EVENTS OF THE PISTON STROKE.
By the aid of [Fig. 10], we will trace the relative movements of the crank and eccentric connections. For the sake of simplicity, the eccentric is represented as connecting directly with the rocker-arm.
The crank-pin being at the point A, or the forward center, the piston must be in the front of the cylinder, or at the beginning of the backward stroke. Owing to the angular advance already referred to, the eccentric center is at F; and, being a certain distance ahead of the middle position, it has pushed the lower arm of the rocker from a to b, drawing back the top arm, which, in its turn, has moved the valve so that it is just beginning to admit steam at the forward port, i. As the crank-pin goes round, the eccentric follows it, opening the steam-port wider till the eccentric reaches the point of its travel nearest A, the limit of the throw. When the eccentric is at this point of its throw, the valve must be at the outside of its travel; and therefore the steam-port is wide open. By this time the crank-pin is getting close up towards the quarter. After passing this point, the forward eccentric begins to draw the bottom rocker-pin towards the axle, and to push the valve ahead, this being the point where the valve changes its direction of motion, just as the piston returns when the crank-pin passes the center. When F reaches the point B, the valve is in the same position it occupied at the beginning of the stroke; but, as it is traveling in the opposite direction, a very small movement more closes the port, cutting off steam. When this happens, the crank-pin has reached the point x. When F gets to g, it is on the central point of its throw; so the valve must then be on the middle point of its travel, with the exhaust cavity just covering the outside edges of the bridges, the forward edge being ready to put the steam-port, i, in communication with the exhaust cavity. This releases the steam from the forward end of the cylinder; and at the same moment the inside edge of the valve covers the back port, k, causing the piston-head to compress any steam left in the back part of the cylinder. When the piston reaches the beginning of the forward stroke, the eccentric F has got to the point f, and the valve is beginning to admit steam for the return stroke, the events of which are similar to those described.
In actual practice, the steam distribution is a little different from the manner that has been followed; for the link-motion provides the means of equalizing the cut-off, making it uniform for both strokes. This changes the events of the stroke a little; but the student who engraves in his mind the movements as they are represented in the diagram, will not be far astray.