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one eccentric only, and it is set, theoretically, at exactly right angles to the main-pin-90 degrees from it-and the lead is derived from the action of the . piston, through suitable connections with the crosshead, the established lead remaining unchanged throughout the different points of cut-off in both forward and backward gear. The engine is of direct valve motion when the reverse lever is in the goahead position, and of indirect motion when the lever in the back-up notch, the reasons for this becoming apparent as the method for reversing the gear is understood.
Now, let's revert our attention to the crude type of engine shown in Fig. 2, again, for we are ready to begin the reconstruction of its valve gear into the Walschaert motion; we will take out the pin at the connection between the valve-stem and the eccentric rod and dropping the end of the latter slightly, connect it to the “combination lever,” —see Fig. 5.
With the combination lever in a plumb vertical position we will also connect the valve-stem to its extreme upper end, and while the combination lever remains at right angles to the valve-stem, the distance from eccentric to valve is not changed and its attachment has had no effect on the valve, which is central on its seat with both admission ports covered; but if we move the lower end of the combination lever
forward it will shorten the distance from eccentric to valve, or move it backward and the distance will be lengthened, and this shortening or lengthening of the line of motion will cause the valve to be displaced,
Fig. 5.-Simple Form of Steam Engine with the Wal
schaert method of Providing Lead.
either forward or backward, and open, somewhat, the front or back steam admission port enough for the lead.
All true levers have three points for the reception and delivery of power: at one point the original force is applied to the lever, while the other two are the resisting and transmitting points. We have the two latter points located as the connections of eccentric rod and valve-stem to the combination lever, while the point of application of power is at the lower end.
As it is the duty of the combination lever to modify the motion that the eccentric delivers to the valve, we must now connect the lower end of this lever to a source 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 (312) 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-pinreaches 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