EARLY APPLICATION OF THE ECCENTRIC.
On the early forms of locomotives, a single eccentric was used to operate the valve for forward and back motion. The eccentric was made with a half circular slot, on which it could be turned to the position needed for forward or back motion. It was held in the required position by a stop-stud fastened on the axle. Several forms of movable eccentrics were invented, and received considerable application during the first decade of railroad operating; but the best of them provided an extremely defective reversing motion. The first engineer to apply two fixed eccentrics as a reversible gear was William T. James of New York, who made a steam carriage in 1829, and worked the engine with four eccentrics,—two for each side. The eccentrics were connected with a link, but the merits of that form of connection were not then recognized here; for it was not applied to locomotives till it became popular in England, and was re-introduced to this country by Rogers. The advantage of the double fixed eccentrics seemed, however, to be recognized from the time James used them; for the plan was adopted by our first locomotive builders. The first locomotive built by Long, who started in 1833 what was afterwards known as the Norris Locomotive Works, Philadelphia, had four fixed eccentrics.
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.
ATTEMPTS TO ABOLISH THE CRANK.
Certain mechanics and inventors have been terribly harassed over this irregular motion of the piston, and numerous devices have been produced for the purpose of securing a uniform motion to the power transmitted. These inventions have usually taken the shape of rotary engines. Probably the fault these people find with the reciprocating engine is one of its greatest merits, for the piston stopping at the end of each stroke permits an element of time for the steam to get in and out of the cylinder.
VALVE MOVEMENT.
The valve travels in a manner similar to the piston; although its stroke is much shorter, and its slow movement is towards the limit of travel. The small circle in the figure shows the orbit of the eccentric’s center, and the valve-travel is equal to the rectilinear line across the circle. If the valve opened the steam-ports at the outside of its travel, the slow movement at that point would be an objection, since the operation of opening would be slow: but the valve opens the ports towards the middle of its travel, when its velocity is greatest; and, the nearer to the mid travel the act of opening is done, the more promptly it will be performed. This has a good deal to do with making an engine “smart” in getting away from a station.