An air brake or, more formally, a compressed-air-brake system, is a type of friction

brake 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. Background ...

for vehicles in which

compressed air Compressed air is air kept under a pressure that is greater than atmospheric pressure. Compressed air is an important medium for transfer of energy in industrial processes, and is used for power tools such as air hammers, drills, wrenches, and o ...

pressing on a piston is used to apply the pressure to the

brake pad Brake pads are a component of disc brakes used in automotive and other applications. Brake pads are composed of steel backing plates with friction material bound to the surface that faces the disc brake rotors. Function Brake pads convert the kin ...

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. A device that is put on a track to slow down railroad cars is also called ...

needed to stop the vehicle. Air brakes are used in large heavy vehicles, particularly those having multiple trailers which must be linked into the brake system, such as

truck A truck or lorry is a motor vehicle designed to transport cargo, carry specialized payloads, or perform other utilitarian work. Trucks vary greatly in size, power, and configuration, but the vast majority feature body-on-frame construction ...

bus A bus (contracted from omnibus, with variants multibus, motorbus, autobus, etc.) is a road vehicle that carries significantly more passengers than an average car or van. It is most commonly used in public transport, but is also in use for cha ...

es,

trailers Trailer may refer to: a Transportation * Trailer (vehicle), an unpowered vehicle pulled by a powered vehicle ** Bicycle trailer, a wheeled frame for hitching to a bicycle to tow cargo or passengers ** Full-trailer ** Semi-trailer **Horse trailer ...

, and

semi-trailers A semi-trailer is a trailer without a front axle. In the United States, the term is also used to refer to the combination of a truck and a semi-trailer; a tractor-trailer. A large proportion of a semi-trailer's weight is supported by a tracto ...

, in addition to their use in railroad trains.

George Westinghouse George Westinghouse Jr. (October 6, 1846 – March 12, 1914) was an American entrepreneur and engineer based in Pennsylvania who created the railway air brake and was a pioneer of the electrical industry, receiving his first patent at the age of ...

first developed air brakes for use in

railway Rail transport (also known as train transport) is a means of transport that transfers passengers and goods on wheeled vehicles running on rails, which are incorporated in tracks. In contrast to road transport, where the vehicles run on a pre ...

service. He patented a safer air brake on March 5, 1872. Westinghouse made numerous alterations to improve his air pressured brake invention, which led to various forms of the automatic brake. In the early 20th century, after its advantages were proven in railway use, it was adopted by manufacturers of trucks and heavy

road vehicle A vehicle (from la, vehiculum) is a machine that transports people or cargo. Vehicles include wagons, bicycles, motor vehicles (motorcycles, cars, trucks, buses, mobility scooters for disabled people), railed vehicles (trains, trams), water ...

Design and function

Air brakes are typically used on heavy trucks and buses. Typical operating pressure is approximately . A compressed-air-brake system is divided into a ''supply system'' and a ''control system''. The supply system compresses, stores and supplies high-pressure air to the control system as well as to additional air operated auxiliary truck systems (gearbox shift control, clutch pedal air assistance

servo Servo may refer to: Mechanisms * Servomechanism, or servo, a device used to provide control of a desired operation through the use of feedback ** AI servo, an autofocus mode ** Electrohydraulic servo valve, an electrically operated valve that c ...

, etc.). The air compressor draws filtered air from the atmosphere and compresses it, storing the compressed air in high-pressure reservoirs. Most heavy vehicles have a gauge within the driver's view, indicating the availability of air pressure for safe vehicle operation, often including warning tones or lights. A mechanical "wig wag" that automatically drops down into the driver's field of vision when the pressure drops below a certain point is also common. The control system consists of service brakes, parking brakes, a control pedal, and an air storage tank. If the vehicle is towing a trailer, it often has a separate trailer-brake system that receives compressed air from the supply system. The parking brakes use a disc or

drum The drum is a member of the percussion group of musical instruments. In the Hornbostel-Sachs classification system, it is a membranophone. Drums consist of at least one membrane, called a drumhead or drum skin, that is stretched over a she ...

arrangement which is designed to be held in the 'applied' position by spring pressure. Air pressure must be produced to release these "spring brake" parking brakes. Setting the parking/emergency brake releases the pressurized air in the lines between the compressed air storage tank and the brakes, thus allowing the spring actuated parking brake to engage. A sudden loss of air pressure would result in full spring brake pressure immediately. The service brakes are used while driving for slowing or stopping the vehicle. When the brake pedal is pushed to apply the service brakes, air is routed under pressure from a supply reservoir to the service brake chamber, causing the brake to be engaged. When the pedal is released, a return spring in the brake chamber disengages the brake, and the compressed air is exhausted to the atmosphere. Most types of truck air brakes are drum brakes, though there is an increasing trend towards the use of disc brakes.

Supply system

The

air compressor An air compressor is a pneumatic device that converts power (using an electric motor, diesel or gasoline engine, etc.) into potential energy stored in pressurized air (i.e., compressed air). By one of several methods, an air compressor forces m ...

(1) is driven by the

engine An engine or motor is a machine designed to convert one or more forms of energy into mechanical energy. Available energy sources include potential energy (e.g. energy of the Earth's gravitational field as exploited in hydroelectric power gen ...

either by

crankshaft A crankshaft is a mechanical component used in a piston engine to convert the reciprocating motion into rotational motion. The crankshaft is a rotating shaft containing one or more crankpins, that are driven by the pistons via the connecting ...

pulley A pulley is a wheel on an axle or shaft that is designed to support movement and change of direction of a taut cable or belt, or transfer of power between the shaft and cable or belt. In the case of a pulley supported by a frame or shell that ...

via a

belt Belt may refer to: Apparel * Belt (clothing), a leather or fabric band worn around the waist * Championship belt, a type of trophy used primarily in combat sports * Colored belts, such as a black belt or red belt, worn by martial arts practit ...

or directly from the engine timing gears. Typically, it is lubricated and cooled by the engine lubrication and cooling systems, but some systems use self-lubricated compressors and/or air-cooled compressors. System pressure is regulated by a governor between a minimum and maximum value; the governor unloads the compressor when the maximum system pressure has been achieved and loads the compressor when it falls below its minimum setpoint. When the compressor is loaded, compressed air is first routed through a cooling coil and into an

air dryer Compressed air dryers are special types of filter systems that are specifically designed to remove the water that is inherent in compressed air. The process of compressing air raises its temperature and concentrates atmospheric contaminants, primar ...

(3) which removes moisture and oil impurities and also may include a

pressure regulator A pressure regulator is a valve that controls the pressure of a fluid or gas to a desired value, using negative feedback from the controlled pressure. Regulators are used for gases and liquids, and can be an integral device with a pressure setti ...

(2),

safety valve A safety valve is a valve that acts as a fail-safe. An example of safety valve is a pressure relief valve (PRV), which automatically releases a substance from a boiler, pressure vessel, or other system, when the pressure or temperature exceeds ...

and smaller purge reservoir (4). As an alternative to the air dryer, the supply system can be equipped with an anti-freeze device and oil separator. The compressed air is then stored in a supply

reservoir A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including contro ...

(6); the supply reservoir, which is physically closest to the compressor, is also called a wet tank because the majority of oil and water from the compressor gather here. A second, downstream reservoir is called the service reservoir, and forms the primary source for brake operation. The supply and service reservoirs are sized to allow several service brake applications if the compressor fails or the engine stops. From the service reservoir, compressed air is then distributed via a four-way protection valve (5) into the primary reservoir (rear brake reservoir) and the secondary reservoir (front/trailer-brake reservoir), a parking-brake reservoir, and an auxiliary air supply distribution point. The supply system also includes various

check Check or cheque, may refer to: Places * Check, Virginia Arts, entertainment, and media * ''Check'' (film), a 2021 Indian Telugu-language film * ''The Checks'' (episode), a 1996 TV episode of ''Seinfeld'' Games and sports * Check (chess), a thr ...

, pressure limiting, drain and

Control system

The control system is further divided into two ''service brake''-circuits, the ''parking brake''-circuit, and the '' trailer brake''-circuit. The dual-service brake circuits are further split into front- and rear-wheel circuits which receive compressed air from their individual supply reservoirs for added safety in case of an air leak. The service brakes are applied by means of a brake pedal air valve (9) which regulates both circuits. Depressing the brake pedal allows compressed air stored in the respective supply reservoirs to enter each service brake chamber (10) and (12), causing the service brakes to actuate. Releasing the brake pedal disconnects the supply reservoirs; a brake return spring forces the service brakes to release, and the compressed air that was used to actuate the service brakes is exhausted to the atmosphere. Repeated application of the service brakes will deplete the air reservoir pressure, prompting the governor to load the compressor again. Relay valve(s) are used to improve brake response with long lines. Routing the lines for the rear and trailer service brakes through the brake pedal air valve could result in unacceptably slow actuation, so these brakes are plumbed directly to the service reservoir through a local relay valve (11) and the line from the brake pedal air valve is used to actuate the relay valve instead of the service brakes. Similarly, quick-release valve(s) are located close to their respective brakes and allow the air to be exhausted more quickly when the brakes are released. The parking brake is the air operated spring brake type where the brake is applied by spring force in the spring brake cylinder (12) and released by compressed air via a hand-control valve (7). The trailer brake consists of a direct two-line system: the supply line and the separate control or service line. The supply line receives air from the

prime mover Prime mover may refer to: Philosophy *Unmoved mover, a concept in Aristotle's writings Engineering * Prime mover (engine), motor, a machine that converts various other forms of energy (chemical, electrical, fluid pressure/flow, etc) into energy ...

park brake air tank via a park brake relay valve and the control line is regulated via the trailer-brake relay valve. The operating signals for the relay are provided by the prime mover brake-pedal air valve, trailer service brake hand control (subject to local heavy vehicle legislation) and the prime mover park brake hand control.

Wig wag

Air-brake systems may include a wig wag device which deploys to warn the driver if the system air pressure drops too low. This device drops a mechanical arm into view when the pressure in the system drops below the threshold of sufficient pressure to reliably deploy the brakes. An automatic wig wag will rise out of view when the pressure in the system rises above the threshold. The manual-reset type must be placed in the out-of-view position manually. Neither will stay in place unless the pressure in the system is above the threshold. The photo to the left shows a manual wig wag which the operator swings to the right out of view when the air pressure is above the threshold where it will remain as long as the pressure is sufficient. Most

U.S. state In the United States, a state is a constituent political entity, of which there are 50. Bound together in a political union, each state holds governmental jurisdiction over a separate and defined geographic territory where it shares its sove ...

commercial driver's license A commercial driver's license (CDL) is a driver's license required in the United States to operate large and heavy vehicles (including trucks, buses, and trailers) or a vehicle of any size that transports hazardous materials or more than 15 p ...

manuals, published by the states’ Departments of Motor Vehicles or equivalents, describe this term. File:Trailer brake relay valve.jpg, Trailer-brake relay valve File:Air brakes relay valve.jpg, Air-brake relay valve

Advantages

Air brakes are used as an alternative to

hydraulic brake A hydraulic brake is an arrangement of braking mechanism which uses brake fluid, typically containing glycol ethers or diethylene glycol, to transfer pressure from the controlling mechanism to the braking mechanism. History During 1904, Frederick ...

s which are used on lighter vehicles such as automobiles. Hydraulic brakes use a liquid (

hydraulic fluid A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backhoe ...

) to transfer pressure from the brake pedal to the brake shoe to stop the vehicle. Air brakes are used in heavy commercial vehicles due to their reliability. They have several advantages for large multi-trailer vehicles: * The supply of air is unlimited, so the brake system can never run out of its operating fluid, as hydraulic brakes can. Minor leaks do not result in brake failures. * Air line couplings are easier to attach and detach than hydraulic lines; the risk of air getting into hydraulic fluid is eliminated, as is the need to bleed brakes when they are serviced. Air-brake circuits on trailers can be easily attached and removed. * Air not only serves as a fluid for transmission of force, but also stores potential energy as it is compressed, so it can serve to control the force applied; hydraulic fluid is nearly incompressible. Air-brake systems include an air tank that stores sufficient energy to stop the vehicle if the compressor fails. * Air brakes are effective even with considerable leakage, so an air-brake system can be designed with sufficient "fail-safe" capacity to stop the vehicle safely even when leaking. * The compressed air inherent in the system can be used for accessory applications that hydraulics are not appropriate for, such as

air horn An air horn is a pneumatic device designed to create an extremely loud noise for signaling purposes. It usually consists of a source which produces compressed air, which passes into a horn through a reed or diaphragm. The stream of air cause ...

s and seat adjusters, doors in case of buses and trolleybuses.

Disadvantages

Although air brakes are considered the superior braking system for heavy vehicles (gross weight ~12 tonnes to 15 tonnes) which would overload hydraulic brakes, they also have the following disadvantages when compared to hydraulic braking systems: * Air brakes generally cost more. * Air-brake systems compress air, which results in moisture that requires air dryers to remove, which also increases the price for air-brake systems and can contribute to higher maintenance and repair costs, particularly in the first five years. Defective air dryers lead to ice in the air-brake system in cold locations. * In the US, commercial drivers are required to obtain additional training in order to legally drive any vehicle using an air-brake system. The

Federal Motor Carrier Safety Administration The Federal Motor Carrier Safety Administration (FMCSA) is an agency in the United States Department of Transportation that regulates the trucking industry in the United States. The primary mission of the FMCSA is to reduce crashes, injuries and f ...

(FMCSA), which regulates the trucking industry in the US, requires that drivers who operate a vehicle equipped with air brakes take their driving test in one. This is because: ** Learning to operate air brakes smoothly has a

learning curve A learning curve is a graphical representation of the relationship between how Skill, proficient people are at a task and the amount of experience they have. Proficiency (measured on the vertical axis) usually increases with increased experience ...

, as they are difficult to operate smoothly. ** Also, since air brakes must be operated differently from hydraulic systems, driving a vehicle with air brakes requires knowledge of proper maintenance. A driver is required to inspect the air pressurization system prior to driving and make sure all tanks are in working order. ** As noted by the

Insurance Corporation of British Columbia The Insurance Corporation of British Columbia (ICBC) is a provincial Crown corporation in British Columbia providing insurance. ICBC was created in 1973 by the NDP government of Premier Dave Barrett. By law, any vehicle registered and driven o ...

(ICBC), "Operating commercial vehicles or vehicles equipped with air brakes requires special knowledge and skill, and the cost of a mistake can be very high. When large vehicles are involved in crashes, the damage—to vehicles, cargo and human lives—can be catastrophic." * Noise pollution: Ranging at 95–115 db for a typical noise level range with levels closer to 115–120 db able to cause immediate damage to hearing. Legislation in countries like Australia & Canada are in the midst of banning trucks with air brakes for use in heavily populated areas. The sound is created by the release of compressed air that has been trapped inside the brake chamber. The squeaking “psss" sound made is caused by air escaping after braking, ensuring air pressure remains at the correct level via opening of the valves reacting to the air compressor.

References

External links

*
''Tractor Protection Systems''
North American Standard Inspection Program,

Commercial Vehicle Safety Alliance The Commercial Vehicle Safety Alliance (CVSA) is a non-profit association of local, state, provincial, territorial and federal commercial motor vehicle safety officials and industry representatives. CVSA focuses on improving driver and vehicle saf ...

(May 19, 2010). {{Railway brakes Vehicle braking technologies de:Bremse (Kraftfahrzeug)#Druckluftbremse