The governor (Fig. 170) of a steam engine is a device which controls the supply of steam by letting into the cylinder just the right quantity. In the pipe which carries the steam from the boiler to the cylinder is a valve called the throttle valve, by which the communication between boiler and engine may be opened or closed. A rod connects this valve to the governor, which is made to turn round by a belt from the crank shaft. The faster the crank shaft turns the faster the governor goes round. At the lower end of the governor are two heavy balls, so hung that as the speed of the governor increases they swing out farther from the center rod and as it slows down they swing nearer to it. This action of the balls is due to centrifugal force. It opens and shuts the throttle valve by raising and lowering the rod which leads from the governor and in this manner the supply of steam is regulated. If the engine moves too fast, the balls of the governor swing out, and this pulls on the rod and partly closes the valve, shutting off some of the steam; if it goes too slowly the balls swing inward and thus open the valve and let in more steam. Thus the speed of the engine is regulated by the governor.
Fig. 170. - Governor.
The speed of the governor should be carefully adjusted, and all its parts kept clean and in perfect working order. When this is done, the engine will always run at a uniform speed, no matter what load or work is on at any time. If any machine is suddenly thrown out of action, the governor should at once control the speed of the engine by cutting off the supply of steam. On the other hand, when a heavy load comes on more steam is admitted by the governor, and thus the speed of the engine is kept nearly constant or uniform.