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.
The valve motion can then be taken in hand. Cut off for the guides, G1 G2, two pieces of stout brass strip, 2-1/2 inches long and 3/4 inch wide. Lay them together in a vice, and bore the holes (Fig. 113) 1-1/4 inches apart, centre to centre, for the 1/8-inch rods, R1 R2. The feet are then turned over and a third hole bored in G1, midway between those previously made, to take the end of the support, PP, of the rocking lever.
[Illustration: FIG. 113.—End view of striking mechanism of steam pump.]
Screw G1 G2 down to the bedplate, 3/4 inch away from the cylinder centre line. G1 is abreast of the mouth of the pump, G2 about half an inch forward of the end of the cylinder.
The striker, S, is a piece of brass strip soldered to 1/2 inch of tubing fitting the piston rod. (See Fig. 113.) Its length is decided by running a rod through the upper holes in G1 G2, allowance being made for the notch in the end. The collar is tapped for two screws, which prevent S slipping on the piston rod. The rods for R1 R2 are now provided with forks, made by cutting and filing notches in bits of brass tubing. The notches should be half as deep again as the rocking lever is wide, to give plenty of room for movement. Solder the forks to the rods, and put the rods in place in the guides, with the forks as far away from G1 as the travel of the slide valve. Then measure to get the length of the rocking lever support. One end of this should be filed or turned down to fit the hole drilled for it; the other should be slotted to fit the lever accurately.
The rocking lever, RL, which should be of steel, is slotted at each end to slide on the pins in the forks, and bored for the pivot pin, which, like those in the forks, should be of hardened steel wire. Assemble the rocking lever in its support and the rod forks, and solder on the support.
To the back end of R2 solder a steel plate, A, which must be bored for the pin in the valve fork, after the correct position has been ascertained by careful measurement.
The stops, T1 T2, are small, adjustable collars, kept tightly in place on
R1 by screws.
Setting the Striker.—Assemble all the parts. Pull out the piston rod as far as it will go, and push the slide valve right back. Loosen the striker and the forward stop, and slide them along in contact until the striker is close to the pump. Tighten up their screws. Then push the piston rod fully in, draw the valve rod fully out, and bring the rear stop up against the striker, and make it fast. Each stop may now be moved 1/16 inch nearer to a point halfway between them to cause “cushioning” of the piston, by admitting steam before the stroke is quite finished.
A pump made by the author on this principle, having a 1-1/4 inch stroke and a 1/2-inch bore, will deliver water at the rate of half a gallon per minute against a head of a few feet.
Note.—To steady the flow and prevent “water hammer,” a small air-chamber should be attached to the delivery pipe.
An Alternative Arrangement.—If the reader prefers a steam pump which will work at slow speeds, and be available, when not pumping, for driving purposes, the design may be modified as shown diagrammatically in Fig. 114. The striker becomes a cross head, and is connected by a forked rod passing on each side of the pump with the crank of a fly wheel overhanging the base. The valve is operated in the ordinary manner by an eccentric on the crankshaft. The steadying effect of the fly wheel and the positive action of the valve make it possible to use a larger pump plunger than is advisable with the striking gear. With a pump piston of considerably greater diameter than the piston rod, the pump may be made double-acting, a gland being fitted at the front end for the piston rod to work through, and, of course, a second set of valves added.
[Illustration: Fig. 114.—Plan of steam pump with fly wheel.]