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of power in such a manner that its "interference” with the valve will be in the proper
direction. We must now disconnect the main rod from the main-pin and move the cross-head forward until the piston is known to be in the exact centre of the cylinder, and then connect the lower end of the combination lever (still in a plumb perpendicular position) with the cross-head by means of a short link-bar; now draw the cross-head to the back end of the guides again, and re-connect the main rod to the main-pin; we have now developed the engine far enough to be represented by the sketch in Fig. 5; the angle assumed by the combination lever has pushed the valve forward a distance that slightly uncovers the back admission port; and thus the Walschaert valve gear derives its lead-permanent lead, too, unaffected by any further changes in the gear that may be necessary in shortening the cut-off by the introduction of reversing mechanism.
If the outside lap of the valve amounts to one (1) inch, and the required port opening for lead is onethirty-second (32) inch, the distance between the vertical lines through the centres of the pin holes in the upper part of the combination lever must—as the engine stands in Fig. 5-be just one and one-thirtysecond (132) inches, equalling lap plus lead.
As the engine starts forward under her own steam
we will note the results at each quarter-turn of the wheel. It must be remembered that the valve is acted upon by two distinct forces: the main propulsion, for its long travel, but this motion is qualified by the influence of the cross-head, exerted through the combination lever, and which would give a short travel to the valve even if the eccentric should be disconnected. When the crank—the main-pin-reaches the upper quarter where its leverage force is greatest the lower end of the combination lever is at the middle of its travel, and it would again stand plumb perpendicular but for the eccentric now being at its farthest point forward, and this has carried the valve to the finish of its forward stroke, leaving the back admission port and front port for exhaust wide open; the crank proceeds in its turning motion and when it has reached the forward dead-centre and the piston is at the front end of the cylinder, the eccentric will be at the lower quarter, and the valve would be once more centred on its seat—but for the alterative effect of that combination lever: its angle will be the reverse to the position of the lever as shown in Fig. 5, with the result that the distance from eccentric to valve will be shortened by that angle, and the valve drawn back exactly one and one-thirty-second (132) inches (we are assuming that the valve has one (1) inch outside, or steam, lap, and one-thirty-second (32)
inch is expected to be the amount of port opening for lead), thus opening the forward port for the pre-admission of steam at the beginning of the back stroke just the same as it was given at the other end of the cylinder.
That is, the lead would be the same at the end of each piston-stroke, if the line of motion from eccentric to valve was a straight, horizontal level, which, in our ideal engine, it is not; in the practical design, however, this error is corrected, and our imagination must supply the deficiency until our valve gear is technically completed.
Just as the main-pin passes the bottom quarter and the eccentric is at its farthest point on the back stroke, the upper end of the combination lever still inclines backward and has drawn the valve to the finish of its backward stroke, with the front port opened for full steam admission and the back port open from the cylinder to the exhaust; a quarter-turn more brings the whole gear back to the original positions of the several parts as presented in Fig. 5.
We have described the action of the gear at four points in the revolution of the driving wheel—a complete cycle of motion--but those four points were taken at times when the steam ports were either wide open or only opened the distance required for lead; intermediately between those points occur the
interesting phases of cut-off and release, and further along in this book diagrams will be found, illustrating in a way most easy of comprehension the positions of the different parts of the Walschaert valve gear at nine particular points: a technical but plain analysis of the gear, in connection with both inside and outside admission valves. (See folder plates, Figs. 35 and 36.) It would be better, however, to defer the study of those diagrams until we have introduced the reversing mechanism, and built up the complete engine.
We have already completed a very nice working engine of the stationary type, but we must supply a method of shortening the travel of the valve so as to restrict the steam port openings when the engine is running fast or under a light load,—this for the sake of economy; while the means provided for so shortening the valve travel will, further, permit the complete reversion of the direction in which the engine
For this we will cut the eccentric rod at about the point marked X in Fig. 5, and that part of the rod that is still connected with the return crank we will continue to call the eccentric rod, but the front section of the rod that connects with the combination lever will be referred to as the “radius rod.” The word radius is applied to the distance from the centre to the circumference of a circle: a spoke of a wheel, for
instance, measured out from the hub centre to the periphery; one end of the radii is a fixed point,—the forward end of the radius rod is “fixed” at a point on the combination lever—with its other end rotating about it,—the back end of our radius rod will have a limited distance for rotation also.
The common double-eccentric gear—the Stephenson motion—has a "floating” link, with no fixed, unyielding point of resistance, and the pin by which it is suspended is not exactly in the centre of the link saddle, and from the latter fact and that at nearly every point in the cycle of motion, at any degree of cut-off, the action of the link is influenced by both eccentrics, the link describes those peculiarly curved lines of motion that are so mystifying to the student of valve gears.
With our single eccentric we are going to do away with a whole lot of those lines, and we will take that same old style of link and put the saddle-pin in the centre of the saddle and the link, and attach the pin to a fixed bracket, so that with no other connections the link could be rotated around on its pivoted centre.
With the Stephenson motion the curve of the link is made on a radius from the main axle upon which the eccentrics are mounted, so that as the link may be raised or lowered any point within it will be at a constant relative distance from the eccentric centre; the Walschaert eccentric, however, does not control