. The steam-engine and other heat-motors. t-point c (sta-tionary temporarily). Any movement of c will be given to cfunchanged in amount, but reversed in direction. If the cut-off valve is put in its midposition in Fig. 192, it willbe found to have a negative lap equal to about half the port-open- 364 THE STEAM-ENGINE AND OTHER HEAT-MOTORS. ing, a. To vary the point of cut-off the position of the cut-offeccentric center is rotated (the radius, OC, being unchanged)about 0. Its three principal positions are shown in Fig. 193. Lay off the diagram for the main valve as usual, BOP beingthe angular a
. The steam-engine and other heat-motors. t-point c (sta-tionary temporarily). Any movement of c will be given to cfunchanged in amount, but reversed in direction. If the cut-off valve is put in its midposition in Fig. 192, it willbe found to have a negative lap equal to about half the port-open- 364 THE STEAM-ENGINE AND OTHER HEAT-MOTORS. ing, a. To vary the point of cut-off the position of the cut-offeccentric center is rotated (the radius, OC, being unchanged)about 0. Its three principal positions are shown in Fig. 193. Lay off the diagram for the main valve as usual, BOP beingthe angular advance, Pe and Pi being the exhaust- and a center C0 and a radius equal to the negative lap of the cut-off valve describe a circle tangent to OA. Draw any crank posi-tion 07. Drop the perpendicular C08. Then from the constructionof the Bilgram diagram (708 +C09 is the distance that the cut-offvalve must move to close the port in the main valve, since we mayconsider the latter stationary. At crank position OCo, the port is. Fig. 193. open, the negative lap, and at OA the port is clored. If the port inthe main valve is closed, it is a matter of indifference whether theport in the cylinder is open or closed. If the eccentric center isrotated to Cu then the cut-off is in crank position 02. The latestdesirable cut-off point of the cut-off valve is 03, or at the point ofcut-off of the main valve. The width of the cut-off blocks must be such that the blockswill not overrun the port and open on the back edge when thevalve is set for the shortest cut-off. The greater the throw of thecut-off eccentric the more rapidly the valve passes over the port REVOLUTION CONTROL. 365 in the main valve and the quicker the cut-off. But this also in-creases the width of the blocks and the consequent friction. Ex. 126. Make a diagrammatic sketch for a Buckeye valve-gearand its Bilgram diagram for a 20X36 engine, making 125 revolu-tions per minute, lead of the main valve, 1/16 in.; maximu
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