brake

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(brāk)

(mechanical engineering) A machine element for applying friction to a moving surface to slow it (and often, the containing vehicle or device) down or bring it to rest.



A disc brake assembly. Wheel rotation is slowed by friction when the hydraulic pistons squeeze the
(click to enlarge)
A disc brake assembly. Wheel rotation is slowed by friction when the hydraulic pistons squeeze the (credit: © Merriam-Webster Inc.)
Device for decreasing the speed of a body or stopping its motion. Most brakes act on rotating mechanical elements and absorb kinetic energy mechanically, hydrodynamically, or electrically. Mechanical brakes are the most common; they dissipate the kinetic energy as heat generated by mechanical friction between a rotating drum or disk and a stationary friction element. A hydrodynamic (fluid) brake has a rotor (rotating element) and a stator (stationary element). Resistance to rotation is created by fluid friction and circulation of the liquid (usually water) from a series of pockets in the rotor to a series of complementary pockets in the stator. air brake.

For more information on brake, visit Britannica.com.

A machine element for applying a force to a moving surface to slow it down or bring it to rest in a controlled manner. In doing so, it converts the kinetic energy of motion into heat which is dissipated into the atmosphere. Brakes are used in motor vehicles, trains, airplanes, elevators, and other machines. Most brakes are of a friction type in which a fixed surface is brought into contact with a moving part that is to be slowed or stopped.

Friction brakes are classified according to the kind of friction element employed and the means of applying the friction forces. See also Friction.

The single-block is the simplest form of brake. It consists of a short block fitted to the contour of a wheel or drum and pressed against its surface by means of a lever on a fulcrum, as widely used on railroad cars. The block may have the contour lined with friction-brake material, which gives long wear and a high coefficient of friction. The fulcrum may be located with respect to the lever in a manner to aid or retard the braking torque of the block. The lever may be operated manually or by a remotely controlled force (Fig. 1a).

Brakes. (<i>a</i>) Single-block brake. The block is fixed to the <a href=operating lever; force in the direction of the top arrow applies the brake. (b) Double-block brake. The blocks are pivoted on their levers; force in the direction of the arrow releases the brake. (c) External shoe brake. Shoes are lined with friction material. (d) Internal shoe brake with lining.">
Brakes. (a) Single-block brake. The block is fixed to the operating lever; force in the direction of the top arrow applies the brake. (b) Double-block brake. The blocks are pivoted on their levers; force in the direction of the arrow releases the brake. (c) External shoe brake. Shoes are lined with friction material. (d) Internal shoe brake with lining.

In double-block brakes, two single-blocks brake in symmetrical opposition, where the operating force on the end of one lever is the reaction of the other, make up a double-block brake (Fig. 1b). External thrust loads are balanced on the rim of the rotating wheel.

An external-shoe brake operates in the same manner as the block brake, and the designation indicates the application of externally contracting elements. In this brake the shoes are appreciably longer, extending over a greater portion of the drum (Fig. 1c). This construction allows more combinations for special applications than the simple shoe, although assumptions of uniform pressure and concentrated forces are no longer possible. In particular, it is used on elevator installations for locking the hoisting sheave by means of a heavy spring when the electric current is off and the elevator is at rest. See also Elevator.

An internal shoe brake has several advantages over an external shoe. Because the internal shoe works on the inner surface of the drum, it is protected from water and grit (Fig. 1d). It may be designed in a more compact package, is easily activated, and is effective for drives with rotations in both directions. The internal shoe is used in the automotive drum brake, with hydraulic piston actuation. See also Automotive brake.

Hoists, excavating machinery, and hydraulic clutch-controlled transmissions have band brakes. They operate on the same principle as flat belts on pulleys. In the simplest band brake, one end of the belt is fastened near the drum surface, and the other end is then pulled over the drum in the direction of rotation so that a lever on a fulcrum may apply tension to the belt.

Disk brakes have long been used on hoisting and similar apparatus. Because more energy is absorbed in prolonged braking than in clutch startup, additional heat dissipation must be provided in equivalent disk brakes. Disk brakes are used for the wheels of aircraft, where segmented rotary elements are pressed against stationary plates by hydraulic pistons. Flexibility, self-alignment, and rapid cooling are inherent in this design. Another application is the bicycle coaster brake.

The caliper disk brake (Fig. 2) is widely used on automotive vehicles. It consists of a rotating disk which can be gripped between two friction pads. The caliper disk brake is hydraulically operated, and the pads cover between one-sixth and one-ninth of the swept area of the disk. See also Automotive brake.

Caliper disk brake. (<i>a</i>) Friction pads on either side of a disk that is free to rotate. (<i>b</i>) Brake applied, hydraulic pressure forces the pistons toward the disk to stop its rotation and hold it stationary. (<i>Automotive & Technical Writing, Charlottesville, Virginia</i>)
Caliper disk brake. (a) Friction pads on either side of a disk that is free to rotate. (b) Brake applied, hydraulic pressure forces the pistons toward the disk to stop its rotation and hold it stationary. (Automotive & Technical Writing, Charlottesville, Virginia)

Railway brakes are normally applied air brakes; if the air coupling to a car is broken, the brakes are applied automatically. To apply the brakes, the brake operator releases the compressed air that is restraining the brakes by means of a diaphragm and linkage. Over-the-road trucks and buses use air brakes. Another form of air brake consists of an annular air tube surrounding a jointed brake lining that extends completely around the outside of a brake drum. Air pressure expands the tube, pressing the lining against the drum.


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noun

    An instrument or means of restraining: bit, bridle, leash, restraint, snaffle. See restraint/unrestraint.

verb

    To control, restrict, or arrest: bit, bridle, check, constrain, curb, hold, hold back, hold down, hold in, inhibit, keep, keep back, pull in, rein (back, in. or up), restrain. See restraint/unrestraint.


n

Definition: stopping device; check
Antonyms: accelerator

v

Definition: check; stop
Antonyms: accelerate, move, start

brake, in technology, device to slow or stop the motion of a mechanism or vehicle.

Types

Friction Brakes

Friction brakes, the most common kind, operate on the principle that friction can be used to convert the mechanical energy of a moving object into heat energy, which is absorbed by the brake. The essential components of a friction brake are a rotating part, such as a wheel, axle, disk, or brake drum, and a stationary part that is pressed against the rotating part to slow or stop it. The stationary part usually has a lining, called a brake lining, that can generate a great amount of friction yet give long wear; it formerly contained asbestos, but this is being replaced by less efficient materials for environmental reasons.

The principal types of friction brake are the block brake, the band brake, the internal-shoe brake, and the disk brake. The block brake consists of a block, the stationary part, that is shaped to fit the contour of a wheel or drum. For example, a wooden block applied to the rim of a wheel has long been used to slow or stop horse-drawn vehicles. A simple band brake consists of a metal band, the stationary part, that can be tightened around a drum by means of a lever. It is found on hoists and excavating machinery. The internal-shoe brake has a drum that contains two stationary semicircular pieces, or shoes, which slow or stop the motion of the drum by pressing against its inner surface. This is the type of brake most often found on automobiles, with an internal-shoe brake drum located on the central part of each wheel. A disk brake of the type used on automobiles has a metal disk and pistons with friction pads that can close on the disk and slow it.

Electric Brakes

A machine that is driven by an electric motor can sometimes use its motor as a brake. Because inertia keeps the machine's shafts moving after the current to the electric motor has been shut off, the machine keeps the motor's armature turning. While this is happening, if the motor's action can be changed to that of a generator, the electric current produced will be drawing its energy from the machine, thus slowing it. However, since such a braking method is not suitable for bringing the machine to a quick stop, it is usually supplemented by friction brakes.

Braking Systems

A manually operated brake pedal or handle is used to activate a brake. With low-power machinery or vehicles the operator can usually apply sufficient force through a simple mechanical linkage from the pedal or handle to the stationary part of the brake. In many cases, however, this force must be multiplied by using an elaborate braking system. In many modern braking systems there no longer is a direct connection between the pedal and the brake; a sensor is used register the force applied to the pedal, and that information is used to determine the pressure to apply to the brake. Automobile braking systems may also include an override that disables the accelerator when the brake is activated. An antilock braking system (ABS) uses sensors to identify when a wheel is locking and then applies and releases the brake automatically several times per second to prevent lockup. ABS can prevent skids, permitting controlled stops, and decreases the amount of time and distance needed to stop a car.

The Air Brake System

An early system for multiplying the braking force, called the air brake system, or air brake, was invented by American manufacturer George Westinghouse and was first used on passenger trains in 1868. It is now widely used on railroad trains. The fundamental principle involved is the use of compressed air acting through a piston in a cylinder to set block brakes on the wheels. The action is simultaneous on the wheels of all the cars in the train. The compressed air is carried through a strong hose from car to car with couplings between cars; its release to all the separate block brake units, at the same time, is controlled by the engineer. An automatic feature provides for the setting of all the block brakes in the event of damage to the brake hose, leakage, or damage to individual brake units. The air brake is used also on subway trains, trolley cars, buses, and trucks.

The Hydraulic Brake System

The hydraulic brake system, or hydraulic brake, is used on almost all automobiles (see hydraulic machine). When the brake pedal of an automobile is depressed, a force is applied to a piston in a master cylinder. The piston forces hydraulic fluid through metal tubing into a cylinder in each wheel where the fluid's pressure moves two pistons that press the brake shoes against the drum.

The Vacuum Brake System

The vacuum brake system, or vacuum brake, depends upon the use of a vacuum to force a piston in a cylinder to hold a brake shoe off a drum; when the vacuum is destroyed, the shoe is released and presses on the drum. In an automotive power brake system, extra pressure can be exerted on the hydraulic master cylinder piston by a vacuum brake's piston.


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A brake is a mechanical device which inhibits motion. Its opposite component is a clutch. The rest of this article is dedicated to various types of vehicular brakes.

Most commonly brakes use friction to convert kinetic energy 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 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 transform kinetic 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 increases quadratically with velocity (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 used the deployed undercarriage as an air brake.

Friction brakes on automobiles store braking heat in the drum brake or disc brake while braking then conduct it to the air gradually. When traveling downhill some vehicles can use their engines to brake.

When the brake pedal of a modern vehicle with hydraulic brakes is pushed, ultimately a piston pushes the brake pad against the brake disc which slows the wheel down. On the brake drum it is similar as the cylinder pushes the brake shoes against the drum which also slows the wheel down.

Contents

Types

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 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, PCC trolley brakes include a flat shoe which is clamped to the rail with an electromagnet; the Murphy brake pinches a rotating drum, and the Ausco 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.
  • 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 a Jake brake to greatly increase pumping losses. Pumping brakes can dump energy as heat, or can be regenerative brakes that recharge a pressure reservoir called a hydraulic 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 a regenerative 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 are eddy current brakes, and electro-mechanical brakes (which actually are magnetically driven friction brakes, but nowadays are often just called “electromagnetic brakes” as well).

Characteristics

Brakes are often described according to several characteristics including:

  • 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" brake.
  • Pedal feel – Brake pedal feel encompasses subjective perception of brake power output as a function of pedal travel. Pedal travel is influenced by the fluid displacement of the brake and other factors.
  • Drag – Brakes have varied amount of drag in the off-brake condition depending on design of the system to accommodate total system compliance and deformation that exists under braking with ability to retract friction material from the rubbing surface in the off-brake condition.
  • Durability – Friction brakes have wear surfaces that must be renewed periodically. Wear surfaces include the brake shoes or pads, and also the brake disc or drum. There may be tradeoffs, for example a wear surface that generates high peak force may also wear quickly.
  • Weight – Brakes are often "added weight" in that they serve no other function. Further, brakes are often mounted on wheels, and unsprung weight can significantly hurt traction in some circumstances. "Weight" may mean the brake itself, or may include additional support structure.
  • Noise – Brakes usually create some minor noise when applied, but often create squeal or grinding noises that are quite loud.

Brake boost

Most modern vehicles use a vacuum assisted brake system that greatly increases the force applied to the vehicle's brakes by its operator.[1] 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

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.[2] Noise can be caused by different things. These are signs that there may be issues with brakes wearing out over time.

Inefficiency

A significant amount of energy is always lost while braking, even with regenerative braking which is not perfectly efficient. Therefore a good metric of efficient 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 the fuel 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 impact fuel economy and vehicle performance.

See also

References

External links


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Dansk (Danish)
1.
n. - bremse
v. tr. - bremse
v. intr. - bremse

2.
n. - brage, brydebænk
v. tr. - brage, bryde

3.
n. - stang

4.
n. - bregne

5.
n. - krat, bevoksning

Nederlands (Dutch)
rem, vertragende factor, stationwagen, adelaarsvaren, kreupelhout, eg, remmen, afremmen, (vlas)braken

Français (French)
1.
n. - (Aut, etc) frein, (fig) frein à
v. tr. - (lit, fig) freiner
v. intr. - (lit, fig) freiner

2.
n. - fourche (pour broyer le lin)
v. tr. - (Tex) broyer (le lin, etc)

3.
n. - pompe

4.
n. - (Bot) fougère

5.
n. - fourré

Deutsch (German)
1.
n. - Bremse, Bremspedal
v. - bremsen

2.
n. - (Tech) Flachsbreche, Bracke
v. - (Flachs) brechen

3.
n. - Pumpenschwengel, Hebelarm

4.
n. - Adlerfarn

5.
n. - Unterholz, Dickicht

Ελληνική (Greek)
v. - φρενάρω, πατώ φρένο
n. - φρένο, (τροχο)πέδη

idioms:

  • air brake    αερόφρενο

Italiano (Italian)
frenare, freno a mano, pedale del freno, freno

Português (Portuguese)
v. - frear, retardar
n. - freio (m)

Русский (Russian)
тормозить, тормоз, тормоза

Español (Spanish)
1.
n. - freno, pedal del freno
v. tr. - frenar
v. intr. - frenarse

2.
n. - agramadera
v. tr. - agramar

3.
n. - manija de bomba

4.
n. - tipo de helecho

5.
n. - matorral

Svenska (Swedish)
v. - bromsa, bråka
n. - broms, bromspedal, buskage, bräken

中文(简体)(Chinese (Simplified))
1. 煞车, 煞住, 抑制, 约束, 制动

2. 揉碎机, 揉面机, 麻梳, 重型耙, 揉碎

3. 阻碍, 约束

4. 草丛, 灌木丛

5. 蕨, 羊齿类

6. 泵柄, 泵杆

中文(繁體)(Chinese (Traditional))
1.
n. - 草叢, 灌木叢

2.
n. - 泵柄, 泵杆

3.
n. - 蕨, 羊齒類

4.
n. - 煞車
v. tr. - 煞住, 抑制, 約束
v. intr. - 煞車, 制動

5.
n. - 阻礙, 約束

6.
n. - 揉碎機, 揉面機, 麻梳, 重型耙
v. tr. - 揉碎

한국어 (Korean)
1.
n. - 브레이크
v. tr. - ~에 브레이크를 걸다
v. intr. - 브레이크를 걸다

2.
n. - 아마를 으깨어 섬유를 분리해 내는 기구
v. tr. - ~을 으깨어 섬유를 분리해 내다

3.
n. - 펌프의 자루

4.
n. - 양치류

5.
n. - 덤불

日本語 (Japanese)
n. - ブレーキ, 制動機, 歯止め, やぶ, ワラビ
v. - ブレーキをかける

العربيه (Arabic)
‏(فعل) أوقف, كبح, فرمل (الاسم) الكابح, البريك, الفرامل, جهاز أيقاف السيارة, سيارة كبيرة‏

עברית (Hebrew)
n. - ‮בלם, מעצור, סבך, איזור שיחים, כרכרה‬
v. tr. - ‮בלם, עצר‬
v. intr. - ‮בלם, עצר‬
n. - ‮כלי לניפוץ פשתן וקנבוס‬
v. tr. - ‮ניפץ (פשתן)‬
n. - ‮סוג של ידית-משאבה‬
n. - ‮סוג של שרך‬
n. - ‮סבך-יער‬


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