THE ECCENTRIC A CRANK. SPECIAL MODEL TO GIVE QUANTITATIVE RESULTS.
The necessary travel of the valve is given to it by means of an
eccentric, which is keyed upon or formed as part of the crank-shaft. The
eccentric is, in effect, a crank, whose throw equals the amount of
eccentricity of the sheave. This may not be obvious; let us investigate
a little. Suppose that we have a big crank-shaft, and want to put a
little crank in the middle of it (for this is what happens in the case
of a steam-engine, the travel of the valve being such that a crank of
the usual ___|‾‾|___ form would be disproportionate to the shaft of
which it formed part—like fig. 7, perhaps). Now, that such a shaft would
be extremely weak at the pin goes without saying. Imagine, that to
lessen the weakness the crank-pin of this little crank is made of
greater diameter, as in fig. 8, or even more so, as in fig. 9; still we
get the same result, which is that the _throw_ of the crank remains the
same, and equals the distance of the centre of the crank-pin from the
centre of the crank-shaft—_i.e._, the amount that this crank-pin is
“_ex-centric_,” out of centre, and _the eccentric sheave being simply an
exaggerated crank-pin_ we get back to our original statement that it is
virtually a crank whose throw equals the half-travel of the valve, which
in turn equals the amount of eccentricity of the sheave.
[Illustration: FIG. 7.—Half-travel of Valve.]
[Illustration: FIG. 8.—Half-travel of Valve.]
[Illustration: FIG. 9.—Half-travel of Valve.]
Being fixed to the crank-shaft and moving with it, always in a fixed
relationship to the main crank, the eccentric operates the valve in the
necessary accordance with the movement of the crank and piston, as will
presently be explained.
Returning now to fig. 5, it must be understood that a valve of the type
exemplified therein must be in the _middle_ of its travel whenever the
piston is at _either end_ of its stroke. The reason for this must be
clearly appreciated for the sake of what follows hereafter; it may be
arrived at very easily with the aid of fig. 1. Put a disc of card in the
circle provided in that figure, and let it turn about a drawing-pin
stuck through its centre; this disc is the equivalent of a crank-shaft.
A crank-arm (C) may be permanently marked upon it in ink, as in fig. 10;
another arm, which may be marked upon it in pencil, will serve to
represent a “valve-crank,” the equivalent of the eccentric by which the
valve is to be operated.
[Illustration: FIG. 10.]
Actual connection between that valve-crank and the valve being dispensed
with, some means for making the movement of the valve correspond
accurately with that of its crank must be provided. This provision is
made by the numbered scales[2] in fig. 1; the circular scale within
which the card disc revolves is numbered in correspondence with scales
on the ports, and to these latter an arrow on the strip of paper which
carries the valve is adjusted (see also fig. 10). To find the position
in which to mark the arrow on the valve, put the latter in the centre of
its travel and mark the arrow-head just above the “0” on the No. 1
scale. When the disc is turned, the end of the eccentric-arm or
valve-crank travels with it around the circular scale. When the
eccentric-arm is moved step by step around the circle to the various
numbered graduations, the valve must be moved step by step to bring its
arrow-head to the corresponding numbers on the horizontal scale on the
ports. Thus the eccentric and valve, although not moved simultaneously,
as they would be if a rod connected them, are kept in accurate
relationship whilst moved independently. If the reader will now read the
preface, which he has probably skipped, he will the more readily
appreciate the principle of this arrangement.
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Footnote 2:
The horizontal scales on the port-faces are obtained by projecting up
equidistant circumferential marks from various circles having
different valve-cranks for their radii. The scale (No. 1) is for the
elementary form of valve, without lap; it serves also for link-motion
in mid-gear; No. 2 is for a valve having outside lap; and No. 3 is for
link-motion “linked up.” The student need not, unless he pleases,
trouble himself to investigate the construction of the diagram, which
is drawn to scale, so that if copied accurately it will serve to give
him the demonstrations desired.
Further, taking the main crank-arm (C) as an index-finger, the position
of its end at the points of cut-off, release, etc., with different
valves and eccentrics, may be marked in pencil on the circular scale, or
better, on a piece of tracing-paper interposed between the disc and that
scale; by this means the student will obtain circular diagrams enabling
him to compare the results due to different settings and proportions of
gear, and he will also discover in what manner those results are
affected by individual elements of the mechanism in any given example.
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