THE Vacuum Brake, as the name implies, is operated by means of a vacuum which is formed in the connections that act the part of the cylinder in the air-brake. With an air-brake, compressed air is made to do the work of applying the brakes by moving a piston to which the brake-lever is attached, the air being carried throughout the train by means of iron pipes and rubber hose: with the vacuum brake the work is done in a similar way with similar connections; but, instead of compressed air being forced inside the pipes and apparatus, all the air is exhausted out, and the natural pressure of the atmosphere is made to do the work.

Under each vehicle of a train, as seen in Fig. 33, a diaphragm is securely fastened which performs the combined duties of cylinder and piston. It consists of a kettle-shaped casting with a loose disk of heavy rubbered duck fastened over its mouth; the center of the disk being provided with an iron plate, through which passes an eye-bolt for forming connection with the brake-lever. The inside of the diaphragm is connected to the pipe which passes along the train, and has its front end connected with the ejector on the locomotive.

The position of the ejector in the cut can be clearly seen in Fig, 34, where there is also a diaphragm to be seen under the deck where it is located when used to operate driver brakes. The ejector is operated on the same principle as the water injector, only it is used to lift air instead of water. A cross-section of the injector is shown in Fig. 34. When the engineer wishes to apply the brake, he pulls the handle 41 (broken off in the cut), which opens the valve B49, and admits steam to the body of the ejector A1. The steam rushes upward round the end of the tube 5, its velocity being accelerated in passing through the contracted opening left round the top of the tube. Passing through tubes 3 and 6, the steam shoots up in the form of a column with a hollow base; the tube 5, which is connected with the pipes and diaphragms on the train, forming this base. The effect of the steam passing out under these conditions is to induce a current through the tube 5, which draws up the valve B7, and sucks the air out of the pipes and diaphragms. A vacuum being thus formed in the diaphragms, the atmosphere presses the flexible ends together. This tendency to collapse is retarded by the brake-rod connections, and the latter receive a pull equal to the combined atmospheric pressure on the diaphragm. The brake-levers are arranged to transmit a proper tension to the brake-shoes for making the brake effective. A vacuum gauge placed on the front of the ejector enables the engineer to regulate the power as he wants it. The brake is released by pushing on the lever 24, which opens the valve 8, and admits air into the brake-pipes. The release-valve attachment is sidewise in vertical section cut through the handle, and is put separate for convenience of illustration.

The valve B7 of the ejector needs grinding occasionally; and, if the lift should be too great, the valve will hammer the seat out of shape. Sometimes when waste or other fibrous impurities are sucked through the pipe, they stick in this valve, keeping it away from the seat. The valve, is very easily reached by taking off the cap O4. The steam-valve B49 needs about the same care as any other steam-valve, and its troubles are of the same nature. The shoulder at the top of the tube 5, which is used to obstruct the steam, thereby increasing the velocity of the quantity that passes, sometimes gets cut into channels with the fast moving steam striking it. This reduces the promptness of the ejector's action, but it is a form of deterioration that proceeds very slowly. Care must be taken to keep the drip-valves A and B16 in order, otherwise there may be trouble with the ejector throwing water, or freezing up if the engine stands where that apparatus will get cold in winter.

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