Locomotive Engine Running and Management

by Angus Sinclair

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.

ANGULAR ADVANCE OF ECCENTRICS.

The change made on the eccentric location is to advance it from e to F, being a horizontal distance equal to the extent of lap and lead, and known as the angular advance of the eccentric. The centers F and B represent the full part, or “belly,” of the forward and back eccentrics in the position they should occupy, where a rocker is employed, when the piston is at the beginning of the backward stroke. It will be perceived that the eccentrics both incline towards the crank-pin, and the eccentric which is controlling the valve follows the crank-pin. Thus, when the engine is running forward, F follows the crank: when she is backing, B follows.

It is a good plan for an engineer to make himself familiar with the proper position of the eccentrics in relation to the crank, for the knowledge is likely to save time and trouble when any thing goes wrong with the valve-motion. With this knowledge properly digested, a minute’s inspection is always sufficient to decide whether or not any thing is wrong with the eccentrics.

ANGULARITY OF CONNECTING ROD.

In following out the relative motion of the piston and crank, we discover a disturbing factor in what is called the angularity of the connecting rod, which has a curiously distorting effect on the harmony of the motion. When the piston stands exactly in the mid-travel point, the true length of the main rod will be measured from the center of the wrist-pin to the center of the driving-axle. If a tram of this length be extended between these points, this will be found correct, as every machinist accustomed to working on rods knows. Now, if the back end of the tram should be raised or lowered towards the points where the center of the crank-pin must be when the crank stands on the top or bottom quarter, it will be found that the tram point will not reach the crank-pin center, but will fall short a distance in proportion to the length of the main rod. The dotted lines a′ and b′ in [Fig. 11] show how far a rod 7½ times the length of the crank falls short. A shorter rod will magnify this obliquity, while a longer rod will reduce it.