The governor for a common D valve engine regulates the engine speed by varying the opening in the bore of the pipe through which the steam passes from the boiler to the steam chest, leaving a wider opening in proportion as the engine runs slower, and reducing the opening when the engine runs faster. Assuming the engine to be running at its slowest, or its load to be so great that a full supply of steam is required in order to keep the engine up to its proper speed, and the governor will be open at its widest, so that all the further action the governor can have is to reduce the steam pipe opening, and thus cause the pressure in the steam chest to be less than that in the steam pipe.
This action is called wire-drawing the steam, and the governor is called a throttling governor.
An engine bed or bed plate is a frame that is seated or bedded to its foundation along its whole length.
An engine frame is seated to its foundations at two or more places, but not continuously throughout its length.
THE CYLINDER.
Cylinders are secured to the engine frames in three principal ways, as follows: by bolting them down to the bed plate; by bolting them to one end of the bed plate, so that they may expand and contract without springing the bed plate; and in vertical engines, by bolting them to the top of the frames.
The bores of cylinders require to be parallel, so that the piston rings may fit to the bore without requiring to expand and contract in diameter at different parts of the stroke.
Cylinders are designated for size by the diameter of the cylinder bore and the length of the stroke; thus, a 10 × 12 cylinder has a piston of ten inches diameter and 12 inches stroke.
The wear of a cylinder bore is (if the engine is kept in proper line and the piston rings, or packing rings as they are sometimes termed, fit to the bore with an equal pressure throughout the stroke) greatest near the middle of the length and least at the ends of the stroke. But when the piston rings are set out by the steam pressure, and the point of cut off occurs early in the stroke, the wear may be greatest at the ends of the cylinder bore, because of the pressure of the steam diminishing during the expansion.
The counterbore of a cylinder is a short length at each end of the cylinder, that is made of larger diameter than the rest of the bore, so that the piston head may travel completely over the working bore, and thus prevent the formation of a shoulder at each end of the cylinder. Such a shoulder forms when there is a part of the bore over which the piston does not pass. The length of the counterbore should exceed the amount of the taper on the connecting rod key, so that as the connecting rod length alters from the wear, the piston shall not strike the cylinder head.