Hydraulic brake

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A schematic illustrating the major components of a hydraulic disc brake system

The hydraulic brake is an arrangement of braking mechanism which uses brake fluid, typically

containing ethylene glycol, to transfer pressure from the controlling unit, which is usually near the operator of
the vehicle, to the actual brake mechanism, which is usually at or near the wheel of the vehicle.

Contents
 [hide]

1 Construction

2 System Operation

3 Component

specifics

o 3.1 Power

brakes

4 Special

considerations

5 See also

6 External links

7 Patents

8 References
[edit]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) ordrum attached to a 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 disc brakes on the front wheels and drum brakes on the
rear wheels. However, because disc brakes have been shown a better stopping performance and are therefore
generally safer and more effective than drum brakes, four-wheel disc brakes have become increasingly
popular, replacing drums on all but the most basic vehicles. Many two-wheel vehicles designs, however,
continue to employ a drum brake for the rear wheel.

For simplicity, the braking system described hereafter uses the terminology and configuration for a simple disc
brake.

[edit]System Operation

Within a hydraulic brake system, as the brake pedal is pressed/ brake lever is squeezed, 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 which results in an increase in the pressure of the entire hydraulic
system. This forces fluid through the hydraulic lines toward one or more calipers where it acts upon one or two
additional caliper pistons secured by one or more seated O-rings which prevent the escape of any fluid from
around the piston.

The brake caliper piston(s) then apply force to the brake pads. This causes them to be pushed 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 from this friction is often dissipated through vents and channels in the rotor
and through the pads themselves which are made of specialized heat-tolerant materials (kevlar,sintered glass,
et al.).
Subsequent release of the brake pedal/ lever allows the spring(s) within the master cylinder assembly to return
that assembly's piston(s) back into position. This relieves the hydraulic pressure on the caliper allowing the
brake piston in the caliper assembly to slide back into its housing and the brake pads to release the rotor.
Unless there is a leak somewhere in the system, at no point does any of the brake fluid enter or leave.

[edit]Component specifics

(For typical light duty automotive braking systems)

The brake pedal is a simple lever. One end is attached to the framework of the vehicle, a pushrod extends
from a point along its length, and the foot pad is at the other end of the lever. The rod either extends to the
master cylinder (manual brakes) or to the vacuum booster (power brakes).

In a four-wheel car, the master cylinder is divided internally into two sections, each of which pressurizes a
separate hydraulic circuit. Each section supplies pressure to one circuit. 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 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 speed reduction) remains intact to stop the mechanically-damaged vehicle. Just before 1970, diagonally
split systems had become universal for automobiles sold in the United States.

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.

[edit]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; these have negligible expansion
under pressure and can give a firmer feel to the brake pedal with less pedal travel for a given braking effort.

[edit]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. If it gets into the brake lines, it
can degrade brake performance dramatically. This is why light oils are often used as hydraulic fluids. Oil
displaces water, protects plastic parts against corrosion, and can tolerate much higher temperatures before
vaporizing.

"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, vaporization of the
hydraulic fluid under temperature extremes, and 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.