Different Types of Valves
In the following are shown some of the valves in common use, being, with the exception of the Corliss, modifications of the plain slide valve, and similar in their action.
Double-Ported Balanced Valve.—A valve of this type has already been shown in [Fig. 2]. This valve is flat in form, with two finished surfaces, and works between the valve-seat and a plate, the latter being prevented from pressing against the valve by special bearing surfaces which hold it about 0.002 inch away. The construction of the valve is such that when open the steam reaches the port through two openings as indicated by the arrows at the left. The object of this is to reduce the motion of the valve and quicken its action in admitting and cutting off steam.
Fig. 17. Engine with Piston Valve
Piston Valve.—The piston valve shown in [Fig. 17] is identical in its action with the plain slide valve shown in [Fig. 8], except that it is circular in section instead of being flat or rectangular. The advantage claimed for this type of valve is the greater ease in fitting cylindrical surfaces as compared with flat ones. The valve slides in special bushings which may be renewed when worn. Piston valves are also made with double ports.
Fig. 18. Section through Cylinder of Engine of the Four-valve Type
Four-Valve Type.—[Fig. 18] shows a horizontal section through the cylinder and valves of an engine of the four-valve type. The admission valves are shown at the top of the illustration and the exhaust valves at the bottom, although, in reality, they are at the sides of the cylinder. The advantage of an arrangement of this kind is that the valves may be set independently of each other and the work done by the two ends of the cylinder equalized. The various events, such as cut-off, compression, etc., may be adjusted without regard to each other, and in such a manner as to give the best results, a condition which is not possible with a single valve.
Fig. 19. Different Types of Corliss Valves
Fig. 20. Longitudinal Section through Corliss Engine
Fig. 21. The Gridiron Valve
Gridiron Valve.—One of the principal objects sought in the design of a valve is quick action at the points of admission and cut-off. This requires the uncovering of a large port opening with a comparatively small travel of the valve. The gridiron valve shown in [Fig. 21] is constructed especially for this purpose. This valve is of the four-valve type, one steam valve and one exhaust valve being shown in the section. Both the valve and its seat contain a number of narrow openings or ports, so that a short movement of the valve will open or close a comparatively large opening. For example, the steam valve in the illustration has 12 openings, so that if they are 1⁄4 inch in width each, a movement of 1⁄4 inch of the valve will open a space 12 × 1⁄4 = 3 inches in length.
Corliss Valve.—A section through an engine cylinder equipped with Corliss valves is shown in [Fig. 20]. There are four cylindrical valves in this type of engine, two steam valves at the top and two exhaust valves at the bottom. This arrangement is used to secure proper drainage. The action of the admission and exhaust valves is indicated by the arrows, the upper left-hand and the lower right-hand valve being open and the other two closed.
Fig. 22. The Monarch Engine with Corliss Valve Gear.—A, Rod to Eccentric; B, Governor;
C, Reach Rod; D, Radial Arm; E, Steam Valve; F, Bell-crank; G, Wrist Plate;
H, Exhaust Valve; K, Dash-pot
Side and sectional views of different forms of this type of valve are shown in [Fig. 19]. They are operated by means of short crank-arms which are attached to a wrist-plate by means of radial arms or rods, as shown in [Fig. 22]. The wrist-plate, in turn, is given a partial backward and forward rotation by means of an eccentric attached to the main shaft and connected to the upper part of the wrist-plate by a rod as indicated. The exhaust valves are both opened and closed by the action of the wrist-plate and connecting rods. The steam valves are opened in this manner, but are closed by the suction of dash pots attached to the drop levers on the valve stems by means of vertical rods, as shown.
Figs. 23 to 26. Action of Corliss Valve Gear
The action of the steam or admission valves is best explained by reference to [Figs. 23 to 26]. Referring to [Fig. 23], A is a bell-crank which turns loosely upon the valve stem V. The lower left-hand extension of A carries the grab hook H, while the upper extension is connected with the wrist-plate as indicated. Ordinarily the hook H is pressed inward by the spring S, so that the longer arm of the hook is always pressed against the knock-off cam C. The cam C also turns upon the valve stem V and is connected with the governor by means of a reach rod as indicated in [Fig. 23] and shown in [Fig. 22]. The drop lever B is keyed to the valve stem V, and is connected with the dash pot by a rod as indicated by the dotted line. This is also shown in [Fig. 22]. The end of the drop lever carries a steel block (shown shaded in [Fig. 23]), which engages with the grab hook H.
Fig. 27. Governor for Corliss Engine
When in operation, the bell-crank is rotated in the direction of the arrow by the action of the wrist-plate and connecting-rod. As the bell-crank rotates, the grab hook engages the steel block at the end of the drop lever B and lifts it, thus causing the valve to open, and to remain so until the bell-crank has advanced so far that the longer arm of the grab hook H is pressed outward by the projection on the knock-off cam, as shown in [Fig. 24]. The drop lever now being released, the valve is quickly closed by the suction of the dash pot, which pulls the lever down to its original position by means of the rod previously mentioned.
Fig. 28. Dash-pot for Corliss Engine
The governor operates by changing the point of cut-off through the action of the cam C. With the cam in the position shown in [Fig. 25], cut-off occurs earlier than in [Fig. 24]. Should the cam be turned in the opposite direction (clockwise), cut-off would take place later. A detailed view of the complete valve mechanism described is shown assembled in [Fig. 26], with each part properly named. A detail of the governor is shown in [Fig. 27]. An increase in speed causes the revolving balls BB to swing outward, thus raising the weight W and the sleeve S. This in turn operates the lever L through rod R and a bell-crank attachment, as shown in the right-hand view. An upward and downward movement of the balls, due to a change in speed of the engine, swings the lever L backward and forward as shown by the full and dotted lines. The ends of this lever are attached by means of reach-rods to the knock-off cams, this being shown more clearly in [Fig. 22]. The connections between the lever L and cam C are such that a raising of the balls, due to increased speed, will reduce the cut-off and thus slow down the engine. On the other hand, a falling of the balls will lengthen the cut-off through the same mechanism.
Mention has already been made of the dash pot which is used to close the valve suddenly after being released from the grab hook. The dash-pot rod is shown in [Fig. 26], and indicated by dotted lines in [Figs. 23] to [25]. A detailed view of one form of dash pot is shown in [Fig. 28]. When the valve is opened, the rod attached to lever B, [Figs. 23] and [24], raises the piston P, [Fig. 28], and a partial vacuum is formed beneath it which draws the piston and connecting rod down by suction as soon as the lever B is released, and thus closes the valve suddenly and without shock. The strength of the suction and the air cushion for this piston are regulated by the inlet and outlet valves shown on the sides of the dash pot.