Engineering Wonders of the World
Volume I

■' ■............................................................................ ■■■ 250 ENGINEERING WONDERS OF THE WORLD. slightly reduced, the piston is pushed further down by the higher pressure of the reservoir air, VVV moves until y comes opposite a, and ports a1 and b1 are closed. Air passes by V2 and through pipe C to the brake cylinder. This reduces the pressure on the upper side of the piston, which moves up- wards, closing V2, but not shifting the main valve, and the brakes are held on gently. A decided reduction of train pipe pressure draws VVV down till x is opposite a1; full reduction pulls the slide quite clear of a1, so opening the brake port a1 full. The brakes go hard on. To release them the pressure must be restored in the train pipe by admitting air from the engine reservoir and pump. The Rapid-acting Triple Valve. to the brake Escape^ 'fr/p/e^ Brake sq. inch Fig. 9. B 80/bs persq. inch BO lbs per cylinder D C Three way cock emergency application of the brake not only connects reservoir and brake cylinder in the manner described above, but also opens a secondary valve, which allows air to flow directly from the train pipe cylinder. The valve remains open until the pressure in the brake cylinder is the same as that in the train pipe, when it closes, preventing a return of air to the train pipe, a. This system has the double advantage of using the train pipe air in the brake cylinders, and of so accelerating the fall of pressure in the pipe, and, consequently, the action of the ordinary triple valve, as to ensure practically simultaneous action throughout trains of almost any length. The air-pump of the Westinghouse brake is controlled automatically by a governor attached pipe leading from the boiler to the steam cylinder of the pump. An excess of pressure in the reservoir opens a port, which admits compressed << i to the steam Main Reser- voir on En- gine. øO/bsper sq. inch. The Air- Pump. valve piston rises, first closes and then pushes up the slide valve, so connecting the brake reservoir with the train pipe through groove G and the brake cylinder with the exhaust through a1, a, b, ö1: the brakes fall off. The operations of charging reservoirs and applying the brakes are shown graphically by Figs. 9 and 10 respectively, wherein, for simplicity’s sake, the triple valve is replaced by a four-way cock. These diagrams explain themselves. In the Westinghouse system the rapid-acting valve of the vacuum brake has its counter- part in the rapid-acting triple valve. The principle of this is, briefly, as follows :—An V2, Fig. 10. to a little cylinder, and B 70/bs tw sq. in. A BO/bsper square inch. air piston carrying the steam valve at its lower end. The depression of this valve on to its seating cuts off the steam. After an application of the brakes and a reduction of reservoir pres- sure, the port referred to is closed, the air escapes from the cylinder, and the steam valve opens again and starts the pump. On electric railways an electric air-compressor, forces down a