Brake

# Brake

Overview

Discussion

Encyclopedia

A brake is a mechanical device which inhibits motion. Its opposite component is a clutch
Clutch
A clutch is a mechanical device which provides for the transmission of power from one component to another...

Most commonly brakes use friction
Friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and/or material elements sliding against each other. There are several types of friction:...

to convert kinetic energy
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

into heat
Heat
In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...

, 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
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

into potential energy
Potential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...

in such stored forms as pressurized air
Compressed air energy storage
Compressed Air Energy Storage is a way to store energy generated at one time for use at another time. At utility scale, energy generated during periods of low energy demand can be released to meet higher demand periods....

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
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...

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.

A quadratic function, in mathematics, is a polynomial function of the formf=ax^2+bx+c,\quad a \ne 0.The graph of a quadratic function is a parabola whose axis of symmetry is parallel to the y-axis....

with velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...

(), an object traveling at 10 meters per second has 100 times as much energy as one traveling at 1 meter per second, and consequently the theoretical braking distance
Braking distance
Braking distance refers to the distance a vehicle will travel from the point where its brakes are fully applied to when it comes to a complete stop...

, 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 wheel
Wheel
A wheel is a device that allows heavy objects to be moved easily through rotating on an axle through its center, facilitating movement or transportation while supporting a load, or performing labor in machines. Common examples found in transport applications. A wheel, together with an axle,...

ed vehicle
Vehicle
A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....

s have a brake of some sort. Even baggage cart
Baggage cart
Baggage carts, luggage carts or trolleys are small vehicles pushed by travelers to carry individual luggage, mostly suitcases. There are two major sizes: One for big luggage and one for small luggage...

s and shopping cart
Shopping cart
A shopping cart is a cart supplied by a shop, especially supermarkets, for use by customers inside the shop for transport of merchandise to the check-out counter during shopping...

s may have them for use on a moving ramp. Most fixed-wing aircraft
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...

are fitted with wheel brakes on the undercarriage
Undercarriage
The undercarriage or landing gear in aviation, is the structure that supports an aircraft on the ground and allows it to taxi, takeoff and land...

. Some aircraft also feature air brakes
Air brake (aircraft)
In aeronautics, air brakes or speedbrakes are a type of flight control surface used on an aircraft to increase drag or increase the angle of approach during landing....

designed to reduce their speed in flight. Notable examples include glider
Glider aircraft
Glider aircraft are heavier-than-air craft that are supported in flight by the dynamic reaction of the air against their lifting surfaces, and whose free flight does not depend on an engine. Mostly these types of aircraft are intended for routine operation without engines, though engine failure can...

s and some World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

-era aircraft, primarily some fighter aircraft
Fighter aircraft
A fighter aircraft is a military aircraft designed primarily for air-to-air combat with other aircraft, as opposed to a bomber, which is designed primarily to attack ground targets...

and many dive bombers of the era. These allow the aircraft to maintain a safe speed in a steep descent. The Saab B 17
Saab 17
-References:* Jane, Fred T. “The Saab-17.” Jane’s Fighting Aircraft of World War II. London: Studio, 1946. ISBN 1-85170-493-0....

dive bomber
Dive bomber
A dive bomber is a bomber aircraft that dives directly at its targets in order to provide greater accuracy for the bomb it drops. Diving towards the target reduces the distance the bomb has to fall, which is the primary factor in determining the accuracy of the drop...

used the deployed undercarriage as an air brake.

Friction brakes on automobile
Automobile
An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

s store braking heat in the drum brake
Drum brake
A drum brake is a brake in which the friction is caused by a set of shoes or pads that press against a rotating drum-shaped part called a brake drum....

or disc brake
Disc brake
The disc brake or disk brake is a device for slowing or stopping the rotation of a wheel while it is in motion.A brake disc is usually made of cast iron, but may in some cases be made of composites such as reinforced carbon–carbon or ceramic matrix composites. This is connected to the wheel and/or...

while braking then conduct it to the air gradually. When traveling downhill some vehicles can use their engines to brake
Engine braking
Engine braking is where the retarding forces within an engine are used to slow a vehicle down, as opposed to using an external braking mechanism, for example friction brakes or magnetic brakes....

.

When the brake pedal
Automobile pedal
An automobile may have two to four foot pedals. The arrangement is the same for both right- and left-hand traffic. From left to right:* normally operated by the left foot:**clutch pedal, not in the case of automatic transmission...

of a modern vehicle with hydraulic brake
Hydraulic brake
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...

s is pushed, ultimately a piston
Piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...

pushes the brake pad against the brake disc
Disc brake
The disc brake or disk brake is a device for slowing or stopping the rotation of a wheel while it is in motion.A brake disc is usually made of cast iron, but may in some cases be made of composites such as reinforced carbon–carbon or ceramic matrix composites. This is connected to the wheel and/or...

which slows the wheel down. On the brake drum it is similar as the cylinder pushes the brake shoe
Brake shoe
A brake shoe is the part of a braking system which carries the brake lining in the drum brakes used on automobiles, or the brake block in train brakes and bicycle brakes.-Automobile drum brake:...

s against the drum which also slows the wheel down.

## 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
Band brake
A band brake is a primary or secondary brake, consisting of a band of friction material that tightens concentrically around a cylindrical piece of equipment to either prevent it from rotating , or to slow it . This application is common on winch drums and chain saws and is also used for some...

; a rotating drum with shoes that expand to rub the inside of a drum, commonly called a "drum brake
Drum brake
A drum brake is a brake in which the friction is caused by a set of shoes or pads that press against a rotating drum-shaped part called a brake drum....

", although other drum configurations are possible; and pads that pinch a rotating disc, commonly called a "disc brake
Disc brake
The disc brake or disk brake is a device for slowing or stopping the rotation of a wheel while it is in motion.A brake disc is usually made of cast iron, but may in some cases be made of composites such as reinforced carbon–carbon or ceramic matrix composites. This is connected to the wheel and/or...

". 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
Murphy brake
The Murphy brake is a drum brake with shoes that pinch the drum via a rotating fork. In contrast, most drum brakes use either shoes that press only on the inside of the drum, or shoes that press only on the outside of the drum, like a band brake. Like many drum brakes, the Murphy brake is...

pinches a rotating drum, and the Ausco Lambert disc brake
Ausco Lambert disc brake
The Ausco Lambert disc brake is an unusual brake where an axially-expanding shoe assembly is sandwiched between two linked rotating discs. It may be thought of as an "inside out" disc brake: instead of pads pinching a disc, the pads expand inside a hollow disc.-History:Ausco Lambert brakes were...

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
Jake brake
A compression release engine brake, frequently called a Jake brake or Jacobs brake, is an engine braking mechanism installed on some diesel engines...

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
Hydraulic accumulator
A 'hydraulic accumulator' is an energy storage device. It is a pressure storage reservoir in which a non-compressible hydraulic fluid is held under pressure by an external source. That external source can be a spring, a raised weight, or a compressed gas...

.

• 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
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...

(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
Vehicle braking system fade, or brake fade, is the reduction in stopping power that can occur after repeated or sustained application of the brakes, especially in high load or high speed conditions...

. 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
Unsprung weight
In a ground vehicle with a suspension, the unsprung weight is the mass of the suspension, wheels or tracks , and other components directly connected to them, rather than supported by the suspension...

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
Vacuum servo
A vacuum servo is a component used on motor vehicles in their braking system, to provide assistance to the driver by decreasing the braking effort. In the USA it is commonly called a brake booster.-Background:...

system that greatly increases the force applied to the vehicle's brakes by its operator. This additional force is supplied by the manifold vacuum
Manifold vacuum
Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere....

generated by the running engine. This force is greatly reduced when the engine is running at full throttle, as the 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
Left-foot braking
Left-foot braking is the technique of using the left foot to operate the brake pedal in an automobile, leaving the right foot dedicated to the throttle pedal...

is used.

Because of low vacuum at high RPM, reports of unintended acceleration
Sudden unintended acceleration
Sudden Unintended Acceleration is the unintended, unexpected, uncontrolled acceleration of a vehicle from a stationary position, low initial speed or at cruising speed, often accompanied by an apparent loss of braking effectiveness...

are often accompanied by complaints of failed or weakened brakes, as the high-revving engine 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 further elevating engine RPM and reducing available braking power while increasing the engine's effective torque.

## 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
Noise pollution
Noise pollution is excessive, displeasing human, animal or machine-created environmental noise that disrupts the activity or balance of human or animal life...

.

For road vehicles, the noise produced varies significantly with tire
Tire
A tire or tyre is a ring-shaped covering that fits around a wheel rim to protect it and enable better vehicle performance by providing a flexible cushion that absorbs shock while keeping the wheel in close contact with the ground...

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.

## Inefficiency

A significant amount of energy is always lost while braking, even with regenerative braking which is not perfectly efficient
Energy conversion efficiency
Energy conversion efficiency is the ratio between the useful output of an energy conversion machine and the input, in energy terms. The useful output may be electric power, mechanical work, or heat.-Overview:...

. Therefore a good metric of efficient energy use
Efficient energy use
Efficient energy use, sometimes simply called energy efficiency, is the goal of efforts to reduce the amount of energy required to provide products and services. For example, insulating a home allows a building to use less heating and cooling energy to achieve and maintain a comfortable temperature...

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
Fuel economy-maximizing behaviors
Fuel economy-maximizing behaviors describe techniques that drivers can use to optimize their automobile fuel economy. The energy in fuel consumed in driving is lost in many ways, including engine inefficiency, aerodynamic drag, rolling friction, and kinetic energy lost to braking...

.

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.

• Anti-lock braking system
Anti-lock braking system
An anti-lock braking system is a safety system that allows the wheels on a motor vehicle to continue interacting tractively with the road surface as directed by driver steering inputs while braking, preventing the wheels from locking up and therefore avoiding skidding.An ABS generally offers...

• Archaic past tense of the verb 'to break' (see brake)
• Air brake (rail)
Air brake (rail)
An air brake is a conveyance braking system actuated by compressed air. Modern trains rely upon a fail-safe air brake system that is based upon a design patented by George Westinghouse on March 5, 1872. The Westinghouse Air Brake Company was subsequently organized to manufacture and sell...

Air brakes are used in trucks, buses, trailers, and semi-trailers. George Westinghouse first developed air brakes for use in railway service. He patented a safer air brake on March 5, 1872. Originally designed and built for use on railroad train application, air brakes remain the exclusive systems...

• Anchor
Anchor
An anchor is a device, normally made of metal, that is used to connect a vessel to the bed of a body of water to prevent the vessel from drifting due to wind or current. The word derives from Latin ancora, which itself comes from the Greek ἄγκυρα .Anchors can either be temporary or permanent...

• Bicycle brake systems
Bicycle brake systems
A bicycle brake is used to slow down or stop a bicycle. There have been various types of brake used throughout history, and several are still in use today...

• Brake-by-wire
Brake-by-wire
Drive-by-wire technology in automotive industry replaces the traditional mechanical and hydraulic control systems with electronic control systems using electromechanical actuators and human-machine interfaces such as pedal and steering feel emulators...

(or electromechanical braking)
• Brake lining
Brake lining
Brake linings are the consumable surfaces in brake systems, such as drum brakes and disc brakes used in transport vehicles.-History:Brake linings were invented by Bertha Benz during her historic first long distance car trip in the world in August 1888.-Structure and function:Brake linings are...

• Brake bleeding
Brake bleeding
Brake bleeding is the procedure performed on hydraulic brake systems whereby the brake lines are purged of any air bubbles...

• Breeching (tack)
Breeching (tack)
Breeching is a strap around the haunches of a draft, pack or riding animal. Both under saddle and in harness, breeching engages when an animal slows down or travels downhill and is used to brake or stabilize a load.-Harness breeching:...

• Bundy tube
Bundy tube
Bundy tube, sometimes called Bundy pipe, is type of double-walled low-carbon steel tube manufactured by rolling a copper-coated steel strip through 720 degrees and resistance brazing the overlapped seam in a process called Bundywelding. It may be zinc- or terne- coated for corrosion protection...

• Counter-pressure brake
Counter-pressure brake
The counter-pressure brake , also named the Riggenbach counter-pressure brake after its inventor, Niklaus Riggenbach, is a dynamic railway brake on steam locomotives that, through a combination of means, brakes the locomotive in the driving cylinders...

• Dynamic braking
Dynamic braking
Dynamic braking is the use of the electric traction motors of a railroad vehicle as generators when slowing the Locomotive. It is termed rheostatic if the generated electrical power is dissipated as heat in brake grid resistors, and regenerative if the power is returned to the supply line...

• Electromagnetic brake
Electromagnetic brake
An eddy current brake, like a conventional friction brake, is responsible for slowing an object, such as a train or a roller coaster. However, unlike electro-mechanical brakes, which apply mechanical pressure on two separate objects, eddy current brakes slow an object by creating eddy currents...

• Electronic Parking Brake
• Hand brake
Hand brake
In cars, the hand brake is a latching brake usually used to keep the car stationary, and in manual transmission vehicles, as an aid to starting the vehicle from stopped when going up an incline - with one foot on the clutch , the other on the accelerator In cars, the hand brake (emergency brake,...

• Band brake
Band brake
A band brake is a primary or secondary brake, consisting of a band of friction material that tightens concentrically around a cylindrical piece of equipment to either prevent it from rotating , or to slow it . This application is common on winch drums and chain saws and is also used for some...

• Line lock
Line lock
A line lock is a device that allows the front brakes to lock independently of the rear brakes via a switch. The device is an electric solenoid that controls a valve which is placed on the front brake line, and is controlled by a switch...

• Overrun brake
Overrun brake
An overrun brake is a brake system commonly used on small trailers, where the motion of the trailer with respect to the towing vehicle is used to actuate the brake.A spring enables the drawbar to move back and forth slightly relative to the trailer chassis...

• Parking brake
• Railway brake
• Threshold braking
Threshold braking
Threshold braking or limit braking is a driving technique most commonly used in motor racing, but also practiced in road vehicles to slow a vehicle at the optimum rate using the brakes....

• Trail braking
Trail braking
Trail braking is a motorcycle riding and driving technique where the brakes are used beyond the entrance to a turn and are gradually released up to the point of apex....

• Vacuum brake
Vacuum brake
The vacuum brake is a braking system employed on trains and introduced in the mid-1860s. A variant, the automatic vacuum brake system, became almost universal in British train equipment and in those countries influenced by British practice. Vacuum brakes also enjoyed a brief period of adoption in...