The terms axis, axle, arbor, and shaft, in mechanics, are generally understood to mean the bar, or rod, which passes through the center of a wheel. A gudgeon is the pin, or support, on which a horizontal shaft turns; the pins upon which an upright shaft turns are called pivots.
The engine proper consists of a hollow cylinder closed at both ends; inside it is the piston, a sliding partition which fits the bore of the cylinder sufficiently close to prevent the steam leaking past it, but having sufficient freedom to allow it to move from end to end of the cylinder with as little friction as possible.
In modern engines the pressure of the atmosphere is not employed to drive the piston down. The steam is admitted into the cylinder above the piston at the same time that it is condensed or withdrawn from below, and thus exerts its expansive force in the returning as well as in the ascending stroke. This results in a great increase of power.
The practical construction of the piston and cylinder, and the arrangement of connecting pipes by which steam is admitted alternately above and below the piston, is fully shown in [Figure A]. This gives a sectional view of the cylinder, of the piston, and of the distribution of steam. The entire engine is of iron. To the piston, T, is fixed a rod, A, which slides with gentle friction in a tubulure, U, placed at the center of the plate which closes the cylinder. As it is very important that no steam shall escape between the piston-rod and this tubulure, the latter is formed of two pieces, one attached to the plate, while the other, which fits in the first, can be pressed as tightly as is desired, so as to compress the material soaked with fat which is between the two tubulures. This arrangement is called a stuffing-box; it prevents the escape of steam without interfering with the motion of the piston.
FIGURE A
Valve-Chest.—This is the arrangement by which steam passes alternately above and below the piston.
[Figure A] presents a vertical section of this valve-chest and shows its relation to the cylinder. The steam enters the valve-chest from the boiler by the brass tube x. From the valve-chest two conduits, a and b, are connected with the cylinder, one above and the other below. If they were both open at once, the steam, acting equally on the two faces of the piston, would keep it at rest. But one of these is always closed by a slide-valve, y, fixed to a rod, i. This moves alternately up and down, by means of an eccentric, e, placed on the horizontal shaft. The [899] slide-valve closes the conduit a, and allowing the steam to enter at b, below the piston, the latter rises. But when it reaches the top of the stroke the rod i sinks, and with it the slide-valve, which then closes the conduit b, and allows the steam to enter at a. The piston then sinks, and so forth at each displacement of the slide-valve.
It now remains to explain what happens when the steam presses below the piston. It must not remain above, otherwise the piston could not move. But while the steam enters below by the conduit b, the top of the cylinder, by means of the conduit a, is connected with a cavity, O, from which passes the tube L. Through this tube the steam which has already acted upon the piston passes into the atmosphere, or else is condensed in a vessel filled with cold water, which is called the condenser. If, on the other hand, the piston sinks, the vapor below the piston passes, by the conduit b, to the cavity O, and to the tube L.
Transmission of Motion.—The alternating rectilinear motion thus generated within the cylinder is transmitted, by means of a rod attached to the piston, to a strong beam ff, movable upon a central axis, a system of jointed rods ee, called the parallel motion, being interposed for the purpose of neutralizing the disturbing action which the circular path of the beam would otherwise exert upon the piston. The reciprocating motion of the beam is now, through the intervention of the connecting-rod g and crank h, converted into a circular or rotatory motion, which is rendered continuous and uniform by the fly-wheel i, to the axis of which the machinery to be impelled is connected.