The point, D, performing a figure which is equal to an ellipse,
with the error to be eliminated added, so neutralizing its effect on the
motion of the fulcrum, F.
The "lap" and "lead" are opened by the action of the valve lever acting as
a lever, and the port opening is given by the incline of the curved slide
in which the center of that lever slides, and the amount of this opening
depends upon the angle given to that incline. When these two actions are
in unison, the motion of the valve is very rapid, and this occurs when the
steam is being admitted. Then follows a period of opposition of these
motions, during which time the valve pauses momentarily, this
corresponding to the time when the port is fully open. Further periods of
unison follow, at which time the sharp "cut-off" is obtained.
The "compression" resulting with this gear is also reduced to a minimum,
owing to the peculiar movement given to the valves (_i. e._, the series of
accelerations and retardations referred to), as, while the "lead" is
obtained later and quicker, the port is also shut for "compression" later
and quicker, doing away with the necessity for a special expansion valve,
with its complicated and expensive machinery, and allowing the main valve
to be used for expansion, as the "compression" is not of an injurious
amount, even with a "cut-off" reduced to 15 per cent.
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