The credit of inventing the ordinary shifting link is due to William Howe of Newcastle, England. This inventor was a pattern-maker in the works of Robert Stephenson & Co., and he invented the link in 1842 in practically its present form. The idea of Howe was to get out an improved reversing motion; and he made a pencil-sketch of the link, to explain his views to his employers. The superintendent of the works was favorably disposed to the invention, and ordered Howe to make a pattern of the motion, which was done; and this was submitted to Stephenson, who approved of the link, and directed that one should be tried on a locomotive. Although Stephenson gave Howe the means of applying his invention, he does not seem to have perceived its actual value, for the link was not patented; and Stephenson never failed to patent any device which he thought worth protecting.
The link-motion was applied to a locomotive constructed for the Midland Railway Company, and proved a success from the day it was put on. Seeing how satisfactorily the invention worked, Robert Stephenson paid Howe twenty guineas (one hundred and five dollars) for the device, and adopted the link as the valve-gear for his locomotives. This is how the shifting link comes to be called the “Stephenson link,” and the credit for this invention was not extravagantly paid for.
The capability which the link possesses of varying the steam admission and release, did not appear to be understood by the inventor; nor was the mechanical world aware, for some time after the link was brought into use, that it could be employed to adjust the inequality of steam distribution, due to the angularity of the connecting rod.
CONSTRUCTION OF THE SHIFTING LINK.
Fig. 12.
As usually constructed for American locomotives, the link is a slotted block curved to the arc of a circle, with a radius about equal to the distance between the center of the driving-axle and the center of the rocker-pin. The general plan of the link-motion is shown in [Fig. 12]. Fitted to slide in the link-slot is the block which encircles the rocker-pin. The eccentric-rods are pinned to the back of the link; the forward eccentric-rod connecting with the top, and the back-up eccentric-rod with the bottom, of the link. Bolted to the side and near the middle of the link is the saddle, which holds the stud to which the hanger is attached; this, in its turn, connecting with the lifting arm, which is operated by the reversing rod that enables the engineer to place the link in any desired position.
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.