[Illustration: FIG. 257.]
The Foster single cylinder pump valve motion is a compound valve piston
and slide valve in one piece and performs the office of both a main
and auxiliary valve. It seats over the main steam cylinder on the web
or casting connecting together the valve pistons, and is provided at
the bottom with vents or openings, _A_ and _B_, Fig. 257, for opening
and closing the main steam ports to the main steam cylinder. The sides
of this valve are cut to form the recesses, _C_ and _D_, which are for
the purpose of opening and closing the small steam ports, which admit
steam to the valve pistons; it is also provided with L shape slots for
the purpose of alternately exhausting steam from the valve pistons
through the steam ports.
[Illustration: FIG. 258.]
[Illustration: FIG. 259.]
[Illustration: FIG. 260.]
In operation the main piston rod commences its forward stroke, motion
is communicated to the vertical arm, which moves forward on the rod
until it engages with one of the cams, _E_, Fig. 259, adjusted on the
valve stem. The arm coming in contact with the inclined faces of the
cam imparts a rolling or oscillating movement to the stem and valve,
opens one of the recesses, _C_ or _D_, cut in the side of the valve
and admits steam through a port to one of the valve pistons. At the
same time this oscillating movement of the valve opens the slot, _F_,
Fig. 260, opposite the first, and exhausts steam through the alternate
port from the other piston. Steam thus admitted to one of the pistons
(while exhausts from the other piston), carries the valve over its
seat, and by means of the vents, _A_ and _B_, steam is admitted through
one of the main ports and exhausts through the other. When the main
piston has completed one stroke, it moves backward on its return; the
arm strikes against the other cam on the valve stem and communicates a
reverse motion to the valve which closes and opens the alternate ports
to admit and exhaust steam to and from the pistons. It also cushions
the piston at the end of each stroke which prevents the piston from
striking the heads.
_To set the valves._ This can best be done by starting the pump slowly
and adjusting the cams, _E_, so as to open the valve at the proper time
to compel the piston to make a full stroke. A little experimenting
combined with good judgment is all that is necessary, unless the piston
valve be badly worn, in which case _a new valve must be substituted_
and fitted to the valve chest.