The eccentric
operating this expansion valve may be set to close this steam port at
any point in the stroke that is desired, the closing occurring when
the expansion valve has covered the steam port. Continuing the
movements of the valves, the two would move together until one or the
other reached its dead center, when the movements would be in opposite
directions.
[Illustration: FIG. 1.]
There are three ways of effecting the cut off in such engines, the
main valve meanwhile being undisturbed, its eccentric fastened
securely so as not to disturb the points of lead, release, and
compression. All that is required is to cause the edge of the
expansion valve to cover the steam port earlier in the stroke, and
this can be done, first, by increasing the angular advance of the cut
off eccentric; second, by adding lap to the cut off valve; and third
by changing the throw of the eccentric. In all these instances the
riding valve is caused to reach the edge of the steam port earlier in
the stroke. We will take first, as the simplest, those methods by
which the lap of the cut off valve is increased.
It will be noted that there is but one edge of this valve that is
required to do any work, and that is to close the valve.
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