Marine engineers, therefore, need some mechanical means of restraining their engines from "running away." The device must be very sensitive and quick acting, since the engines would increase their rate threefold in a second if left ungoverned when running "free"; while on the other hand it must not throttle the steam supply a moment after the work has begun again when the screw takes the water.
Many mechanisms have been invented to curb the marine engine. Some have proved fairly successful, others practically useless; and the fact remains that, owing to the greater difficulty of the task, marine governing is not so delicate as that of land engines. A great number of steamships are not fitted with governors, for the simple reason that the engineers are sceptical about such devices as a class and "would rather not be bothered with them."
But whatever may have been its record in the past, the marine governor is at the present time sufficiently developed to form an item in the engine-rooms of many of our largest ships. We select as one of the best devices yet produced that known as Andrews' Patent Governor; and append a short description.
It consists of two main parts--the pumps and the ram closing the throttle. The pumps, two in number, are worked alternately by some moving part of the engine, such as the air-pump lever. They inject water through a small pipe into a cylinder, the piston-rod of which operates a throttle valve in the main steam supply to the engines. At the bottom of this cylinder is a by-pass, or artificial leak, through which the water flows back to the pumps. The size of the flow through the by-pass is controlled by a screw adjustment.
We will suppose that the governor is set to permit one hundred revolutions a minute. As long as that rate is not exceeded the by-pass will let out as much water as the pumps can inject into the cylinder, and the piston is not moved. But as soon as the engines begin to race, the pumps send in an excess, and the piston immediately begins to rise, closing the throttle. As the speed falls, the leak gets the upper hand again, and the piston is pushed down by a powerful spring, opening the throttle.
It might be supposed that, when the screw "races," the pumps would not only close the throttle, but also press so hard on it as to cause damage to some part of the apparatus before the speed had fallen again. This is prevented by the presence of a second control valve (or leak) worked by a connecting-rod rising along with the piston-rod of the ram. The two rods are held in engagement by a powerful spring which presses them together, so that a hollow in the first engages with a projection on the second. Immediately the pressure increases and the piston rises, the second valve is shut by the lifting of its rod, and so farther augments the pressure in the cylinder and quickens the closing of the throttle valve. This pressure increase must, however, be checked, or the piston would overrun and stop the engines. So when the piston has nearly finished its stroke the connecting-rod comes into contact with a stop which disengages it from the piston-rod and allows the second control valve to be fully opened by the spring pulling on its rod. The piston at once sinks to such a position as the pressure allows, and the action is repeated time after time.