122. Q. What is the duty of the distributing valve?

A. To admit air from the main reservoir to the locomotive brake cylinders when applying the brake, to automatically maintain the brake cylinder pressure against leakage, to develop the proper brake cylinder pressure regardless of piston travel and to exhaust the air from the brake cylinders when releasing the brake.

123. Q. To what is the distributing valve attached?

A. To the distributing valve reservoir.

124. Q. How many chambers has the distributing valve reservoir?

A. Two; pressure chamber and application chamber.

Fig. 8: Release, Automatic or Independent.
Connections: MR, Main Reservoir Pipe; IV, Distributing Valve Release Pipe; II, Application Cylinder Pipe; CYLS, Brake Cylinder Pipe; BP, Brake Pipe.
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125. Q. Name the different pipe connections to the distributing valve reservoir.

A. Referring to Fig. 8, the connection marked "MR" is the main reservoir supply pipe; "II", application cylinder pipe; "IV", distributing valve release pipe; "BP", brake pipe; "CYLS", brake cylinder pipe.

126. Q. To what do these different pipes connect?

A. The main reservoir supply pipe connects the distributing valve with the main reservoir pipe. The application cylinder pipe connects the application cylinder of the distributing valve with the independent and automatic brake valves. The distributing valve release pipe connects the application cylinder exhaust port in the distributing valve with the independent brake valve, and through it, when in running position, to the automatic brake valve. The brake cylinder pipe connects the distributing valve with the different brake cylinders on the locomotive. The brake pipe branch pipe connects the distributing valve with the brake pipe.

127. Q. Explain the operation of the distributing valve when making an automatic service application of the brake.

A. When the brakes are fully charged, the brake pipe and pressure chamber pressures are equal, and when a gradual reduction of brake pipe pressure is made it will be felt in chamber "p" at the right of the equalizing piston 26, creating a difference in pressure on the two sides of the piston, causing it to move to the right. The first movement of the piston closes the feed groove "v", also moves the graduating valve 28, uncovering the service port "z" in the equalizing slide valve 31; this movement of the piston also causes the shoulder on the end of its stem to engage the equalizing slide valve, and the continued movement of the piston moves the valve to service position, in which port "z" connects with port "h" in the seat of the valve, as shown in Fig. 9. As the equalizing slide valve chamber is at all times connected to the pressure chamber, air can now flow from this chamber to both the application cylinder and chamber through ports "z" and "h", cavity "n" and port "w" until the pressure on the left or pressure chamber side of the equalizing piston 26 becomes slightly less than that in the brake pipe, when the piston and graduating valve will move to the left until the shoulder on the piston stem strikes the slide valve; this movement of the graduating valve closes the service port "z", thus closing the communication between the pressure chamber and application chamber and cylinder, also closing port "l" which leads to the safety valve. The distributing valve is now said to be in service lap position. (See Fig. 10.)

128. Q. Upon what does the pressure in the application chamber and cylinder depend when making a service application of the brake?

A. On the amount of brake pipe reduction; and as the relative volume of the pressure chamber and application cylinder and chamber is practically the same as that of an auxiliary reservoir and brake cylinder, it will be understood that one pound from the pressure chamber will make two and one-half pounds in the application chamber and cylinder; in other words, with the pressure chamber charged to seventy pounds and no pressure in the application chamber and cylinder, if they were connected and the pressure allowed to equalize it would do so at about fifty pounds; that is, twenty pounds from the pressure chamber will make fifty pounds in the application chamber and cylinder.

Fig. 9: Automatic Service.

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129. Q. How is the application piston 10 affected by the air pressure in the application cylinder "g"?

A. Pressure forming in this cylinder will force the piston to the right; the piston in moving will carry with it the exhaust valve 16, closing the exhaust ports "e" and "d", at the same time moving the application valve 5, opening the supply port "b", allowing main reservoir air from chamber "a" to flow through ports "b" and "C" to the connection marked "CYLS", and on to the different brake cylinders of the locomotive until the pressure in the brake cylinders and at the right of the application piston becomes slightly greater than that in chamber "g" when the application piston and valve will move back to lap position as shown in Figures 9 and 10.

130. Q. With the application valve in lap position, if there be brake cylinder leakage, will the locomotive brake leak off?

A. No; any drop in brake cylinder pressure will be felt in chamber "b" at the right of the application piston 10, causing a difference in pressure on the two sides of the piston, thus allowing the pressure in the application cylinder to move the application piston and valve to the right, again opening the supply port "b" allowing a further flow of main reservoir air from chamber "a" to the brake cylinders until the pressure is again slightly greater than that in the application cylinder "g", when the application piston and valve will move back to lap position. Thus in this way air will be supplied to the brake cylinders of the locomotive, holding the brake applied regardless of leakage.

131. Q. What effect will piston travel have on the pressure developed in the brake cylinders?

A. None; as the pressure in the brake cylinders is entirely dependent on the pressure in the application cylinder, which is not affected by piston travel.

132. Q. Explain the movement of the parts in the distributing valve when the automatic brake valve is moved to release position, after an automatic application of the brake.

A. In release position of the brake valve, air from the main reservoir flows direct to the brake pipe, causing a rise of pressure which is felt in chamber "p" on the right or brake pipe side of the equalizing piston 26; this increase of pressure will cause the piston to move toward the left, carrying the graduating valve 28 and slide valve 31 to release position.

Fig. 10: Service Lap.

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This allows the air from the application chamber and cylinder to flow to the distributing valve release pipe "IV" and on through the independent brake valve to the automatic brake valve, where the port to which this pipe leads is blanked by the automatic rotary valve, thus preventing the air from leaving the application chamber and cylinder, holding the locomotive brake applied while the train brakes are being released. The movement of the parts, and the results obtained are the same where the release is made in holding position.

133. Q. Explain the movement of the parts in the distributing valve when the brake valve is moved to running position after having first been moved to release or holding position, following a brake application.

A. In this position of the brake valve the port to which the distributing valve release pipe is connected is open to the exhaust, thus allowing the air to escape from the application chamber and cylinder. The reduction of pressure in chamber "g", will allow the brake cylinder pressure in chamber "b" to force the application piston and its valves to release position, thus allowing the brake cylinder air to escape to the atmosphere, through the exhaust ports "e" and "d". (See Fig. 8.)

134. Q. Explain how an independent release of the locomotive brake is obtained after an automatic application has been made.

A. If the brakes have been applied throughout the train, by means of the automatic brake valve, and it is desired to release the locomotive brakes without releasing the train brakes, the handle of the independent brake valve is placed in release position. In this position of the independent brake valve, the application cylinder in the distributing valve is con[Pg 207]nected through the application cylinder pipe to the direct exhaust port of the independent brake valve; thus exhausting the air from the application cylinder, causing a release of the locomotive brake. This independent release of the locomotive brake does not cause the equalizing piston and its slide valve in the distributing valve to change their position.

135. Q. Explain what takes place when an automatic emergency application is made.

A. Any sudden reduction of brake pipe pressure is felt on the brake pipe side of the equalizing piston 26 and will cause it and the slide valve 31 to move to the extreme right, compressing the graduating spring 60. (See Fig. 11.) In this position pressure chamber air can flow to the application cylinder only as the application chamber is now cut off. This will cause a quick rise of pressure in the application cylinder, forcing the application piston and its valves to full application position, admitting main reservoir air to the brake cylinders and applying the brake. In emergency position of the automatic brake valve there is a small port in the rotary valve, called the blow-down timing port, through which main reservoir air is free to flow to the application cylinder "g" through the application cylinder pipe "II", causing a rise of pressure equal to the adjustment of the safety valve.

136. Q. At what pressure is the safety valve adjusted?

A. At sixty-eight pounds.

137. Q. What is the purpose of the quick action cap, and where is it located?

A. Its purpose is to assist the brake valve in venting brake pipe air when an emergency application of the brake is made, and is located on the brake pipe side of the distributing valve in place of the plain cap. (See Figs. 8 and 11.)

138. Q. Explain the operation of the quick action cap.

Fig. 11: Emergency Position of No. 6 Distributing Valve with Quick-Action Cap.
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A. In an emergency application, the equalizing piston 26 moves to the extreme right, the knob on the piston strikes the graduating stem 59, causing it to compress the graduating spring 46, and move the slide valve 48 to the right, opening port "j".

Brake pipe pressure in chamber "p" flows to chamber "X", pushes down check valve 53, and passes to the brake cylinders through port "m" in the cap and distributing valve body. When the brake cylinders and brake pipe pressures equalize, check valve 53 is forced to its seat by spring 54, thus preventing air in the brake cylinders from flowing back into the brake pipe. When a release of the brake occurs and piston 26 is moved back to its normal position, spring 46 forces graduating stem 59 and slide valve 48 back to release position.

139. Q. Explain the operation of the distributing valve when making an independent application of the brake.

A. When the independent brake valve handle is moved to application position, air is admitted from the reducing valve pipe through the application cylinder pipe to the application chamber and cylinder. Pressure forming in the application cylinder will move the application piston 10 to the right, carrying with it the exhaust valve 16 and the application valve 5, closing the exhaust port and opening the supply port, admitting main reservoir air from chamber "a" to the brake cylinders (see Fig. 12) until the pressure in the brake cylinders and chamber "b" slightly exceeds that in chamber "g", when the application piston 10 and valve 5 will move back to lap position. By moving the brake valve handle to either release or running position, the air is exhausted from the application cylinder and chamber, thus reducing the pressure in chamber "g", allowing the pressure in chamber "b" to force the piston to the left, carrying with it the exhaust valve 16, opening the exhaust ports "e" and "d", allowing the air from the brake cylinders to escape to the atmosphere, thus releasing the brake.