brake, in technology
brake, in technology, device to slow or stop the motion of a mechanism or vehicle.Types
Friction Brakes
Friction brakes, the most common kind, operate on the principle that friction can be used to convert the mechanical energy of a moving object into heat energy, which is absorbed by the brake. The essential components of a friction brake are a rotating part, such as a wheel, axle, disk, or brake drum, and a stationary part that is pressed against the rotating part to slow or stop it. The stationary part usually has a lining, called a brake lining, that can generate a great amount of friction yet give long wear; it formerly contained asbestos asbestos, common name for any of a variety of silicate minerals within the amphibole and serpentine groups that are fibrous in structure and more or less resistant to acid and fire. Chrysotile asbestos, a form of serpentine , is the chief commercial asbestos.
..... Click the link for more information. , but this is being replaced by less efficient materials for environmental reasons.
The principal types of friction brake are the block brake, the band brake, the internal-shoe brake, and the disk brake. The block brake consists of a block, the stationary part, that is shaped to fit the contour of a wheel or drum. For example, a wooden block applied to the rim of a wheel has long been used to slow or stop horse-drawn vehicles. A simple band brake consists of a metal band, the stationary part, that can be tightened around a drum by means of a lever. It is found on hoists and excavating machinery. The internal-shoe brake has a drum that contains two stationary semicircular pieces, or shoes, which slow or stop the motion of the drum by pressing against its inner surface. This is the type of brake most often found on automobiles, with an internal-shoe brake drum located on the central part of each wheel. A disk brake of the type used on automobiles has a metal disk and pistons with friction pads that can close on the disk and slow it.
Electric Brakes
A machine that is driven by an electric motor can sometimes use its motor as a brake. Because inertia keeps the machine's shafts moving after the current to the electric motor has been shut off, the machine keeps the motor's armature turning. While this is happening, if the motor's action can be changed to that of a generator, the electric current produced will be drawing its energy from the machine, thus slowing it. However, since such a braking method is not suitable for bringing the machine to a quick stop, it is usually supplemented by friction brakes.
Braking Systems
A manually operated brake pedal or handle is used to activate a brake. With low-power machinery or vehicles the operator can usually apply sufficient force through a simple mechanical linkage from the pedal or handle to the stationary part of the brake. In many cases, however, this force must be multiplied by using an elaborate braking system.
The Air Brake System
An early system for multiplying the braking force, called the air brake system, or air brake, was invented by American manufacturer George Westinghouse and was first used on passenger trains in 1868. It is now widely used on railroad trains. The fundamental principle involved is the use of compressed air acting through a piston in a cylinder to set block brakes on the wheels. The action is simultaneous on the wheels of all the cars in the train. The compressed air is carried through a strong hose from car to car with couplings between cars; its release to all the separate block brake units, at the same time, is controlled by the engineer. An automatic feature provides for the setting of all the block brakes in the event of damage to the brake hose, leakage, or damage to individual brake units. The air brake is used also on subway trains, trolley cars, buses, and trucks.
The Hydraulic Brake System
The hydraulic brake system, or hydraulic brake, is used on almost all automobiles (see hydraulic machine hydraulic machine, machine that derives its power from the motion or pressure of water or some other liquid.
Hydraulic Engines
Water falling from one level to a lower one is used to drive machines like the water wheel and the turbine .
..... Click the link for more information. ). When the brake pedal of an automobile is depressed, a force is applied to a piston in a master cylinder. The piston forces hydraulic fluid through metal tubing into a cylinder in each wheel where the fluid's pressure moves two pistons that press the brake shoes against the drum.
The Vacuum Brake System
The vacuum brake system, or vacuum brake, depends upon the use of a vacuum to force a piston in a cylinder to hold a brake shoe off a drum; when the vacuum is destroyed, the shoe is released and presses on the drum. In an automotive power brake system, extra pressure can be exerted on the hydraulic master cylinder piston by a vacuum brake's piston.
brake, in botany
brake, in botany: see bracken bracken or brake, common name for a tall fern (Pteridium aquilinum) with large triangular fronds, widespread throughout the world, often as a weed...... Click the link for more information. .
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Brake
A machine element for applying a force to a moving surface to slow it down or bring it to rest in a controlled manner. In doing so, it converts the kinetic energy of motion into heat which is dissipated into the atmosphere. Brakes are used in motor vehicles, trains, airplanes, elevators, and other machines. Most brakes are of a friction type in which a fixed surface is brought into contact with a moving part that is to be slowed or stopped.
Friction brakes are classified according to the kind of friction element employed and the means of applying the friction forces. See Friction
The single-block is the simplest form of brake. It consists of a short block fitted to the contour of a wheel or drum and pressed against its surface by means of a lever on a fulcrum, as widely used on railroad cars. The block may have the contour lined with friction-brake material, which gives long wear and a high coefficient of friction. The fulcrum may be located with respect to the lever in a manner to aid or retard the braking torque of the block. The lever may be operated manually or by a remotely controlled force (Fig. 1a).
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In double-block brakes, two single-blocks brake in symmetrical opposition, where the operating force on the end of one lever is the reaction of the other, make up a double-block brake (Fig. 1b). External thrust loads are balanced on the rim of the rotating wheel.
An external-shoe brake operates in the same manner as the block brake, and the designation indicates the application of externally contracting elements. In this brake the shoes are appreciably longer, extending over a greater portion of the drum (Fig. 1c). This construction allows more combinations for special applications than the simple shoe, although assumptions of uniform pressure and concentrated forces are no longer possible. In particular, it is used on elevator installations for locking the hoisting sheave by means of a heavy spring when the electric current is off and the elevator is at rest.
An internal shoe brake has several advantages over an external shoe. Because the internal shoe works on the inner surface of the drum, it is protected from water and grit (Fig. 1d). It may be designed in a more compact package, is easily activated, and is effective for drives with rotations in both directions. The internal shoe is used in the automotive drum brake, with hydraulic piston actuation. See Automotive brake
Hoists, excavating machinery, and hydraulic clutch-controlled transmissions have band brakes. They operate on the same principle as flat belts on pulleys. In the simplest band brake, one end of the belt is fastened near the drum surface, and the other end is then pulled over the drum in the direction of rotation so that a lever on a fulcrum may apply tension to the belt.
Disk brakes have long been used on hoisting and similar apparatus. Because more energy is absorbed in prolonged braking than in clutch startup, additional heat dissipation must be provided in equivalent disk brakes. Disk brakes are used for the wheels of aircraft, where segmented rotary elements are pressed against stationary plates by hydraulic pistons. Flexibility, self-alignment, and rapid cooling are inherent in this design. Another application is the bicycle coaster brake.
The caliper disk brake (Fig. 2) is widely used on automotive vehicles. It consists of a rotating disk which can be gripped between two friction pads. The caliper disk brake is hydraulically operated, and the pads cover between one-sixth and one-ninth of the swept area of the disk. See Automotive brake
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Railway brakes are normally applied air brakes; if the air coupling to a car is broken, the brakes are applied automatically. To apply the brakes, the brake operator releases the compressed air that is restraining the brakes by means of a diaphragm and linkage. Over-the-road trucks and buses use air brakes. Another form of air brake consists of an annular air tube surrounding a jointed brake lining that extends completely around the outside of a brake drum. Air pressure expands the tube, pressing the lining against the drum.
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