A brake is a mechanical device that inhibits motion by absorbing energy from a moving system. It is used for slowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction.
1 Background 2 Types
2.1 Frictional 2.2 Pumping 2.3 Electromagnetic
3.1 Foundation components
4 Noise 5 Fires 6 Inefficiency 7 See also 8 References 9 External links
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Most brakes commonly use friction between two surfaces pressed
together to convert the kinetic energy of the moving object into heat,
though other methods of energy conversion may be employed. For
example, regenerative braking converts much of the energy to
electrical energy, which may be stored for later use. Other methods
convert kinetic energy into potential energy in such stored forms as
pressurized air or pressurized oil.
Eddy current brakes
K = m
displaystyle K=mv^ 2 /2
), an object moving at 10 m/s has 100 times as much energy as one
of the same mass moving at 1 m/s, and consequently the
theoretical braking distance, when braking at the traction limit, is
100 times as long. In practice, fast vehicles usually have significant
air drag, and energy lost to air drag rises quickly with speed.
Almost all wheeled vehicles have a brake of some sort. Even baggage
carts and shopping carts may have them for use on a moving ramp. Most
fixed-wing aircraft are fitted with wheel brakes on the undercarriage.
Some aircraft also feature air brakes designed to reduce their speed
in flight. Notable examples include gliders and some World War II-era
aircraft, primarily some fighter aircraft and many dive bombers of the
era. These allow the aircraft to maintain a safe speed in a steep
descent. The Saab B 17 dive bomber and
Vought F4U Corsair
Rendering of a drum brake
Single pivot side-pull bicycle caliper brake.
Brakes may be broadly described as using friction, pumping, or electromagnetics. One brake may use several principles: for example, a pump may pass fluid through an orifice to create friction: Frictional
typical braking system for cars:
Frictional brakes are most common and can be divided broadly into
"shoe" or "pad" brakes, using an explicit wear surface, and
hydrodynamic brakes, such as parachutes, which use friction in a
working fluid and do not explicitly wear. Typically the term "friction
brake" is used to mean pad/shoe brakes and excludes hydrodynamic
brakes, even though hydrodynamic brakes use friction. Friction
(pad/shoe) brakes are often rotating devices with a stationary pad and
a rotating wear surface. Common configurations include shoes that
contract to rub on the outside of a rotating drum, such as a band
brake; a rotating drum with shoes that expand to rub the inside of a
drum, commonly called a "drum brake", although other drum
configurations are possible; and pads that pinch a rotating disc,
commonly called a "disc brake". Other brake configurations are used,
but less often. For example,
Peak force – The peak force is the maximum decelerating effect that
can be obtained. The peak force is often greater than the traction
limit of the tires, in which case the brake can cause a wheel skid.
Continuous power dissipation – Brakes typically get hot in use, and
fail when the temperature gets too high. The greatest amount of power
(energy per unit time) that can be dissipated through the brake
without failure is the continuous power dissipation. Continuous power
dissipation often depends on e.g., the temperature and speed of
ambient cooling air.
Fade – As a brake heats, it may become less effective, called brake
fade. Some designs are inherently prone to fade, while other designs
are relatively immune. Further, use considerations, such as cooling,
often have a big effect on fade.
Smoothness – A brake that is grabby, pulses, has chatter, or
otherwise exerts varying brake force may lead to skids. For example,
railroad wheels have little traction, and friction brakes without an
anti-skid mechanism often lead to skids, which increases maintenance
costs and leads to a "thump thump" feeling for riders inside.
Power – Brakes are often described as "powerful" when a small human
application force leads to a braking force that is higher than typical
for other brakes in the same class. This notion of "powerful" does not
relate to continuous power dissipation, and may be confusing in that a
brake may be "powerful" and brake strongly with a gentle brake
application, yet have lower (worse) peak force than a less "powerful"
Pedal feel –
Foundation components are the brake-assembly components at the wheels
of a vehicle, named for forming the basis of the rest of the brake
system. These mechanical parts contained around the wheels are
controlled by the air brake system.
The three types of foundation brake systems are “S” cam brakes,
disc brakes and wedge brakes.
Most modern vehicles use a vacuum assisted brake system that greatly increases the force applied to the vehicle's brakes by its operator. This additional force is supplied by the manifold vacuum generated by air flow being obstructed by the throttle on a running engine. This force is greatly reduced when the engine is running at fully open throttle, as the difference between ambient air pressure and manifold (absolute) air pressure is reduced, and therefore available vacuum is diminished. However, brakes are rarely applied at full throttle; the driver takes the right foot off the gas pedal and moves it to the brake pedal - unless left-foot braking is used. Because of low vacuum at high RPM, reports of unintended acceleration are often accompanied by complaints of failed or weakened brakes, as the high-revving engine, having an open throttle, is unable to provide enough vacuum to power the brake booster. This problem is exacerbated in vehicles equipped with automatic transmissions as the vehicle will automatically downshift upon application of the brakes, thereby increasing the torque delivered to the driven-wheels in contact with the road surface. Noise
Main article: Roadway noise
Although ideally a brake would convert all the kinetic energy into
heat, in practice a significant amount may be converted into acoustic
energy instead, contributing to noise pollution.
For road vehicles, the noise produced varies significantly with tire
construction, road surface, and the magnitude of the deceleration.
Noise can be caused by different things. These are signs that there
may be issues with brakes wearing out over time.
^ Bhandari, V.B. (2010). Design of machine elements. Tata McGraw-Hill. p. 472. ISBN 9780070681798. Retrieved 9 February 2016. ^ "Definition of brake". The Collins English Dictionary. Retrieved 9 February 2016. ^ "Foundation Brakes". ontario.ca. Retrieved 2017-07-22. ^ Nice, Karim (2000-08-22). "How Power Brakes Work". Howstuffworks.com. Retrieved 2011-03-12. ^ C.Michael Hogan, Analysis of highway noise, Journal of Water, Air, & Soil Pollution, Volume 2, Number 3, Biomedical and Life Sciences and Earth and Environmental Science Issue, Pages 387-392, September, 1973, Springer Verlag, Netherlands ISSN 0049-6979 ^ David Hench (May 8, 2014). "Train-sparked fires cause explosions, destroy trailers, force evacuations". Portland Press Herald.
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