[Illustration: FIG. 112--View of steam pump, showing details.]
An arm, S, fixed at right angles to the piston rod, has a forked end which
moves along the rod. This rod is connected with the slide valve through the
rocking arm, R1 and the rod, R2. On it are two adjustable stops, T1 T2,
which S strikes alternately towards the end of a stroke, causing the valve
to shift over and expose the other side of the piston to steam pressure.
The absence of the momentum of a fly wheel makes it necessary for the
thrust exerted by the piston to be considerably greater than the back
pressure of the water, so that the moving parts may work with a velocity
sufficient to open the valve. If the speed falls below a certain limit, the
valve opens only part way, the speed falls, and at the end of the next
stroke the valve is not shifted at all.
The diameter of the plunger must be decided by the pressure against which
it will have to work. For boiler feeding it should not exceed one-third
that of the piston; and in such case the piston rod and plunger may well be
one.
A piston valve, being moved more easily than a box valve, is better suited
for a pump of this kind, as friction should be reduced as much as possible.
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