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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 reerection 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 boxes.
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
long. If the valve-stem crosshead is pulled toward the link the radius rod is too long; if it is pushed toward the steam chest, the radius rod is too short. With the link fixed vertically as directed, and the reverse lever in mid-gear with the link-block pin in even register with the link fulcrum pin, use an adjustable tram and find a point along the horizontal line of the pin connecting the radius rod with the combination lever, that will have the same distance from the centre of the linkblock pin as from marks scratched on each end of the link indicating the middle of the link slot; this distance will equal the correct length of the radius rod between centres of pin holes connecting it with the link block and combination lever.
Any rule for correcting the length of the radius rod, proportioned on the distance the valve stem is shifted in moving the link block from full gear to the mid-gear position, would be too complicated for consideration here.
The Eccentric Rod: The effective length of the eccentric rod may become untrue after the engine has been in service for some time, and is the only piece of the gear of which an inaccuracy should ordinarily be suspected. With the main rod connected up, place the main crank-pin on the exact dead centre; this position is found as follows (see Fig. 38).
To locate Crank Pin on the Dead Centre: Turn the main driving wheel until the crank pin is nearing the centre that is desired, and when the crosshead is within about one inch of its extreme point of travel make an intersecting mark on crosshead and guide bar; with a tram resting upon some fixed point of the enginepreferably the guide yoke-locate the point a (see Fig. 38) on the side of the driving-wheel tire; then continue the rotation of the wheel until the crosshead has
Fig. 38.—Tramming Driving Wheel to Locate the Dead Centre.
passed its extreme point of travel, and starting back has again reached the point where the marks on crosshead and guide bar are in exact register, stop, and with the tram locate a second point, b, on the tire at an equal distance from the centre of the axle as point a; with a pair of dividers locate the point c on the arc midway between a and b; now rotate the wheel to the exact dead centre, which has been secured when the two points of the tram rest on the marks on guide yoke and point c on the wheel. Either dead centre may be established in the same way.
With the main crank-pin on (say the forward) exact dead centre on one side, have the reverse lever moved from the go-ahead corner notch up toward the centre of the quadrant; while the link block is rising, if the valve stem is pushed forward slightly the eccentric rod should be lengthened; but if the valve stem should be drawn backward by the rise of the link block, the eccentric rod needs to be shortened. In either case make but slight changes in the length of the rod, and keep on testing until hooking up the lever to the centre has no effect on the valve stem; the effective length of the eccentric rod is then correct. Then test, and alter if necessary, the other side in the same way, but with the main pin on the back or opposite-dead centre.
To determine the proper proportions of the Combination Lever (“lap and lead lever"): Whenever it is desired to increase or diminish the lead without changing the steam lap of the valve, it can only be done by altering the proportions of the combination lever; and after the lap may have been reduced or increased, the proportions of the combination lever must be changed to suit, if the original amount of lead is to be retained.
Any change in the proportions of the combination lever should be in shortening or lengthening the long (lower) portion. The short (upper) portion should