By Prof. C.W. MacCord.
The accompanying engravings illustrate the arrangement of a propeller engine of 20 inch bore and 22 inch stroke, whose cylinder and valve gear were recently designed by the writer, and are in process of construction by Messrs. Valk & Murdoch, of Charleston, S.C.
In the principal features of the engine, taken as a whole, as will be perceived, there is no new departure. The main slide valve, following nearly full stroke, is of the ordinary form, and reversed by a shifting link actuated by two eccentrics, in the usual manner; and the expansion valves are of the well known Meyer type, consisting of two plates on the back of the main valve, driven by a third eccentric, and connected by a right and left handed screw, the turning of which alters the distance between the plates and the point of cutting off.
The details of this mechanism, however, present several novel features, of which the following description will be understood by reference to the detached cuts, which are drawn upon a larger scale than the general plan shown in Figs. 1 and 2.
IMPROVED STEAM ENGINE.--BY PROF MACCORD.
The first of these relates to the arrangement of the right and left handed screw, above mentioned, and of the device by which it is rotated.
Usually, the threads, both right handed and left handed, are cut upon the cut-off valve stem itself, which must be so connected with the eccentric rod as to admit of being turned; and in most cases the valve stem extends through both ends of the steam chest, so that it must both slide endwise and turn upon its axis in two stuffing boxes, necessarily of comparatively large size.
All this involves considerable friction, and in the engine under consideration an attempt has been made to reduce the amount of this friction, and to make the whole of this part of the gear neater and more compact, in the following manner:
Two small valve stems are used, which are connected at their lower ends by a crosstail actuated directly by the eccentric rod, and at their upper ends by a transverse yoke. This yoke, filling snugly between two collars formed upon a sleeve which it embraces, imparts a longitudinal motion to the latter, while at the same time leaving it free to rotate.
This sleeve has cut upon it the right and left handed screws for adjusting the cut-off valves; and it slides freely upon a central spindle which has no longitudinal motion, but, projecting through the upper end of the valve chest, can be turned at pleasure by means of a bevel wheel and pinion. The rotation of the spindle is communicated to the sleeve by means of two steel keys fixed in the body of the latter and projecting inwardly so as to slide in corresponding longitudinal grooves in the spindle.
Thus the point of cutting off is varied at will while the engine is running, by means of the hand wheel on the horizontal axis of the bevel pinion, and a small worm on the same axis turns the index, which points out upon the dial the distance followed. These details are shown in Figs. 3, 4, and 5; in further explanation of which it may be added that Fig. 3 is a front view of the valve chest and its contents, the cover, and also the balance plate for relieving the pressure on the back of the main valve (in the arrangement of which there is nothing new), being removed in order to show the valve stems, transverse yoke, sleeve, and spindle above described. Fig. 4 is a longitudinal section, and Fig. 5 is a transverse section, the right hand side showing the cylinder cut by a plane through the middle of the exhaust port, the left hand side being a section by a plane above, for the purpose of exhibiting more clearly the manner in which the steam is admitted to the valve chest; the latter having no pipes for this service, the steam enters below the valve, at each end of the chest, just as it escapes in the center.
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.