Ford Explorer, Руководство по техническому обслуживанию и ремонту

Страница: 753 / 1119

CLUTCH
Understanding the Clutch
The purpose of the clutch is to disconnect and connect engine power at the transmission. A vehicle at rest
requires a lot of engine torque to get all that weight moving. An internal combustion engine does not develop
a high starting torque (unlike steam engines) so it must be allowed to operate without any load until it builds
up enough torque to move the vehicle. To a point, torque increases with engine rpm. The clutch allows the
engine to build up torque by physically disconnecting the engine from the transmission, relieving the engine
of any load or resistance.
The transfer of engine power to the transmission (the load) must be smooth and gradual; if it weren't, drive
line components would wear out or break quickly. This gradual power transfer is made possible by gradually
releasing the clutch pedal. The clutch disc and pressure plate are the connecting link between the engine and
transmission. When the clutch pedal is released, the disc and plate contact each other (the clutch is engaged)
physically joining the engine and transmission. When the pedal is pushed in, the disc and plate separate (the
clutch is disengaged) disconnecting the engine from the transmission.
Most clutch assemblies consists of the flywheel, the clutch disc, the clutch pressure plate, the throw out
bearing and fork, the actuating linkage and the pedal. The flywheel and clutch pressure plate (driving
members) are connected to the engine crankshaft and rotate with it. The clutch disc is located between the
flywheel and pressure plate, and is splined to the transmission shaft. A driving member is one that is attached
to the engine and transfers engine power to a driven member (clutch disc) on the transmission shaft. A driving
member (pressure plate) rotates (drives) a driven member (clutch disc) on contact and, in so doing, turns the
transmission shaft.
There is a circular diaphragm spring within the pressure plate cover (transmission side). In a relaxed state
(when the clutch pedal is fully released) this spring is convex; that is, it is dished outward toward the
transmission. Pushing in the clutch pedal actuates the attached linkage. Connected to the other end of this is
the throw out fork, which hold the throw out bearing. When the clutch pedal is depressed, the clutch linkage
pushes the fork and bearing forward to contact the diaphragm spring of the pressure plate. The outer edges of
the spring are secured to the pressure plate and are pivoted on rings so that when the center of the spring is
compressed by the throw out bearing, the outer edges bow outward and, by so doing, pull the pressure plate in
the same direction − away from the clutch disc. This action separates the disc from the plate, disengaging the
clutch and allowing the transmission to be shifted into another gear. A coil type clutch return spring attached
to the clutch pedal arm permits full release of the pedal. Releasing the pedal pulls the throw out bearing away
from the diaphragm spring resulting in a reversal of spring position. As bearing pressure is gradually released
from the spring center, the outer edges of the spring bow outward, pushing the pressure plate into closer
contact with the clutch disc. As the disc and plate move closer together, friction between the two increases and
slippage is reduced until, when full spring pressure is applied (by fully releasing the pedal) the speed of the
disc and plate are the same. This stops all slipping, creating a direct connection between the plate and disc
which results in the transfer of power from the engine to the transmission. The clutch disc is now rotating with
the pressure plate at engine speed and, because it is splined to the transmission shaft, the shaft now turns at the
same engine speed.
The clutch is operating properly if:
It will stall the engine when released with the vehicle held stationary. 1.
The shift lever can be moved freely between 1st and reverse gears when the vehicle is stationary and
the clutch disengaged. 2.
CLUTCH 763