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TM 9-8000
Because the liquid will always occupy the same volume,
34-23. Principles of the Hydraulic System. In hydraulic
the large piston will move one-fourth as far as the small
braking systems, the pressure applied at the brake pedal
piston. Thus, a mechanical advantage is obtained very
is transmitted to the brake mechanism by a liquid. To
similar to that obtained from a simple lever.
understand how pressure is transmitted by a hydraulic
braking system, it is necessary to understand the
With four jars, all of the same diameter, connected to a
fundamentals and principles of hydraulics. Hydraulics is
central jar (E, fig. 34-19), an approximation of the action
the study of liquids in motion, or the pressure exerted by
in four-wheel brakes is obtained. A force exerted on the
liquids conveyed in pipes or conduits.
piston in the central jar will be transmitted to each of the
other jars so that the piston in each will receive an
One well-known hydraulic principle is that liquids cannot
identical force but will move only one-fourth as far as the
be compressed under ordinary pressures. This may be
central piston.
demonstrated by placing a weight on top of a piston fitted
to a jar (A, fig. 34-19). The force of the weight does not
If the four jars have a larger diameter than the central jar,
change the level of the liquid, therefore, it does not
the total pressure on each of the four pistons is greater
diminish the volume or compress the liquid.
than that applied to the central one, and each of the four
pistons moves less than one-fourth as far as the central
Another well-known hydraulic principle is Pascal's law
piston. Hydraulic brake systems operate in such a
(para 20-2), which states that force exerted at any point
upon a confined liquid is distributed equally through the
liquid in all directions. That is, if a total force of 20
34-24. Operation. In a hydraulic brake system, the force
pounds, including piston and weight, is placed upon
is applied to a piston in a master cylinder that
liquid in a jar, and if the piston in the jar has an area of 5
corresponds to the central jar (fig. 34-20). The brake
square inches, the unit hydraulic pressure is increased
pedal operates the piston by linkage. Each wheel brake
by 20/5, or 4 psi. This is illustrated in B, figure 34-19. A
is provided with a cylinder fitted with opposed  pistons
gage inserted at any point in the jar will indicate the
connected to the brake shoes.
same pressure of 4 psi, because the liquid transmits the
pressure equally throughout the jar.
The brake pedal, when depressed, moves the piston
within the master cylinder, forcing the brake liquid or fluid
Use of these hydraulic principles may be illustrated by
from the master cylinder through tubing and flexible hose
interconnecting  two  jars,  of  the  same  diameter,
into the four wheel cylinders. A diagram of a hydraulic
containing liquid (C, fig. 34-19). If a force is exerted on a
brake system is shown in figure 34-20.
piston in one jar, as in the left jar in C, figure 34-19, a
piston placed in the other jar will receive the same
The brake fluid enters each of the wheel cylinders
amount of force, due to the transmission of pressure by
between opposed pistons, making the pistons move the
the liquid. When the areas of the two pistons are equal,
brake shoes outward against the brake-drum. As
moving one piston produces identical movement in the
pressure on the pedal is increased, greater hydraulic
other piston because the liquid is not compressible and
pressure is built up within the wheel cylinders and,
therefore maintains the same volume.
consequently, greater force is exerted against the ends
of the shoes.
By connecting one jar with another jar that has twice the
diameter and therefore four times the area of the first jar
When pressure on the pedal is released, retracting
(D, fig. 34-19), the results are somewhat different,
springs on the brake shoes return the wheel cylinder
although the same basic facts apply. When a force is
pistons to their release position, forcing the brake fluid
exerted on the piston in the small jar, the piston in the
back through the flexible hose and tubing to the master
large jar will receive four times as much force because
the hydraulic pressure acts on four times the area.


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