The second noteworthy feature consists in this: that the cut-off eccentric is not keyed fast, as is customary when valve gear of this kind is employed, but is loose upon the shaft, the angular position in relation to the crank being changed when the engine is reversed; two strong lugs are bolted on the shaft, one driving the eccentric in one direction, the other in the opposite, by acting against the reverse faces of a projection from the side of The eccentric pulley.
The loose eccentric is of course a familiar arrangement in connection with poppet valves, as well as for the purpose of reversing an engine when driving a single slide valve. Its use in connection with the Meyer cut-off valves, however, is believed to be new; and the reason for its employment will be understood by the aid of Fig. 6.
For the purposes of this explanation we may neglect the angular vibrations of the connecting rod and eccentric rod, considering them both as of infinite length. Let O be the center of the shaft; let L O M represent the face of the main valve seat, in which is shown the port leading to the cylinder; and let A be the edge of the main valve, at the beginning of a stroke of the piston. It will then be apparent that the center of the eccentric must at that instant be at the point, C, if the engine turn to the left, as shown by the arrow, and at G, if the rotation be in the opposite direction; C and G then may be taken as the centers of the "go-ahead" and the "backing" eccentrics respectively, which operate the main valve through the intervention of the link.
Now, in each revolution of the engine, the cut-off eccentric in effect revolves in the same direction about the center of the main eccentric. Consequently, we may let R C S, parallel to L O M, represent the face of the cut-off valve seat, or, in other words, the back of the main valve, in which the port, C N, corresponds to one of those shown in Fig. 4; and the motion of the cut-off valve over this seat will be precisely, the same as though it were driven directly by an eccentric revolving around the center, C.
In determining the position of this eccentric, we proceed upon the assumption that the best results will be effected by such an arrangement that when cutting off at the earliest point required, the cut-off valve shall, at the instant of closing the port, be moving over it at its highest speed. And this requires that the center of the eccentric shall at the instant in question lie in the vertical line through C.
Figs. 3-12
IMPROVED STEAM ENGINE.--BY PROF MACCORD.
Next, the least distance to be followed being assigned, the angle through which the crank will turn while the piston is traveling that distance is readily found; then, drawing an indefinite line C T, making with the vertical line, G O, an angle, G C T. equal to the one thus determined, any point upon that line may be assumed as the position of the required center of the cut-off eccentric, at the beginning of the stroke.
But again, in order that the cut-off may operate in the same manner when backing as when going ahead, this eccentric must be symmetrically situated with respect to both C and G; and since L O M bisects and is perpendicular to G C, it follows that if the cut-off eccentric be fixed on the shaft, its center must be located at H, the intersection of C T with L M. This would require the edge of the cut-off valve at the given instant to be at Q, perpendicularly over H; and the travel over the main valve would be equal to twice C H, the virtual lever arm of the eccentric, the actual traverse in the valve chest being twice O H, the real eccentricity.
This being clearly excessive, let us next see what will occur if the lever arm, CH, be reduced as in the diagram to CK. The edge of the cut-off valve will then be at N; it instantly begins to close the port. CN, but not so rapidly as the main valve opens the port, AB.