The simplest form and also the most common form of single valve is the 1)-shaped slide valve. In the diagram, Fig. 6, W' represents a por tion of the top wall of the cylinder, S and S, being the steam ports to the two ends of the cylinder. At the top these steam ports open into the inclosed space A, B, C, I), which represents the steam chest. The valve proper is represented by V. which is operated by the rod P, passing out through a suitable orifice in the forward end of the steam chest. The steam from the boiler is admitted into the space above the valve. It will be observed that in the position shown the valve exactly covers the ports S and S„ and steam from the steam chest cannot enter the cylinder, nor can any steam contained in the cylinder escape. If the valve is moved either to the right or left of the position shown, both ports are immediately uncovered. if the move ment be toward the right, port S, is opened to the live steam space above valve V, and port S is opened to the exhaust space E, beneath the valve V. A reverse movement of the valve opens port S to the live steam space. and port S, to the exhaust space. The movement of the valve from 'centre' to the right and back again to 'centre' accomplishes the right-hand stroke of the piston; its movement from 'centre' to the left and back again to 'centre' accomplishes the left-hand stroke of the piston. These alternate right and left movements of the valve are ac complished by means of the rod B, and are ec centric. (See ECCENTRIC.) When the valve is at 'centre' the axis of the eccentric is at right angles to the fly-wheel crank, providing the valve is constructed, as shown, to cover exactly the steam ports, which is the construction for non expansive working of the steam. This is the simplest form of slide valve. To provide for the expansive working of the steam, its use as a compression cushion and other desirable varia tions in admission and exhaust, the construction of the valve and the position of the eccentric have to be modified in various ways, an analysis of which is a matter of considerable mechanical intricacy.
The train of mechanism by which the opening and closing movements of the valves are effected is called the valve gear. This varies in con struction and operation. The precise character of the gear adopted in any case is determined partly by the form of valve and partly by the nature of the general problem presented for solu tion. In sonic cases it is merely required that a fairly good steam distribution shall be se cured; in other instances a good method of ex pansion must be obtained; in still other cases the system must be capable of adjustment with a view to wise regulation of speed, and in all locomotive and marine engines easy and prompt reversal of the direction of motion is required. Different inventors have invented different forms of valve gear designed particularly to meet one or the other of these conditions. Of these the link motion invented by Howe in 1843, and first i used on Stephenson 's engines, is perhaps the most familiar, it being the gear very generally used, either as originally designed or in some modified form, on locomotives and marine engines. Fig. 7 shows the Howe link, or, as it is more com monly called, the Stephenson reversing link valve gear. The two eccentrics E and E, are set on the crank shaft, 5, at right angles to the crank, C. These eccentrics carry two rods r and r,
diverging from each other at a slight angle and with their forward ends connected by the slotted, curved link, L. The valve stein, V, has a block or pin which connects it to the link by means of a sliding fit in the slot. If the link be lifted so that the rod r lies nearly in line with V, then the valve receives its motion merely as if driven directly by the single eccentric E; if the link be lowered so that rod r, lies nearly in line with V, then the valve acts practically as if driven by the single eccentric Now eccentric E, being set for forward motion of the engine and eccen tric E being set for backward motion, it will be seen that the movement of the link gives a ready means for reversing the engine. It does more than this, however, since by putting the link in intermediate positions between full forward and full backward gear the cut-off of the valve can be adjusted for expansive working of the steam. The train of levers v, w, x, y, and z in Fig. 7 shows the means of operating this link in an ordinary locomotive engine.
The governor of a steam engine is a device whose function is to regulate the energy de veloped in accordance with the load propelled. If the admission and pressure of the steam be constant, the speed will vary as the load varies; with a light load the speed will be high and with a heavy load it will be slow. These fluctuations in speed are always undesirable and may easily become dangerous, as, for example, when the engine 'races' or 'runs away,' causing the fly wheel to burst from centrifugal force. Gov ernors act to regulate the energy in two ways, first by cutting off steam from the boiler as the speed increases, which amounts to a reduction in boiler pressure, and second by re ducing the quantity of steam admitted into the cylinder. The first are called throttling govern ors and the second are called cut-off governors. Govern ors are also classi-' fied according to their method of ac tion and according to their form. The most common form of governor is the fly-ball or pendulum governor, which may operate either by throttling or by cut-off action. The diagram (Fig. 8) shows a form of pendulum governor. The stem or shaft, 5, is given rotary motion by means of a belt embracing the pulley, P. Keyed to the top of the shaft is the plate a, carrying, by means of the arms b and the heavy balls c and and hung to the arms b and b, are the arms d and d, which connect with the plate e, which is free to slide up and down the shaft S. The balls and their connecting arms revolve with the shaft, and because of the familiar law of eeutri fugal force they tend to fly apart as the speed of ro tation increases and to come closer together as this speed decreases. As the balls Ily apart, due to increased speed, they lift the plate c, and this operates the train of levers x, y, and z in such a way as either to shut off steam from the boiler or from the cylinders. That is, as the speed in creases the energy is decreased, and conversely as the speed decreases the energy is increased. In this way the energy is proportioned to the load at every instant, the exactness with which this proportion is maintained depending upon the sensitiveness of the governor.