Page:The New International Encyclopædia 1st ed. v. 18.djvu/612

* STEAM ENGINE. 526 STEAM ENGINE. to prevent the condensation of steam in the cyl- inder by the loss of heat due to radiation from its outer walls. The piston is a circular disk of metal of sufficient strength to resist distortion by the steam pressure. In the edge of this disk are «et rings of metal so arranged that they are constantly pressed outward against the walls of the cylinder, either by their own elasticity or by the elasticity of springs pressing against them from behind. Tliese rings are called packing rings, and their function is to make the piston fit steam-tight in the cylinder and at the same time render its movement as free from friction as possible. The function of the valves and valve gear is to control the admission and exhaust of the steam. The functions of the valve are both primary and secondary. The primary function is to admit the .steam from the boiler to one side of the piston, while the exhaust steam filling the other end of the cylinder is permitted to escape ■with the least possible resistance. The secondary functions are to close the admission of steam at the point necessary to give the expansion de- sired and to close the exhaust orifice at such a point in the return stroke that a certain vol- ume of steam shall be caught and compressed behind the piston to serve as an elastic cushion. It is necessary also that the opening of the in- lets and outlets of the cylinder should be so timed with respect to the stroke of the piston that pressure may not be brought too soon against the piston head, nor the exhaust opened until the expanding steam has done its entire work for that stroke. The valves for admitting and distributing steam in an engine cylinder may open the ports which they control, eitlier by lifting from their seats or by sliding upon their seats. When the engine is a double-acting one there must be provision to connect each end of the cylinder with the boiler and each end with the exhaust pipe. When the engine is single- acting it is only necessary to connect one end to the boiler and exhaust pipe alternately. Ap- parently the simplest arrangement would be to have four valves, as shown bv Fig. 2, one at each end for steam, and one at each end for ex- haust. This is the arrangement in the Corliss engine. An important type of engines makes use of a separate valve for admitting steam at each end, while the exhaust is controlled by a single valve for both ends. Such engines are called three-valve engines. Another arrangement is to have one valve for admitting steam to both ends and another controlling the exhaust for both ends. Such engines are called two-valve en- gines. A fourth class has one single valve, so designed as to perform the inlet and outlet func- tions for both ends. Such are the great majority of stationary engines and all locomotive and marine engines. The simplest form and also the most common form of single valve is the D-shaped slide valve. In the diagram. Fig. 6, W represents a por- tion of the top wall of the cylinder, S and Si 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, D, which represents the steam chest. The valve projier is represented by V, which is operated ))y the rod R 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 Si, 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, l)oth jjorts are immediately uncovered. If tlie move- ment be toward the right, port Si 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 vSi to the e.xhaust space. The movement of the valve from 'centre' to the right and back again to 'centre' accomplishes the right-liand 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 R, 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 some 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