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upon it are attached links of unequal length, connected at their upper extremities with the ends of the eccentric-rods, one of which is above and one below the studs on the lever of the valve-shaft, so that the upper eccentric-rod, being in gear, gives the forward motion, and the lower gives the backward motion. In other engines, forks are situated above and below the stud of the eccentric levers; the forward eccentric-rod is lifted up out of gear by a link depending from the lever on the reversing shaft, and by the same movement the backing eccentric is lifted into gear by a longer link connecting it to a lever, not upon the reversing shaft, but upon a shaft below it. Stephenson and Hawthorn have both used a similar arrangement, but admitting of the eccentric-rods being both under the studs of the lever on the valve-shaft, so that there is no danger, in the event of a disengaged rod falling down, or of any part of the gearing being bent or twisted by both rods being in gear at the same time. The motion of the eccentrics is now frequently transmitted directly to the valves. In Pauwel's arrangement of valve gearing, the valve works on the side of the cylinder, and the valve-rod is prolonged in the form of a deep flat blade of a lozenge section, on each side of which a stud is fixed,-one being intended for the notch of the forward eccentric-rod, and the other for that of the reversing eccentric. Above them is fixed the reversing shaft, from a
lever on which depend two links of unequal length, which are jointed to the ends of the eccentric-rods. By working this lever up or down, the eccentricrods will be alternately engaged and disengaged, and will communicate their respective motions to the valve; or if the lever be kept in its mid position, both eccentrics will be out of gear, and the valve of course will remain stationary. Pauwel's engines are difficult to work, and are subject to shocks from going suddenly into gear: this arises from the whole weight of levers and rods being on the front of the reversing shaft, but the evil might be remedied by attaching a counterbalance to the shaft. Valves situated upon the sides of the cylinders are in many cases more easily connected with the eccentric, but they require springs to keep them up to the face, so that it appears preferable to make the faces of the two cylinders inclined to one another rather than upright, if valves on the sides of the cylinders are preferred. Stephenson's link motion is the most elegant, and one of the most eligible modes of connecting the valve with the eccentric yet introduced. The nature of this arrangement will be made plain by a reference to fig. 61, where e is the valve-rod which is attached by a pin to an open curved link connected at the one end with the driving eccentric-rod d, and at the other with the backing eccentric-rod d'. The link with the eccentric-rods is capable of being moved up or
down by the rod f and bell crank f", situated on the shaft g, while the valve-rod remains in the same
horizontal plane. It is very clear that each end of the link must acquire the motion of the eccentricrod in connection with it, whatever course the central part of the link may pursue, and the valve-rod will partake most of the motion of the eccentric-rod that is nearest to it. When the link is lowered down, the valve-rod will acquire the motion of the upper eccentric-rod, which is that proper for going ahead; when raised up, the valve-rod will acquire the motion of the reversing eccentric, while in the central position the valve-rod will have no motion, or almost none. The link motion therefore obviates the necessity of throwing the eccentric-rod out of gear; it also enables the engine to be worked to a certain extent expansively, though as a contrivance for working expansively, we cannot hold it as de
serving of much commendation. The dead point of the link motion is where the line of the valve-rod bisects the angle formed by the eccentric-rods. The maximum forward motion is when the rods are as figured, and the maximum backward motion when the rods d and d' are in the position h" and h'. The best forms of the link motions have side studs, to which the eccentric-rods are connected, and these. are placed so that at the greatest throw, whether Fig. 62.
backward or forward, the valve-rod and eccentricrod are in the same straight line, and the valve receives the full throw of the eccentric. A counterweight is also attached to the shaft to balance the weight of the link and rods. The second eccentric and eccentric-rod of the link motion might, it appears to us, be beneficially dispensed with by placing the shaft g in the plane of the valve-rod, and attaching a pin to the centre of the link, which would work in the eye of the horizontal arm of the lever f. This lever would in such case require to be made much stronger than at present, as it would have to withstand the thrust of the eccentric, and
the link would then virtually be a double-ended lever with a movable centre. Where more convenient, the pin in the centre of the link might be moved in vertical or curved guides, instead of being attached to the lever f. The act of raising the link, and with it the eccentric-rod, would in effect alter the position of the eccentric on the shaft, and, if the eccentric-rod were properly proportioned in length, would make the lead right on the reversing side.
How to set the valves of locomotives.-When the cylinder is horizontal, the crank is horizontal at the ends of the stroke; but it is not vertical when the piston is at the middle of its stroke, owing to the deviation from parallelism introduced from the connecting-rod being compelled to move at one of its extremities in a straight line. When the piston is at the end of the bottom stroke, and is gradually advanced towards the middle of the stroke, the end of the connecting-rod is carried round by the crank in a curve opposed to that which it would naturally describe round the cross-head as centre; but when the piston has approached the end of the top stroke, the curvature of the path in which the end of the connecting-rod is moved by the crank is in the same direction as that of the circle which it would describe round the cross-head, and these curves would coincide if the connecting-rod were equal in length to the crank it will be easily seen, therefore, that at