RELATIVE MOTION OF PISTON AND CRANK, SLIDE-VALVE, AND ECCENTRICS.
Fig. 9.
When a locomotive is running, the wheels turn with something near a uniform speed; but any part which receives a reciprocating motion from a crank or eccentric travels at an irregular velocity. [Fig. 9] shows the relative motion of the crank-pin and piston during a half revolution. The points in the path of the crank-pin marked A, 1, 2, B, 3, 4, C, are at equal distances apart. The vertical lines run from them to the points a, b, c, d, e, represent the position of the piston in relation to the position of the crank-pin. That is, while the crank-pin traverses the half-circle, A B C, to make a half revolution, the piston, guided by the cross-head, travels a distance within the cylinder equal to the straight line A C. The crank-pin travels at nearly uniform speed during the whole of its revolution, but the piston travels with an irregular motion. Thus, while the crank-pin travels from A to 1, the piston travels a distance equal to the space between A and a. By the space between the lines, it will be seen that the piston travels slowly at the beginning of the stroke, gets faster as it moves along, reaches its highest velocity about half stroke, then slows down towards the end till it stops, and is ready for the return stroke.