14-05-2014, 03:42 PM
Basic Hydraulic System Theory
Introduction
Automotive brake systems use the force of hydraulic
pressure to apply the brakes. Because automotive brakes
use hydraulic pressure, we need to study some basic
hydraulic principles used in brake systems. These include
the principles that fluids cannot be compressed, fluids can
be used to transmit movement and force, and fluids can be
used to increase force.
LAWS OF HYDRAULICS
The liquid must be available from a continuous source, such as
the brake fluid reservoir or a sump. In a hydraulic brake sys-
tem, the master cylinder serves as the main fluid pump
and moves the liquid through the system. The lines used
to carry the liquid may be pipes, hoses, or a network of
internal bores or passages in a single housing, such as
those found in a master cylinder. Valves are used to regu-
late hydraulic pressure and direct the flow of the liquid.The
output device is the unit that uses the pressurized liquid to
do work. In the case of a brake system, the output devices
are brake drum wheel cylinders (Figure 1) and disc brake
calipers.
Fluid Characteristics
If a liquid is confined and a force applied, pressure is
produced. In order to pressurize a liquid, the liquid must be
in a sealed container. Any leak in the container will decrease
the pressure.
The basic principles of hydraulics are based on certain
characteristics of liquids. Liquids have no shape of their
own; they acquire the shape of the container they are put
in. They also always seek a common level. Therefore, oil in a
hydraulic system will flow in any direction and through any
passage, regardless of size or shape. Liquids are basically
incompressible, which gives them the ability to transmit
force. The pressure applied to a liquid in a sealed container
is transmitted equally in all directions and to all areas of the
system and acts with equal force on all areas. As a result, liq-
uids can provide great increases in the force available to do
work. A liquid under pressure may also change from a liquid
to a gas in response to temperature changes.
HYDRAULIC BRAKE SYSTEMS
Engineers must consider these principles of force,
pressure, and motion when designing a brake system for
any vehicle. If an engineer chooses a master cylinder with
relatively small piston areas, the brake system can develop
very high hydraulic pressure, but the pedal travel will be
extremely long. Moreover, if the master cylinder piston
travel is not long enough, this high-pressure system will not
move enough fluid to apply the large-area caliper pistons
regardless of pressure. If, on the other hand, the engineer
selects a large-area master cylinder piston, it can move a
large volume of fluid but may not develop enough pres-
sure to exert adequate braking force at the wheels.
The overall size relationships of master cylinder pis-
tons, caliper pistons, and wheel cylinder pistons are bal-
anced to achieve maximum braking force without grabbing
or fading. Most brake systems with front discs and rear
drums have large-diameter master cylinders (a large piston
area) and a power booster to increase the input force.
Summary
In a hydraulic brake system, the master cylinder moves
brake fluid through the system.The lines used to carry the
liquid may be pipes,hoses,or a network of internal bores
or passages in a single housing, such as those found in
a master cylinder. Valves are used to regulate hydraulic
pressure and direct the flow of the liquid. The output
devices are brake drum cylinders and disc brake calipers.
Hydraulics is the study of liquids in motion.
Liquids are considered noncompressible fluids.
Pascal’s Law says that pressure at any one point in a
confined liquid is the same in every direction and
applies equal force on equal areas.
If a liquid is confined and a force applied, pressure is
produced. If the pressure on the fluid is applied to a
movable output piston, it creates output force.
In a brake system, a small master cylinder piston is
used to apply pressure to larger pistons at the wheel
brake units to increase braking force.