The
eccentrics are so placed that the passage in the main valve is opened
long before the main valve itself is ready to admit steam to the
cylinder, so that only the outer edges are the ones to be considered,
and it will be readily seen that the two valves traveling in opposite
directions, any lap added to the working edge of the cut off valve
will cause it to reach the edge and therefore close the port earlier
than it would if there was less lap. And we might carry it to the
extreme that we could add lap enough that the steam passage would not
be opened at all.
In Fig. 2 is shown the method by which this is accomplished, in what
is called Meyer's valve, and such as is used in the Kendall & Roberts
engine. We have only one point to look after, the cut off, so we can
add all the lap we wish without disturbing anything else. In this
engine the lap is changed by hand by means of a little hand wheel on a
stem that extends out of the rear of the steam chest. The valve is in
two sections, and when it is desired to cut off earlier, the hand
wheel is turned in such a direction that the right and left hand
screws controlling the cut off valve move one valve portion back and
the other forward, which would, if they were one valve and they should
be so considered, have the effect of lengthening them, or adding lap
to them.
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