The automatic air-brake was very gen erally adopted for the passenger trains of all important railroads, and fully met all the requirements of its day. When, however, in the development of railroad transportation, the necessity for the use of an automatic power brake upon freight trains became apparent, new conditions were discovered which the automatic air brake was not qualified to meet.
It was clearly evident that the useful ness of the automatic air-brake upon freight trains became contingent upon the discovery of some means whereby the in terval of time elapsing between the ap plication 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 con ditions of operation made it equally evi dent that but two methods could be uti lized for securing a more nearly simul taneous 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 approxi mately the same time. While the first of these two methods proves entirely satis factory for ordinary application of the brakes in regular service, so much time is occupied by it that it is wholly unsuit able for applying the brakes when emer gencies 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.
'rho quick-action air-brake was in troduced 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 dis tinct brake systems, one of moderate power and smooth and gentle application for all the customary operations of every day train service, and the other of 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 pres sure in the train pipe takes place. When the air pressure in the train pipe is re duced comparatively slowly, the leftward 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 con tinued reduction of the air pressure in the train pipe (acting upon the left face of the piston), so that such a preponder ance of air pressure upon the right face of the piston, as is necessary to compress the spring of the stem, does not oc cur. 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 neces sary, 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 re sult in a service application, or to dis charge 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 res ervoirs 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 three way 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 fol lows: 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 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 ap plication of the brakes, so that the brakes may be kept applied with the force cor responding 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 mechan ism 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 posi tions of a rotary disk valve, the various positions of which are defined and se cured by the movement of a handle op erated by the engineer.
For detailed descriptions of the oper ation of the air-brake consult "The De velopment of the Electro-Pneumatic Brake," and other publications of the Westinghouse Air-Brake Company.