Monday, January 28, 2008

Brake (2)

Hydraulic Brake

Automobiles were originally braked mechanically. By about 1930, however, modern hydraulic system were developed. In a hydraulic system, depression of the brake pedal moves a piston in a master cylinder, forcing hydraulic fluid through piping to a slave cylinder at each wheel. These cylinders are each fitted with pistons moved by the pressure of the fluid, which brings the brake lining into contact with the rotating brake drum or disk, producing a breaking force.

As vehicle became heavier and faster, the pedal pressure required to brake the vehicle increased beyond a comfortable, safe level. In automobiles power brakes use the engine vacuum to increased the pressure applied to the piston in the master cylinder, reducing the required pedal pressure.

Power-assisted brake systems are also needed in such heavy vehicle as bases, trucks and railroad trains. One such system is the pneumatically operated Westinghouse air brake patented in 1869 by George Westinghouse, an American manufacturer. Each railroad car has its own reservoir, called an auxiliary reservoir, connected by means of a valve with a brake pipe extending the length of the train. To apply the brakes, the engineer lowers the pressure and the brake cylinder. This allows the compressed air in the reservoir to enter the brake cylinder, braking the train. To release the brakes the engineer builds up the pressure in the brake pipe. Any sudden drop in brake-pipe air pressure, such as that caused by cars uncoupling, will automatically apply the brake, making this a fail-safe system.
Other types of Brake

A machine powered by an electric motor may be designed with dynamic braking. The circuitry of the motor can be switched so that the motor operates as a generator driven by the rotating axle. This convert the rotational energy into electricity, which slows the machines.Braking can also be accomplished by reversing the thrust. In propeller-driven aircraft and ships, the pitch of the propeller blade can be reversed so as to produced thrust in the opposite direction, in some ships the direction of propeller rotation can be reversed.

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