The engravings, Figs. 287, 288, show the valve gear, also section of
steam cylinder and steam chest. The piston and valves are in their
central position, which condition never occurs in the operation of this
pump; if it did the pump would stop. The valves and pistons being at
one end or the other of the stroke uncovers the ports, and the moment
steam is admitted the pump will start. Referring to the engravings, A,
is the main steam pipe, and B, the auxiliary steam pipe. These pipes
are one, inside the casting, so that one pipe supplies both. Assume the
valve, C, moved to the left so that the port, D, is uncovered.
[Illustration: FIG. 287.]
[Illustration: FIG. 288.]
Live steam then flows through the port, D, and pushes the balanced
piston valve, E, to the left, carrying the slide valve, F, with it to
the left, so that the port, G, is opened to the steam cylinder. The
steam enters through the port, G, pushes the main piston, H, to the
left, completing one stroke of the piston. As the piston travels to
the left the lever, J, pushes the crosshead, I, to the right, which
opens corresponding ports in the left-hand end of the cylinder, and
the piston valve, E, is pushed to the right, which admits steam on the
left-hand side of the main piston through the port, K, pushing the
piston to the right, completing the second stroke of the piston. The
auxiliary slide valve, C, is operated by the crosshead, I, coming in
contact with the tappets, L L.
The auxiliary tappets and stem, M M, theoretically could be dispensed
with, but they are put in place for the reason that occasionally the
valve, E, might stick, due to the pump standing for some time unused
or from some other cause. In such a contingency the crosshead, I, is
pushed to the right by the action of the main piston and comes in
contact with the tappets, M M, which causes the piston valve to start,
after which steam will complete the work. When the pump is running, the
crosshead, I, never quite touches the tappets, M M, because it engages
the tappets, L L, admitting steam to the piston valve and shifts it
before the tappets, M M, are touched.
The reason of the double ports in the auxiliary steam chest is to
have one port, D, for steam, and one port, N, for the exhaust. Steam
being imprisoned between these two ports forms a cushion, preventing
the piston valve from striking the heads of the chest. The tappets,
L L, set closer together or farther apart control the stroke of the
main piston, H. When the pump is running very fast the momentum of
the moving parts increases and the tappets will have to be set closer
together for high speed than for slow. The tappets, M M, are adjustable
to their right relation with the tappets, L L. The general design and
easy means of adjustment make a reliable single cylinder valve motion.
_To set the valves._ There are no complicated internal parts requiring
adjustment, and almost all parts requiring manipulation can be handled
while the pump is running.