AIR-BRAKE, a mechanical device for regulating the speed of railroad trains and for stopping them. In the course of its development, the air-brake has been known in three different forms—the straight air-brake, the automatic air brake and the quick-action automatic air-brake, each in its turn fulfilling the requirements of its day and laying the foundation for the succeeding form.
The straight air-brake was the easiest and simplest form and was introduced by Mr. Westinghouse about the year 1869.
The automatic air-brake, introduced by Mr. Westinghouse in 1873, was de signed to remedy the defects of the earlier system and to meet the advancing re quirements of the time. The apparatus consisted of that already employed in the straight air-brake system, with the ad dition upon each vehicle of a storage res ervoir, of sufficient capacity to supply the brake cylinder upon that vehicle, and a valve mechanism, called a triple valve, operated by variations in the air pressure in the train pipe, to control the operation of the brake cylinder. This triple valve was placed in the branch pipe leading from the train pipe to the brake cylinder, and was also supplied with a pipe lead ing to the new storage reservoir. It was called a triple valve because it performed the three functions of (1) permitting air to flow from the train pipe into the stor age reservoir, for the purpose of charging the latter with air pressure; (2) permit ting the compressed air to flow from the reservoir into the brake cylinder, for the purpose of applying the brakes, and (3) permitting the compressed air to flow from the brake cylinder to the atmosphere to remove the pressure from the brake cylinders and thereby release the brakes. The storage reservoir upon the locomotive became thereafter known as the main reservoir, and those upon the individual cars became known as auxiliary reser voirs. The characteristic feature of the automatic air-brake is the triple valve, under the immediate control of which are all the operations of the brakes upon in dividual cars.
The automatic air-brake is applied by an operation of the triple valve which results from the discharge of air from the train pipe to the atmosphere, The application of the brakes need not be confined to the manipulation of the op erating valve by the engineer, but will result from any cause by which the train pipe air pressure may become sufficiently reduced. It was this feature of the ap paratus which gave it the designation au tomatic. Should any portion of the train become detached, or should the train pipe or hose become ruptured, a reduction of air pressure in the train pipe immediately follows, and the brakes become automati cally applied upon all the cars of the train. The importance of this feature of the automatic brake is very marked. Of all the operations of the air-brake apparatus, the necessity of prompt and reliable action, when the full retarding effect of the brakes is needed, stands pre eminent. Of all the various manipula tions of the air pressure, that of per mitting the air pressure in the train pipe to be discharged to the atmosphere is the simplest and most surely attainable. In this way the prompt response of the brake apparatus, when emergency calls for its operation, is most fully assured, and the .automatic air-brake has, there fore, taken a most conspicuous place in the front rank of railroad safety ap pliances. No accidental disorder of the apparatus can prevent the application of the brakes in emergencies. By means of the engineer's operating valve, or of a valve called the conductor's valve, con nected with the train pipe in each pas senger car, or by the occurrence of any disorder which dissipates the air pres sure in the train pipe, the apparatus automatically causes the train to come to a stop—in the latter case calling attention to the disorder and giving opportunity for such repair as shall again insure safety before the train proceeds.