A. To admit air from the main reservoir to the locomotive brake
cylinders when applying the brakes; 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, in all automatic applications
of the brake.
174. Q. Explain the operation of the control valve when making an
automatic service application of the brake.
A. Air enters the control valve at the connection marked "BP" (Fig. 21),
which leads to chamber "F" above the piston 3, forcing it down,
uncovering the feed groove "G" in the bushing, allowing air to feed past
the piston into the slide valve chamber, and then through port "H" to
the auxiliary reservoir. The air will feed through in this manner until
the auxiliary reservoir and brake pipe pressure equalize. When a gradual
reduction of brake pipe pressure is made, it will be felt in chamber
"F", above piston 3, creating a difference in pressure on the two sides
of the piston, which will cause it to move upward.
[Illustration: Fig. 21. Automatic Control Valve. Full Release.]
The first movement of the piston closes the feed groove "G", also moves
the graduating valve 10, uncovering the service port "J" in the slide
valve 4, and the continued movement of the piston moves the slide valve
to service position, in which the service port "J" connects with port
"E" in the valve seat. (See Fig. 22.) As the slide valve chamber and
auxiliary reservoir are connected at all times, air can now flow from
the auxiliary to the control cylinder "D" and control reservoir, through
ports "H", "J" and "E", until the pressure on the lower or auxiliary
side of piston 3 becomes slightly less than that in chamber "F" or brake
pipe side, when the piston and graduating valve will move down until the
shoulder on the piston strikes the slide valve; this movement of the
graduating valve closes the service port "J", thus closing the
communication between the auxiliary and control cylinder and reservoir,
also closing port "W", which leads to the safety valve. (See Fig. 23.)
175. Q. How is piston 2 affected by the air pressure in the control
cylinder "D"?
A. Pressure forming in this cylinder will force the piston downward. The
piston in moving down will carry the exhaust valve 7 with it, closing
the exhaust port "N" and moves the preliminary admission valve "1A" from
its seat against the tension of spring 8, allowing the pressure in
chamber "O" to pass to the brake cylinders, thus creating a balancing
effect on valve 1, which allows it to be opened against main reservoir
pressure, thus allowing main reservoir air to flow from chamber "A" to
chamber "B" and the brake cylinders on the locomotive (see Fig. 22)
until the pressure in the brake cylinders and chamber "B", below piston
2, becomes slightly greater than that in the control cylinder "D" when
the piston will move up just far enough to allow the valves "1" and
"1A" to be seated, or to lap position. (See Fig. 23.)
[Illustration: Automatic Control Valve. Service Position. Fig. 22.]
176. Q. With the control valve now in lap position, will the brake
release on account of brake cylinder leakage?
A. Any drop in brake cylinder pressure will be felt in chamber "B" below
the piston 2, causing a difference in pressure on the two sides of the
piston, allowing the pressure in the control cylinder "D" to move the
piston 2 down, unseating the admission valves, allowing a further flow
of main reservoir air from chamber "A" to chamber "B" and the brake
cylinders until the pressure is again slightly greater than that in the
control cylinder "D", when the piston 2 will again move up, allowing the
admission valves to close. Thus in this way air will be supplied to the
brake cylinders of the locomotive, holding the brakes applied regardless
of leakage.
177. Q. Explain the movement of the parts in the control valve, when the
automatic brake valve is moved to release position, following 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 an increase of pressure, which
is felt in chamber "F" on the upper side of piston 3; this increase of
pressure will cause the piston to move down, carrying with it the
graduating valve 10 and slide valve 4 to release position. This allows
air from the control cylinder "D" and control reservoir to flow through
the release pipe "IV" and on to the automatic brake valve, where the
port to which this pipe leads is blanked by the automatic rotary valve,
which prevents the air leaving the control cylinder and reservoir, thus
holding the locomotive brake applied while the train brakes are being
released. The movement of the parts are the same where the release is
made in holding position.
178. Q. Explain the movements of the parts in the control valve when the
automatic brake valve is moved to running position, after having first
been moved to release or holding position.
[Illustration: Automatic Control Valve. Service Lap Position. Fig. 23.]
A. In this position of the brake valve the port to which the release
pipe "IV" is connected is open to the exhaust, thus allowing the air in
the control cylinder and reservoir to escape to the atmosphere. The
reduction of pressure in the control cylinder "D" below that in chamber
"B" causes the control piston 2 to move up, carrying with it the exhaust
valve 7 to release position, opening the exhaust port "N", thus allowing
the air to return from the brake cylinders through ports "C" and "N" to
the atmosphere, releasing the brake. (See Fig. 21.)
179. Q. Explain what takes place in the control valve when an automatic
emergency application of the brake is made.
A. Any sudden reduction of brake pipe pressure will be felt on the brake
pipe side of piston 3, and will cause it and the valve 4 to move to
their extreme upper position, the knob on the piston striking the
graduating stem 13, causing it to compress the spring 14, moving the
emergency valve 15 upward, opening port "Q"; this allows brake pipe air
to flow against valve 16, unseating it, then through port "T" to the
brake cylinder. (See Fig. 24.) In the meantime auxiliary reservoir air
can flow past the end of the slide valve through port "E" to the control
cylinder "D" and control reservoir, forcing piston 2 downward unseating
valves "I" and "IA", thus allowing main reservoir air to flow to the
brake cylinders, applying the brake.
180. Q. At what pressure will the auxiliary reservoir and control
reservoir equalize when using seventy pounds brake pipe pressure?
A. At about fifty pounds; however, with the automatic brake valve in
emergency position, there is a small port in the rotary valve (called
the blow-down timing port) opened to the control reservoir pipe and
control reservoir which allows main reservoir air to flow to the control
reservoir and cylinder, raising the pressure to the adjustment of the
safety valve.
[Illustration: Fig. 24. Automatic Control Valve. Emergency Position.
(With Quick Action Cylinder Cap.)]