[Illustration: FIG. 273.]
[Illustration: FIG. 274.]
The steam chest of this pump differs somewhat from other pumps in
which valve pistons are employed to operate the main valve. See
accompanying engraving, Fig. 272. This pump has a steam chest with a
cover for inspection and repairs. This chest is bored at each end to
form suitable cylinders to receive the valve piston, E. By the side of
the valve piston, E, in the steam chest is an auxiliary slide valve, G,
Figs. 273 and 274, which admits and releases the steam from the ends of
the valve piston. The valve piston, E, has two slots at the center, the
lower one receiving the lug on the back of the main valve and the upper
one the toe on the rocker shaft, D. The rocker shaft has two toes, the
larger one, F, engaging with the valve piston, and the smaller one with
the auxiliary slide valve, G, as shown in Fig. 274. The auxiliary, as
well as the main, valves are plain slides designed to take up the wear
automatically. The pendulum lever, J, Fig. 273, is connected with the
piston rod as shown and rotates the shaft, D, and by means of the toes
the valves move together.
The auxiliary slide valve, G, admits and exhausts the steam to and from
the ends of the valve piston. Steam being admitted, the piston moves
toward one end. The two valves also move in the same direction by means
of the rocker shaft and the toes. This movement continues until the
piston has nearly completed its stroke, when the auxiliary valve opens
one of the small ports leading to the end of the valve piston, thus
admitting steam at one end and exhausting it from the other. The valve
piston moving from one end of the steam chest to the other, shifts the
position of the main valve and reverses the motion of the main piston.
The valve piston is moved the greater part of its travel by the toe
on the rocker shaft, thus reversing the steam distribution, by steam
pressure, which brings the opposite end of the slot in the driver to
a second engagement by the toe on the rocker shaft which begins the
return stroke.
The steam acts to throw the main valve only near the ends of the valve
travel, and being already in motion the valve requires but very slight
help to complete its stroke quickly without shock.
The motion of the auxiliary valve is synonymous with the main piston
so there can be no practical dead center. The valves are very durable
under such easy motion.
_With this arrangement of valves no setting is necessary_ because when
assembling the parts the centers of the pendulum lever and the toes on
the rocker shaft are adjusted parallel with one another.
THE McGOWAN.
The McGowan single cylinder steam pump and section is shown in Fig.
275. Its main valve is of the ^B^ form and is driven by a valve piston.
Steam enters the central port in valve seat and into the cylinder
through one of the cavities in the valve and exhausts through the
opposite. The two tappet valves cover the auxiliary ports, shown by
dotted lines, leading to the ends of the steam chest and connect with
the main exhaust ports. These tappet valves are raised by means of
levers, the ends of which project downward and into the cylinder so
that when the piston nears the end of a stroke it comes in contact with
the lever and raises it sufficiently to lift the tappet valves.
Each tappet valve lever is pivoted on a pin which fits into a recess
near the main ports, as indicated by the dotted lines.
[Illustration: FIG. 275.]
When the piston reaches the end of its stroke it lifts one of the
tappet levers and with it the corresponding valve is raised from its
seat, opening the port leading from the end of the steam chest to
the main exhaust port. The pressure is thus relieved on one end of
the valve piston and the steam pressing on the opposite end forces
the valve piston to the opposite end of its stroke, thus reversing
the distribution of steam to the cylinder and starting the piston on
its return stroke. The valve piston thus moves back and forth, the
ends of the steam chest being filled with steam at initial pressure.
Steam escaping from one end of the steam chest causes a difference of
pressure on the two ends of the valve piston, from which we realize the
power to move the main valve. The tappet valves, having a very slight
lift, operate without shock or noise. The main valve is connected with
the valve piston so that all lost motion is taken up automatically.
A rocker shaft, extending through the steam chest carries a toe moving
in a slot in the top of the valve piston, so that the valve can be
moved by hand.
_To set the valves of this pump._ Simply keep the valves in order. The
motion of the piston as it nears the end of the stroke opens and closes
the valves.