116

MODERN STEAM PRACTICE.

necting rod for the valve spindle. As the circular valve depends onits fit to make it steam-tight, it is evident that it should be arrangedvertically, so that the wear may be as little as possible. To set outthis valve-gear the line of the main crank is placed level, the crankpin being at the commencement of the IN stroke; the valve is set fullopen, and at a point on the valve spindle, with a certain radius tosuit, describe an arc: this is the curve of the link. It is evidentthat when the sliding block in the link is moved to and fro alongwith the radius rod in connection with the valve spindle, the line ofthe crank being level, that no motion is imparted to the valve, oreven when the crank is revolving the radius link can be drawn to thecentre of Vibration of the lever shaft; thus the valve always remainsopen when required, the lever and link simply vibrating to and fro.The motion of the lever is always constant, travelling in an arc dueto the throw of the eccentric, while the pin and block working inthe slotted link can be moved out at pleasure, giving a varyingthrow. Hence the valve can cut off the steam supply up to one-halfof the stroke of the piston; but from the nature of the motion, theeccentric passing the dead centre of its throw, while the crank forthe engine is gaining rapidly, the valve does not commence to openuntil the piston has travelled about five-eighths of its stroke, whenthe main slide-valve has come into action, and consequently nomore steam can be admitted into the cylinder. Thus the expansion-valve gives a varying cut off from the valve-casing, while that of themain slide-valve admits a constant volume into the cylinder, de-veloping the full power of the engine while so doing. The blockand pin for the separate expansion-valve can be drawn back to thecentre of Vibration, leaving the passages in the expansion-valvealways full open. This single eccentric and link motion for work-ing an expansion-valve separately from the main slide-valvecan be arranged for any dass of valve, as likewise the locality ofthe valve in relation to the eccentric on the main shaft, the arrange-ment given being originally designed by the author for marineengines.

The geometry of the steam-engine next calls for attention, atleast that portion of it which immediately bears on the subject ofvalve motions. This must be always carefully studied, in order todetermine all the points requisite in arranging the slide-valve, thelap and lead of the valve to suit a given cut-off in the cylinder beingof the first importance.