Brake
From Wikipedia, the free encyclopedia
- For the band, see Brakes (band).
- For other uses, see Brake (disambiguation).
A brake is a device for slowing or stopping the motion of a machine or vehicle, and to keep it from starting to move again. The kinetic energy lost by the moving part is usually translated to heat by friction. Alternatively, in regenerative braking, much of the energy is recovered and stored in a flywheel, capacitor or turned into alternating current by an alternator, then rectified and stored in a battery for later use.
Note that kinetic energy increases with the square of the velocity (E = ½m·v2 relationship). This means that if the speed of a vehicle doubles, it has four times as much energy. The brakes must therefore dissipate four times as much energy to stop it and consequently the braking distance is four times as long.
Brakes of some description are fitted to most wheeled vehicles, including automobiles of all kinds, trucks, trains, motorcycles, and bicycles. Baggage carts and shopping carts may have them for use on a moving ramp. Some aeroplanes are fitted with wheel brakes on the undercarriage. Some aircraft also feature air brakes designed to slow them down in flight. Notable examples include gliders and some WWII-era fighter aircraft. 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.
Deceleration and avoiding acceleration when going downhill, can also be achieved by using a low gear, see engine braking.
Friction brakes on cars store the heat in the rotating part (drum brake or disc brake) during the brake application and release it to the air gradually.
[edit] History
Early braking systems, used to stop vehicles with steel rimmed wheels, consisted of a curved wooden block designed to bear against the steel tire when manipulated by a single leverage system from the drivers seat.
This "brake shoe" was the normal way of braking either a horse drawn vehicle or steam locomotive. Many varieties of arrangements of levers, rods and pivots were utilised to bring them into operation.
In 1895 the Michelin Brothers had begun the move towards replacing steel rimmed wheels with the pneumatic rubber tire forcing them to think of a new braking system as "brake shoes" were no longer satisfactory.
A new method of braking was required and two early devices attempted to apply the force of friction to the axle or to a drum on the axle or transmission shaft. This type of brake was actuated by the driver depressing a pedal or operating a lever. Heavier pressure caused the bands to contract more tightly around the drum giving greater retardation.
One included the use of a wooden block inside a flexible contracting metal band which when pressed together would tighten around the drum causing friction between the drum, which is connected to the wheel, and the wooden blocks and therefore slowing down the wheel.
The other was an inner wheel or brake drum which was added with an external contracting band meant to bear against the drum to retard the vehicle. However, continuous replacement of drum and band combined with poor friction quality, soon led to the lining the band with a replacement friction material. Lead, cotton and camel hair were used as lining, but they burned out too quickly which led Herbert Frood to produce an asbestos fabric in 1908.
In 1899 Daimler had a cable wound around a drum and anchored to the chassis so that when the cable was tightened while the car was moving forwards the rotation of the drum increased the tightness and grip of the cable, therefore reducing the amount of force required to pull the lever or press on the pedal in order to stop the vehicle. However, in reverse it tended to work against the pull of the cable and loosen its grip.
The "added" braking efficiency called "servo assistance" is still an important factor in the design of drum brakes today. Most modern cars have vacuum assisted braking.
The external band brake was vulnerable to road dirt and weathering which caused rapid wear of lining, loss of efficiency and on occasions "automatic" brake application due to drum expansion. To overcome these problems the internal expanding shoe brake was developed, in which the brake shoes were inside a 'brake drum' (protected from weather and dust).
Its first appearance seems to have been with Louis Renault in 1902 and remained the basic principle for the next fifty years.
Originally, motor car brakes were operated by mechanical means and became known as "mechanical" brakes i.e. a mechanical system was used to transform the effort of the driver's foot on the brake pedal into expansion of the brake shoes against the drum. (On depressing the brake pedal, the cam is rotated by a lever connected to the pedal and forces the shoes into contact with the brake drum. Springs attached to both shoes return the shoes to the original "off" position when the brakes are released.)
To this day, bicycles have mechanical brakes, operated by hand lever and cable. This closes calipers, containing the friction pads, onto the rim of the wheel.
One solution, by Maurice Farman 1920, to the challenge of increasing the "servo action" was to connect two shoes with a pivot and secure the other end of the "trailing" shoe, with a pivot, to the back plate. This in effect made both shoes "leading shoes".
Early brakes were operated by a linkage system of fixed rods and levers supplement by Bowden cables. (Cables were invented in 1906 and were developed for the bicycle.) The linkage system of rods and levers were not easy to keep in good operating order. Equalising brake pressure on the wheels also presented a number of problems, many of which were solved by the introduction of the hydraulic system, using fluid to transfer the force applied to the brake pedal.
Hydraulic systems make use of the fact liquids cannot by compressed to any appreciable degree and that pressure applied at any points within a closed system is transmitted equally throughout (Pascal's law).
In a basic hydraulic braking system all the cylinders and brake lines form one closed system filled with brake fluid. The master cylinder has a single piston, whiles each wheel cylinder has two opposed pistons. All pistons have rubber cups to maintain pressure and prevent loss of fluid. The pressure generated in the master cylinder is transmitted with equal and undiminished force to the pistons of each wheel cylinder so that pressures applied to all brake shoes are identical.
Most modern cars now have disc brakes. The brake pads are mounted within the jaws of a caliper, which grips a brake disc, providing the necessary friction. Performance cars are fitted with larger wheels, to permit larger brake discs.
