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brake,in botany: see brackenbracken
common name for a tall fern (Pteridium aquilinum) with large triangular fronds, widespread throughout the world, often as a weed. It is considered poisonous to livestock when eaten in quantity, but the rootstocks and the young shoots, cooked,
..... Click the link for more information. .
brake,in technology, device to slow or stop the motion of a mechanism or vehicle.
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 asbestosasbestos,
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.
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.
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. In many modern braking systems there no longer is a direct connection between the pedal and the brake; a sensor is used register the force applied to the pedal, and that information is used to determine the pressure to apply to the brake. Automobile braking systems may also include an override that disables the accelerator when the brake is activated. An antilock braking system (ABS) uses sensors to identify when a wheel is locking and then applies and releases the brake automatically several times per second to prevent lockup. ABS can prevent skids, permitting controlled stops, and decreases the amount of time and distance needed to stop a car.
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 machinehydraulic 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.
a complex device for decelerating or stopping a machine or mechanism; in hoisting and conveying machines, the brake also serves to hold the load in a raised position.
Depending on their action, brakes are classified as mechanical (friction), hydraulic, or electric (electromagnetic and induction) brakes. Distinctions are also made between shoe, band, disk, and cone brakes, depending on the design of the working elements.
The most common type of brake in machines and mechanisms, including hoisting and conveying machines and the mechanisms of machine tools and railroad trains, is the external shoe brake, in which the shoes are attached to levers on fulcrums, usually in symmetrical opposition relative to the brake drum. Internal shoe brakes are used in motor vehicles. The various structural designs of shoe brakes are largely distinguished by the lever system used and the type of actuation. Shoe-rail brakes are used in the drive mechanism of some vehicles, railroad cars, and locomotives; their action is based on the compression of the brake shoes against the rails. Such brakes are especially effective in emergency braking.
In band brakes, a flexible band is used instead of a shoe. The band surrounds the drum, permitting an increase in the frictional torque, which also increases with increasing angles of wrap. Band brakes are used in the hoisting, transport, and turning mechanisms of hoisting and conveying machines. Their disadvantages include the significant force that deforms the shaft of the brake drum, the uneven distribution of stress and wear over the arc of friction material in contact with the drum, and the relatively large effect of changes in the coefficient of friction on the braking torque.
In disk brakes, the frictional torque is created as a result of the compression of disks rotating together with a shaft mechanism against nonrotating disks. Disk brakes may be used to achieve high frictional torques, which increase with an increasing number of disks. Disks also have the advantages of compactness and relatively easy protection from the environment, including complete hermetic sealing if necessary. Their disadvantages include poor heat dissipation from the frictional surfaces, especially in multi-disk brakes. Disk brakes find use in various mechanisms in vehicles and metal-cutting machine tools.
Combination disk-shoe brakes create friction between the edge surfaces of a disk and the end surfaces of friction shoes, which are pressed against the disk from both sides. The shoes cover only a small part of the disk’s friction surface, which provides better heat dissipation and increases the life of the shoes. Such a design shows promise for future development. A significant advantage of the design is the relatively low moment of inertia of the disk in comparison with the moment of inertia of the brake drum of a shoe brake or band brake. This decreases the load on the engine when the mechanism is actuated and lessens the amount of kinetic energy converted into heat during braking. Such brakes are especially effective in the braking systems of heavy vehicles, such as trucks.
In load-bearing brakes, which are used in the mechanisms of hoisting and conveying machines, the braking torque arises from the action of the load being transported. Such brakes are used as lowering brakes in winches and boom hoists and as emergency brakes in escalators. The safety grips used in manual hoisting machines are load-bearing brakes equipped with a ratchet gear, which prevent the twisting or unwinding of the driving crank caused by the action of the load being lifted.
Speed-limiting brakes, or limiters, are used to ensure the safe operation of various machines and mechanisms. These devices prevent increases in the speed of a mechanism above a predetermined speed but cannot stop the mechanism or load. They are used in the drive mechanisms of various hoists and conveyors to control the speed at which heavy loads are lowered; they also find applications in testing devices. Types of limiters include centrifugal, dynamic (hydraulic), and induction limiters and small explosive devices. In centrifugal brakes, when the speed increases above a given limit, the centrifugal force of the rotating elements of the brake increases. This creates pressure on the stationary part of the brake, which generates the required braking torque.
The frictional torque created by a brake depends on the force with which the friction elements—shoes, bands, or disks—are pressed against the friction surface of the component connected to the drum or disk and on the properties of the materials of the contacting pair. In order to increase the compressing force in some brakes, the self-energizing effect is used, in which the force of friction arising between contacting surfaces facilitates the additional compression of these surfaces. Friction materials that can be glued or riveted to the working elements make it possible to reduce the size of brakes, lessen the power required for actuation, and obtain higher braking torques.
Brake actuation may be by mechanical, hydraulic, pneumatic, vacuum, electromagnetic, electrohydraulic, or electromechanical linkage. With mechanical actuation, as is usually used for the emergency brakes of motor vehicles, the actuation force is transferred from a lever or pedal through a system of rods, levers, and connections. Mechanical actuation becomes complex when the brake is significantly removed from the control. Hydraulic and pneumatic actuation linkages are more advanced. Hydraulic actuation is used in passenger cars and cranes, and pneumatic actuation is used in trucks, buses, streetcars, railroad trains, and airplane landing gear. In pneumatic and electropneumatic actuation systems, the major power components are the brake power cylinders, which are connected by an air line to a compressor through a brake valve, and a lever system for the friction shoes; such brakes are used in railroad rolling stock (seeKAZANTSEV BRAKE and MATROSOV AIR BRAKE).
Electric actuation uses special AC or DC electromagnets that act on the lever system of the brake. Electrohydraulic and electromechanical plungers are also used. These are devices consisting of an energy converter with an independent motor and the plunger itself with a rod that moves back and forth in conjunction with the lever system. Brake plungers are insensitive to overloads, and they make it possible to limit travel of the rod in both directions without the danger of overloading the motor or the components of the plunger. Because they can operate at high frequencies, they can be used in speed-control systems for the working parts of machines. In some brake designs, actuation is by a short-circuited servomotor connected to a lever system through a gear or crank transmission.
Other types of braking include electrical and aerodynamic braking, for example, the use of drag parachutes and high-lift devices on an airplane, and braking accomplished as a result of a change in the operating mode of an engine, for example, compression braking in motor vehicles.
REFERENCESAleksandrov, M. P. Tormoznye usiroistva v mashinostroenii. Moscow, 1965.
Mashchenko, A. F., and V. G. Rozanov. Tormoznye sistemy avtotransportnykh sredstv. Moscow, 1972.
Borisov, S. M. Friktsionnye mufty i tormoza stroitel’nykh i dorozhnykh mashin. Moscow, 1973.
Krylov, V. I., E. V. Klykov, and V. F. Iasentsev. Avtomaticheskie tormoza. Moscow, 1973.
Kazarinov, V. M., V. G. Inozemtsev, and V. F. Iasentsev. Teoreticheskie osnovy proektirovanüa i ekspluatatsii avtolormozov. Moscow, 1968.
Gavrilenko, B. A., V. A. Minin, and L. S. Olovnikov. Gidravlicheskie tormoza. Moscow, 1961.
Ioganson, R. A. Induktornye tormoza. Moscow-Leningrad, 1966.
M. P. ALEKSANDROV, IU. K. ESENOVSKII-LASHKOV, V. G. INOZEMTSEV, and E. V. KLYKOV.
Under the general editorship of M. P. ALEKSANDROV
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).
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
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.