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BRAKE
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BRAKE . (I) A See also:term for rough-tangled undergrowth, connected, according to the New See also:English See also:Dictionary, with " break," to See also:separate. The" brake-See also:fern " (Pieria aquilina) is the See also:common " bracken," and is a shortened See also:form of that See also:northern Eng. word, derived from a Scand. word for " fern " (cf. Swed. broken), though often confused with " brake," undergrowth. (v) A term 414 applied to many implements and See also:mechanical and other appliances, often spelled " break." Here there are probably several words, difficult to separate in origin, connected either with "break," to separate, and its derived meanings, or with the Fr. braquer (appearing in such expressions as braquer un See also:canon, to turn or point a See also:gun), from O. Fr. brat, See also:modern See also:bras, an See also:arm, See also:Lat. bracchium. The word is thus used of a toothed See also:instrument for separating the fibre of See also:flax and See also:hemp; of the " break-rolls " employed in See also:flour manufacture; of a heavy wheeled vehicle used for " breaking in " horses, and hence of a large See also:carriage of the wagonette type; of an arm or See also:lever, and so of the winch of a crossbow and of a See also:pump handle, cf. " brake-pump "; of a curb or bridle for a See also:horse; and of a mechanical appliance for checking the See also:speed of moving vehicles, &c. It is noteworthy that the two last meanings are also possessed by the Fr. frein and the Ger. Bremse. Brakes, in See also:engineering, are See also:instruments by means of which mechanical See also:energy may be expended in overcoming See also:friction. They are used for two See also:main classes of purpose: (I) to limit or decrease the velocity of a moving See also:body, or to bring it completely to See also:rest; and (2) to measure directly the amount of frictional resistance between two bodies, or indirectly the amount of energy given out by a body or bodies in See also:motion. See also:Machines in which brakes are employed for purposes of the second class are commonly known as dynamometers (q.v.). The other class is exemplified in the brakes used on wheeled vehicles and on See also:cranes, lifts, &c. Here a body, or See also:system of bodies, originally at rest, has been set in motion and has received See also:acceleration up to a certain velocity, the See also:work which has been done in that acceleration being stored up as " actual energy " in the body itself. Before the body can be brought to rest it must See also:part with this energy, expending it in overcoming some See also:external resistance. If the energy be See also:great in proportion to the usual resistance tending to stop the body, the motion will continue for a See also:long See also:time, or through a long distance, before the energy has been completely expended and the body brought to rest. But in certain cases considerations of safety or convenience require that this time or distance be greatly shortened, and this is done by artificially increasing the external resistance for the time being, by means of a brake. A See also:simple method of obtaining this increased resistance is by pressing a See also:block or See also:shoe of See also:metal or See also:wood against the rim of a moving See also:wheel, or by tightening a flexible strap or See also:band on a rotating See also:pulley or See also:drum. In wheeled road vehicles, a wheel may be prevented from rotating by a See also:chain passed through its spokes and attached to the body of the vehicle, when the resistance is increased by the substitution of a rubbing for a See also:rolling See also:action; or the same effect may be produced by fixing a slipper or skid under the wheel. Other forms of brake depend, not on the friction between two solid bodies, but on the frictional resistance of a fluid, as in " See also:fan " and " pump " brakes. Thus the motion of revolving See also:blades may be opposed by the resistance of the See also:air or of a liquid in which they are made to work, or the motion of a plunger fitting tightly in a See also:cylinder filled with a fluid may be checked by the fluid being prevented from See also:escape except through a narrow orifice. The See also:fly used to regulate the speed of the striking See also:train in a See also:clock is an example of a fan brake, while a pump brake is utilized for controlling the recoil of guns and in the See also:hydraulic buffers sometimes fitted at terminal railway stations to stop trains that enter at excessive speed. On electric tramcars a braking effect is sometimes obtained by arranging the connexions of the See also:motors so that they See also:act as generators driven by the moving See also:car. In this way a See also:counter-See also:torque is exerted on the axles. The current produced is expended by some means, as by being made to operate some frictional braking See also:device, or to magnetize See also:iron shoes carried on the car just over, but clear of, the See also:running rails, to which they are then magnetically attracted (see See also:TRACTION). The simplest way of applying a brake is by See also:muscular force, exerted through a See also:hand or See also:foot lever or through a See also:screw, by which the brake block is pressed against the rim of the wheel or the band brake tightened on its drum. This method is sufficient in the See also:case of most road vehicles, and is largely used on railwayvehicles. But the See also:power thus available is limited, and becomes inadequate for heavy vehicles moving at high speeds. Moreover, on a train consisting of a number of vehicles, the hand brakes on each of which are See also:independent of all others, either a brakesman must be carried on each, or a number of the brakes must be See also:left unused, with consequent loss of stopping power; while even if there is a brakesman on every vehicle it is impossible to secure that all the brakes throughout the train are applied with the promptness that is necessary in case of emergency. Considerations of this sort led to the development of power brakes for railway trains. Of these there are five main classes: (I) Mechanical brakes, worked by springs, friction wheels on the See also:axle, chains See also:wound on drums, or other mechanical devices, or by the force produced when, by See also:reason of a sudden checking of the speed of the See also:locomotive, the momentum pow Railwayer of the cars causes pressure on the draw-bars or buffing brakes. devices. (2) Hydraulic brakes, worked by means of See also:water forced through pipes into proper mechanism for transmitting its force to the brake-shoes. (3) Electric brakes. (4) Air and vacuum brakes, worked by compressed air or by air at atmospheric pressure operating on a vacuum. (5) Brakes worked by See also:steam or water from the See also:boiler of the See also:engine, operating by means of a cylinder; the use of these is generally limited to the locomotive. Of this See also:kind is the counter-pressure or water brake of L. le Chatelier. If the See also:valve See also:gear of a locomotive in motion be reversed and the steam regulator be left open, the cylinders act as compressors, pumping air from the exhaust See also:pipe into the boiler against the steam pressure. A retarding effect is thus exercised, but at the cost of certain inconveniences due to the passage of hot air and cinders from the See also:smoke See also:box through the cylinders. To remedy these, le Chatelier arranged that a See also:jet of hot water from the boiler should be delivered into the exhaust pipe, so that steam and not the hot flue gases should be pumped back.
Power brakes may be either continuous or independent—continuous if connected throughout the train and with the locomotive by pipes, wires, &c., as the compressed air, vacuum and electric brakes; independent if not so connected, as the buffer-brakes and hand-brakes. Continuous brakes may be divided into two other great classes—automatic and non-automatic. The former are so arranged that they are applied automatically on all the coaches of the train if any important part of the apparatus is broken, or the couplings between cars are ruptured; in an emergency they can be put on by the guard, or (in some cases) by a passenger. Non-automatic brakes can be applied only by the See also:person (usually the engine-See also:driver) to whom the management of them is given; they may become inoperative on all the coaches, and always on those which have become detached, if a coupling or other important and generally essential part is broken. Many mechanical and several hydraulic and See also:electrical continuous brakes have been invented and tried; but experience has shown them so inadequate in practice that they have all practically disappeared, leaving the See also: (3) It must be capable of being applied by the engine-driver or by any of the officials in See also:charge of the train, either in See also:concert or independently. (4) The motion of the train must be arrested in the shortest possible distance. (5) The failure of a vital part must declare itself by causing the brake to be applied and to remain applied until the cause of failure is removed. (6) The breaking of the train in two or more parts must cause immediate automatic application of the brakes on all the coaches. (q) When used in See also:ordinary service stops it must be capable of See also:gradual and See also:uniform application (followed, if necessary, by a full emergency application at any part of the service application) and of prompt See also:release under all conditions of application. (8) It must be simple in operation and construction, not liable to derangement, and inexpensive in See also:maintenance. The Westinghouse non-automatic or " straight " air-brake, patented in 1869, consists in its simplest form of a See also:direct-acting, Simple steam-driven air-pump, carried on the locomotive, which air-brake. forces compressed air into a See also:reservoir, usually placed under the foot-See also:plate of the locomotive. From this reservoir a pipe is led through the engine See also:cab, where it is fitted with a three-way See also:cock, to the See also:rear of the locomotive See also:tender, where it terminates in a flexible See also:hose, on the end of which is a coupling. The coaches are furnished with a similar pipe, having hose and coupling at each end, which communicates with one end of a cylinder containing a See also:piston, to the See also:rod of which the brake-rods and levers are connected. The application of the brakes is effected by the engine-driver turning the three-way cock, so that compressed air flows through the pipe and, acting against one See also:side of the brake-cylinder piston, applies the brake-shoes to the wheels by the See also:movement of this piston and the rods and levers connected to it. To release the brakes the three-way cock is turned to cut off communication between the main reservoir and the train-pipe, and to open a See also:port permitting the escape of the compressed air in the train-pipe and brake-cylinders. This brake was soon found defective and inadequate in many ways. An appreciable time was required for the air to flow through the pipes from the locomotive to the car-cylinders, and this time increased quickly with the length of the trains. Stilldischarges air from the train-pipe, this See also:equilibrium is destroyed, and the greater pressure in the See also:auxiliary reservoir forces the triple-valve to a position which allows air from the auxiliary reservoir to pass directly into the brake-cylinder. This air forces out the piston of the brake-cylinder and applies the brakes, connexion being made with the brake-See also:rigging at A. The purpose of the small groove n which establishes communication between the two sides of the piston when the brakes are off, is to prevent their unintended application through slight leakage from the train-pipe. To release the brakes, the driver, by moving the handle of his valve to the release position, admits air from the main reservoir to the train-pipe, the pressure in which thus becomes greater than that in the auxiliary reservoir; the piston and slide-valve of the triple-valve are thereby forced back to their normal position, the compressed air in the brake-cylinder is discharged, and the piston is brought back by the coiled See also:spring, thus releasing the brakes. At the same time the auxiliary reservoir is recharged. With this " ordinary " brake, since an appreciable time is required for the reduction of pressure to travel along the train-pipe from the engine, the brakes are applied sensibly sooner at the front Quick• than at the end of the train, and with long trains this See also:Quick air' difference in the time of application becomes a See also:matter of brake. importance. The " quick-acting" brake was introduced to remedy this defect. For it the triple valve is provided with a supplementary mechanism, which, when the air pressure in the train-pipe is suddenly or violently reduced, opens a passage whereby air from the train-pipe is permitted to enter the brake-cylinder directly. The result is twofold: not only is the pressure from the auxiliary reservoir acting in the brake-cylinder reinforced by the pressure in the train-pipe, but the pressure in the train-pipe is reduced locally in every vehicle in extremely rapid :See also:succession instead of at the engine only, and See also:Section through Triple-Valve and Brake-Cylinder. more objectionable, however, was the fact that on detached coaches the air-brakes could not be applied, the result being sometimes serious collisions between the front and rear portions of the train. In the Westinghouse " ordinary " automatic air-brake a main air reservoir on the engine is kept charged with compressed air at Automatic 80 lb per sq. in. by means of the steam-pump, which may Auto ake. be controlled by an automatic See also:governor. On electric See also:railways a pump, driven by an electric motor, is generally employed; but occasionally, on trains which run See also:short distances, no pump is carried, the main reservoir being charged at the terminal points with sufficient compressed air for the See also:journey. Conveniently placed to the driver's hand is the driver's valve, by means of which he controls the flow of air from the main reservoir to the train-pipe, or from the train-pipe to the See also:atmosphere. A reducing-valve is attached to the driver's valve, and in the normal or running position of the latter reduces the pressure of the air flowing from the main reservoir to the train-pipe by to or 15 lb per sq. ip. From the engine a train-pipe runs the whole length of the train, being rendered continuous between each vehicle and between the engine and the rest of the train by flexible hose couplings. Each vehicle is provided with a brake-cylinder H (fig. 1), containing a piston, the movement of which applies the brake blocks to the wheels, an " auxiliary air- reservoir " G, and an automatic " triple-valve " F. The auxiliary reservoir receives compressed air from the train-pipe and stores it for use in the brake-cylinder of its own vehicle, and both the auxiliary reservoir and the triple-valve are connected directly or indirectly with the train-pipe through the pipe E. The automatic action of the brake is due to the construction of the triple-valve, the See also:principal parts of which are a piston and slide-valve, so arranged that the air in the auxiliary reservoir acts at all times on the side of the piston to which the slide-valve is attached, while the air in the train-pipe exerts its pressure on the opposite side. So long as the brakes are not in operation, the pressures in the train-pipe, triple-valve and auxiliary reservoir are all equal, and there is no compressed air in the brake-cylinder. But when, in See also:order to apply the brake, the driver in consequence all the brakes are applied almost simultaneously throughout the train. The same effect is produced should the train break in two, or a hose or any part of the train-pipe burst; but during ordinary or " service " stops the triple-valve acts exactly as in the ordinary brake, the quick-acting portion, that is, the See also:vertical piston and valve seen in fig. 1, not coming into operation. 
When the handle Z is turned to the position X the quick-acting mechanism is rendered inoperative, and when it is at Y the brake on the vehicle concerned is wholly cut out of action. A further improvement introduced in the Westinghouse brake in 1906 was designed to give quick action for service as well as emergency stops. In this the triple-valve is substantially the same as in the ordinary brake. The additional mechanism of the quick-acting portion is dispensed with, but instead, a small chamber, normally containing air at atmospheric pressure, is provided on each vehicle, and is so arranged that it is put into communication with the train-pipe by the first movement of the triple-valve. As soon, therefore, as the driver, by lowering the pressure in the train-pipe, causes the triple-valve in the foremost vehicle of the train to operate, a certain quantity of air rushes out of the train-pipe into the small chamber; a further See also:local reduction in the pressure of the train-pipe in that vehicle is thereby effected, and this almost instantaneously actuates the triple-valve of the succeeding vehicle, and so on throughout the train. In this way, on a train 1800 ft. long, consisting of sixty 3o-ft. vehicles, the brake-blocks may be applied, with equal force, on the last vehicle about 21 seconds later than on the first. Brake-blocks can be applied, without skidding the wheels, with greater pressure at high speeds than at See also:low. See also:Advantage is taken of this fact in the See also:design of the Westinghouse " high-speed " brake, invented in 1894, which consists of Nigh-attachments enabling the pressure in the train-pipe and speedairreservoirs to be increased at the will of the driver. The brake. increased pressure acting in the brake-cylinder increases in the same proportion the pressure of the brake-shoes against the wheels. Attached to the brake cylinder is a valve for automatically reducing the pressure therein proportionately to the reduction in speed, until the maximum pressure under which the brakes are operated in making ordinary stops is reached, when this valve closes and the maximum safe pressure for operating the brakes at ordinary speeds is retained until a stop is made. In the automatic vacuum-brake, the exhausting apparatus generally consists of a combined large and small ejector (a form of jet- p worked drivepr, though sometsteam and under See also:control of the imes s a me h nicalhair-pump, driven vacuum- from the crosshead of the locomotive, is substituted for brake. the small ejector. These ejectors, of which the small one is at work continuously while the large one is only employed when it is necessary to create vacuum quickly, e.g. to take off the brakes after a short stop, produce in the train-pipe a vacuum equal to about 20 in. of See also:mercury, or in other words reduce the pressure within it to about one-third of an atmosphere. The train-pipe extends the whole length of the train and communicates under each vehicle with a cylinder, to the piston of which, by suitable rods and levers, the brake-shoes are connected. The communication between the train-pipe and the cylinder is controlled by a See also:ball-valve, one form of which is shown in fig. 2. The release-valve is for the purpose of unmoved; but with a sudden one the vacuum below the valve is destroyed more quickly, and with the difference of pressure the See also:diaphragm lifts the valve and admits air. A rapid-acting valve (fig. 3) is sometimes interposed between the train-pipe and the cylinder on each vehicle. In the normal or running position, a vacuum is maintained below the valve A and above the diaphragm B, while the chamber below B and above A is at atmospheric pressure.' For an emergency application of the brake, air is suddenly admitted to the train-pipe and thus to the See also:lower side of A, and the pressure acting on the under side of B is sufficient to cause it to lift the valve A, and to admit air from the atmosphere, both to the brake-cylinder and the train-pipe, through the clappet-valve D, which also rises because of the difference of pressure on its two sides. In a graduated application, neither D nor A rises from its seat, but air from the train-pipe finds See also:access to the brake-cylinder by passing around the peg C, which is so proportioned as to allow the necessary amount of air to enter the brake-cylinder, and so obtain simultaneous action of the brake throughout the train. When the handle E is turned so as to prevent the clappet D from rising, the rapid action is cut out and the brake acts as an ordinary vacuum automatic brake. A modification of the device for obtaining accelerated action, described withdrawing the ball from its seat when it is necessary to take off the brakes by hand; it is made air-tight by a small diaphragm, the pressure of which, when there is vacuum in the pipe, pulls in the spindle and allows the ball to fall freely into its seat. When air is exhausted through the train-pipe it travels out from below the piston direct, and from above it past the ball, which is thus forced off its seat, to See also:roll back again when the exhaustion is See also:complete. In this See also:state of affairs the piston is held in equilibrium and the brake-blocks are See also:free of the wheels. To apply them, air is admitted to the train-pipe, either purposely by the guard or driver, or accidentally by the rupture of the train-pipe or coupling-hose between the vehicles. The air passes to the lower side of the piston, but is prevented from gaining access to the upper side by the ball-valve which blocks the passage ; hence the pressure becomes different on the two sides of the piston, which in consequence is forced upwards and thus applies the brakes. They are released by the re-See also:establishment of equilibrium (by the use of the large ejector if necessary); when this is done the piston falls and the brakes drop off. The See also:general arrangement of the apparatus is shown in fig. 2. To render the application of the brakes nearly simultaneous throughout a long train, the valve in the guard's See also:van is arranged to open automatically when the driver suddenly lets in air to the train-pipe. This valve has a small hole through its See also:stem, and is secured at the See also:top by a diaphragm to a small See also:dome-like chamber, which is exhausted when a vacuum is created in the train-pipe. A gradual application destroys the vacuum in the chamber as quickly as in the pipe and the diaphragm remains above in connexion with the Westinghouse brake, is also applicable. Accelerating See also:chambers, again containing air at atmospheric pressure, are provided on each vehicle and are connected with the train-pipe by valves which open as the vacuum in the latter begins to decrease with the operation of the driver's valve. The air thus admitted into the train-pipe effects a still further local reduction of the vacuum, which is sufficient to actuate the accelerating valve of each next succeeding vehicle and is thus rapidly propagated throughout the train. Famous tests of railway brakes were those made by See also:Sir See also:Douglas See also:Galton and Mr George Westinghouse on the See also:London, See also:Brighton and See also:South See also:Coast railway, in See also:England, in 1878, and by Brake a.See also:committee of the See also:Master Car Builders' Association, trials. near See also:Burlington, See also:Iowa, in 1886 and 1887. The See also:object of the former See also:series (for accounts of which see Proc. Inst. Mech. Eng., 1878, 1879) was to determine the co-efficient of friction between the brake-shoe and the wheel, and between the wheel and See also:rail at different velocities when the wheels were revolving and when skidded, i.e. stopped in their rotation and caused to slide. These experiments were the first of their kind ever undertaken,and for many years their results furnished most of the trustworthy data obtainable on the friction of motion. It was found that the co-efficient of friction between See also:cast-iron shoes and See also:steel-tired wheels increased as the speed of the train decreased, varying from 0.111 at 55 m. an See also:hour to 0.33 when the train was just moving. It also decreased with the time during which the brakes were applied; thus at 20 m, an hour the -co-efficient was at the beginning o•182, after ten seconds 0.133, after twenty seconds 0.099. Generally speaking, especially at moderate speeds the decrease in the co-efficient of friction due to time is less than} the increase due to decrease of speed, although when the time is long the See also:reverse may be true. When the wheels are skidded the retardation of the train is always reduced; therefore, for the greatest braking effect, the pressures on the brake-shoes should never be sufficient to cause the wheels to slide on the rails. The Burlington brake tests were undertaken to determine the practicability of using power brakes on long and heavy See also:freight trains. In the 1886 tests there were five competitors—three buffer-brakes, one compressed-air brake, and one vacuum-brake. The tests comprised stops with trains of twenty-five and fifty vehicles, at 20 and Flo. 3.—Rapid-acting Vacuum-Brake Valve. 40 M. an hour, on the level and on gradients oft in too. They demonstrated that the buffer-brakes were inadequate for long trains, and that considerable improvements in the continuous brakes, both compressed-air and vacuum, would be needed to make them act quickly enough to avoid excessive shocks in the rear vehicles. In 1887 the trials of the See also:year before were repeated by the same committee, and at the same See also:place. Trains of fifty vehicles, about 2000 ft. long and fitted with each brake, were again provided, and there were again five competitors, but they all entered continuous brakes—three compressed-air brakes, one vacuum and one electric. The results of the first See also:day's test of the train equipped with Westing-house brakes are shown in Table I., the distances in which are the feet run by the train after the brakes were set, and the times the seconds that elapsed from the application of the brakes to full stop. Speed in Distance in Time in See also:Equivalent Distance See also:Miles per Feet. Seconds. at 20 M. and 40 M. Hour. 191 186 91 196 693 191 215 II 233 361 588 17 -- The remarkable shortness of these stops is the more evident when they are compared with the best results obtained in 1886, as shown in Table II. Speed in Distance in Time in Equivalent Distance Miles. Feet. Seconds. at 20 M. and 40 M. 23.5 424 171 307 20.3 354 16 340 40 922 221 922 40 927 221 -- 927 The time that elapsed between the application of the brakes on the engine and on the fiftieth vehicle was almost twice as great in 1886 as in 1887, being in the latter tests only five to six seconds, and in 1887 the stops were made in less than two-thirds the distance required in 1886. Still, violent shocks were caused by the rear vehicles running against those in front, before the brakes on the former were applied with sufficient force to hold them, and these shocks were so severe as to make the use of the brakes in practice impossible on long trains. When the triple-valves were actuated electrically, however, the stops were still further improved, as shown in Table III. IV. 14TABLE III. Stops of a Train of Fifty Empty Cars— Electric Application of Air-Brakes. Speed in Distance in Time in Equivalent Distance Miles. Feet. Seconds. at 20 M. and 40 m. 211 16o 7 139 .. 23 183 8 138 -. 38 475 142 519 361 460 14 545 Although the same levers, shoes, rods and other connexions were used, there were no shocks in the fiftieth car of the train on any stop, whether on the level or on a gradient. The committee in charge reported that the best type of brake for long freight trains was one operated by air, in which the valves were actuated by See also:electricity, but they expressed doubt of the practicability of using electricity on freight trains. The Westinghouse See also:Company then proceeded to quicken the action of the triple-valve, operated by air only, so that stops with fifty-car trains could be made without See also:shock, and without electrically operated valves; and they were so successful in this respect that, towards the end of the same year, 1887, with a train of fifty vehicles, stops were made without shock, fully equalling in quickness and shortness of distance run any that had been made at the trials by the electrically operated brakes. In 1889 some further tests were made by Sir Douglas Galton with the automatic vacuum-brake, on a practically level portion of the See also:Manchester, See also:Sheffield & See also:Lincolnshire railway (now the Great Central). The train was composed of an engine, tender and See also:forty carriages, the See also:total length over buffers being 1464 ft., and the total See also:weight 574 tons, of which 423 tons were braked. At a speed of about 32 m. an hour this train was brought to a standstill in twelve seconds after the application of the brakes, in a distance of 342 ft. Additional information and CommentsThere are no comments yet for this article.
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