DEFINITION OF AN ECCENTRIC.
The reciprocating motion which causes the valves to open and close the steam-ports at the proper periods, is, with most locomotives, imparted from eccentrics fastened upon the driving-axle. An eccentric is a circular plate, or disk, which is secured to the axle in such a position that it will turn round on an axis which is not in the center of the disk. The distance from the center of the disk to the point round which it revolves is called its eccentricity, and is half the throw of the eccentric. Thus, if the throw of an eccentric requires to be 5 inches, the distance between the center of the driving-axle and the center of the eccentric will be 2½ inches. The movement of an eccentric is the same as that of a crank of the same stroke, and the eccentric is preferred merely because it is more convenient for the purposes to which it is applied than a crank would be.