Braking Systems
Many variations of braking systems have evolved over the years. Some have been more successful than others depending on the distance required to stop, the speed of the vehicle, and the person operating the brake.
The first truly effective and reliable brakes were made for trains and were patented by George Westinghouse in 1869. He was met with great resistance, however, from railroad brakemen who made their living by listening for whistle signals from the engineer. Westinghouse's brake was originally activated by opening valves to release compressed air into tubes that ran the length of the train. He soon realized, however, that a leak or break in the lines rendered the entire system ineffective. In 1872, he improved the mechanism so that air pressure held the brake open until it was released.
While steam, compressed air, vacuum, and hydraulic brakes were all in use in various kinds of transportation at the turn of the century, early automobiles reverted to an older brake type known as band brakes. Band brakes simply consist of a flexible band that loops around a drum that revolves with the engine. In early automobiles, a leather band was connected to a wheel. When the vehicle needed to be stopped, a lever was pulled or a pedal pushed to activate the brake, the band then tightened, and the wheel slowed through friction. Variations of band brakes are still used in large, slow-moving machinery such as cranes, hoists, and tractors.
An interesting phenomenon related to band brakes is the principle of self-energization. This is the tendency of the band to tighten itself, without more pressure being applied, as it is pulled forward by the drum. Self-energization works only if the band is allowed to tighten in the same direction as the drum is rotating, so unless the drum changes rotation directions, the brake takes much more pressure to stop reverse movement.
Louis Renault's 1902 invention of the standard drum brake provided more consistently reliable braking in both the forward and reverse directions. The drum is an iron casting that rotates with the wheel. Two curved brake shoes are hinged together and fastened to a backing plate that doesn't turn. When the brakes are applied, the shoes are pushed against the inside of the spinning drum, and friction slows the drum and the wheel. Renault's brake relied on two hinged shoes that were forced apart by a cam. In 1917, R. Stevens patented adjustable shoes to compensate for wear on the shoes or brake drum. Many other patents over the years have improved the drum brake system. Pistons and cylinders have replaced cams, for example, but the same basic design is still used on many cars, particularly on rear wheels.
Hydraulic, or fluid-activated, brakes came into use in the mid-1930s. A master cylinder is the primary component of a hydraulic system. This master cylinder serves as a central reservoir that contains a thick brake fluid. It is connected to drum brakes by tubes called lines which are also filled with brake fluid. When pressure is applied to the brakes, the master cylinder pressurizes the fluid trapped in the system, which in turn applies pressure to close shoes against the brake drum. The emergency brake is a manual method of bypassing the hydraulic system so the brakes can be engaged. A wire cable from the parking brake lever or pedal is attached to the brakes (usually on the rear wheels) and sets the brakes when it is pulled.
Disc brakes were first patented for use on sports cars where frequent, high-pressure braking overheated the shoes on drum brakes, reducing their effectiveness. Disc brakes consist of a caliper, or clamp, that fits over the thin, vertical edge of a wheel, known as a rotor. Bicycle brakes often operate with calipers that squeeze bicycle rims in much the same way. When pressure is applied to engage disc brakes, the caliper (which does not rotate) grips each side of the rotor (which moves as the wheel turns). As the caliper closes, it pushes pads against the rotor disc, and friction again causes the wheel to slow. These brakes apply more equal pressure to wheels for safer stops regardless of vehicle speed or amount of pressure applied. Because these brakes are not self-energizing, more initial pressure is required to squeeze the calipers; therefore, servo-assist (relay) mechanisms are often integrated into the brake systems.
Antilock braking systems (ABS systems) have evolved during the 1970s and 1980s in aircraft. On land, they also provide safer stops, especially in front-wheel-drive cars with heavy front loads and on wet or icy pavements. The main component of an antilock system is a small mechanism that senses and regulates the fluid pressure to each wheel and maintains a steady pressure to prevent wheel lockup and skidding. During the braking process, sensors compare pressure in the hydraulic system to wheel movement. If a sensor detects that a wheel is beginning to lockup or skid, the computer activates a small, electric valve that reduces the fluid pressure to a particular wheel and allows it to continue rotating alone. Pressure is blocked until the tire regains traction or the brake pedal is released. These measurements and changes are calculated as often as fifteen times each second in some antilock systems.
In the 1990s, ABS brakes have been improved through the development of the so-called "self-steering" ABS. This system is equipped with a yaw sensor that evaluates the rate at which the car is turning; when oversteering or understeering (spinning or loss of steering control) begin, the system modifies the braking force applied to the individual wheels. The driver can slow the vehicle while steering to avoid an object without losing control of the car. Electronic sensors in new ABS systems also assist the driver by measuring the speed at which the brake pedal is depressed. If the driver stabs at the brake, the vehicle is fully stopped by the ABS system that recognizes an emergency situation even if the driver has backed off the brake.
Other braking systems in use include regenerative brakes on electric trains and air brakes on aircraft. Regenerative brakes involve an electric motor, an accumulator, and a generator; the motor "steals" the energy supply and slows a vehicle through energy loss, the accumulator stores the stolen electricity, and the generator releases the electrical power after braking is completed. Airplanes are slowed in-flight by air brakes or flaps that extend at right angles above and below their wings to create drag. On the ground, wheel brakes are aided by reverse thrust of the engines, which, in effect, uses the engine exhaust stream to push the aircraft in an opposite direction.
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