When in operation, the bell-crank is rotated in the direction of the arrow by the action of the wrist-plate and connecting-rod. As the bell-crank rotates, the grab hook engages the steel block at the end of the drop lever B and lifts it, thus causing the valve to open, and to remain so until the bell-crank has advanced so far that the longer arm of the grab hook H is pressed outward by the projection on the knock-off cam, as shown in [Fig. 24]. The drop lever now being released, the valve is quickly closed by the suction of the dash pot, which pulls the lever down to its original position by means of the rod previously mentioned.
Fig. 28. Dash-pot for Corliss Engine
The governor operates by changing the point of cut-off through the action of the cam C. With the cam in the position shown in [Fig. 25], cut-off occurs earlier than in [Fig. 24]. Should the cam be turned in the opposite direction (clockwise), cut-off would take place later. A detailed view of the complete valve mechanism described is shown assembled in [Fig. 26], with each part properly named. A detail of the governor is shown in [Fig. 27]. An increase in speed causes the revolving balls BB to swing outward, thus raising the weight W and the sleeve S. This in turn operates the lever L through rod R and a bell-crank attachment, as shown in the right-hand view. An upward and downward movement of the balls, due to a change in speed of the engine, swings the lever L backward and forward as shown by the full and dotted lines. The ends of this lever are attached by means of reach-rods to the knock-off cams, this being shown more clearly in [Fig. 22]. The connections between the lever L and cam C are such that a raising of the balls, due to increased speed, will reduce the cut-off and thus slow down the engine. On the other hand, a falling of the balls will lengthen the cut-off through the same mechanism.
Mention has already been made of the dash pot which is used to close the valve suddenly after being released from the grab hook. The dash-pot rod is shown in [Fig. 26], and indicated by dotted lines in [Figs. 23] to [25]. A detailed view of one form of dash pot is shown in [Fig. 28]. When the valve is opened, the rod attached to lever B, [Figs. 23] and [24], raises the piston P, [Fig. 28], and a partial vacuum is formed beneath it which draws the piston and connecting rod down by suction as soon as the lever B is released, and thus closes the valve suddenly and without shock. The strength of the suction and the air cushion for this piston are regulated by the inlet and outlet valves shown on the sides of the dash pot.
Engine Details
[Figs. 29] to [37] show various engine details, and illustrate in a simple way some of the more important principles involved in steam engine design.
A partial cross-section of an adjustable piston is shown in [Fig. 29], and a longitudinal section of the same piston in [Fig. 30]. The principal feature to be emphasized is the method of automatic expansion employed to take up any wear and keep the piston tight. In setting up the piston a hand adjustment is made of the outer sleeve or ring R by means of the set-screws AA. Ring R is made in several sections, so that it may be expanded in the form of a true circle. Further tightness is secured without undue friction by means of the packing ring P which fits in a groove in R and is forced lightly against the walls of the cylinder by a number of coil springs, one of which is shown at S. As the cylinder and piston become worn, screws A are adjusted from time to time, and the fine adjustment for tightness is cared for by the packing ring P and the coil springs S.