The movement of this valve very much resembles that produced by means of an eccentric. The movements of the valves are so timed that as the main piston nears the end of the stroke its movement is sufficiently retarded to permit the water valves to seat quietly and without jar, also on the return stroke, the steam is so gradually admitted that the piston starts with ease and gradually increases its speed to the middle of the stroke, from which point it gradually decreases toward the opposite end. A proper adjustment of the cut-off valve allows the piston to stop momentarily at the ends of the stroke without any possibility of striking the cylinder heads.
To Set the Valve. Adjust all joints so that there will be no lost motion in the valve gear. Then move the crosshead to the end of its stroke, and see that the cut-off valve opens the valve chamber one-sixteenth of an inch, and that the steam valve closes the port leading to the cylinder to within one-sixteenth of an inch. Next move the crosshead to the other end of its stroke and note that the valves are in the same relative position. If, from any cause the cut-off valve does not open correctly to admit steam to the valve chamber at the ends of the stroke of the piston, the valve can be shortened by filing off the ends. This should not be done, however, until all lost motion has been taken up in the valve gear.
THE HOOKER, WITH OUTSIDE VALVE GEAR.
In this pump (see [Figs. 266], [267]) the steam is admitted to the center of the valve chamber which contains the main valve, A, and the supplemental slide valve, B. The recess in the center of the valve piston, C, receives the main valve and moves it when steam is supplied to or exhausted from either end of the valve piston.
In operation live steam passes through the left-hand ports, D and E, drives the main piston to the right and the exhaust passes out of the right-hand port, F, into the cavity in the main valve, A, thence through the exhaust port, G, into the atmosphere.
Fig. 265.
As the main piston nears the right-hand port, the valve lever H, attached to and moving with piston rod brings the dog, I, on plate, J, in contact with the valve arm, K. This moves the supplemental valve, B, to the right and supplies live steam to the right of the valve piston, C, and exhausts the steam from the left-hand end through the ports, L. The main valve, being enclosed by the valve piston, moves with it to the left. As the steam enters the right hand main port and exhausts from the left hand port, the main piston commences its return stroke and the operation just described becomes practically continuous. As the main piston closes the main port, F, to the right, it cushions on compressed exhaust steam, because the main valve, A, has then closed the auxiliary port, M, leading to that end of the main cylinder. The steam in this case is supplied through the main and auxiliary ports, but exhausts through the main port only.