AIR BRAKE. A brake operated through the medium of compressed air, and extensively applied to railroad trains and street railway engineer and inventor, Mr. George Westing house. It consisted of a steam-driven, direct acting air compressor (see Fig. 1), carried on the locomotive, which forced compressed air into a reservoir. This reservoir was connected through a three-way valve, in the engine cab, to the rear of the locomotive tender where it terminated in a flexible hose with a coupling. Throughout the length of every car a pipe was extended, with a similar hose and coupling at each end. To combine a number of cars and a locomotive into a train the respective couplings were united, making the brake pipe continuous throughout the train. The brake pipe at the rear of the train was closed by a cock. On each car, the brake pipe communicated with one face of its respective brake cylinder piston. On the other face were located brake rods and levers holding the brake shoes almost up against the treads of the car wheels. Normally the three-way valve connected the brake pipe to the atmosphere. To apply the brakes, the engineer turned the three-way valve to a posi tion which cut off the brake pipe from the at mosphere and permitted compressed air to flow into it from the reservoir (now known as the main reservoir) on the locomotive. This air, thereupon, forced the brake cylinder pistons on each car outward, actuating the brake rods and levers, and thereby forcing the shoes against the treads of the car wheels.
Whenever the desired pressure was developed in the brake pipe, the engineer turned the three way valve into a position (lap position) which cut off the main reservoir from the brake pipe as well as continuing to hold the communica cars. Originally this brake was devised merely as a safety device, but is now considered in valuable as a dividend-earning asset also. This latter consideration is based on the principle, that the better the brake, the higher the sched uled speed; consequently, the more frequently the car or train can traverse a given road so that fewer cars need be purchased and less labor employed. The air brake permits high sched uled speed because of the short emergency stopping distance made possible from high velocities and because a train or car may be run at high speed within a short distance of the stopping point, which is a highly important feature where stops are frequent. The first type of this appliance, designated as the straight-air, or non-automatic, brake, was in vented in 1869 by the well-known American tion between the brake pipe and atmosphere closed. In order to release the brakes, the three-way valve was moved into a position which cut off the main reservoir from the brake pipe and, at the same time, vented the latter to the atmosphere. This operation removed the pressure from the faces of the brake cylinder pistons and permitted the brake cylinder release springs to pull the shoes from the treads of the wheels, through the medium of the rods and levers.
This brake, while being far superior to any vehicle brake heretofore invented, required im provements in the following respects: (1) The brake pipe pressure, being higher on the front than on the rear cars during the period the brake pipe pressure was being built up, caused the rear cars to run violently into the head cars and produced a rough stop. (2) The stop with a long train was prolonged through a considerably greater distance than with a single car because of the time necessary to transmit the required air through the long brake pipe to the respective brake cylinders. (3) For a
long train, an impracticably large size main reservoir was required to fill all brake cylinders to standard maximum pressure without the as sistance of the compressor. (4) In case a train broke in two, or a hose burst, the brake would be lost. This would be especially danger ous on grades.
In order to improve these conditions, Mr. Westinghouse invented the automatic brake in 1872 (see Fig. 2). With this scheme, air was through a valve which reduced its pressure, was admitted by the brake valve into the brake pipe. The air then passing to the outer face of each triple valve piston held each in its innermost position, where this air was able to flow by the piston through a groove into a storage reservoir (auxiliary reservoir), charging it until the pres sure on the two faces of the triple valve piston became balanced. In this innermost position of the triple valve piston, a slide valve, controlled by the piston, locally connected its respective brake cylinder to the atmosphere. To retard partially, or stop a train under ordinary con ditions, the brake valve was moved into "seri. ices position whereby the main reservoir air supply was cut off from the brake pipe and the stored in a reservoir on each car at a time when a brake was not required. When a brake appli cation was desired the stored air, being at the spot ready for use, was caused to flow from the reservoir on each car to its respective brake cylinder. In order to apply the brakes with this arrangement, it became necessary to employ a device for controlling the flow of this stored air, known as a plain triple valve, on each car. This device had to be operated just the reverse of the straight air brake, viz., to release the brakes, air was put into the brake pipe, and to apply them, the brake pipe air was vented to the atmosphere. This scheme had the added advantage of operating on the °closed circuit° principle, in that, whenever a train broke in two, or a hose ruptured, the resultant reduction in brake pipe pressure caused the brakes to apply automatically. Experience with the air brake to this time indicated that two rates of applying the brakes were desirable for train control :— one, a comparatively slow rate (service rate) for ordinary use, through which the train would be stopped as quickly as con sistent with the comfort to passengers and free dom from damage to lading and equipment ; the other, a rapid rate (emergency rate) through which the train would be stopped at a rate As great as conditions would permit, to be used only when a short, quick stop became necessary. To utilize these new functions, the engineer's three-Way valve was developed into a form designated as a brake valve. With this automatic system, as with the straight air system, compressed air was forced into the main reservoir by a compressor. Normally, however, when the train was running free over the road, the compressed air, after passing air in the latter vented to the atmosphere. The rate of reduction in brake pipe pressure, while slow, was nevertheless faster than air could flow from the storage reservoir through the groove past the triple valve piston back into the brake pipe. Consequently, a differential pres sure, thus developed in the triple valve pinion, forced it outward to a position where its ac companying slide valve cut off the brake cylinder from the atmosphere and connected the brake cylinder to the auxiliary reservoir in such 2 manner that the pressure in the auxiliary reser voir dropped at the same rate as the pressure in the adjacent section of the brake pipe.