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instance, measured out from the hub centre to the periphery; one end of the radii is a fixed point,—the forward end of the radius rod is "fixed” at a point on the combination lever—with its other end rotating about it,—the back end of our radius rod will have a limited distance for rotation also.
The common double-eccentric gear—the Stephenson motion-has a “floating” link, with no fixed, unyielding point of resistance, and the pin by which it is suspended is not exactly in the centre of the link saddle, and from the latter fact and that at nearly every point in the cycle of motion, at any degree of cut-off, the action of the link is influenced by both eccentrics, the link describes those peculiarly curved lines of motion that are so mystifying to the student of valve gears.
With our single eccentric we are going to do away with a. whole lot of those lines, and we will take that same old style of link and put the saddle-pin in the centre of the saddle and the link, and attach the pin to a fixed bracket, so that with no other connections the link.could be rotated around on its pivoted centre.
With the Stephenson motion the curve of the link is made on a radius from the main axle upon which the eccentrics are mounted, so that as the link may be raised or lowered any point within it will be at a constant relative distance from the eccentric centre; the Walschaert eccentric, however, does not control the radius of the link, and because the link does not change its position in relation to the axle, the curve need not be as it was, and we have reasons for curving it in the opposite direction-reasons soon to be shown.
First, we will hitch-up the link; we have suspended it at a point where, by setting it in a vertical position, its lower end can be connected to the eccentric rod, and having made this connection we have given a definite motion to the link; it is somewhat as though we had left the "back-up” eccentric, only, of the Stephenson gear still in connection with the link.
The link-block too has been retained, and to it we will attach the back end of the radius rod (see Fig. 6); now it will be apparent why we curve the ends of the link forward: with the link in a vertical position, as it stands, it must be possible to slide the link-block up and down from one end of the link to the other, carrying the end of the radius rod with it without producing any motion whatever at the front end connection of the radius rod with the combination lever. To make this possible the forward curve of the link must have a radius equal to the length of the radius rod. We have now completed the design of the Walschaert valve gear so far as it is represented by Fig. 6, except in supplying the mechanism by which the reverse lever may shorten the cut-off and reverse the motion.
In Fig. 6 the link-block and radius rod are at the lower end of the link, and there is, in effect, a straight line of motion from eccentric to valve, by which we have for the time a direct motion engine. As the wheel turns forward the eccentric will move the valve forward, just as if the link was not in the gear at all. If, however, the radius rod should be raised to the upper end of the link, the link itself would perform
Fig. 6.-Building up the Walschaert Valve Gear.
the functions of a rocker-arm and the forward motion of the eccentric would draw the valve back; that would admit steam to the front end of the cylinder, and the wheel would have to turn the other way-backwardand then the eccentric would also turn backward, the link would change that direction of motion and again push the valve forward, admitting steam to the back end of the cylinder as it ought, showing that to reverse
the direction in which the engine runs it is only necessary to change the position of the back end of the radius rod to the opposite end of the link.
This raising and lowering of the radius rod and link-block is accomplished by connecting the bar that was once the link hanger to the radius rod, and when the reverse lever is thrown in forward gear the linkblock goes down in the link, instead of the link itself sliding down on the block as it does with the Stephenson gear; and when the reverse lever is placed in a notch back of the centre of the quadrant the link-block rises to a point above the centre of the link, carrying the radius rod with it, so that when the eccentric moves in any given direction it will move the valve in the same or the opposite way according to whether the radius rod is below or above the centre of the link.
In Fig. 6 the engine is in full gear ahead, and the valve is given its full travel; in order to shorten the travel and thereby secure earlier cut-offs of admission and release, the reverse lever is hooked up nearer to the centre of the quadrant, and the closer the link-block will be placed to the fulcrum pin at the centre of the link, the shorter will be its travel forth and back, with the consequent shortening of the motion of the valve until, with the reverse lever in the centre notch and the link-block pin centred exactly with the fulcrum pin of the link saddle, the radius rod has ceased its motion, and there will be no movement of the valve except that which is received from the motion of the combination lever.
With the Stephenson link motion the amount of lead given to the valve in full gear is steadily increased as the reverse lever is hooked up, and that is considered by many men to be one of its bad features; hooking-up with the Walschaert gear does not increase the lead, because it is obtained from the cross-head, and that is one of the desirable features that recommend this gear. If the motion work has been correctly designed and “set up,” and particularly if the eccentric rod is of the correct length, when the engine is standing on either dead centre the reverse lever can be shifted from the farthest go-ahead position clear over to the back corner notch of the quadrant without disturbing the position of the valve, and this is because the eccentric at a point directly above or below the centre of the main axle holds the link in a vertical position, and any point in the link at which the link-block may be stationed will be at a common distance from the point represented by the pin connecting the front end of the radius rod with the combination lever—due to the curve of the link.
If we should set the reverse lever in the centre notch of the quadrant and have the engine moved by "pinching,” or towing, as before stated, the eccentric would