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In locating the longitudinal position of the link fulcrum, consideration must be given to the lengths of the eccentric and radius rods so that both may be approximately of the same length. When these lengths fall below three and one-half times the total vertical sweep of the link-block the radius rod should be favored in preference to the eccentric rod. The exact position of the eccentric crank must be plotted as well as the longitudinal location of point K. The former must bear such relation to the main crank that it brings the link in its middle position when the main crank is on either side of its dead-centres and the connecting point K must be so located that it swings the link in the required angle d on either side of the middle position of the link; that is, in other words, the point K should be so located on the curve it must follow with fulcrum O as a centre that its deviation from the tangent of the eccentric rod to this curve is such that it, as near as practicable, compensates for the irregularities brought about by the angularities of the main and eccentric rods which in ordinary cases brings it from 2 inches to 5 inches in the rear of the tangent to the link drawn through the fulcrum 0.
The locus of the suspension point of the radius rod lifting bar must also be plotted so that the link-block is at the same point of the link in its extreme positions at all cut-offs. This locus is a curve with its centre in the vicinity of the point F when in its mid-gear position. It would be impracticable, however, to have a lift arm of this length, and a curve of smaller radius must be substituted and so applied that it intersects with the former curve at points giving the least possible distortion to the motion, favoring the position of the link-block in which it is mostly used in service.
The sliding lifter shown in Fig. II meets these conditions better than any other method of suspension, but, due to wheel arrangements of various designs of engines, this is not always applicable but must be substituted by swinging lifters, as appear in use with nearly all the designs of valve gear that are illustrated in this book, and which, when properly plotted, give for all practical purposes equally good results.
The vertical height of lower connection m of the combination lever in relation to the crosshead connection has a slight influence on the port opening and should, therefore, in the centre position of the lever, be about in the same level as the crosshead connecting point n (see Fig. 11).
In Fig. 32 is the motion of the valve graphically represented by the Long diagram at different cut-offs, where the horizontal lines represent the port opening edges in the cylinder face, and the curves of the steam inlet edges of the valve, showing the opening and cut-off points where the latter intersect the former and the port openings at the various points of the stroke are measured by the height of these curves over and under the opening edges of the ports at both ends of the valve, respectively.
It will be noticed that these ellipses are slightly flattened on one side, which is caused by the slower lineal motion imparted to the valve relative to the angular
-LAPPLEAD FRONT END
BACK END LEAD Cup
Fig. 32.—-Long's Diagram of Valve Events Effected by
motion when the eccentric passes its back centre compared with that of the front centre, due to the angularity of the eccentric rod, and is more marked the shorter the rod. Fully symmetrical ellipses are not obtainable, as this would require the eccentric and main rods to be of infinite length; this angularity, however, is of but little detriment to the distribution
of the steam as long as the relation between the length of the eccentric rod and the throw of the eccentric is not less than the given limitations, and is present in all kinds of continuous valve motions derived from uniformly rotating cranks or eccentrics.
GENERAL NOTES FOR ADJUSTING WAL
1. Ascertain by the following method the position of the eccentric crank: Mark the position of the link relative to its middle position on both of the deadcentres of the main crank. If the position of the link is the same in both cases, the eccentric crank position is correct; if not, the eccentric crank should be shifted until this occurs, or as near so as possible.
2. After the eccentric crank has been correctly set the eccentric rod should be lengthened or shortened as may be required to bring the link in its middle position so that the link-block can be moved from its extreme forward to its extreme backward position without imparting any motion to the valve. It may be noted that the link position may be observed by the usual tram marks on the valve-stem, or direct by marks on the link pin, as may be found most convenient with the link-block in full gear-preferably ahead.
3. The difference between the two positions of the valve on the forward and back centres of the engine is the lap and lead doubled: it is the same in any position of the link-block and cannot be changed by changing the leverage relations of the combination lever.
4. The tram marks of the opening moments at both ends of the valve should be marked on the valve-stem and the latter lengthened or shortened until equal leads at both ends are obtained.
5. Within certain limits this lengthening or shortening may be made on the radius rod if it should prove more convenient, but it is desirable that its length should be so nearly equal to the radius of the link that no apparent change in the lead should occur in moving the link-block as stated in note No. 2.
6. The lead may be increased by reducing the lap, and the cut-off points will then be slightly advanced. Increasing the lap produces the opposite effect on the cut-off and reduces the lead the same amount. With good judgment these quantities may be varied to offset the irregularities inherent in transforming rotary into lineal motions.
7. The valve events are to a great extent dependent on the location of the suspension point of lifter of the rear end of the radius rod, when swinging lifter is used, which requires that this point should be properly laid