The expansion valve is V-shaped, and circular to fit its
circular-seat. The valve rod of the expansion valve has a sector upon
it and operated by a gear upon the governor stem, which rotates the
valve rod, and the edge of the valve rod is brought farther over the
steam port, thus practically adding lap to the valve. Little movement
is found necessary to make the ordinary change in cut-off, and it is
found to be much easier to move the riding valve across the valve than
in a direction directly opposite. It would require considerable force
to move the upper valve by the governor faster than the lower, or in a
direction opposite to that in which it is moving, but very little
force applied sideways at the same time it is moving forward will give
it a sideways motion. In this device the governor has only to exert
this side pressure and therefore has less to do than if it were called
upon to move the upper valve directly against the movement of the
lower.
Something similar is the valve of the Woodbury engine, of Rochester,
N.Y. The cut-off valve is cylindrical, covering diagonal ports
directly opposite, and is caused to be rotated by the action of the
governor that operates a rack in mesh with a segment.
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