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 airbrake 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 designed to remedy the defects of the earlier system and to meet the advancing requirements of the time. The apparatus consisted of that already employed in the straight air-brake system, with the addition upon each vehicle of a storage reservoir, 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 leading 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 storage reservoir, for the purpose of charging the latter with air pressure; (2) permitting 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 AIR-BRAKE 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 reservoirs. 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 individual cars. The automatic air-brake is applied by an operation of the triple valve which results from the discharge of air from the train pine to the atmosphere. The application of the brakes need not be confined to the manipulation of the operating valve by the engineer, but will result from any cause by by which the trainpipe air pressure may become sufficiently reduced. It was this feature of the apparatus which gave it the designation automatic. 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 automatically 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 preeminent. Of all the various manipulations of the air pressure, that of permitting 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, therefore, taken a most conspicuous place in the front rank of railroad safety appliances. 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, connected with the train pipe in each passenger car, or by the occurrence of any disorder which dissipates the air pressure 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.
The automatic air-brake was very generally adopted for the passenger trains of all important railroads, and fully met all the requirements of its day. When, however, in the the development of railroad transportation, the necessity for the use of an automatic power brake upon freight AIR-BRAKE trains became apparent, new conditions were discovered which the automatic airbrake was not qualified to meet. It was clearly evident that the usefulness of the automatic air-brake upon freight trains became contingent upon the discovery of some means whereby the interval of time elapsing between the application of the brakes upon the cars of the forward end of the train and of those at the rear end of the train could be so diminished that no damaging shocks should result from any operation of the brakes. An examination of the conditions of operation made it equally evident that but two methods could be utilized for securing a more nearly simultaneous application of the brakes to all the cars, one of which is to reduce the air pressure in the train pipe so gradually that such reduction is nearly uniform throughout the train, and the other is to provide a series of openings in the train pipe, in addition to that through the engineer's brake valve, so that the train pipe air may be discharged at different points throughout the train at approximately the same time. While the first of these two methods proves entirely satisfactory for ordinary application of the brakes in regular service, so much time is occupied by it that it is wholly unsuitable for applying the brakes when emergencies require prompt and efficient action. The second method, therefore, became the only practical solution of the use of the compressed air-brake as an effective safety appliance upon freight trains.
The quick-action air-brake was introduced by Mr. Westinghouse about 1888, and was the result of the development of this principle.
The quick-action automatic air-brake system virtually consists of two distinct brake systems, one of moderate power and smooth and gentle application for all the customary operations of everyoperations ns of of everye day train service, and the other of high the other of hig high power and violent application for use only when emergencies require most energetic means to avert destruction of life and property. It has practically succeeded all other forms of power brake upon railroad trains, and in 1900 was in use upon about 1,000,000 cars.
It has already been noted that the condition which determines whether a service or an emergency application of the brakes will result from a reduction of the air pressure in the train pipe is the rate of rapidity or the suddenness with which the reduction of the air pressure in the train pipe takes place. When the air pressure in the train pipe is reduced comparatively slowly, the leftward AIR-BRAKE movement of the triple valve piston is terminated by the resistance of the spring supporting the stem in such a position that the compressed air of the auxiliary reservoir becomes discharged into the brake cylinder, thereby reducing the air pressure of the auxiliary reservoir (which acts upon the right face of the triple valve piston) co-ordinately with the continued reduction of the air pressure in the train pipe (acting upon the left face of the piston), so that such a preponderance of air pressure upon the right face of the piston, as is necessary to compress the spring of the stem, does not occur. It is only when the air pressure, acting upon the left face of the triple valve piston, is reduced much more rapidly than the discharge of auxiliary reservoir air to the brake cylinder, will permit the air pressure upon the right face of the piston to be reduced, that the piston makes its complete movement to the left and causes a quick application of the brakes throughout the train. It is necessary, therefore, that the engineer's brake operating valve shall be provided with such means as shall readily enable the engineer to discharge air from the train pipe with only such rapidity as shall result in a service application, or to discharge the air with such greater rapidity as shall cause the emergency application of the brakes.
It is found also that, inasmuch as it is necessary to elevate the air pressure in the train pipe as rapidly as possible, to a point somewhat above the pressure of the air remaining in the auxiliary reservoirs after an application of the brakes, in order to force the triple valve piston to the right and release the brakes, the provision of a stored pressure in the main reservoir upon the locomotive, higher than that ordinarily charged into the train pipe and brake apparatus, is very desirable for temporary use in effecting a prompt release of the brakes. It has thus occurred that the primitive threeway cock, used for an engineer's brake operating valve, with the earlier forms of the air-brake, has given place to a more complicated device, now employed for effecting the various operations of the quick-action air-brake.
The functions of the modern engineer's brake valve may be enumerated as follows: To supply air to the train pipe and the auxiliary reservoirs throughout the train, at a certain definitely determined pressure for the proper operation of the brakes, the standard pressure adopted for this purpose by the railroads being 70 pounds; to discharge air from the train pipe to the atmosphere at such a rate of rapidity that all the applications AIRD of the brakes in customary service may be effected without the operation of the quick-action mechanism of the triple valves; to maintain any reduced train pipe air pressure resulting from an application of the brakes, so that the brakes may be kept applied with the force corresponding to such reduced train pipe pressure; to discharge air from the train pipe to the atmosphere with such rapidity, in emergency applications of the brakes, as shall cause the quick-action mechanism of the triple valves to operate with certainty; and to temporarily supply the train pipe with an unusually high air pressure whenever the brakes are to be released. These various operations are in practice controlled by different positions of a rotary disk valve, the various positions of which are defined and secured by the movement of a handle operated by the engineer. AISNE erally in use is a considerable improvement. A bell-formed receiver of glass is made to rest on a horizontal plate of thick glass ground perfectly smooth. In the center of that plate, under the receiver, is an opening into a tube which, passing for some distance horizontally, ultimately branches at right angles into two portions, entering two upright cylinders of glass. The cylinders are firmly cemented to the glass plate, and within them are two pistons fitting them so closely as to be air-tight. Each piston is worked by a rack and pinion, turned by a handle; while each cylinder is fitted with a valve, so contrived that, when the piston is raised, communication is opened between the cylinder and the receiver, which communication is again closed as the piston falls. It is evident that when anyone commences to work the machine, the air in the cylinders will be immedi-
For detailed descriptions of the operately expelled the first upward motion ation of the air-brake consult "The Development of the Electro-Pneumatic Brake," and other publications of the Westinghouse Air-Brake Company.