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the connecting-rod. In engines which have been in use for some time, however, there is generally a good deal of end play in the bearings of the axles themselves, and this slackness contributes to make the oscillation of the engine more violent.
Connecting-rods.-It is very desirable that the length of the connecting-rod should remain invariable, in spite of the wear of the brasses, for there is a danger of the piston striking against the cover of the cylinder, if it be shortened, as the clearance is left as small as possible, in order to economize steam. In some engines the strap encircling the crank-pin is fixed immovably to the connecting-rod by dovetailed keys, as shown in fig. 58, and a bolt passes through the keys, rod, and strap, to prevent the dovetail keys from working out. The brass is tightened by a gib and cutter, which is kept from working loose by three pinching screws, and a cross-pin or cutter through the point. The effect of this arrangement is to lengthen the rod, but at the cross-head end of the rod the elongation is neutralized, by making the strap loose, so that in tightening the brass the rod is shortened by an amount equal to its elongation at the crank-pin end. The tightening here is also effected by a gib and cutter, which is kept from working loose by two pinching screws pressing on the side of the cutter. Both journals of the connecting-rod are furnished with oil-cups, having a small tube in the
centre, with siphon wicks. The connecting-rod, represented in figs. 58, 57, is a thick flat bar, with its edges rounded. Stephenson's connecting-rod is
made at the crank end; a strap of round iron passes over both brasses, and is attached to the T end of the connecting rod by means of nuts upon the ends of the bent iron, which is made thickest in the middle, to resist the strain. This plan has the defect of shortening the connecting-rod when the
brasses are screwed up, and the brasses require to be very strong and heavy. Hawthorn's connectingrod has a strap at each end, tightened by a gib and cutter; but, to obviate the tendency to shorten the rod, the piston-rod end is furnished with a cutter for tightening the brass outwards. The point of the cutter is screwed, and goes through a lug attached to the gib, and is tightened by a nut. It would be preferable to attach the lug to the cutter and the screw to the gib, as the projection of the screw, when the cutter is far in, would not then be so great. In the engines on the Rouen Railway the piston-rod end of the connecting-rod has neither strap nor brass, but simply embraces the crosshead, while the crank end is hollowed out to admit brasses, which are tightened by a gib and cutter. The length of the connecting-rod varies from four times the length of the crank to seven times. The long connecting-rod has the advantage of diminishing the friction upon the slides.
Eccentrics and eccentric-rod.-The eccentrics are made of cast iron; and when set on the axle between the cranks, they are put on in two pieces. held together by bolts, as shown in figs. 59, 60: but in straight-axle engines they are cast in a piece, and are secured on the shaft by means of a key. The eccentric, when in two pieces, is retained at its proper angle on the shaft by a pinchingscrew, which is provided with a jam-nut to prevent
it from working loose. A piece is left out of the eccentric in casting it, to allow of the screw being inserted, and the void is afterwards filled by inserting a dovetailed piece of metal. Stephenson and Hawthorn leave holes in their eccentrics on each side of the central arm, and they apply pinchingscrews in each of these holes. The screws sometimes slacken and allow the eccentric to shift, unless they are provided with jam-nuts. In the Rouen engines with straight axles, the four eccentrics are cast in one piece.
Eccentric straps are best made of wrought iron, as inconvenience arises from the frequent breakage of brass ones. When made of malleable iron, onehalf of the strap is forged with the rod, the other half being secured to it by bolts, nuts, and jamPieces of brass are in some cases pinned within the malleable iron hoop, but it appears to be preferable to put brasses within the strap to encircle the eccentric, as in the case of any other bearing.
When brass straps are used, the lugs have generally nuts on both sides, so that the length of the eccentric-rod may be adjusted; but it is better for the lugs of the hoops to abut against the necks of the screws, and if any adjustment is necessary from the wear of the straps, washers can be interposed. In some engines the adjustment is effected by screwing the valve-rod, and the cross-head through which it passes has a nut on either side of it by which its position upon the valve-rod is determined. The forks of the eccentric-rod are steel. The length of the eccentric-rod is the distance between the centre of the crank axle and the centre of the valve-shaft. Valve motions.-In locomotives the eccentrics are now always fixed upon the axle, and two are used, one for the forward, the other for the backward motion: the loose pulleys have been given up on account of their liability to get out of order from the shocks to which they were subjected by sudden change of direction when worked at a quick speed. The arrangement whereby the motion of the eccentric is transmitted to the valve, is either direct or indirect. In cases of indirect attachment the motion is given through the intervention of levers, and there is some variety in the arrangements by which the reversing is accomplished. Alcard and Buddicome use a pair of eccentrics at the end of the axle, which is straight; the reversing shaft is placed below the level of the piston-rod, and to a lever keyed