G" and "H"
are then no longer open to chamber "L" and the feed valve pipe, and
these chambers being small, the pressure raises quickly to main
reservoir pressure due to the leakage of air past the supply piston 8,
which forms but a loose fit in its bushing. When the pressure in chamber
"G" becomes nearly equal to that in chamber "B", the piston spring "6"
forces the piston 8 and its slide valve 9 to closed position, which
prevents further flow of air from the main reservoir to the feed valve
pipe (see Fig. 6). The feed valve will remain in closed position until
the pressure in chamber "L" is slightly reduced so that the pressure on
the diaphragm 14 is no longer able to withstand the pressure of the
regulating spring 17, which then forces the diaphragm to the left,
lifting the regulating valve 12 from its seat and again opening port "K"
to chamber "L", thus dropping the pressure at the left of piston 8 below
that of the main reservoir acting on the opposite side of the piston.
[Illustration: Fig. 7. Diagram of B-6 Feed Valve, Open.]
Main reservoir pressure then forces the supply piston and valve over
into open position, as shown in Fig. 7, and allows a further flow of air
through port "c" to the feed valve pipe to again raise its pressure to
the adjustment of the feed valve, when the valve will again close.
102. Q. What is the duty of the feed valve?
A. To control and maintain a constant pressure in the brake pipe when
the brake valve is in running or holding position.
103. Q. What defect in the feed valve will cause the brake pipe pressure
to equalize with that in the main reservoir?
A. This may be caused by a defective feed valve case gasket, permitting
main reservoir air to leak into the feed valve pipe, or leakage past the
supply valve, or the regulating valve held from its seat, or the supply
valve piston too tight a fit in its cylinder.
104. Q. If the brake pipe charges too slowly when nearing the maximum
pressure, where is the trouble?
A. This may be caused by a loose-fitting supply valve piston 8, or the
port past the regulating valve 12 partly stopped up.
105. Q. How should the feed valve be tested?
A. With the brakes released, and charged to the adjustment of the feed
valve, create a brake pipe leak of from seven to ten pounds and note the
black hand on the brake pipe gauge. The fluctuation of this hand will
indicate the opening and closing of the feed valve, which should not
permit a varia
|