The single cylinder pumps of this make are equipped with the gear
illustrated in Fig. 252, in sizes varying from 4 inches in diameter by
5 inches stroke to 28 inches in diameter by 24 inches stroke.
The arrangement of valves and ports is shown in the engravings, Figs.
253 and 254.
The admission of live steam to the cylinder and of exhaust steam to the
atmosphere is controlled by a valve piston, A, shown in Fig. 252.
[Illustration: FIG. 252.]
Assume that the piston is in position shown, Fig. 253, and that both
the main and auxiliary valves cover their respective steam ports. By
means of a starting bar, operating through a stuffing-box in the valve
chest, the piston valve, A, is moved toward the head of the steam
chest, D, thus opening the ports, E and L, and admitting live steam
through L, from the cavities, S, of the valve piston to the housing end
of the main steam cylinder, through the port, F, Fig. 255, forcing the
main piston, P, toward the opposite end of the stroke, or toward the
left in the figure. The port, E, Fig. 253, being open, the exhaust
steam escapes from front of the main piston through the port, F, Fig.
255, into the main exhaust port, G, through the port, E. The piston, P,
travels to its extreme left position and the auxiliary slide valve has
been drawn to such a position in the direction indicated by the arrow
in the smaller drawing in Fig. 252, as to bring valve piston, A, toward
the opposite end; the exhaust steam from the steam chest escapes from
before it, through the exhaust port, K, the opening of which into the
chest is at such a distance from the head as will permit sufficient
exhaust steam to remain to afford a cushion to the valve piston.
[Illustration: FIG. 253.]
[Illustration: FIG. 254.]
[Illustration: FIG. 255.]
[Illustration: FIG. 256.]
With the auxiliary slide valve in position to bring the hole, H, over
the port, J, Fig. 256, it is plain that the exhaust through the port,
K, will pass into the main exhaust through the port, L. With the main
piston at its extreme travel toward the right, the ports, E and L,
which correspond to F and F, respectively, in Fig. 255, are opened in
such a manner as to exhaust steam to the atmosphere from the housing
end of the steam cylinder through the port, F, and live steam from the
chest to the head end of the main cylinder, through the port, F, thus
driving the main piston, P, toward the housing end of the cylinder, or
toward the right. The piston and reciprocating parts traveling in this
direction move the auxiliary slide valve to its maximum point of travel
in the opposite direction, thus opening the opposite auxiliary steam
and exhaust ports and again driving the valve piston toward the head,
D, of the steam chest, whence a new stroke begins.
Lost motion in the valve gear is taken up by adjustable links, on all
sizes above 7 inches diameter by 10 inches stroke and on some smaller
sizes.
Cushioning of the steam pistons in the larger sizes and upwards is
accomplished by means of suitable valves called cushion valves. In the
smaller sizes sufficient cushioning is done by exhaust steam passing
from the clearance space next the head through a small hole drilled
into the main steam port.
_To set the valve_ of this pump it is only necessary to place the
piston in its central position and adjust the lever so that the valve
will occupy its central position. By this proceeding the travel of the
valve is equalized.