Figs. 51 and 52 illustrate a very simple form of fixed-cylinder engine controlled by a slide valve.
An open-ended “trunk” piston, similar in principle to that used in gas engines, is employed; and the valve is of the piston type, which is less complicated than the box form of valve, though less easily made steam-tight in small sizes. The engine is single-acting, making only one power stroke per revolution.
The cylinder is a piece of brass tubing; the piston another piece of tubing, fitting the first telescopically. Provided that the fit is true enough to prevent the escape of steam, while not so close as to set up excessive friction, a packing behind the piston is not needed; but should serious leakage be anticipated, a packing of thick felt or cloth, held up by a washer and nuts on the gudgeon G, will make things secure. Similarly for the built-up piston valve P may be substituted a piece of close-fitting brass rod with diameter reduced, except at the ends, by filing or turning, to allow the passage of steam.
CONSTRUCTION.
[Illustration: FIG. 51.—Elevation of simple reciprocating steam engine.]
The bed is made of wood, preferably oak, into the parts of which linseed oil is well rubbed before they are screwed together, to prevent the entry of water. A longitudinal groove is sawn in the top of the bed, as indicated by the dotted line in Fig. 51, to give room for the connecting rod in its lowest position, and a cross groove is scooped in line with the crank shaft to accommodate the lower part of the crank disc and the big end of the rod. (If the wing W under the cylinder is screwed to the side of the bed, instead of passing through it, as shown, a slight cutting away of the edge will give the necessary clearance in both cases. )
[Illustration: FIG. 52.—Plan of simple reciprocating steam engine.]
The cylinder and valve tube A should be flattened by filing and rubbing on emery cloth, so that they may bed snugly against one another and give a good holding surface for the solder. A steam port, S P, should next be bored in each, and the “burr” of the edges cleaned off carefully so as not to obstruct valve or piston in the slightest degree. “Tin” the contact surfaces thinly, and after laying valve tube and cylinder in line, with the portholes corresponding exactly, bind them tightly together with a turn or two of wire, or hold them lightly in a vice, while the solder is made to run again with the aid of a spirit lamp. If it seems necessary, run a little extra solder along the joint, both sides, and at the ends.
The valve, if built up, consists of a central rod, threaded at the rear end, four washers which fit the tube, and a central spacing-piece. The forward washer is soldered to the rod. Behind this is placed a felt packing. Then come in order the central spacing-piece, with a washer soldered to each end, a second packing, and a fourth washer. The series is completed by an adjusting nut to squeeze the packings, and a lock nut to prevent slipping. The back end of the valve must be wide enough to just more than cover the steam port. If the felt proves difficult to procure or fit, one may use a ring or two of brass tubing, with an external packing of asbestos cord.
The cylinder wing W should have the top edge turned over for an eighth of an inch or so to give a good bearing against the cylinder, and be held in position by a wire while the soldering is done. It is important that the line of the wing should be at right angles to a line passing through the centres of the valve tube and cylinder.