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.
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.
The former motion increases in rapidity, while the latter decreases; therefore at some point they will become equal in velocity, and the openings of the two ports will be the same; and the question is, Will this maximum effective port area give a sufficient supply of steam?
This diagram is the same as the one actually used in the engine under consideration, in which it was required to follow a minimum distance of 5 inches in the stroke of 22. Under these conditions it is found that the actual port opening for that point of cutting off is three-fifths of that allowed when following full stroke, whereas the speed of the piston at the time when this maximum opening occurs is less than half its greatest speed.
This, it would seem, is ample; but we now find the eccentric, K, no longer in the right position for backing; when the engine is reversed it ought to be at, P, the angle, POL, being equal to the angle, KOL. By leaving it free, therefore, to move upon the shaft, by the means above described, through the angle, KOP, the desired object is accomplished. The real eccentricity is now reduced in the proportion of OK to OH, while the lengths of the cut-off valves, and what is equally important, their travel over the back of the main valve, are reduced in the proportion of CK to CH, in this instance nearly one-half; a gain quite sufficient to warrant the adoption of the expedient.
The third, and perhaps the most notable, peculiarity is the manner of suspending and operating the main link. As before stated, this link is used only for reversing, and is therefore always in "full gear" in one direction or the other; and the striking feature of the arrangement here used is that, whether going ahead or backing, there is no slipping of the link upon the link block.
The link itself is of the simplest form, being merely a curved flat bar, L, in which are two holes, A and B (Fig. 7), by which the link is hung upon the pins, which project from the sides of the eccentric rods at their upper ends.
This is most clearly shown in Fig. 8, which is a top view of the reversing gear. The link block is a socket, open on the side next to the eccentric rods, but closed on the side opposite, from which projects the journal, J, as shown in Fig. 9, which is a vertical section by the plane, XY. This journal turns freely in the outer end of a lever, M, which transmits the reciprocating motion to the valve, through the rock-shaft, O, and another lever, N. Connected with the lever, M, by the bridge-piece, K, and facing it, is a slotted arm, G, as shown in the end view, Fig. 10. The center line of this slot lies in the plane which contains the axes of the journal, J, and of the shaft, O.
A block, E, is fitted to slide in the slotted arm, G; and in this block is fixed a pin, P. A bridle-rod, R, connects P with the pin, A, of one of the eccentric-rods, prolonged for that purpose as shown in Fig. 8; and a suspension-rod, S, connects the same pin, P, with the upper end of the reversing lever, T, which is operated by the worm and sector. The distance, JO, in Fig. 10, or in other words the length of the lever, M, is precisely equal to the distance, AB, in Fig. 7, measured in a right line; and the rods, R and S, from center to center of the eyes, are also each of precisely this same length. Further, the axis about which the reversing lever, T, vibrates is so situated that when that lever, as in Fig 11, is thrown full to the left, the pin in its upper end is exactly in line with the rock-shaft, O.
When the parts are in this position, the suspension-rod, S, the arm, G, and the lever, M, will be as one piece, and their motions will be identical, consisting simply of vibration about the axis of the rock-shaft, O. The motion of the lever, M, is then due solely to the pin, B, which is in this case exactly in line with the journal, J, so that the result is the same as though this eccentric rod were connected directly to the lever; and the pin, P, being also in line with B and J, and kept so by the suspension-rod, S, it will be seen that the bridle-rod, R, will move with the link, L, as though the two were rigidly fastened together.
When the reversing lever, T, is thrown full to the right, as in Fig. 12, the pin, P, is drawn to the inner end of the slot in the arm, G, and is thus exactly in line with the rock-shaft, O. The suspension-rod, S, will, therefore, be at rest; but the pin, A, will have been drawn, by the bridle-rod, R, into line with the journal, J, and the bridle-rod itself will now vibrate with the lever, M, whose sole motion will be derived from the pin, A.
There is, then, no block slip whatever when the link thus suspended and operated is run in "full gear," either forward or backward.
If this arrangement be used in cases where the link is used as an expansion device, there will be, of course, some block slip while running in the intermediate gears. But even then, it is to be observed that the motion of the pin, A, relatively to the rocker arm is one of vibration about the moving center, J; and its motion relatively to the sliding block, E, is one of vibration about the center, P, whose motion relatively to E is a small amount of sliding in the direction of the slot, due to the fact that the rocker arm itself, which virtually carries the block, E, vibrates about O, while the suspension-rod, S, vibrates about another fixed center. It will thus be seen that, finally, the block slip will be determined by the difference in curvature of arcs which curve in the same direction, whether the engine be running forward or backward; whereas in the common modes of suspension the block slip in one direction is substantially the half sum of the curvatures of two arcs curving in opposite directions.
Consequently it would appear that the average action of the new arrangement would be at least equal to that of the old in respect to reducing the block slip when running in the intermediate gears, while in the full gears it entirely obviates that objectionable feature.