CONNECTING ECCENTRIC-RODS TO LINK.
Fig. 15.
Fig. 16.
In looking over the motion, it is well to note that the eccentric-rods are properly connected,—the forward eccentric-rod with the top, the backward eccentric-rod with the bottom, of the link. When the crank-pin is on the forward center, the eccentrics will occupy the position they appear in, in [Fig. 15], where the rods are open, and nearly horizontal. The full parts of both eccentrics are advanced towards the crank-pin, so that the centers of the eccentrics are advanced from a perpendicular line drawn through center of axle, a horizontal distance equal to the lap and lead. When the crank-pin is on the back center, the eccentric centers will be behind the axle, and the rods will be crossed as they are seen in [Fig. 16]. The reason why the rods must be crossed when the crank is in this position, is, that the forward eccentric center is below the axle, and the backward eccentric center is above. As the forward eccentric-rod maintains its connection with the top of the link, and the backward eccentric-rod is at the opposite end, crossing of the rods is inevitable. This fact is worth imprinting on the memory, for I have known of several cases where men got the rods up wrong by putting them open when the engine stood with the crank on the back center.
MARKING THE VALVE-STEM.
Fig. 17.
In ordinary practice, valves are set with the steam-chest cover down, and the position of the valve on the seat is identified by marks on the valve-stem. Before the cover is put down, the valve is placed as in [Fig. 17], just beginning to open the forward steam-port; a thin piece of tin being generally used to gauge the opening. When the valve stands in this position, a tram is extended from a center punch-mark c, on the stuffing-box, straight along the valve-stem as far as it will reach; and the point, here located at a, is marked. The valve is then moved forward till it begins to uncover the back port, when another measurement is made with the tram, which locates the point b on the valve-stem. Whatever position the valve may stand on, it may now be identified by the tram. When the tram cuts the space half way between a and b, the valve stands in the middle of the seat.