ACTION OF THE LINK.
Regarded in its simplest form, the action of the link in full gear is the same upon the valve movement as a single eccentric. When the motion is working, as in the figure, with the eccentric-rod pin in line with the rocker-pin, it will be perceived that the movement can not differ much from what it would be were the eccentric-rod attached to the rocker. Here the forward eccentric appears as controlling the movement of the valve. Putting the link in back motion brings the end of the backing eccentric-rod opposite the rocker-pin, the effect being that the back-up eccentric then operates the valve. When the link-block is shifted toward the center of the link, the horizontal travel of the rocker-pin is decreased; consequently, the travel of the valve is reduced; for, with ordinary engines, the travel of the valve in full gear equals the throw of the eccentrics, the top and bottom rocker-arm being of the same length. The motion transmitted from the eccentrics, and their means of connection with the link, make the latter swing as if it were pivoted on a center which had a horizontal movement equal to the lap and lead of the valve. The extremities of the link, or rather the points opposite the eccentric-rods, swing a distance equal to the full throw of the eccentric. The variation of valve-travel that can be effected by the link, is from that of the eccentric throw in full gear down to a distance in mid gear which agrees with the extent of lap and lead. The method of obtaining these various degrees of travel is by moving the link so that the block which encircles the rocker-pin shall approach the middle of the link.
When an engine is run with the lever in the center notch, the supply of steam is admitted by the lead-opening alone. In full gear the eccentric, whose rod-end is in line with the rocker-pin, exerts almost exclusive control over the valve movement; but, as the link-block gets hooked towards the center, it comes to some extent under the influence of both eccentrics.
A thoughtful examination of [Fig. 12] will throw light on the reason why the proper position of a slipped eccentric can be determined by the other eccentric when the engine is on the center. In the cut, the crank-pin is represented on the forward center; and in that position the eccentric centers are both an equal distance in advance of the main shaft center. It will be evident now that the valve must occupy practically the same position for forward or back gear, as each of the eccentric-rods reaches the same distance forward. Putting the motion in back gear would bring the backup eccentric-rod pin to the position now occupied by the pin belonging to the forward eccentric-rod.