[Illustration: FIG. 264.]
The accompanying engraving, Fig. 264, shows a section of the slide
valve chamber, which is fitted to the top of cylinder. The slide valve,
B, is fitted in the valve chamber, A, the cut-off valve, C, works on
top of the slide valve and is operated by the valve stem, D. The lever,
F, which moves the valve, is attached to the crosshead on the piston
rod. While the crosshead moves the length of the stroke, the cut-off
valve, C, moves twice the width of the steam ports in the cylinder.
The opening in the cut-off valve, C, is equal to the length of the dog,
E, plus the throw of the slide valve, B, plus one-sixteenth inch. The
length of the valve chamber on the inside is equal to the length of the
cut-off valve minus the throw of the cut-off valve. The valve stem,
D, is connected to the cut-off valve, C, so that it moves the latter
valve a distance proportional to the movement of the steam piston. The
opening through the cut-off valve is equal in width to the distance
from inside to inside of the ports on top of the slide valve. As the
piston moves to the right, as shown in the drawing, the cut-off valve
moves to the left, so as to open the right-hand end of the valve box
and admits steam, which acts upon the slide valve and moves it to the
left-hand end of the valve box. This opens the port wide in the slide
valve, which corresponds with the port leading to the cylinder while
in the upper side of the slide valve the port is nearly closed by
the cut-off valve, so that little steam is admitted to the cylinder;
consequently, the piston is gradually started on its return stroke. On
the return stroke the cut-off valve begins to move and opens one of the
ports on top of slide valve, which stands still until the piston has
made a half stroke.
By this time the valve is wide open “full port” to the cylinder. The
cut-off valve, C, coming in contact with the dog, E, both the slide
valve and cut-off valve move together to the end of the stroke. Notice
that as soon as the cut-off valve, C, engages the slide valve and it
begins to move, the port to the cylinder closes so that at the end of
the stroke but little steam can get to the piston. The cut-off valve
opens the valve box, A, to admit steam to complete the stroke of slide
valve, and should the steam fail to throw the slide valve, the piston,
by means of the cut-off valve, C, coming in contact with the dog, E,
moves the slide valve, thereby opening the opposite port and preventing
the piston from striking the cylinder head.
The lower corner of the slide valve, B, is removed and a slot cut to
the exhaust port, the slot being of sufficient size to release the
steam from the ends of the valve box and exhaust pressure of it into
the pocket of the slide valve.
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.