Fig. 241.
In [Fig. 240] we have as an example the construction of a cam to operate the slide valve of an engine which is to have the steam supply to the cylinder cut off at one-half the piston stroke, and that will admit the live steam as quickly as a valve having steam lap equal to, say, three-fourths the width of the port. In [Fig. 240] let the line a represent a piston stroke of 24 inches, the outer circle b the path of the outer edge of the cam, and the inner circle c the inner edge of the cam, the radius between these circles representing the full width of the steam port. Now, in a valve having lap equal to three-fourths the width of the steam port, and travel enough to open both ports fully, the piston of a 24-inch-stroke engine will have moved about 2 inches before the steam port is fully opened, and to construct a cam that will effect the same movement we mark a dot d, distant from the end e of piston stroke 2⁄26 of the length of the line a, and by erecting the line f we get at point g, the point at which the cam must attain its greatest throw. It is obvious, therefore, that as the roller is at r the valve will be in mid-position, as shown at the bottom of the figure, and that when point g of the cam arrives at e the edge p of the valve will be moved fair with edge s of the steam port t, which will therefore be full open. To cut off at half stroke the valve must again be closed by the time point n of the cam meets the roller r; hence we may mark point n. We may then mark in the cam curve from n to m, making it as short as it will work properly without causing the roller to fail to follow the curve or strike a blow when reaching the circle c. To accomplish this end in a single cam, it is essential to make the curve as gradual as possible from point m to o, so as to start the roller motion easily. But once having fairly started, its motion may be rapidly accelerated, the descent from o to q being rapid. To prevent the roller from meeting circle c with a blow, the curve from q to n is again made gradual, so as to ease and retard the roller motion. The same remarks apply to the curve from r to g, the object being to cause the roller to begin and end its passage along the cam curve as slowly as the length of cam edge occupied by the curve will permit. There is one objection to starting the curve slowly at g, which is that the port s will be opened correspondingly slowly for the live steam. This, however, may be overcome by giving the valve an increased travel, as shown in [Fig. 241], which will simply cause the valve edge to travel to a corresponding amount over the inside edge of the port. The increased travel is shown by the circles y and z, and it is seen that the cam curve from w to r is more gradual than in [Fig. 240], while the roller r will be moved much more quickly in the position shown in [Fig. 241] than it will in that shown in [Fig. 240], both positions being that when the piston is at the end of the stroke and the port about to open. While that part of the cam curve from g to m in [Fig. 241] is moving past the roller r, the valve will be moving over the bridge, the steam port remaining wide open, and therefore not affecting the steam distribution. After point m, [Fig. 241], has passed the roller, we have from m to t to start the roller gradually, so that when it has arrived at t and the port begins to close for the cut-off it may move rapidly, and continue to do so until the point n reaches the roller and the cut-off has occurred, after which it does not matter how slowly the valve moves; hence we may make the curve from n to the circle y as gradual as we like.
Fig. 242.
[Fig. 242] represents a cam for a valve having the amount of lap represented by the distance between circles c and y, the cam occupying the position it would do with the piston at one end of the stroke, as at e. Obviously, a full port is obtained when point g reaches the roller, and as point n is distant from e three-quarters of the diameter of the outer circle, the cut-off occurs at three-quarter stroke, and we have from n to y to make the curve as gradual as we like, and from w to r in moving the valve to open the port. We cannot, however, give more gradual curves at g and at m without retarding the roller motion, and therefore opening and closing the port slower, and it would simply be a matter of increase of speed to cause the roller to fail to follow the cam surface at these two points unless a return cam be employed.
We have in these engine cams considered the steam supply and point of cut-off only, and it is obvious that a second and separate cam would be required to operate the exhaust valves.
