Jeep Cherokee 2000, Service Manual

Page: 1027 / 1528

pressure will be created if the fluid is not confined. It
will simply “leak” past the piston. There must be a
resistance to flow in order to create pressure. Piston
sealing is extremely important in hydraulic operation.
Several kinds of seals are used to accomplish this
within a transmission. These include but are not limited
to
O–rings, D–rings, lip seals, sealing rings, or
extremely close tolerances between the piston and the
cylinder wall. The force exerted is downward (gravity),
however, the principle remains the same no matter
which direction is taken. The pressure created in the
fluid is equal to the force applied, divided by the piston
area. If the force is 100 lbs., and the piston area is 10
sq. in., then the pressure created equals 10 PSI.
Another interpretation of Pascal’s Law is that regard-
less of container shape or size, the pressure will be
maintained throughout, as long as the fluid is confined.
In other words, the pressure in the fluid is the same
everywhere within the container.
FORCE MULTIPLICATION
Using the 10 PSI example used in the illustration
(Fig. 41), a force of 1000 lbs. can be moved with a
force of only 100 lbs. The secret of force multiplica-
tion in hydraulic systems is the total fluid contact
area employed. The illustration, (Fig. 41), shows an
area that is ten times larger than the original area.
The pressure created with the smaller 100 lb. input
is 10 PSI. The concept “pressure is the same every-
where” means that the pressure underneath the
larger piston is also 10 PSI. Pressure is equal to the
force applied divided by the contact area. Therefore,
by means of simple algebra, the output force may be
found. This concept is extremely important, as it is
also used in the design and operation of all shift
valves and limiting valves in the valve body, as well
as the pistons, of the transmission, which activate
the clutches and bands. It is nothing more than
using a difference of area to create a difference in
pressure to move an object.
PISTON TRAVEL
The relationship between hydraulic lever and a
mechanical lever is the same. With a mechanical
lever it’s a weight–to–distance output rather than a
pressure–to–area output. Using the same forces and
areas as in the previous example, the smaller piston
(Fig. 42) has to move ten times the distance required
to move the larger piston one inch. Therefore, for
every inch the larger piston moves, the smaller pis-
ton moves ten inches. This principle is true in other
instances also. A common garage floor jack is a good
example. To raise a car weighing 2000 lbs., an effort
of only 100 lbs. may be required. For every inch the
car moves upward, the input piston at the jack han-
dle must move 20 inches downward.
Fig. 40 Pressure on a Confined Fluid
Fig. 41 Force Multiplication
Fig. 42 Piston Travel
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AUTOMATIC TRANSMISSION—30RH 21 - 119
DESCRIPTION AND OPERATION (Continued)