The Locomotive Of Today

98 The Engine: Straight Link Motion. This motion has two eccentrics upon the axle similar to the previous types described, and the eccentric rods are coupled clirect to the top and the bottom of the link respectively. There is also a radius rod with a block swinging in a forked end, and working in the slot of the link, and a reversing shaft which has arms on opposite sides of it; these arms have suspension links pivotted on them, which are coupled to the link on one side and to the radius rods on the other, so that a partial revolution of the reversing shaft moves them both simultaneously in opposite directions. The shaft may be placed either below the motion as illustrated, or above, as found most suitable for the other details of the engine to be fitted. The respective lengths of radius and eccentric rods, &c., must be so proportioned that the ares described by the radius rod and link shall be always tangential to each other so that the block shall move in a straight line when the engine is reversed if it is desired to get an accurate distribution of steam ; hence a straight link is necessary, and well-designed examples have a practically equal lead in all gears, but all possess at times slight inequalities in one or both of the full gears, this is, however, very little with long radius rods and relatively short travel. The different lengths of the arms upon the reversing shaft, for supporting the links and radius rods are essential to a proper suspension of these parts, the exact ratio between them depending upon the dimensions of the other portions of the gear, and being coupled up to the heavier links and eccentric rods upon the short arm, they balance the lighter radius rod upon the longer arm, equalising the weight and thus greatly aiding the reversing of the motion from one full gear to the other without requiring the aid of balance weights or springs. The open or box link may be used, and the point of sus- pension be either at the top, bottom, or centre of the link as the other proportions, etc., may render it necessary, and open or crossed rods may be employed. By this gear the inequality of distribution of steam in forward or backward strokes, incidental to both the shifting .and stationary links, is greatly reduced as the slipper block is never far from the centre line of motion. The straight link motion is not employed in America, as, like the stationary link, it requires a long distance for the radius rod, making it diflicult to use in conjunction with a rocker. It is, however, extensively used both in Britain and on the Continent.