I propose in this chapter to give a few practical hints showing how to build a perfect model of an inverted-cylinder direct-action engine with link-motion reversing gear, like the [sketch] below, which represents a type in daily use on the river and sea. Such a model, having a fixed cylinder, has not the friction of other types, and therefore it gives more power, size for size, than an oscillating engine, and does not get so easily out of order.
You must of course have a lathe, which I will therefore suppose you to possess; but should there not be a slide-rest to it, you must get the cylinder bored by a professional turner, for which he will charge about two shillings, according to the size of your castings.
Let me first briefly explain the action of the steam in the engine by a diagram ([Fig. 1], p. 139). The cylinder A is bolted into the standard, B; the ports or steam-passages are shown at C; and the slide-valve that allows the steam to pass alternately to each side of the piston is marked D, in its case F. G G are the stuffing-boxes, which have to be packed with lamp-cotton greased to make them steam-tight, H is the piston, with its rod finishing in a cross-head J, which is cut with a groove to slide up and down the standards to guide it and prevent the piston-rod being bent out of shape. K shows the connecting-rod, attached at its lower end to the crank L. M is one of the eccentrics working the slide-valve. N is the main shaft, resting on the plummer blocks O O, having a heavy fly-wheel at P and the coupler at Q. R is the top cylinder plate, drilled to screw in the grease-cock, of which I will presently give a drawing on an enlarged scale, S is the bed-plate, T the steam supply, and X the exhaust.
You will observe that the steam is coming in at the top of the cylinder, through the top port, as shown by the arrow, pressing the piston down and allowing the waste steam that has already raised the piston to escape through the lower port, and so into the exhaust. By that time the slide-valve is raised (by the eccentric) sufficiently to cut the steam off from the top port, which by that means is in its turn put in communication with the exhaust, and allows the steam to pass out of the top part of the cylinder, whilst it admits it to the lower portion, and so on alternately.
FIG 1
FIG 2
And now to the practical work. After having the cylinder bored, as already mentioned, get a piece of oak or other hard wood 11⁄2 inch square and about 6 inches long. Turn one end of it in the lathe, so that it fits the inside of cylinder, and drive it on. Then put it in the lathe again, and turn the flanges A ([Fig. 2]) down, and be very careful that they are quite true and square.
