Brake (device)

2016-06-18

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Introduction; Automobile Brakes; Truck, Bus, and Rail Brakes; Other Braking Systems
I  Introduction
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Brake (device), device used to slow and stop a rotating wheel and thus a moving vehicle. Brakes such as those on automobiles, trucks, trains, and bicycles use friction between a wheel and another object to slow the motion of the vehicle. The friction created by the rubbing together of two objects generates a large amount of heat. A brake system must be capable of dissipating the heat as rotating wheels slow, because excess heat can cause the brakes to lose their grip and fail.
II  Automobile Brakes
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Passenger cars and light trucks use a hydraulic brake system to stop motion (see Hydraulics). Such a system uses a chemical-based liquid known as brake fluid to transmit pressure from a brake pedal to the brakes on each wheel. Aviator and inventor Malcolm Loughead, one of the founders of the Lockheed Martin Corporation, invented hydraulic brakes in 1918. Four-wheel hydraulic brakes were introduced on the 1921 Duesenberg and the 1924 Chrysler automobile models.
To apply the brakes, the driver steps on a brake pedal. The pedal pushes a piston inside an assembly called the master cylinder, which is filled with brake fluid. The master cylinder is connected to the wheel brakes by hollow steel tubes called brake lines, which are also filled with brake fluid. Pushing the piston squeezes the fluid inside the master cylinder, creating hydraulic pressure. Since liquid cannot be compressed, the pressure is transmitted through the brake lines to additional pistons inside each brake. These pistons push brake linings against drums and discs attached to the wheels in order to slow the vehicle down. For safety purposes, the brake system for the four wheels of a car or truck is divided into two separate circuits (each with its own piston inside the master cylinder). If a fluid leak in either circuit causes a loss of pressure, the two brakes in the other circuit will still be operational to stop the vehicle. Cars and trucks use two types of brakes, called drum brakes and disc brakes, to stop motion.
Prior to 1965, all cars and trucks had drum brakes on the front and rear wheels. Drum brakes consist of curved brake shoes that rest within a rotating iron cylinder, or drum, connected to the axle and the wheel. When drum brakes are applied, hydraulic pressure from the master cylinder pushes a pair of pistons in the drum against the brake shoes. The shoes then press against the wall of the drum, slowing the wheel. When the brakes are released, springs pull the shoes back away from the drum. Various types of self-adjusting mechanisms within drum brakes help maintain the correct amount of distance between the shoes and drum.
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In 1965 disc brakes were introduced on automobiles. Disc brakes have greater stopping power than drum brakes and are usually installed on the front wheels to improve braking during sudden stops. Disc brakes consist of a metal disc, or rotor, that is connected to the wheel. A device called a caliper rests on the edge of the rotor and holds two friction pads on either side of the rotor. Applying the brakes causes fluid to push a piston within the caliper, which pinches the brake pads against the rotor and slows the wheel. Disc brakes do not have return springs, like those in drum brakes, to disengage the brakes. Instead, a seal around the piston bends slightly when the brakes are applied and then retracts to pull the piston back away from the rotor when the brake pedal is released. Also, disc brakes rely on a very small amount of wobble, called runout, that is normally present in the rotor. When the brakes are released, the runout of the rotor simply pushes the pads away from the rotor.
Disc brakes are considered superior to drum brakes, because disc brakes can handle higher braking temperatures and dissipate heat more quickly. Also, disc brakes do not trap water as drum brakes can. When drum brakes become wet, they suffer a decrease in braking, called brake fade, which can happen when driving through deep puddles. Most cars and trucks use disc brakes on the front wheels and drum brakes on the rear wheels, although some cars now feature disc brakes on all wheels.
Disc brakes generally require added pedal pressure, so most vehicles equipped with these brakes have power-braking systems to reduce a driver’s pedal effort. Most power-braking systems use a vacuum to increase braking power. An engine’s pistons create a vacuum as they draw air into the engine. This vacuum is connected by a tube to both sides of a special spring-loaded diaphragm located near the master cylinder. When the brake pedal is pressed, ordinary air is allowed to enter on one side of the vacuum diaphragm. The vacuum on the other side then pulls the diaphragm to one direction with added force. This added force is sent to the master cylinder, increasing braking power.
Both drum and disc brakes contain several features to dissipate the large amount of heat produced by friction. If the heat is not dissipated, the brakes may malfunction. To dissipate heat more quickly, many rotors are vented and have cooling fins sandwiched between the faces of the rotor. Most disc brakes use semimetallic brake pads that contain chopped steel-wool fiber to aid heat dissipation. The brake linings on drum brakes are made of heat-resistant material. Prior to the introduction of disc brakes, most vehicles had brake linings that contained asbestos fiber. Asbestos brake linings were mostly discontinued in the late 1980s because of the health risks posed by asbestos. Even so, some replacement brake linings made by parts manufacturers still contain asbestos.
Every car and truck also has a parking brake, sometimes called an emergency brake, for locking the wheels when the vehicle is parked. The parking brake uses levers and cables, rather than hydraulics, to activate the brakes. The parking brake is usually connected to the rear brakes only. In the event of a total hydraulic failure, the parking brake could be used to stop the vehicle.
In 1985 the first antilock brake system (ABS) was introduced for motor vehicles in the United States as a safety feature to give drivers more control when braking. ABS uses a microprocessor and individual wheel-speed sensors to monitor the brakes. Hydraulic control valves for each brake circuit prevent skidding during panic stops or when braking hard on wet or slippery surfaces. By 1990 ABS was available on about 25 percent of all new cars and trucks. ABS is now available on over 90 percent of all new vehicles.
Wheel-speed sensors monitor the rotation of each wheel. When the brakes are applied, the ABS microprocessor compares wheel speeds. If one or more wheels are rotating more slowly than the others are (a situation that causes wheel lockup and loss of driver control), the system energizes control valves to isolate the affected brake circuit. Brake pressure is held momentarily and is then released before it is reapplied. This cycle allows the wheel to regain traction and prevents skidding. The hold-release-reapply cycle is repeated rapidly for as long as needed or until the vehicle comes to a stop. The cycling of the ABS control valves and pulsating hydraulic pressure can usually be heard and felt through the brake pedal. These indicators are designed intentionally to alert the driver that the ABS is assisting braking. The driver should maintain firm pedal pressure while the ABS is active, as pumping the pedal can defeat the action of the ABS and increase the stopping distance. The ABS does not operate during normal braking and does not engage unless one or more wheels start to lose traction.
Improvements in ABS technology now allow some systems to prevent wheel spin when accelerating on wet or slippery surfaces. This capability is known as traction control. When the wheel-speed sensors detect that a drive wheel is starting to spin, the ABS applies the brake on the affected wheel to slow it down. Some of these newer systems also provide additional control when cornering or changing lanes.
III  Truck, Bus, and Rail Brakes
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Large, heavy-duty trucks, as well as buses and trains, use compressed air pressure rather than hydraulic fluid to operate their brakes. Constant air pressure is maintained in the braking system, keeping the brakes released. When the pressure is released, the brakes are applied. A loss of pressure from a leaky system will actually apply the brakes rather than result in a total loss of braking, as might be expected. This safety feature was invented for use on railroad trains in 1869 by George Westinghouse. Most large trucks and buses have drum or disc brakes, just like automobiles have. Trains use specially designed brakes for slowing and stopping.
Tractor-trailers use two types of brake systems. The first is the normal slowing and stopping system, which uses compressed air to activate the brakes. The second system is the emergency brake system. Inside the drum brakes of a truck, the brake shoes are connected to a spring held in place by a diaphragm that is filled with air. When air pressure is applied to the diaphragm, the pressure overcomes the spring and allows the brake to release. However, when there is no air in the diaphragm, the spring pushes the shoes against the drum and applies the brake. As long as constant air pressure is maintained in the emergency system, the brakes remain released. But if there is a leak in the brake lines or if the air compressors fail, the air pressure inside the diaphragm decreases, and the spring brake mechanism automatically applies the drum brakes.
A typical truck air-brake system includes an engine-driven compressor to generate air pressure, several air tank storage reservoirs, and various control valves. If air pressure is lost for any reason while the truck is in motion, the brakes will automatically apply and bring the vehicle to a halt. Likewise, if the air system cannot generate the required pressure to release the brakes, the truck cannot be driven. Another reason why trucks use air brakes is because the air-pressure brake lines on a truck tractor can be easily connected to those on a semitrailer so both can work together.
Some trucks also have a device that uses engine compression to help slow the vehicle when decelerating. The pistons in an engine compress air and fuel in order to produce combustion. When the compression brake is activated, the valves that normally regulate the air intake and exhaust flow from the engine are adjusted to produce air resistance within the engine cylinders. The added resistance slows the pistons down and thus reduces the speed of the engine. Compression brakes do not involve the wheel brakes at all. Use of a compression brake saves wear on the brake linings and is helpful when driving down steep inclines.
The brake system used on trains is similar to the air system used on trucks, but the design of the individual wheel brakes is different. On a train, the shoes are pressed directly against the wheel rim. A compressor generates air pressure that is stored in air tanks. Air hoses connect the brakes on all the train cars into one system. Applying air pressure into the system releases the brakes, and releasing air pressure from the system applies the brakes.
IV  Other Braking Systems
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Bicycles have one of three types of brakes. Coaster brakes are used on the rear wheel of single-speed bikes, which do not have shiftable gears. A mechanism inside the rear wheel hub creates a binding action that slows or locks the rear wheel when the pedals are operated in the backward direction. On bicycles with multiple gears, caliper and cantilever brakes (also called side-pull and center-pull brakes, respectively) are used on the wheels. Both of these types of brakes use hand levers and cables to operate the brake mechanism, which consists of two levers that squeeze a pair of rubber pads against both sides of the wheel rim. Both types are spring-loaded to retract the pads when the pull cable is released. Cantilever brakes are considered better for mountain bikes and for bicycle racing, because these brakes provide more leverage for increased braking force.
Aircraft have hydraulic brakes on their landing gear for stopping after they have landed. The antilock brake system was first developed in 1947 for use on the B-47 bomber. Many aircraft also have special flaps or spoilers called air brakes that can be extended from the wings to increase aerodynamic drag. These flaps may be used to slow the aircraft when it is diving or maneuvering in flight and to help slow it after it has landed. Other means of braking aircraft include propeller blades that can change pitch (operating angle) and thrust reversers that redirect the jet blast sideways or forward in jet engines. Many high-performance military aircraft also have special parachutes called drogue chutes that deploy upon landing to assist braking. The space shuttle uses several drogue chutes for braking, because it lands at speeds in excess of 480 km/h (300 mph). Some types of racing cars also use drogue chutes to assist braking (see Automobile Racing).
Electric cars and other electric vehicles use drum and disc brakes to stop, but some vehicles also make use of magnetic brakes, which create opposing magnetic fields to resist motion. Called regenerative braking, this technique recaptures some of the vehicle’s momentum as electrical energy. Regenerative braking uses the magnets within the electric motor itself to slow the vehicle. When the driver releases the accelerator pedal, the electric motor changes into a generator, recapturing the energy from the moving car and transforming it back into electricity. The extra electricity is then used to recharge and extend the driving range of the batteries.