EFFECT OF LAP ON THE ECCENTRIC’S POSITION.
Fig. 10.
With the short valve without lap used on the earliest forms of locomotives, the eccentric was set at right angles to the crank or “square” on the dotted line e, [Fig. 10]. The least movement of the eccentric from its middle position had the effect of opening the steam-ports. One advantage about an eccentric set in this position, was that it opened and closed the ports when moving the valve at its greatest velocity. Lengthening the valve-face by providing lap entails a change in the location of the eccentric; for, were it left in the right-angle position, the steam-port would remain covered till the eccentric had moved the valve a distance equal to the extent of the lap on one end, and the piston would begin its stroke without steam.