07-12-2012, 01:15 PM
Basic Hydraulic System Theory
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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
Automotive brake systems are complex hydraulic circuits.
To better understand how the systems work, a good
understanding of how basic hydraulic circuits work is
needed.A simple hydraulic system has liquid, a pump, lines
to carry the liquid, control valves, and an output device.The
liquid must be available from a continuous source, such as
the brake fluid reservoir or a sump. In a hydraulic brake system,
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 regulate
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.
Pascal’s Law
More than 300 years ago a French scientist, Blaise
Pascal, determined that if you had a liquid-filled container
with only one opening and applied force to the liquid
through that opening, the force would be evenly distributed
throughout the liquid. This explains how pressurized
liquid is used to operate and control the brakes on a
vehicle. The action of the brake pedal on the pistons
inside the master cylinder pressurizes the brake fluid and
the fluid is delivered to the various wheel brake units
(Figure 3).
Pascal constructed the first known hydraulic device,
which consisted of two sealed containers connected by a
tube.The pistons inside the cylinders seal against the walls
of each cylinder and prevent the liquid from leaking out of
the cylinder and prevent air from entering into the cylinder.
When the piston in the first cylinder has a force
applied to it, the pressure moves everywhere within the
system.
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, liquids
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.
Fluids Can Transmit Movement
Liquids can be used to transmit movement. Two cylinders
of the same diameter are filled with a liquid and connected
by a pipe as shown in Figure 4. If you force piston A
downward, the liquid will push piston B upward. Because
piston A starts the movement, it is called the apply piston.
Piston B is called the output piston. If the apply piston
moves 10 inches, the output piston also will move 10
inches. This principle works not only for one output piston,
but for any number of output pistons.
The principle that motion can be transmitted by a liquid
is used in hydraulic brake systems. A master cylinder
piston is pushed when the driver applies the brakes. The
master cylinder piston is the apply piston.The brake fluid in
the master cylinder is connected by pipes to pistons in
each of the car’s front and rear wheel brake units. Each of
the wheel brake pistons is an output piston. They move
whenever the master cylinder input piston moves.
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 pressure
to exert adequate braking force at the wheels.
The overall size relationships of master cylinder pistons,
caliper pistons, and wheel cylinder pistons are balanced
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.
HYDRAULIC BRAKE FLUID
The liquid used in a hydraulic brake system is brake fluid.
The specifications for all automotive brake fluids are defined
by Society of Automotive Engineers (SAE) Standard J1703
and Federal Motor Vehicle Safety Standard (FMVSS) 116. Fluids
classified according to FMVSS 116 are assigned United
States Department of Transportation (DOT) numbersOT 3,
4, and 5. Basically, the higher the DOT number (Figure 10),
the more rigorous the specifications for the fluid.These specifications
list the qualities that brake fluid must have, such as:
Free flow at low and high temperatures.
A high boiling point (over 400°F or 204°C).
A low freezing point.
Ability to not deteriorate metal or rubber brake parts.
Ability to lubricate metal and rubber parts.
Ability to absorb moisture that enters the hydraulic
system.
Choosing the right fluid for a specific vehicle is not based
on the simple idea that if DOT 3 is good,DOT 4 must be better,
and DOT 5 better still. The domestic carmakers all specify
DOT 3 fluid for their vehicles, but Ford calls for a heavy-duty
variation that meets the basic specifications for DOT 3 but has
the higher boiling point of DOT 4. Import manufacturers are
about equally divided between DOT 3 and DOT 4.
Other Brake Fluid Requirements
A high-temperature boiling point is not the only
requirement brake fluid must meet.Brake fluid must remain
stable throughout a broad range of temperatures, and it
must retain a high boiling point after repeated exposure to
high temperatures. Brake fluid must also resist freezing and
evaporation and must pass specific viscosity tests at low
temperatures. If the fluid thickens and flows poorly when
cold, brake operation will suffer.
Besides temperature requirements, brake fluid must
pass corrosion tests. It also must not contribute to deterioration
of rubber parts and must pass oxidation-resistance
tests. Finally, brake fluid must lubricate cylinder pistons and
bores and other moving parts of the hydraulic system.
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.