From the upper end of the lever, E, the motion is carried
direct to the valve by the rod, G. It will be evident thus that by one
revolution of the crank the lower end of the lever, E, will have imparted
to it two different movements, one along the longer axis of the ellipse,
traveled by the point, A, and one through its minor axis up and down,
these movements differing as to time, and corresponding with the part of
the movement of the valve required for lap and lead, and that part
constituting the port opening for admission of steam.
[Illustration: JOY'S REVERSING AND EXPANDING VALVE GEAR.]
The former of these is constant and unalterable, the latter is
controllable by the angle at which the curved slide, J, may be set with
the vertical.
It will further be evident that if the lever, E, were pinned direct to
the connecting rod at the point, A, which passes through a practically
true ellipse, it would vibrate its fulcrum, F, unequally on either side of
the center of the curved slide, J, by the amount of the versed sine of the
arc of the lever, E, from F D; it is to correct this error that the lever,
E, is pinned at the point, D, to a parallel motion formed by the parts, B
and C.
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