AIR BRAKE. A brake worked by com pressed air, which is extensively applied to rail way ears in the United States, and also to a less extent in other countries. Air brakes are also used on street railway ears. The air brake in its first form was invented by George Westing house, Jr., an American engineer, in 1S69, and is known' as the straight air brake. This brake consisted of an air pump, a main reservoir, and an engineer's valve on the locomotive, and of a train pipe and brake cylinder on each ear. The air pump served to keep the main reservoir filled with air under pressure, and the brakes were ap plied by throwing the engineer's valve so as to allow the air from the main reservoir to enter the train pipe and thence into the brake cylin ders on the cars, thus forcing the pistons out and applying the brakes on eaeh car. The train pipe of one ear was connected to that of the next by flexible hose, with a coupling between cars. This form of brake had several objections, the more important of which were that the brakes on the forward ears were applied so much sooner than those on the rear ears that the rear cars bunted up against the forward ears, causing shocks and damage; and in ease a hose burst or a coupling parted, the air pressure would es cape without setting the brakes. These objec tions to the straight air brake led Mr. Westing house to invent, in 1873, the automatic air brake.
In this brake each car was equipped with an auxiliary reservoir and a triple valve in addition to the train pipe and brake cylinder. The triple valve was located at the junction of the train pipe and the two pipes leading to the brake cylinder and to the auxiliary reservoir. The principle of operation of this improved brake is as follows: Air pressure is maintained in the an_xiliary reservoirs and in the train pipe at all times when the brakes are not applied, the pressure in the train pipe being exactly equal to that in the reservoirs, and there being no pres sure in the brake cylinder, owing to the fact that as long as the train pipe and auxiliary reservoir pressures are equal, the triple valve is held in a position closing the air inlet to the brake cylinder. To apply the brakes, the equili brium between the train pipe and the auxiliary reservoir pressures is disturbed by allowing air to escape from the train pipe: as soon as this is done, the excess air pressure in the auxiliary reservoir throws the triple valve so that it admits pressure front the reservoir into the brake cylin der and applies the brakes. To release the brakes, air pressure is retained in the train pipes by admitting air to it from the main reservoir on the locomotive. This gives an ex cess pressure in the train pipe above the pres sure in the auxiliary reservoir, which throws the triple valve so as to close the inlet to the brake cylinder and open the inlet to the auxiliary reservoir from the train pipe, thus allowing the two to attain equal pressures again. To permit air to escape from the train pipe, and thus apply the brakes, there is the engineer's valve pre viously mentioned, and also a conductor's valve on each ear, the latter being used only in case of emergency. It is evident also that should break occur in the train pipe, or its hose connec tions, through any accident, the pressure is re lieved and the brakes are applied automatically.
It will readily be appreciated from what has been said that the triple valve is an exceedingly important part of the mechanism of the auto matic air brake. It performs three duties: (I)
Charge): the auxiliary reservoirs; (2) applies the brakes; and (3) releases the brakes. These duties are. moreover, performed automatically, and, as experience has shown, with almost abso lute certainty as long as the valve mechanism is kept in good order. The triple valve is, how ever, not the only automatic feature of the air brake. The operation of the air pump is eon trolled automatically by a pump governor, which shuts the steam off from the air pump as soon as the pressure in the main reservoir has reached a eertain amount, and admits it again when the pressure falls below this amount. There is also an automatic contrivance for closing the ends of the coupling hose when they are disconnected; this valve opens automatically when the hose is coupled. This describes briefly the construction and operation of the plain automatic air brake. It was, as will be obvious to all, a vast improve ment over the straight air brake. Its chief ob jection was that in an emergency application on a long train the forward brakes were applied so much sooner than those in the rear that the slack of the train ran ahead and often did great damage. To remedy this objection Mr. Westing house invented, in 1887, the quick action triple valve, by which the application was so much hastened at the rear of the train that the slack had no chance to run ahead. At present the quick action brake is the prevailing equipment of railway trains in America, it having replaced practically entirely the plain automatic brake. The very high passenger train speed; of recent years led Mr. Westinghouse, in 1897, to place on the market a high speed brake. This brake is designed to use very high air pressure when the brake is applied with the train at full speed, which pressure is gradually reduced by an anto matie reducing. valve on the brake cylinder as the speed diminishes. This brake has not been extensively used. Tests made with the regular high speed brake attached to a fifty•car train showed the following among other results: Emergency stop of train running at 40 miles per hour made in about 075 feet, in 20 seconds; breaking the train in two at a speed of from 20 to 25 miles per hour, the two sections stopped at distances of from 32 feet to 180 feet apart ; applying brakes with train stamling to show rapidity of action. all brakes applied within two seconds; comparison of emergency air brake stop and band brake stop at 20 miles per hour; air brake stop in 158 to 194 feet, hand brake stop in 1000 feet to 1720 feet ; service stop test to de termine time of release of brakes, all brakes released in four seconds. Several forms of air brake besides the Westinghouse have been em ployed to some extent in America, but they are exactly similar in their principles of operation. The air brake has been applied to electric street ears and to cable ears. In this application the air pump is operated from one of the axles, and usually the straight air system is used, in ex actly similar form as it was formerly used for steam railway trains. Those desiring further and more technical information on this subjeet are referred to Blackall, Air Brake Catechism (New York, 1901)) ; Synnestvedt. Air Brake Dis eases (New York. 1900); Proceedings Master Car Builders' Association.