The length of the connecting rod is the distance from the center of crank-pin hole to center of cross-head pin-hole.
The length of link-hanger is the distance from center to center of holes.
CONDITIONS.
Since this article treats only on the adjustment of the link-motion, the following items are supposed to be known and established: The lap of valve, which in this case will be three-fourths of an inch; the throw of eccentrics, 5 inches; the stroke of the piston, 24 inches; the position of the rocker, as per [Fig. 19]; the length of the rocker-arms, which are in this case of equal length; length of link-hanger and all dimensions of link, complete, as shown in [Fig. 19]; and also the length of the connecting rod.
The adjustment of the link-motion may at first sight appear to be a difficult problem, as we must have a knowledge of the relative motions of the piston and slide-valve; but by reducing this problem to several elementary problems, so that the laws governing the relative motions may be discovered and clearly defined, a clear conception of our subject can be gained, and the solution of our original problem can be accomplished with comparative ease.
In order to find what kind of elementary problems are applicable, let us suppose that we are looking at a locomotive with a link-motion, as shown in [Fig. 19], applied and correctly adjusted. Now let us examine it, commencing with the valve. We find that the valve receives its motion from the upper rocker-arm, and this receives its motion from the lower rocker-arm. According to our conditions, previously stated, both of the rocker-arms are of the same length; and, therefore, the arc described by the upper rocker-arm will be the same length as the arc described by the lower one. We also notice that the link which moves the lower rocker-arm is held in position by the lifting-shaft arm. The question, then, will naturally arise, Must this lifting-shaft arm have some particular length, and the center of lifting-shaft have some particular position? We answer, “Yes;” and this is one of our elementary problems to solve. Again, we notice that the saddle-pin is not in the center of the link; and we ask again, “Why?” To answer this will be another elementary problem. The next we notice are our eccentric-rods. These we find, on examination, to have some particular length; and to find this length is another elementary problem. Next we examine our eccentrics: these, we find, are fastened to the axle; and, since the crank is also fastened to the same axle, it follows that there are some relative positions between them; to find these positions is another elementary problem. Now let us look once more at the rocker, and we find that the two rocker-arms are not in the same straight line: hence, to find the amount of offset is another elementary problem. And, lastly, we must be able to find the position of crank-pin to correspond with the position of piston when at full stroke at either end of the cylinder, and also when at half stroke moving in either direction.
Here, then, we have all the elementary problems that are necessary to be understood for the solution of our original problem.
We will now explain all these problems, in an order the reverse to that in which we stated them; hence we have the following order:—
1st, To find position of crank at full and half stroke.