Hydraulic brake

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A schematic illustrating the major components of a

hydraulic disc brake system.
A hydraulic brake is an arrangement of
braking mechanism which uses brake fluid,
typically containing glycol ethers or
diethylene glycol, to transfer pressure from
the controlling mechanism to the braking
mechanism.

History

Knox Motors Co. used hydraulic brakes in 1915, in a

Tractor unit[1] .
Malcolm Loughead (who later changed the
spelling of his name to Lockheed) invented
hydraulic brakes, which he would go on to
patent in 1917.[2][3] "Lockheed" is a
common term for brake fluid in France.

Fred Duesenberg used Lockheed hydraulic

brakes on his 1914 racing cars[4] and his
car company, Duesenberg, was the first to
use the technology on a passenger car, in
1921.

Knox Motors Company of Springfield, MA

was equipping its tractors with hydraulic
brakes, beginning in 1915.[5]
The technology was carried forward in
automotive use and eventually led to the
introduction of the self-energizing
hydraulic drum brake system (Edward
Bishop Boughton, London England, June
28, 1927) which is still in use today.

Construction
The most common arrangement of
hydraulic brakes for passenger vehicles,
motorcycles, scooters, and mopeds,
consists of the following:

Brake pedal or lever

A pushrod (also called an actuating rod)
 A master cylinder assembly containing a
piston assembly (made up of either one
or two pistons, a return spring, a series
of gaskets/ O-rings and a fluid reservoir)
Reinforced hydraulic lines
Brake caliper assembly usually
consisting of one or two hollow
aluminum or chrome-plated steel
pistons (called caliper pistons), a set of
thermally conductive brake pads and a
rotor (also called a brake disc) or drum
attached to an axle.

The system is usually filled with a glycol-

ether based brake fluid (other fluids may
also be used).
At one time, passenger vehicles commonly
employed drum brakes on all four wheels.
Later, disc brakes were used for the front
and drum brakes for the rear. However
disc brakes have shown better heat
dissipation and greater resistance to
'fading' and are therefore generally safer
than drum brakes. So four-wheel disc
brakes have become increasingly popular,
replacing drums on all but the most basic
vehicles. Many two-wheel vehicle designs,
however, continue to employ a drum brake
for the rear wheel.

The following description uses the

terminology for / and configuration of a
simple disc brake.

System operation

U.S. Army training film: Hydraulic Brake

Operations (circa 1983)

Play media

In a hydraulic brake system, when the

brake pedal is pressed, a pushrod exerts
force on the piston(s) in the master
cylinder, causing fluid from the brake fluid
reservoir to flow into a pressure chamber
through a compensating port. This results
in an increase in the pressure of the entire
hydraulic system, forcing fluid through the
hydraulic lines toward one or more
calipers where it acts upon one or more
caliper pistons sealed by one or more
seated O-rings (which prevent leakage of
the fluid).

The brake caliper pistons then apply force

to the brake pads, pushing them against
the spinning rotor, and the friction between
the pads and the rotor causes a braking
torque to be generated, slowing the
vehicle. Heat generated by this friction is
either dissipated through vents and
channels in the rotor or is conducted
through the pads, which are made of
specialized heat-tolerant materials such
as kevlar or sintered glass.

Alternatively, in a drum brake, the fluid

enters a wheel cylinder and presses one or
two brake shoes against the inside of the
spinning drum. The brake shoes use a
similar heat-tolerant friction material to the
pads used in disc brakes.

Subsequent release of the brake

pedal/lever allows the spring(s) in the
master cylinder assembly to return the
master piston(s) back into position. This
action first relieves the hydraulic pressure
on the caliper, then applies suction to the
brake piston in the caliper assembly,
moving it back into its housing and
allowing the brake pads to release the
rotor.

The hydraulic braking system is designed

as a closed system: unless there is a leak
in the system, none of the brake fluid
enters or leaves it, nor does the fluid get
consumed through use. Leakage may
happen, however, from cracks in the O-
rings or from a puncture in the brake line.
Cracks can form if two types of brake fluid
are mixed or if the brake fluid becomes
contaminated with water, alcohol,
antifreeze, or any number of other
liquids.[6]

An example of a hydraulic
brake system
Hydraulic brakes transfer energy to stop an
object, normally a rotating axle. In a very
simple brake system, with just two
cylinders and a disc brake, the cylinders
could be connected via tubes, with a
piston inside the cylinders. The cylinders
and tubes are filled with incompressible
oil. The two cylinders have the same
volume, but different diameters, and thus
different cross-section areas. The cylinder
that the operator uses is called the master
cylinder. The spinning disc brake will be
adjacent to the piston with the larger
cross-section. Suppose the diameter of
the master cylinder is half the diameter of
the slave cylinder, so the master cylinder
has a cross-section four times smaller.
Now, if the piston in the master cylinder is
pushed down 40 mm, the slave piston will
move 10 mm. If 10 newtons (N) of force
are applied to the master piston, the slave
piston will press with a force of 40 N.

This force can be further increased by

inserting a lever connected between the
master piston, a pedal, and a pivot point. If
the distance from the pedal to the pivot is
three times the distance from the pivot to
the connected piston, then it multiplies the
pedal force by a factor of 3, when pushing
down on the pedal, so that 10 N becomes
30 N on the master piston and 120 N on
the brake pad. Conversely, the pedal must
move three times as far as the master
piston. If we push the pedal 120 mm down,
the master piston will move 40 mm and
the slave piston will move the brake pad by
10 mm.

Component specifics
(For typical light duty automotive braking
systems)

In a four-wheel car, the FMVSS Standard

105, 1976;[7] requires that the master
cylinder be divided internally into two
sections, each of which pressurizes a
separate hydraulic circuit. Each section
supplies pressure to one circuit. The
combination is known as a dual master
cylinder. Passenger vehicles typically have
either a front/rear split brake system or a
diagonal split brake system (the master
cylinder in a motorcycle or scooter may
only pressurize a single unit, which will be
the front brake).
A front/rear split system uses one master
cylinder section to pressurize the front
caliper pistons and the other section to
pressurize the rear caliper pistons. A split
circuit braking system is now required by
law in most countries for safety reasons; if
one circuit fails, the other circuit can still
stop the vehicle.

Diagonal split systems were used initially

on American Motors automobiles in the
1967 production year. The right front and
left rear are served by one actuating piston
while the left front and the right rear are
served, exclusively, by a second actuating
piston (both pistons pressurize their
respective coupled lines from a single foot
pedal). If either circuit fails, the other, with
at least one front wheel braking (the front
brakes provide most of the braking force,
due to weight transfer), remains intact to
stop the mechanically damaged vehicle.
By the 1970s, diagonally split systems had
become common among automobiles
sold in the United States. This system was
developed with front-wheel-drive cars'
suspension design to maintain better
control and stability during a system
failure.

A Triangular split system was introduced

on the Volvo 140 series from MY 1967,
where the front disc brakes have a four-
cylinder arrangement, and both circuits act
on each front wheel and on one of the rear
wheels. The arrangement was kept
through subsequent model series 200 and
700.

The diameter and length of the master

cylinder has a significant effect on the
performance of the brake system. A larger
diameter master cylinder delivers more
hydraulic fluid to the caliper pistons, yet
requires more brake pedal force and less
brake pedal stroke to achieve a given
deceleration. A smaller diameter master
cylinder has the opposite effect.
A master cylinder may also use differing
diameters between the two sections to
allow for increased fluid volume to one set
of caliper pistons or the other.

A proportioning valve may be used to

reduce the pressure to the rear brakes
under heavy braking. This limits the rear
braking to reduce the chances of locking
up the rear brakes, and greatly lessens the
chances of a spin.

Power brakes

The vacuum booster or vacuum servo is

used in most modern hydraulic brake
systems which contain four wheels. The
vacuum booster is attached between the
master cylinder and the brake pedal and
multiplies the braking force applied by the
driver. These units consist of a hollow
housing with a movable rubber diaphragm
across the center, creating two chambers.
When attached to the low-pressure portion
of the throttle body or intake manifold of
the engine, the pressure in both chambers
of the unit is lowered. The equilibrium
created by the low pressure in both
chambers keeps the diaphragm from
moving until the brake pedal is depressed.
A return spring keeps the diaphragm in the
starting position until the brake pedal is
applied. When the brake pedal is applied,
the movement opens an air valve which
lets in atmospheric pressure air to one
chamber of the booster. Since the
pressure becomes higher in one chamber,
the diaphragm moves toward the lower
pressure chamber with a force created by
the area of the diaphragm and the
differential pressure. This force, in addition
to the driver's foot force, pushes on the
master cylinder piston. A relatively small
diameter booster unit is required; for a very
conservative 50% manifold vacuum, an
assisting force of about 1500 N (200n) is
produced by a 20 cm diaphragm with an
area of 0.03 square meters. The
diaphragm will stop moving when the
forces on both sides of the chamber reach
equilibrium. This can be caused by either
the air valve closing (due to the pedal
apply stopping) or if "run out" is reached.
Run out occurs when the pressure in one
chamber reaches atmospheric pressure
and no additional force can be generated
by the now stagnant differential pressure.
After the run out point is reached, only the
driver's foot force can be used to further
apply the master cylinder piston.

The fluid pressure from the master cylinder

travels through a pair of steel brake tubes
to a pressure differential valve,
sometimes referred to as a "brake failure
valve", which performs two functions: it
equalizes pressure between the two
systems, and it provides a warning if one
system loses pressure. The pressure
differential valve has two chambers (to
which the hydraulic lines attach) with a
piston between them. When the pressure in
either line is balanced, the piston does not
move. If the pressure on one side is lost,
the pressure from the other side moves
the piston. When the piston makes contact
with a simple electrical probe in the center
of the unit, a circuit is completed, and the
operator is warned of a failure in the brake
system.
From the pressure differential valve, brake
tubing carries the pressure to the brake
units at the wheels. Since the wheels do
not maintain a fixed relation to the
automobile, it is necessary to use hydraulic
brake hose from the end of the steel line at
the vehicle frame to the caliper at the
wheel. Allowing steel brake tubing to flex
invites metal fatigue and, ultimately, brake
failure. A common upgrade is to replace
the standard rubber hoses with a set which
are externally reinforced with braided
stainless-steel wires. The braided wires
have negligible expansion under pressure
and can give a firmer feel to the brake
pedal with less pedal travel for a given
braking effort.

The term 'power hydraulic brakes' can also

refer to systems operating on very
different principles where an engine-driven
pump maintains continual hydraulic
pressure in a central accumulator. The
driver's brake pedal simply controls a valve
to bleed pressure into the brake units at
the wheels, rather than actually creating
the pressure in a master cylinder by
depressing a piston. This form of brake is
analogous to an air brake system but with
hydraulic fluid as the working medium
rather than air. However, on an air brake air
is vented from the system when the brakes
are released and the reserve of
compressed air must be replenished. On a
power hydraulic brake system fluid at low
pressure is returned from the brake units
at the wheels to the engine-driven pump as
the brakes are released, so the central
pressure accumulator is almost instantly
re-pressurised. This makes the power
hydraulic system highly suitable for
vehicles that must frequently stop and
start (such as buses in cities). The
continually circulating fluid also removes
problems with freezing parts and collected
water vapour that can afflict air systems in
cold climates. The AEC Routemaster bus
is a well-known application of power
hydraulic brakes and the successive
generations of Citroen cars with
hydropneumatic suspension also used
fully powered hydraulic brakes rather than
conventional automotive brake systems.

Special considerations
Air brake systems are bulky, and require air
compressors and reservoir tanks.
Hydraulic systems are smaller and less
expensive.

Hydraulic fluid must be non-compressible.

Unlike air brakes, where a valve is opened
and air flows into the lines and brake
chambers until the pressure rises
sufficiently, hydraulic systems rely on a
single stroke of a piston to force fluid
through the system. If any vapor is
introduced into the system it will
compress, and the pressure may not rise
sufficiently to actuate the brakes.

Hydraulic braking systems are sometimes

subjected to high temperatures during
operation, such as when descending steep
grades. For this reason, hydraulic fluid
must resist vaporization at high
temperatures.
Water vaporizes easily with heat and can
corrode the metal parts of the system.
Water which enters brake lines, even in
small amounts, will react with most
common brake fluids (i.e., those which are
hygroscopic[8][9]) causing the formation of
deposits which can clog the brake lines
and reservoir. It is almost impossible to
completely seal any brake system from
exposure to water, which means that
regular changing out of brake fluid is
necessary to ensure that the system is not
becoming overfilled with the deposits
caused by reactions with water. Light oils
are sometimes used as hydraulic fluids
specifically because they do not react with
water: oil displaces water, protects plastic
parts against corrosion, and can tolerate
much higher temperatures before
vaporizing, but has other drawbacks vs.
traditional hydraulic fluids. Silicone fluids
are a more expensive option.

"Brake fade" is a condition caused by

overheating in which braking effectiveness
reduces, and may be lost. It may occur for
many reasons. The pads which engage the
rotating part may become overheated and
"glaze over", becoming so smooth and
hard that they cannot grip sufficiently to
slow the vehicle. Also, vaporization of the
hydraulic fluid under temperature extremes
or thermal distortion may cause the linings
to change their shape and engage less
surface area of the rotating part. Thermal
distortion may also cause permanent
changes in the shape of the metal
components, resulting in a reduction in
braking capability that requires
replacement of the affected parts.

See also
Air brake (road vehicle)
Anti-lock braking system
Bicycle brake systems
Brake bleeding
Brake-by-wire
 Fuse (hydraulic)
Hydraulics
Hydraulic circuit
Railway air brake
Torque converter
Vehicle brake

References
1. Automobile Engineering, Vol. II., p.
183. American Technical Society,
Chicago, 1919
2. Loughhead, Malcolm, "Braking
apparatus," U.S. Patent no. 1,249,143
(filed: 1917 January 22 ; issued: 1917
December 4).
3. Csere, Csaba (January 1988), "10 Best
Engineering Breakthroughs", Car and
Driver, 33 (7), p. 61
4. http://www.autonews.com/article/199
60626/ANA/606260745/stopping-
power-put-duesenbergs-forever-in-
industrys-winners-circle
5. "Motor Age" . 1915.
6. Sean Bennett (3 November 2006).
Modern Diesel Technology: Brakes,
Suspension & Steering . Cengage
Learning. p. 97. ISBN 978-1-4180-
1372-1.
7. "Federal Motor Vehicle Safety
Standards and Regulations" .
 www.nhtsa.gov. Retrieved 2016-10-01.
8. "CDC - NIOSH Pocket Guide to
Chemical Hazards - Ethylene glycol" .
www.cdc.gov. Retrieved 11 April 2018.
9. "CDC - NIOSH Pocket Guide to
Chemical Hazards - Propylene glycol
monomethyl ether" . www.cdc.gov.
Retrieved 11 April 2018.

External links
Nice, Karim. "How Brakes Work" . How
Stuff Works. Retrieved 18 June 2010.
"Hydraulic Brakes" . Integrated
Publishing. Archived from the original
 on 30 March 2010. Retrieved 18 June
2010.
Erjavec, Jack (2004). Automotive
Technology: A Systems Approach,
Delmar Cengage Learning. ISBN 1-4018-
4831-1
Allan and Malcolm Loughead
(Lockheed) Their Early Lives in the Santa
Cruz Mountains including the invention
of the hydraulic brake.

Patents

US 2746575 Disc brakes for road and

other vehicles. Kinchin 1956-05-22
US 2591793 Device for adjusting the
return travel of fluid actuated means.
Dubois 1952-04-08
US 2544849 Hydraulic brake automatic
adjuster. Martin 1951-03-13
US 2485032 Brake apparatus. Bryant
1949-10-08
US 2466990 Single disk brake. Johnson
Wade C, Trishman Harry A, Stratton
Edgar H. 1949-04-12
US 2416091 Fluid pressure control
mechanism. Fitch 1947-02-12
US 2405219 Disk brake. Lambert Homer
T. 1946-08-06
US 2375855 Multiple disk brake.
Lambert Homer T. 1945-05-15
US 2366093 Brake. Forbes Joseph A.
1944-12-26
US 2140752 Brake. La Brie 1938-12-20
US 2084216 V-type brake for motor
vehicles. Poage Robert A. and Poage
Marlin Z. 1937-06-15
US 2028488 Brake. Avery William
Leicester 1936-02-21
US 1959049 Friction Brake. Buus Niels
Peter Valdemar 1934-05-15
US 1954534 Brake. Norton Raymond J
1934-04-10
 US 1721370 Brake for use on vehicles.
Boughton Edward Bishop 1929-07-16
DE 695921 Antriebsvorrichtung mit
hydraulischem Gestaenge.... Borgwar
Carl Friedrich Wilhelm 1940-09-06
GB 377478 Improvements in wheel
cylinders for hydraulic brakes. Hall
Frederick Harold 1932-07-28
GB 365069 Improvements in control
gear for hydraulically operated devices
and particularly brakes for vehicles.
Rubury John Meredith 1932-01-06

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