FIG. 78.—WATT’S AUTOMATIC GOVERNOR AND THROTTLE VALVE.
Steam engines are divided into two principal classes, the low pressure engine, using steam usually under 40 pounds to the square inch, and the high pressure engine, using steam from 50 to 200 pounds. In the low pressure engine there is the expansive pressure of the steam on one side of the piston, aided by the suction of a vacuum on the opposite side of the piston, which vacuum is created by the condensation of the discharging, or exhaust steam, by cold water. As there are two factors at work impelling the piston, only a relatively low pressure in the boiler is required. In the high pressure engines there is no condensation of the exhaust steam, but it is discharged directly into the air, and this type was originally called “puffers.” Familiar examples of the low pressure type are to be found in our side wheel passenger steamers, and of the high pressure type in the steam locomotive.
FIG. 79.—PRINCIPLE OF CUT-OFF.
One of the most important steps in the development of the steam engine was the addition of the cut-off. Prior to its adoption steam was admitted to the cylinder during the whole time the piston was making its stroke from one end of the cylinder to the other. In the cut-off (see [Fig. 79]), when steam is being admitted through the port p, and the piston is being driven in the direction of the arrow, it was found that if the steam were cut off when the piston arrived at the position 1, the expansive action of the steam behind it in chamber a would continue to carry the piston with an effective force to the end of its stroke, or to position 2. This of course effected a great saving in steam. Various cut-offs have been devised. Perhaps that most easily recognized by most persons is the one seen in the engine room of our side wheel steamers, of which illustration is given in [Fig. 80]. This was invented in 1841 by F. E. Sickels, and was the first successful drop cut-off. It was covered by his patents, May 20, 1842, July 20, 1843, October 19, 1844, No. 3,802, and September 19, 1845, No. 4,201. A rock shaft s is worked by an eccentric rod e from the paddle wheel shaft. The rock shaft has lifting arms a that act upon and alternately raise the feet c on rods b b. One of these rods b works the valves that admit steam, and the other the valves that discharge steam. The valve rod that admits steam has a quick drop, or fall, to cut off the live steam before the piston reaches the end of its stroke. In [Fig. 81] is shown the celebrated Corliss cut-off and valve gear, in which a central wrist plate and four radiating rods work the valves. This valve gear was covered in Corliss patents, No. 6,162, March 10, 1849, and No. 8.253, July 29, 1851.
