This section is from the book "Scientific American Reference Book. A Manual for the Office, Household and Shop", by Albert A. Hopkins, A. Russell Bond. Also available from Amazon: Scientific American Reference Book.
A governor of a steam engine is a device for automatically operating the throttle, or for shortening the stroke of the slide valve when the engine attains a dangerous speed.
When a dangerous speed is acquired, the centrifugal force acting upon a pair of balls tends to lift a sleeve which, through a bell crank, operates the throttle.
The operation is very similar to that of Watt, but the balls are required to lift a weight which may be adjusted as desired.
The degree of sensitiveness is governed by the length of the cross arms, and also by an adjustable weight, which is lifted by the balls.
Two pairs of balls are used, one pair acting to counterbalance the other.
The balls when thrown out by centrifugal action depress a rod in the hollow central shaft and this rod acts directly on the block in the link thus shortening the stroke of the slide valve
230 and 231. Proell's Governor. - In 230 the balls, aside from lifting a weight, act to compress a spiral spring. In 231 the outward movement of the balls is controlled by an air dashpot.
A cross arm governor which acts to raise a weight.
The balls move on parabolic guide arms, which modify the effect of the centrifugal force, and produce equal valve movement, which is exactly proportional to the speed of the engine.
The balls are secured to the ends of a lever, which assumes a more horizontal position as the speed of the engine increases. A spring normally holds the arm in the tilted position illustrated.
The centrifugal action of the ball moves the eccentric toward the center, thus reducing the stroke of the slide valve. A leaf spring resists the centrifugal action of the ball.
The balls are thrown out by centrifugal force against the action of a spring raising the block in the link and thus varying the stroke of the valve.
The weights operate against the spring to move a toothed sector, which moves the eccentric toward the center of the crank shaft, thus varying the stroke of the slide valve.
The weights have bearings in the side plates of the governor. They also carry pins by which they are connected to the eccentric. When the weights are thrown out by centrifugal action, they move the eccentric toward the center of the crank shaft.
239 and 240. Vane Governors. - The shaft is prevented from rotating too rapidly by the atmospheric resistance acting on a pair of vanes. This resistance may be varied by adjusting the vanes to different angles. In some types of vane governors the inclined vanes serve to lift a sleeve, cutting off the supply of power.