340
BRAKES
current furnishing the brake resistance. The current generated The Westinghouse non-automatic air-brake consists in varies in potential from zero to a maximum, and the speed of fall its simplest form of a direct-acting, steam-driven air-pump, of the cage is regulated by varying the electrical resistance. carried on the locomotive, which forces comElectric lifts similarly operated are used also on warships for pressed air into a reservoir, usually placed under supply of ammunition to the guns. jj_ Ba ^ the foot-plate of the locomotive. From the (3) Kailway.—From the very beginning of steam reservoir a pipe is led through the engine cab, where it is railways, hand-brakes of various forms have been em- fitted with a three-way cock, to the rear of the locomotive ployed, and they are still largely used on goods trucks. tender, where it terminates in a flexible hose, on the end Each hand-brake is independent of all others, and the of which is a coupling. The coaches are furnished with a time required to bring a number of them into operation similar pipe, having hose and coupling at each end, which depends upon the activity of the brakemen. The neces- is connected to one end of a cylinder containing a piston, sity for prompt and effective application of brakes in an to the rod of which the brake-rods and levers are connected. emergency led to the invention of means to operate them The application of the brakes is effected by the enginewith power superior to muscular force. Such brakes are driver turning the three-way cock, so that compressed air known as power brakes. These may be divided into four flows through the pipe and acting against one side of the classes :—(1) Mechanical brakes, worked by springs, by brake-cylinder piston, applies the brake-shoes to the wheels friction wheels on the axle, by chains wound on drums, by the movement of this piston and the rods and levers and other mechanical devices, or by the force produced connected to it. To release the brakes the three-way cock when, by reason of a sudden checking of the speed of the is turned to cut off communication between the main locomotive, the momentum of the cars causes pressure on reservoir and the train-pipe, and to open a port permitting draw-bars or buffing devices. All such momentum or the escape of the compressed air in the train-pipe and buffer brakes were proved radically defective by tests made brake-cylinders. This brake was soon found defective and by a committee of the Master Car Builders’ Association of inadequate in many ways. An appreciable time was rethe United States at Burlington, Iowa, in 1886. (2) quired for the air to flow through the pipes from the Hydraulic brakes, worked by means of water forced locomotive to the car-cylinders, and this time increased through pipes into proper mechanism for transmitting its quickly with the length of the trains. Still more serious, force to the brake-shoes. (3) Electric brakes, worked by however, was the fact that on detached coaches the airmagnets acting on discs or similar appliances fastened to brakes could not be applied, the result being sometimes the wheels or axles; or, more recently, by magnets using serious collisions between the front and rear portions of the rail as a keeper and utilizing the drag thus produced the train. Those respects in which the non-automatic to force ordinary brake-shoes against the wheels. (4) Air- brake is inadequate will be understood from the following and vacuum-brakes, worked by compressed air, or by air summary of the requirements most important in a trainat atmospheric pressure operating on a vacuum. To these braking apparatus. (1) It must be capable of application might be added brakes worked by steam or water from to every wheel throughout the train. (2) It must be so the boiler of the engine, operating by means of a cylinder, prompt in action that the shortest possible time shall the use of which is generally limited to the locomotive. elapse between its first application and the moment when Prominent among these is the Le Chatelier or water-brake the full power can be exerted throughout the train. (3) It so called, in which a pipe is led from the boiler below the must be capable of being applied by the engine-driver or water line to the locomotive cylinder exhaust ports, the by any of the officials in charge of the train, either in concylinders acting as compressors. The arrangement also cert or independently. (4) The motion of the train must prevents hot gases and cinders from being drawn into the be arrested in the shortest possible distance. (5) The failure of a vital part must declare itself by causing the cylinders from the smoke-box. Power brakes may be either continuous or independent— brake to be applied and to remain applied until the cause continuous if connected throughout the train and with the of failure is removed. (6) The breaking of the train in two locomotive by pipes, wires, &c., as the compressed air, or more parts must cause immediate automatic application vacuum and electric brakes; independent if not so con- of the brakes on all the coaches. (7) When used in nected, as the buffer-brakes and hand-brakes. Continuous ordinary service stops it must be capable of gradual and brakes may be divided into two other great classes— uniform application (followed, if necessary, by a full automatic and non-automatic. The former are so arranged emergency application at any part of the service applithat they are applied automatically on all the coaches of cation) and of prompt release under all conditions of the train if any important part of the apparatus is broken, application. (8) It must be simple in operation and or the couplings between cars are ruptured; in an emer- construction, not liable to derangement, and inexpensive gency they can be put on by the guard, or (in some cases) in maintenance. In the automatic vacuum-brake, the exhausting apparaby a passenger. Non-automatic brakes can be applied tus generally consists of a combined large and small ejector, only by the person (usually the engine-driver) to whom the management of them is given. They may become inopera- worked by steam and under the control of the tive on all the coaches, and always on those which have driver, though in some forms of the vacuumbecome detached, if a coupling or other important and brake, such as are used on the Great Western brake. generally essential part is broken. Many mechanical and and London and North-Western railways, an airseveral hydraulic and electrical continuous brakes have pump, driven from the main axle of the locomotive, is been invented and tried. Experience has shown them so substituted for the small ejector. These ejectors, of which inadequate in practice that they have all practically dis- the small one is at work continuously while the large one appeared, leaving the field to the air- and the vacuum- is only employed when it is necessary to create vacuum brakes. At first these were non-automatic, but in 1872 quickly, e.g., to take off the brakes after a short stop, prothe automatic air-brake was invented by Mr George West- duce a vacuum of 18 or 20 inches in the train-pipe, which inghouse, the many advantages of which over the non- extends the whole length of the train and communicates automatic brake quickly led to its wide adoption. An under each vehicle with a cylinder, to the piston of which, automatic vacuum-brake, invented a few years later, is by suitable rods and levers, the brake-shoes are connected. used largely in the United Kingdom and the British The communication between the train-pipe and the cylinder | is controlled by a ball-valve, which consists merely of a colonies.
