until the pressure is the same both above and below the piston. As long as the pressure remains the same on both sides of the piston it will not move, but when this condition is changed so that the pressure on one side is enough greater than on the other side to overcome the resistance or friction of the

Fig. 6. Plain Triple Valve, with Operative Parts Removed

movable parts against the stationary parts, the piston will move away from the higher toward the lower pressure. This principle must be thoroughly understood, as upon it practically all the automatic devices of the air brake operate. The devices used in connection with the air brake, which are automatic in their action, depend for their operation upon the movements of one or more

pistons or diaphragms. The piston or diaphragm is a movable partition separating two sources of pressure. As long as these pressures are equal, no movement occurs; but as soon as the pressure on one side becomes higher than that on the

other, the piston or diaphragm tends to move toward the lower pressure; and as soon as the pressures again become equal, the tendency to move ceases. This is true whether the pressurs are due to compressed air, or springs, or a combination of the two. With the triple valve the changes in pressure which cause it to

operate are due to the increase or decrease in brake-pipe pressure caused by a movement of the brake-valve handle, opening of a conductor's valve, a burst hose or broken pipe, leakage, etc.; or due to a decrease in auxiliary reservoir pressure caused by the flow of air from the auxiliary reservoir to the brake cylinder during an application of the brakes. We will now see how these occur.

Release Position, Fig. 7.-This is the position that the operating parts of the triple valve take (a) when the system is first being charged up, and (b) when the brakes are released after an application. As was just stated above, the air pressure in the brake pipe and chamber h forces the piston, together with slide valve and graduating valve, upward as far as they will go. A ground seat on the upper face of the piston fits tightly against the lower end of the slide-valve bushing; the feed groove k is cut in this seat. The size of the two feed grooves i and k are carefully determined to give a certain rate of flow of air from chamber to chamber m, and in cleaning or repairing a triple valve care should be taken not to alter the size of these grooves. This will be more fully explained later. Chamber and the auxiliary reservoir receive air through the two feed grooves and chamber h until their pressure is equal to that in the brake pipe. The brake cylinder is connected through port (the drilled holes in valves and valve seats are usually called "ports")-with port p and the exhaust opening to the atmosphere by a milled cavity in the face of the slide valve; if there was any pressure in the cylinder when the operating parts took this position, it would escape to the atmosphere. Consequently in release position, the brake pipe is connected with the auxiliary reservoir; and the brake cylinder is connected to the atmosphere.

Service Position, Fig. 8.-This is the first position that the operating parts take during an appliction of the brakes in an ordinary stop or slow down. It is brought about by a moderate and gradual reduction of brake-pipe pressure. This destroys the equality of pressures on the two sides of the piston-(the feed grooves i and k are not large enough to allow the air to flow back from the auxiliary reservoir to the brake pipe as rapidly as the brake-pipe pressure reduces) and according to our principle above stated, the piston will move toward the lower pressure as soon as the difference between brake-pipe and auxiliary-reservoir pressures is enough to overcome the friction of the moving parts. It moves downward (closing the feed groove i connecting brake pipe and auxiliary reservoir pressures) until the knob projecting below the piston strikes the graduating stem 8, which stops it. Just here it should be noticed that the slide valve fits into a cut-away portion of the piston stem; also that the length of the slide valve is a little less than that of the cutaway portion, permitting the piston to move slightly without moving the slide valve. A very slight reduction of pressure under the piston causes it to move down until the shoulder on the top end of the piston stem strikes the slide valve, thus closing the feed groove i and pulling the graduating valve off its seat in the slide valve. Consequently, when the piston starts downward, the slide valve does not move until the top end of the piston stem strikes it, and forces it also downward. Also, the graduating valve 7 is attached by a pin to the piston stem, so that it does move whenever the piston moves. The graduating valve is a small plug valve having a seat almost in the center of the slide valve. Two ports are drilled in the slide valve, one from each side, shown in Figs. 6, 7, 8, 9, and 10, by a dotted curve and marked IV, which connect with the graduating valve below its seat; while the port Z connects the upper side of its seat with the face of the slide

valve. Consequently the first movement of the piston downward draws the graduating valve away from its seat before the slide valve starts to move; this allows auxiliary-reservoir air to pass into port Z through ports W. When

the piston is brought to a stop by the graduating stem, port Z in the slide valve just registers with (or matches) port r in the seat; thus the movement of the slide valve first closes the connection between the brake cylinder and atmosphere, then makes connection between the anxiliary rservoir and brake

cylinder, allowing air to flow from the former to the latter and causing the brakes to apply. Consequently in service position, the auxiliary reservoir is connected to the brake cylinder. No other connection is made.

Lap Position, Fig. 9.-This is the second position that the operating parts take during an application of the brakes in an ordinary stop or slow down. There is one condition of operation in service applications where Lap Position may not occur; this will be discussed in the next paragraph. The amount of reduction made by the engineer in brake-pipe pressure to apply the brakes depends upon the conditions of the stop; after a sufficient reduction is made the brake-pipe pressure is held at that pressure until either a heavier application is required or a release of the brakes is desired. The operating parts of the triple valve being in service position, air flows from the auxiliary reservoir to the brake cylinder, reducing the pressure in the former and increasing it in the latter, until the auxiliaryreservoir pressure above the piston becomes enough less than the reduced brake-pipe pressure below the piston to overcome its friction, when the usual effect of unequal pressures causes the piston and graduating valve to move upward until the graduating valve strikes its seat in the slide valve, assuming what is termed lap position, and cutting off the flow of air to the brake cylinder. As the friction of piston and graduating valve is small compared with that of the slide valve, which is forced against its seat by the air pressure, as well as by the spring 14, a smaller difference in pressure on the two sides of the piston will move it and the graduating valve than would be required to also move the slide valve. So when the piston forces the graduating valve to its seat, it stops, because the graduating valve prevents any further flow of air from the auxiliary reservoir to the brake cylinder, and therefore the auxiliary-reservoir pressure does not get enough lower than brake-pipe pressure to move the slide valve also. Thus in Lap Position, all communication between the four connections to the triple valve-(brake pipe, auxiliary reservoir, brake cylinder and atmosphere)—is closed.

Full Service Application. This is the particular case of service application mentioned in the last paragraph where the operating parts of the triple valve do not go to lap position, but remain in service position, as shown in Fig. 8. You will remember that when the operating parts go to service position, the auxiliary reservoir is connected to the brake cylinder; air flows from the auxiliary reservoir, which is under pressure, to the brake cylinder to apply the brakes; the pressure in the auxiliary reservoir falls, while that in the brake cylinder rises; consequently if these two volumes are allowed to remain connected, it is evident that the falling pressure of one and the rising pressure of the other will finally become equal, or as we usually say, the two pressures "equalize." After that, there is no tendency for air to flow from one to the other in either direction. For example, the most common brake-pipe pressure carried with the ordinary quick-action brake is 70 pounds; the highest pres sure desired in the brake cylinder in ordinary service is 50 pounds; consequently the auxiliary reservoir is made of such a size that when it is filled with air at 70 pounds, and connected to a brake cylinder in which the piston moves out a certain distance, its pressure will fall and the brake-cylinder pressure rise until they "equalize" at 50 pounds. This will be more fully discussed later, but we may say here that the amount of brake cylinder piston travel obtained in a brake application determines the pressure at which the cylinder and auxiliary reservoir will equalize, and it is very important that