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BRAKES

341

half-inch brass ball, having a rolling action in a horizontal pressure on which, when there is vacuum in the pipe, pulls direction (see Figs. 1 and 2). The spindle with the release in the spindle and allows the ball to fall freely into its seat. When air is exhausted through the train-pipe it travels out from the top of the cylinder direct, and from the bottom past the ball, which is thus forced up the incline of the spindle, to roll back again to its seating when the exhaustion is complete. In this state of affairs the piston is held in equilibrium and the brake-blocks are free of the wheels. To apply them, air is admitted to the train-pipe, either purposely by the guard or driver, or accidentally by the rupture of the train-pipe or coupling hose between the vehicles. The air passes to the lower side of the piston, but is prevented from gaining access to the upper side by the ball valve which blocks the passage ; hence the pressure becomes different on the two sides of the piston, which in consequence is forced upwards and thus applies the brakes (see Fig. 1). They are released by the re-establishment of equilibrium (by the use of the large ejector if necessary); when this is done the piston falls and the brakes drop off. The general arrangement of the apparatus is shown in Fig. 2. To render the application of the brakes nearly simultaneous throughout a long train, the valve in the guard’s van is arranged to open automatically when the driver suddenly lets in air to the train-pipe. This valve has a small hole through its Fm. 1.—Automatic Vacuum-Brake. (The dots indicate atmospheric pressure.) stem and is secured at the top by a diaphragm to a small valve is added for the purpose of withdrawing the ball dome-like chamber, which is exhausted when a vacuum is from its seat when it is necessary to take off the brakes by created in the train-pipe. A gradual application destroys hand; it is made air-tight by a small diaphragm, the the vacuum in the chamber as quickly as in the pipe, but Steam Combination Ejector

Dnp Trap Fig. 2.—Automatic Vacuum-Brake, showing its general arrangement. with a sudden one the vacuum below the valve is de- the pump, controlled by an automatic governor. Constroyed more quickly, and so the pressure of the atmosphere veniently placed on the locomotive is a valve called the driver’s valve, which is connected by a pipe Automatic on the diaphragm lifts the valve and admits air. In the Westinghouse automatic quick-action air-brake a with the main reservoir; by means of this the ait.mbrakem main air reservoir on the engine is kept charged with driver controls the flow of air from the main, compressed air at 90 lb per square inch by means of reservoir to the train-pipe, or from the train-pipe to the