The steam cylinder and the pump are set in line with one another (in the case shown, horizontally), and half as far apart again as the stroke of the cylinder.  The plunger is either a continuation of the piston rod, or attached to it.

[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.

CONSTRUCTION.

The cylinder will not be described in detail, as hints on making a slide-valve cylinder have been given on earlier pages.  The piston rod should be three times as long as the stroke of the cylinder, if it is to serve as pump plunger; and near the pump end an annular groove must be sunk to take a packing.

The pump, if designed to work horizontally, will have the valves arranged like the pump illustrated in Fig. 65; if vertically, like the pump shown in Fig. 109.  Both suction and delivery pipes should be of ample size, as the pump works very fast.  The pump is mounted on a foot, F, made by turning up the ends of a piece of brass strip, and filing them to fit the barrel.

The bed can be fashioned out of stout sheet brass or zinc.  Let it be of ample size to start with, and do not cut it down until the pump is complete.  Rule a centre line for cylinder and pump, and mount the cylinder.  Pull out the piston rod plunger as far as it will go, and slip the pump barrel on it.  The foot of the pump must then be brought to the correct height by filing and spreading the ends until the plunger works quite easily in the pump, when this is pressed down firmly against the bed.  When adjustment is satisfactory, mark the position of the foot on the bed, solder foot to barrel, and drill and tap the foot for the holding-down screws.  Don’t forget that the distance between pump and cylinder gland must be at least 1-1/3 times the stroke.