The Locomotive Of Today

The Engine : Link Motions. 97 radius rod and does not move the valves, hence ttøe lead is con- stant forali gears with this motion, and open or crossed eccentric rods can be employed without affecting its peculiarities. To minimise the slip of the block in the link which this gear is liable to suffer from if allowed to get slack and worn, it is usual to modify the method of setting so as to make the distribution of steam most correct in the forward gear, especially in express or main line goods engines, % and in practice this can be best done by hanging the link with its centre just below the centre line of motion, and lengthening the back gear eccentric rod, at the same time reducing the angle of advance of the back gear eccentric—this arrangement being shown in the illustration. Slip cannot be entirely obvi- ated, as the radius rod has not the power to hold the quadrant block with the same vertical rigidity as the valve spindle guides employed with the shifting link motion have, and for this reason the open link with the eccentric rod eyes at the ends of the link cannot be employed with this gear; some- times, however, the eyes are formed immecliately beliind the position the quadrant block occupies when it is in its extreme position at the top or bottom of the link. This form of motion at one time found great favour in this country, but it has now been almost entirely superseded by other gears, although it is still used very largely on the Continent. It is seldom, if ever, met with in America, for one reason, it requires such a long distance for the radius rod that it cannot well be used in combination with a rocking shaft, with which all engines there are equipped, for actuating the valves above the cylinders. It is much easier in reversing than the shifting link motion, as the radius rod only requires to be moved, whereas in the other the link, eccentric rods, etc., have to be lifted, and the friction of the eccentric straps over- come; therefore no balance weight or springs are required, unless for special reasons the radius rods are exceptionally heavy. An objection to both the stationary and shifting link motion is, that as the centre of motion of the valve is moved farther and farther from the centre of the link, due to the link or radius rod respectively being raised or lowered, the distri- bution of steam in the forward stroke is different to that in the backward stroke. The Allan or “straight” link motion is shown in Fig. 20 as applied to an inside cylindered engine; in this both the link and the radius rod are moved when reversing, but in opposite directions, and as the action is more direct the slip upon the block is greatly reduced. 7