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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 never be altered in length; one reason being that such alteration would change the angularity of the radius rod; and, further, the upper end of the combination lever is so extremely short in comparison with the lower end, that to make the modification there the calculations would indicate fractions so small as to be impossible for the mechanic to work with; while a seemingly slight error in the length of the short end would introduce a noticeable irregularity in the valve's motion, yet an error of the same distance in the length of the long end of the lever would probably have no appreciable influence on the valve.
The “long travel" of the valve is the motion imparted by the eccentric; and the “short travel” is that derived from the main crosshead and imparted to the valve by the angularity of the combination lever. Place the reverse lever in the centre notch of the quadrant, link block in the exact centre of the link; now rotate the main drivers a full revolution and the valve will travel between its limits in each direction, a distance equal to twice the lap plus the lead.
With outside-admission valves the valve stem is .connected with the upper end of the combination lever, while with inside-admission valves the radius rod is connected at the top, and the valve-stem connection is the intermediate one.
For Outside-Admission Valves.--Divide the piston