The result would be that the riding valve would reach the
edge of the steam port earlier in the stroke, bringing about an
earlier cut off. If the cut off is desired to be later, the hand wheel
is so turned that the right and left hand screws will bring the valve
sections nearer together, thus practically taking off lap. Now this
may be done by hand or it may be done by the action of a governor.
[Illustration: FIG. 2.]
In the latter case the governor at each change of load turns the right
and left hand screws to add or take away lap, as the load demands an
earlier or later cut off; in other cases the governor moves a rack in
mesh with a gear by which the valve sections are brought closer
together or are separated. The difficulty with the case where the hand
wheel is turned by hand is that the cut off is fixed where you leave
it, and governing can only be at the throttle. For this reason
anywhere near full boiler pressure would not be obtained in the
cylinder of the engine. If the load was a constant one, and the cut
off could be fixed at about one-third, causing the throttle to open
its widest, very good results would be obtained, but there is no
margin left for governing.
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