Fig. 50.—Cross-section of a Westinghouse air brake.
68. The Air Brake.—Compressed air is used to do work in many machines, such as pneumatic drills, hammers, and air brakes. The Westinghouse air brake (Fig. 50) uses air at a pressure of about 70 lbs. to the square inch. The essential parts as shown are a reservoir R, the brake cylinder C and a triple valve V, placed under each car with an air pipe P, leading to the engine. This is connected to R by the triple valve V. When the pressure in P is reduced by the engineer or by accident, the triple valve operates so as to admit air from R into the cylinder C pushing the piston H to the left. H is connected to the brakes by levers which press the brake shoes strongly against the wheels. When the air pressure in P is restored the triple valve acts so as to permit the air in C to escape while R is filled again from P. The hissing sound heard when a train stops is caused by air escaping from cylinder C. The spring in C keeps the brakes from the wheels except when the "air is on."
Fig. 51.—Cross-section of a gas meter showing its construction and action.
69. The Gas Meter.—The gas meter consists of a box divided into two parts by a vertical partition (Fig. 51). Two bellows are attached to this partition, one on each side. The valves that regulate the flow of gas to and from the bellows and the chambers A and D are opened and closed by levers connected with the bellows. These levers also operate the hands upon the dials. When the inlet to the bellows B is opened, the outlet of A is also opened. Gas entering B opens the bellows and forces the gas in A out into the house-pipe E. When B is full its inlet valve closes and its outlet valve opens. The inlet of A also opens and its outlet closes. Gas now flows into A, compressing the bellows and B, and forcing the gas from it into the house-pipe. At each filling of the bellows B there will be displaced from A and forced into the house-pipe as much gas as enters B. It is evident that at each emptying of B an equal amount of gas enters A. Thus we have A and B alternately filling and emptying as long as the gas burner is open. To have a continuous flow of gas in the house-pipes two pipes and two chambers are necessary, one being filled while the other is being emptied.
Fig. 52 represents the dials upon a gas meter showing a reading of 54,600 cu. ft.
Fig. 52.—Dials of a gas meter.
70. Centrifugal Pumps. Fluids, such as water and air, are often put in motion by devices called centrifugal pumps (see Art. 78). These pumps contain a revolving part, like a wheel without a rim, whose spokes are replaced by thin blades. This revolving part resembles the paddle wheel of some steam boats and is enclosed in a case or cover having one opening at the rim and another opening on one side about the axle.