CHAPTER II.
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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