping
force of the disc, and power transfer from the spinning flywheel is not fully applied to the disc. The
result is premature wear of the disc friction material.
Waiting in traffic with the vehicle in gear and the clutch disengaged loads the release bearing
excessively. Over time, this shortens release bearing life and can eventually cause noise. Lugging the
clutch occurs when the driver selects the wrong gear for the vehicle speed and load. Under low
speed/high load conditions, a lower gear should be used to reduce the torque applied to the clutch.
Selecting a higher gear causes excessive loading of the disc that can damage the disc hub and
torsion springs as well as the clutch drive straps. Over-revving the engine and high-speed downshifting
can burst the friction material.
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