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of the eccentric—the diameter of the circular path in which it travels—must be of the same length as the full travel of the valve, as there is nothing intervening to shorten or lengthen the motion; and this equality of motion is carried on, through Fig. 4 where both rocker-arms are of the same length to the nearly finished construction in Fig. 6—providing the engine shown in the latter plate is in full gear either way; and the correct full travel of the valve should be of the same distance as the throw of the eccentric, but note the error:
If this engine should be moved one-quarter turn ahead, placing the main-pin on the upper quarter, the lower end of the combination lever would then be at about the middle of its path of motion, but the eccentric being at its farthest stroke ahead, the upper end of the combination lever would also have its farthest inclination ahead and the resultant angle would lengthen the line of motion between the eccentric and valve; this increases the travel of the valve at its forward stroke, and is the result of the method of obtaining that advance of the valve necessary in overcoming the lap, even if the lead, as pre-admission of steam, is not desired.
Move the engine so that the main-pin is on the lower quarter and the eccentric at the end of its backward stroke and the opposite effect will be produced;
the combination lever, centred at the lower end, will incline backward at its top end and the angle assumed in this case will shorten the distance from eccentric to valve, and this, on the backward stroke, increases the travel of the valve in that direction.
So, if the full travel of the valve and the full throw of the eccentric are to be maintained as equal distances—and they are—then the valve in Fig. 6 overtravels in each direction; we have not allowed for that and the question is, How can the travel of the valve be correctly shortened up and not create any other interference with the motion? Shortening the throw of the eccentric by changing it to a point nearer the hub centre would take away the increase of travel that was developed in securing the advance of the valve,—would it not?
Certainly; that would be a good way out of the difficulty—if we hadn't a better one: double result in overcoming the error—“kill two birds with one stone.”
With this gear the link generally sets rather highhigher than appears in the sketch—and when the eccentric is on the lower quarter there is a great and undesirable angle between the eccentric rod and the line of the valve-stem; horizontal lines through the centres of the main axle and the pin connecting the eccentric rod to the link are too widely apart, and
we can secure a
the result is an angle great enough to disturb the correct transmission of motion. So, let's leave the throw of the eccentric and the required full travel of the valve remain equal, as they should be, and now extend an arm down from the lower end of the link and connect the eccentric rod to its lowest point instead of to the link itself. We have completed the Engine Practical now, as it appears in Fig. 7, in which it is seen that with the reverse lever in full forward gear, the radius rod as far down in the link as it will go is not in line with the eccentric rod; this change has shortened the swing of the link and the carry of the link-block, and on some engines this foot of the link is so long that the reduction in link-block travel amounts to a great deal, but it only affects the long motion of the valve and does not interfere with the amount of lead supplied by the combination lever.
We have completed our task of erecting a design of Walschaert's valve gear that is applicable to an ordinary locomotive, and it is presented in Fig. 7 for technical study; it is of the same general type as the one we built up, in Fig. 6, corrected, with the position of the piston changed to the forward end of the cylinder. A vertical line is drawn through the exact centre of the valve seat and another similar line through the centre of the valve. The valve is shown displaced the distance required to open the admission port the
amount required for lead by the angle of the combination lever, and this advance of the valve is equal to the distance between the vertical lines through valve and seat; if now the combination lever were disconnected at its lower end from the cross-headcut off from the influence of the piston-and drawn to an exactly vertical position, both pin holes at its upper end would be vertically even, and the centre lines through valve and seat would coincide, the valve being central on its seat and the lead eliminated. With an engine standing as in Fig. 7, the proportions of the combination lever must be such that the distance between vertical lines through the two pinholes at its upper end must be the same as the distance between the vertical lines through the valve and seat as shown, with the valve moved back far enough to open up the admission port the resolved amount for lead; and this distance between centres of valve and seat must cover the measurements of both lap and lead.
Ideally, the position of the Walschaert eccentric is fixed at 90 degrees of the wheel circle from the main-pin, and if the foot, or lower extension that we have supplied to the link to which the eccentric rod is connected, was long enough that when the main-pin was on a dead point and the link was standing vertically the pin in the link-foot connection of the eccentric rod would be on a horizontal line through the centre of the
main axle, then the relative positions of eccentric and main-pin would be as stated; but the link foot seldom extends so far down, and to correct the results of the angle so produced in its effect on the motion of the link and valve, the eccentric is set closer to the mainpin than 90 degrees; this point will be more clearly explained and illustrated later on.
The oscillating link of the Walschaert valve gear differs from the shifting link of the Stephenson type constructively as well as in the direction from which its curve radiates. Fig. 7a illustrates a style of the Walschaert link that approximates the standard type that has been adopted by locomotive builders in this country, and is used, with only such variations as variance in the general engine design may render necessary, by the American Locomotive Company on all of their engines equipped with the Walschaert gear at this time, exactly as shown in the plate.
While the Stephenson link is an open, one-piece affair, and is simple in construction, suspended by a pin in the link-saddle on one side only, it is usually considered necessary for the Walschaert link to be supported by a trunnion on each side as a furtherance of stability. In Fig. 7a, the piece that forms the link is shown in side elevation A as ia, and in end view B as 1b, and is forged from wrought iron that is afterward case-hardened. Referring to view B, the