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Cardboard models of each of those valves will be found in the pocket on the inside of the front cover, and these models may be placed on the horizontal lines representing the valve seats of the folder plates and a good idea can be obtained of the work done by the Walschaert gear in either of the two methods of applying it, namely, to actuate inside and outside admission valves.
Place one of the cardboard valves on its seat and imagine the main pin to be at any one of the nine numbered points of the first half of the wheel's revolution, turning forward, and then move the valve model until its index points even with the mark at the corresponding number of the scale that shows the different positions of the combination lever; this will be the correct location of the valve at that time, and the relative positions of a point on the piston rod-say the wrist pin-both pins in the union bar that connects the lower end of the combination lever with the crosshead, both ends of the radius rod, the reversing link, its point of connection with the eccentric rod, and the position of the eccentric are all shown by the same numbers.
These diagrams arc—with the exception of the reduction in size-reproduced exactly from models used by the Baldwin Locomotive Works, through the courtesy of that company, and are quite interesting, not only in telling the story of the operation of the Walschaert valve gear, but also in settling to a certain extent any questions as to differences that may be thought to exist in the results obtained in the use of valves of internal admission vs. external admission.
Theoretically, the Walschaert eccentric is placed exactly 90 degrees of the wheel's circle from the main crank-pin, and it has been referred to in this book, in each instance, as so placed; but the diagrams, Figs. 35 and 36, show the eccentric as further and nearer to the main-pin than go degrees. This advance or recession of the Walschaert eccentric from a right angle to the main-pin is not for the purpose of securing lead, as it is with the Stephenson motion; if the connection of eccentric rod with the foot of the link was on a horizontal line through the centre of the main axle, then the eccentric would be set at just 90 degrees from the main-pin; and it is endeavored to get the eccentric rod to lie in as nearly a horizontal position as possible when the main-pin is on the upper or lower quarter, but this generally results in so great a length of the lower extension or foot of the link—the distance from link trunnion, or fulcrum, to the pin connecting the link foot and eccentric rod-as to cause an overshortening of the throw of the link. So, there must be a compromise; the foot of the link is not brought down, usually, as low as theory would place it; its connection with the eccentric rod is in most cases above a horizontal line through the driving-wheel hub centre, and to overcome the errors introduced by the resultant angular position of the eccentric rod the location of the eccentric in its relation toward the main-pin is changed accordingly. In Fig. 35, representing an inside admission valve, it is seen that the eccentric and main-pin are more than 90 degrees apart, while with the outside admission valve of Fig. 36 the eccentric is less than a quarter turn from the main-pin. The location of the eccentric as it is governed by the point of connection between the eccentric rod and link is exemplified in Fig. 37.
Squaring the centres of axle and link fulcrums, the angle indicates the point of connection from the eccentric rod to link foot with the link in a vertical position and main-pin on a dead-centre; continuing, the third side of the incomplete square furnishes the radius upon which the eccentric shall be placed, and it now depends upon which dead centre the main-pin stands and whether the valve is of inside or outside admission whether the eccentric shall be on one or the other side of the axle; in Fig. 37 the valve is of ouiside adm ssion and the main-pin is on the back centre, so we place the eccentric on the wheel radius that brings it 45 degrees nearer to the main-pin than 90 degrees from it, because we find that a line from axle centre
to the point of connection of eccentric rod and link foot is 45 degrees above the horizontal line through the hub centre. In other words, the wheel radius on
Centre Line of Main Rod
Fig. 37.—Hypothetical Design of Walschaert Gear.
which the eccentric is placed must always be 90 degrees from a line through the hub centre and link-foot pin when the link is in an exactly vertical position—a position in which the reverse lever can be moved
between the farthest go-ahead and back-up notches on the quadrant without displacing the valve, and with the main-pin on a dead-centre.
With outside admission valves the eccentric and main-pin must be 90 degrees apart minus the number of degrees represented by the elevation of the linkfoot pin above its normal position on the horizontal line through the hub centre. With inside admission valves, raising the link foot above the theoretically normal position places the location of the eccentric 90 degrees plus the number of degrees that the link-foot pin is raised, from the main-pin; thus, in Fig. 37 the position is shown in dotted lines in which the eccentric would be placed if the valve was of inside admission and the eccentric should follow the main-pin.
Fig. 37 represents an extreme case; but it would be possible to raise the link until the line through the hub centre and link-foot pin was perpendicular to the horizontal line through hub centre; in that case, and with outside admission valves, the eccentric would be o degrees from the main-pin: That is, the main-pin and eccentric would be on the same wheel radius. With inside admission valves the eccentric would be 180 degrees from the main-pin, or, just opposite it.
This supposititious case of extreme link elevation most plainly illustrates why irregularities in the valve motion are produced when the link-foot pin is raised