Page:Collier's New Encyclopedia v. 01.djvu/115

AIR-BRAKE trains became apparent, new conditions were discovered which the automatic air-brake 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 everyday 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 pressure in the train pipe takes place. When the air pressure in the train pipe is reduced 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 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 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 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