There are many modifications of the long D-valve, but the principle of all is the same; I shall therefore describe the short slide-valve which is nearly always used in the models which are purchased at the shops. This, too, is the usual form of valve in locomotives, traction-engines, and the majority of those in use for agricultural and similar purposes. A, Fig. 62, is the cylinder as before in section with piston. A thick piece is cast with the cylinder, on one side of it, having steam ports also cast in it, which are here left white. The two as before go to the top and bottom of the cylinder, and have no communication with the central one, which is bored straight into the boss, and generally is turned at right angles and connected with the condenser, or with a pipe opening into the chimney of the engine to increase the draught by means of the jets of steam, as is the case always in locomotives, or into the air, which is less usual. Seen from behind, these ports are like B, being cast and cut rectangular; and the face, B, is planed quite level, which is absolutely necessary to the proper action of the slide-valve which has to work upon it. This valve is a box of iron, C, with a wide flange or rim, this flange being of sufficient width to close either port. If this valve is placed as it stands when the engine is at rest, b covers the upper steam port, and a the lower; while the exhaust or middle port is open to the hollow part of the box. Now, if we slide the valve downwards until the upper port is open, the other two will be in communication, being united by being both together in the inside of this box or valve. Suppose the valve then cased in, and that steam is admitted from the boiler into the case, it is evident that such steam could freely pass to the top of the cylinder above the piston to force it downwards, while that which was below would escape by the lower port into the box, and thence pass to the condenser. If, instead of pushing down the valve, we had drawn it upwards, the lower port would have been opened, and the upper and middle would have been brought into communication inside the valve, and the contrary effect would have been produced upon the piston.
This is the arrangement adopted, and which will be clearly understood from the following sectional drawing, D. a, a, is the thick casting upon the cylinder, with the upper and lower steam ports, which end towards the middle of the cylinder, with the third port lying between; then b is a section of the valve, in such a position that the flange of it no longer covers the lower steam port, while the other two are open together on the inside of the valve. The latter is cased in by the valve-box, e e, in the back of which is the steam pipe f coming from the boiler. The valve-rod, which is moved by the engine, passes at o through a stuffing-box. It is evidently necessary that this slide-valve should fit, and work very smoothly and correctly against the face of the ports, so as not to allow any escape of the steam. It is not, however, packed in any way at the back (although springs have been sometimes added), because, as the back is subjected to the full pressure of the steam from the boiler, this keeps it quite close to its seat. The rod, however, by which it is worked, might prevent this close contact of the two surfaces if it was screwed into the valve; it is therefore made with a cross, E, at the end, which falls into a notch in a boss cast upon the back of the valve as seen at F. This allows a certain degree of play in one direction, and permits the steam to press it close even after it has become worn by use.
You will, I think, now clearly understand how steam can be admitted alternately to the top and bottom of a cylinder, and how the exhausted steam that has done its work escapes. I must therefore now tell you how the rod of the slide-valve is moved up and down by the engine, but to do this, I must draw such engine complete.
Fig. 63.
The cylinder, A, is screwed down on its side upon the bed-plate, R R, out of which are cut two holes, one for the fly-wheel, P, of which part only appears for want of space, the other for the crank, L, on the end of the axle, M M, running through bearings, N N. The slide-valve-box is at B, C being the steam-pipe from the boiler. The piston-rod has necessarily to move only in a straight line in the direction of its length, but the crank which it has to work to turn the fly-wheel must needs move round in a circle. Hence, a poker-and-tongs joint, F O F, is arranged. The connecting-rod, H, which is attached to the crank by brasses at K, divides or is attached to a forked piece, at the lower end of which are a pair of bearings or brasses, F F. The piston-rod carries the piece O, the cross-bar of which is turned, being, in fact, the pin which passes into these bearings at F F. This forms, therefore, a hinge-joint at this place, so that although the piston-rod cannot leave the right line, and can only slide in the guide, E, the rod, H, has an up-and-down motion upon this hinge, allowing the revolution of the crank-pin to take place. D is the valve-rod, in which is a hinge at S, which suffices for the slight movement required in the rod, as it rises and falls by the action of the eccentric, T, the motion and effect of which I now have to explain.
V is a round disc of metal with a recess on its edge, so that it is like an ordinary pulley, but large in proportion to its thickness. A hole for the main crank axle, to which it has to be firmly keyed, is made through it, but not in its centre (hence its name, eccentric—out of the centre). As the axle revolves, it is evident that this disc revolving with it will carry any point, Y, of its surface round in a circle; the centre of which is on the central line or axis of the crank-shaft. I have drawn such circle as described by the point Y, farthest from the axis; but any and all points describe larger or lesser circles round the same centre. The point Y may, therefore, be considered as the centre of a crank-pin; and the eccentric might, so far as its effects are concerned, be replaced by a crank. Now, if you turn the fly-wheel of your lathe by hand, the crank will revolve, but the treadle will rise and fall only in a straight line; and you will presently see how the eccentric, in its revolution, gives just such a to-and-fro motion to the rod D, and consequently also to the slide-valve, which it has to move.
Round the disc V, closely encircling it, is a flat ring, shown separately at X, with a rod, W, attached to and part of it. This ring is generally made in separate halves, united by bolts passing through projecting lugs or ears. The ring also fits into the groove turned on the edge of the disc V, so that it cannot slip off sideways. This outer ring is turned quite smooth and true on the inside, so that the eccentric disc can revolve within it. In doing so, it is plain that the whole ring will rise and fall, and that the rod W will move up and down, or to and fro, like the treadle of the lathe, thereby giving motion to the valve-rod, which is a continuation of the rod W. As the upper end, however, of this rod has an oscillating, or up-and-down motion, this is imparted, in a certain degree, to its other end, at the farthest distance from the eccentric; and hence the necessity for a hinged joint at S, to prevent the valve-rod from partaking of this movement. It is, however, very slight, so that the rod of the valve is not often made to pass through guides like the piston. The whole movement of the valve-rod is very limited, its traverse only being required to be sufficient to shift the valve the width of one of its ports at each stroke. The length of stroke or traverse which can be obtained by the eccentric is always equal to twice the distance between its real centre, and that on which it turns, which will always be a guide to you in making an engine.