The valves, as can easily be seen, are very simple affairs, the steam pressure in the steam chest holding the valve either open or shut until it is moved by the pawl on the rock-shaft. The amount of travel on the rock-shaft is fixed by the design, but the proportionate travel above and below the horizontal is controlled by the length of the connecting-rods from the crank to the rock-shaft. There are besides the mechanical valve-gear the electric and hydraulic, but these will be left for a future article.
The Governor
The speed of the machine is controlled by the automatic opening and closing of the admission valves under the control of a governor (Fig. [19]), of the spring-weighted type attached directly to the top end of the turbine shaft. The action of the governor depends on the balance of force exerted by the spring, and the centrifugal effort of the rectangular-shaped weights at the lower end; the moving weights acting through the knife-edge suspension tend to pull down the lever against the resistance of the heavy helical spring. The governor is provided with an auxiliary spring on the outside of the governor dome for varying the speed while synchronizing. The tension of the auxiliary spring is regulated by a small motor wired to the switchboard. This spring should be used only to correct slight changes in speed. Any marked change should be corrected by the use of the large hexagonal nut in the upper plate of the governor frame. This nut is screwed down to increase the speed, and upward to decrease it.
The Stage Valves
Fig. [20] represents one of the several designs of stage valve, sometimes called the overload valve, the office of which is to prevent too high pressure in the first stage in case of a sudden overload, and to transfer a part of the steam to a special set of expanding nozzles over the second-stage wheel. This valve is balanced by a spring of adjustable tension, and is, or can be, set to open and close within a very small predetermined range of first-stage pressure. The valve is intended to open and close instantly, and to supply or cut off steam from the second stage, without affecting the speed regulation or economy of operation. If any leaking occurs past the valve it is taken care of by a drip-pipe to the third stage.
