Abrake is amechanical device that inhibits motion by absorbing energy from a moving system.[1] It is used forslowing or stopping a moving vehicle, wheel, axle, or to prevent its motion, most often accomplished by means of friction.[2]
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Most brakes commonly usefriction between two surfaces pressed together to convert thekinetic energy of the moving object intoheat, though other methods of energy conversion may be employed. For example,regenerative braking converts much of the energy toelectrical energy, which may be stored for later use. Other methods convertkinetic energy intopotential energy in such stored forms aspressurized air or pressurized oil.Eddy current brakes use magnetic fields to convert kinetic energy into electric current in the brake disc, fin, or rail, which is converted into heat. Still other braking methods even transformkinetic energy into different forms, for example by transferring the energy to a rotating flywheel.
Brakes are generally applied to rotating axles or wheels, but may also take other forms such as the surface of a moving fluid (flaps deployed into water or air). Some vehicles use a combination of braking mechanisms, such as drag racing cars with both wheel brakes and a parachute, or airplanes with both wheel brakes and drag flaps raised into the air during landing.
Since kinetic energy increasesquadratically withvelocity (), 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 theoreticalbraking distance, when braking at the traction limit, is up to 100 times as long. In practice, fast vehicles usually have significant air drag, and energy lost to air drag rises quickly with speed.
Almost allwheeledvehicles have a brake of some sort. Evenbaggage carts andshopping carts may have them for use on amoving ramp. Mostfixed-wing aircraft are fitted withwheel brakes on theundercarriage. Some aircraft also featureair brakes designed to reduce their speed in flight. Notable examples includegliders and someWorld War II-era aircraft, primarily somefighter aircraft and manydive bombers of the era. These allow the aircraft to maintain a safe speed in a steep descent. TheSaab B 17dive bomber andVought F4U Corsair fighter used the deployed undercarriage as an air brake.
Friction brakes onautomobiles store braking heat in thedrum brake ordisc brake while braking then conduct it to theair gradually. When traveling downhill some vehicles canuse their engines to brake.
When the brakepedal of a modern vehicle withhydraulic brakes is pushed against themaster cylinder, ultimately apiston pushes thebrake pad against thebrake disc which slows the wheel down. On thebrake drum it is similar as the cylinder pushes thebrake shoes against the drum which also slows the wheel down.
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 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 aband 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,PCC trolley brakes include a flat shoe which is clamped to the rail with an electromagnet; the Murphy brake pinches a rotating drum, and theAusco Lambert disc brake uses a hollow disc (two parallel discs with a structural bridge) with shoes that sit between the disc surfaces and expand laterally.
Adrum brake is a vehicle brake in which the friction is caused by a set ofbrake shoes that press against the inner surface of a rotating drum. The drum is connected to the rotating roadwheel hub.
Drum brakes generally can be found on older car and truck models. However, because of their low production cost, drum brake setups are also installed on the rear of some low-cost newer vehicles. Compared to modern disc brakes, drum brakes wear out faster due to their tendency to overheat.
Thedisc brake is a device for slowing or stopping the rotation of a road wheel. A brake disc (or rotor in U.S. English), usually made ofcast iron orceramic, is connected to the wheel or the axle. To stop the wheel,friction material in the form ofbrake pads (mounted in a device called abrake caliper) is forcedmechanically,hydraulically,pneumatically orelectromagnetically against both sides of the disc. Friction causes the disc and attached wheel to slow or stop.
Pumping brakes are often used where a pump is already part of the machinery. For example, an internal-combustion piston motor can have the fuel supply stopped, and then internal pumping losses of the engine create some braking. Some engines use a valve override called aJake brake to greatly increase pumping losses. Pumping brakes can dump energy as heat, or can beregenerative brakes that recharge a pressure reservoir called ahydraulic accumulator.
Electromagnetic brakes are likewise often used where an electric motor is already part of the machinery. For example, many hybrid gasoline/electric vehicles use the electric motor as a generator to charge electric batteries and also as aregenerative brake. Some diesel/electric railroad locomotives use the electric motors to generate electricity which is then sent to a resistor bank and dumped as heat. Some vehicles, such as some transit buses, do not already have an electric motor but use a secondary "retarder" brake that is effectively a generator with an internal short circuit. Related types of such a brake areeddy current brakes, andelectro-mechanical brakes (which actually are magnetically driven friction brakes, but nowadays are often just called "electromagnetic brakes" as well).
Electromagnetic brakes slow an object throughelectromagnetic induction, which createsresistance and in turn either heat or electricity. Friction brakes apply pressure on two separate objects to slow the vehicle in a controlled manner.
Brakes are often described according to several characteristics including:
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.[3]
Most modern passenger vehicles, and light vans, use avacuum assisted brake system that greatly increases the force applied to the vehicle's brakes by its operator.[4] This additional force is supplied by themanifold 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 - unlessleft-foot braking is used.
Because of low vacuum at high RPM, reports ofunintended 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.
Heavier road vehicles, as well as trains, usually boost brake power withcompressed air, supplied by one or more compressors.
Although ideally a brake would convert all the kinetic energy into heat, in practice a significant amount may be converted intoacoustic energy instead, contributing tonoise pollution.
For road vehicles, the noise produced varies significantly withtire construction,road surface, and the magnitude of the deceleration.[5] Noise can be caused by different things. These are signs that there may be issues with brakes wearing out over time.
Railway brake malfunctions can produce sparks and causeforest fires.[6] In some very extreme cases, disc brakes can become red hot and set on fire. This happened in the Tuscan GP, when the Mercedes car, the W11 had its front carbon disc brakes almost bursting into flames, due to low ventilation and high usage.[7] These fires can also occur on someMercedes Sprinter vans, when the load adjusting sensor seizes up and the rear brakes have to compensate for the fronts.[8]
A significant amount of energy is always lost while braking, even withregenerative braking which is not perfectlyefficient. Therefore, a good metric ofefficient energy use while driving is to note how much one is braking. If the majority of deceleration is from unavoidable friction instead of braking, one is squeezing out most of the service from the vehicle. Minimizing brake use is one of thefuel economy-maximizing behaviors.
While energy is always lost during a brake event, a secondary factor that influences efficiency is "off-brake drag", or drag that occurs when the brake is not intentionally actuated. After a braking event, hydraulic pressure drops in the system, allowing the brake caliper pistons to retract. However, this retraction must accommodate all compliance in the system (under pressure) as well as thermal distortion of components like the brake disc or the brake system will drag until the contact with the disc, for example, knocks the pads and pistons back from the rubbing surface. During this time, there can be significant brake drag. This brake drag can lead to significant parasitic power loss, thus impacting fuel economy and overall vehicle performance.
Due to the high cost of traditional manufacturing techniques for brake pad production, it requires a significant amount of development and production time, including multiple precision machining processes. To avoid these limitations, 3D printing technology can be used to manufacture. This method can increase some economic costs and improve environmental protection.[9]
In the 1890s, Wooden block brakes became obsolete when Michelin brothers introduced rubber tires.[10]
During the 1960s, some car manufacturers replaced drum brakes with disc brakes.[10]
In 1966, theABS was fitted in theJensen FF grand tourer.[10]
In 1978, Bosch and Mercedes updated their 1936 anti-lock brake system for theMercedes S-Class. That ABS is a fully electronic, four-wheel and multi-channel system that later became standard.[10]
In 2005, ESC — which automatically applies the brakes to avoid a loss of steering control — become compulsory for carriers of dangerous goods without data recorders in the Canadian province of Quebec.[11]
Since 2017, numerousUnited Nations Economic Commission for Europe (UNECE) countries useBrake Assist System (BAS) a function of the braking system that deduces an emergency braking event from a characteristic of the driver's brake demand and under such conditions assist the driver to improve braking.[12]
In July 2013[12] UNECE vehicle regulation 131 was enacted. This regulation definesAdvanced Emergency Braking Systems (AEBS) for heavy vehicles to automatically detect a potential forward collision and activate the vehicle braking system.
On 23 January 2020[12] UNECE vehicle regulation 152 was enacted, defining Advanced Emergency Braking Systems for light vehicles.
From May 2022, in the European Union, by law, new vehicles will have advanced emergency-braking system.[13]
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