The exhaust openings are determined in the same manner, and are shown on opposite side of FG, where the crank passes through the arc DJE during the exhaust period with a positive exhaust lap of the size EF. When the exhaust edge of the valve is line and line this arc becomes GJF, or 180 degrees, and when a negative lap (clearance) occurs, the duration of the exhaust period exceeds the half revolution of the crank. The various events are indicated around the eccentric circle on the figure as they take place during a complete turn of the crank. In Fig. 34 the eccentric and admission valve circles are shown at different cut-offs where each set of lines and circles is governed by the same explanation as those of Fig. 33 where the admission points S, S', S2, and S correspond to the closing positions C, C, C, and C3, cut-off points R, R1, R2, R3, etc. On OH we have the full-travel valve circle, and OL the lap, or radius, of the lap circle, the latter being the same for all cut-offs as well as the lead, the radii H1, H2, H3, etc., of the eccentric circles, or diameters of the corresponding valve circles, terminate on a line HI drawn perpendicular to AB and at a distance from O equal to that of lap and lead. When the reverse lever is in its centre position the diameter of the valve circle falls on the line AB, and is equal to lap and lead. Continuing in back position we have the same method repeated, and OI would be the full-travel valve-circle diameter, or the same as the eccentric radius for the valve travel. Any desired cut-off position may be laid out in same manner as that in Fig. 33, which shows all of the valve events for a complete revolution of the axle. In actual practice the movements are not so regular as the circles indicate, as it is impracticable to obtain the various loci in their theoretical positions; besides, we have the angularities both of the main rods and the eccentric rods to contend with, and whereby irregularities are entering in the problem that must be compensated for, as referred to in the general description. It is not to be considered that a uniform circular motion is the best, but an approximation to it works with less shocks or jerks, and is therefore more desirable for so high-speed an engine as a locomotive. A few advantages can be taken, however, in selecting the suspensions and various connections, so that better results can be obtained than from a true circular motion, which are principally affected by three union points, and are, first, the connecting point of eccentric rod and link; second, the locus of the lifting-link suspension point; and third, the relative height of the crosshead-connection point of the union bar to the corresponding point of the combination lever. It is not necessary to lay out the valve diagrams except where a given cut-off per cent is wanted. This is the most convenient way to find the required lap. Fig. 35 and 36, the diagrams on the folding plates, represent the positions of the valve with the main crank at nine different points of the revolution of the driving wheel. In Fig. 35 the valve is of the piston type, with inside admission, and in Fig. 36 an outside admission valve is shown, of the D-slide pattern. |