HOW LAP AND LEAD ARE REGULATED.
The lap and lead together are dependent on the distance between the center F of lever E, and the center from which the motion is taken for the valve.
ADVANTAGES CLAIMED FOR THE MOTION.
The first advantage claimed for Joy’s valve-gear over the link-motion is, that it is considerably less costly, and that the working-parts are lighter. The saving is not only in weight, however, but also in the greater simplicity of parts, allowing increased facility for tooling and fitting.
It may be needless to point out how readily this valve-motion is adaptable to the requirements of the typical American locomotive, considerably simplifying the present form of valve-motion, by dispensing with the eccentrics and other working-parts, and bringing the gear with all its strains into a direct line, and also leaving the whole of the under part of the engine clear, and permitting a considerable extension of the fire-box,—a most valuable feature.
All the parts are such that they can be readily made by machinery, dispensing with hand labor except in putting them together.
ACTION OF THE MOTION.
The action transmitted to the valve is altogether different from that transmitted by the link-gear, as by this gear it results from the two distinct motions of the connecting rod imparted to the lever E; and these two motions work variously, with and against each other, thus giving to the valve a resultant motion somewhat partaking of the nature of that produced by cams, it really being an accelerated and retarded motion; these accelerations and retardations being so arranged as to come at the right time on the revolution to give the desired result.
Thus, as the crank is passing over the center, the lever E is not affecting the valve at all as a lever, but the motion imparted to the valve is due to the center of E slipping down the curve of the slot-link J; and this gives a smart opening of the lead with a continued smart opening of the port. When the valve is fully open, as the crank travels on, the lever E has commenced acting on the valve; but its center is still sliding down the slot-link JK; and the two movements are opposite and destroying, or partially destroying, each other, producing a retardation of the valve-motion almost amounting to an arrest; and thus is given a nearly straight line on a valve-path diagram where the port is kept fully open, the crank still traveling on, and approaching the time for the cut-off to be effected. The lever E is in the best position to act as a lever at the same time that its center is completing its movement in the slot JK, and is commencing to return, being practically still: thus the lever action of E, having full effect, rapidly closes the port. And so on for the revolution, the time of release also getting its benefit.