Зображення сторінки






An understanding of the method of valve setting, or “squaring the valves," was a very real necessity to roundhouse men, and enginemen as well, in the days when the Stephenson link-motion was exclusively used on locomotives, and this has influenced a great many to think that about the first knowledge an engineman or shopman should obtain of any newly adopted style of valve gear is “how to set the valves.” It is not remarkable that this idea should so widely prevail, for it was usual that any engine that came out of the back shop with valves squarely set, would not continue in service very long until its irregular exhausts, in either or both points of full gear, indicated that one or both of the valves were "out.”

On the preceding pages of this book, the many causes that may separately or collectively throw the valves of the link motion out of true have been mentioned; but this fact has been repeatedly emphasized, that the valves of the Walschaert gear do not get out of square in service, nor if they did would it be possible for them to be squared

outside of the back shop. And if a locomotive with Walschaert gear should enter the back shop with her valves somewhat “out,” a general trueing-up of the whole engine—the frames, rebushing pin holes in the valve gear, refitting driving boxes, etc., will in nearly every case be all of the “valve setting” that is required. If, in a wreck, part of the valve gear should be destroyed, the repairs should be a job for the back shop, where the manufacture of the new pieces of the gear and the recrection of the locomotive should be from the original blueprints—and the valves will invariably set themselves.

Directions for valve setting with the Walschaert gear have appeared in the railroad journals from time to time, but generally these assume inaccuracies in the location of the eccentric crank in its relation to the main crank-pin, incorrect lengths of the radius rods, etc.—conditions almost impossible to obtain in service; but here again we have—or should have our blueprints. The directions for calculating the exactly true location of the Walschaert eccentric, and for trueing incorrect lengths of the radius rod, are too complicated and confusing for the average roundhouse man and engineman; and in the great number of cases those parts only seem to be incorrect in lengths or location, on account of displacements outside of the valve motion.

However, a few directions will be given here for testing and correcting in the easiest manner such possible inaccuracies, for equalizing the valve travel, and for adjusting the “short travel” of the valve after altering the steam lap, or when wishing to change the lead of the valve.

First, see to it that the wedges are set up snugly, and that there is no undue lost motion in the driving boxcs.

To Test the Length of the Radius Rod: It is best to do this with the main rod disconnected. . Place the reverse lever in the centre notch of the quadrant, and move the crosshead of the piston until the combination lever is standing perpendicular-its upper two connection pins on the one vertical line (by sight; you need not plumb it at this time). Now rotate the main pair of wheels until the link is in the strictly vertical position in which the valve will stand at exactly the same spot with the reverse lever in either full gear position-fix the position of the link by shifting the reverse lever from one corner notch to the other, and noting finally that the valve-stem crosshead remains in, or returns to, the same position in each corner notch; having fixed the position of the link, have the reverse lever moved from either corner notch to the centre of the quadrant, and if this movement of the link block to the centre of the link should shift the position of the valve-stem crosshead, the radius rod is too short or too

« НазадПродовжити »