Fig. 2.
The crank-shaft is 1⁄4 in. steel, and should be turned smooth and parallel to fit the hole in the standard. This hole should also be smooth and parallel, which it will be if properly bored with a suitable tool. A long bearing has no more friction than a short one, though a contrary opinion seems to be prevalent. A small hole for supplying the oil necessary for lubrication should be made near the middle of this bearing. The same remarks apply to the bearing H through which the trunnion passes.
The fly-wheel is marked A in [Fig. 1]; it is cast-iron, 21⁄2 inches in diameter and 3⁄4 in. wide on the rim. The rim should be half an inch thick at least, and the boss in the centre as wide as the rim. If bored fairly true, the casting need not be turned on its edge, though it will look better if bright. A small key should be used to fix the fly-wheel on the shaft. This latter, shown broken off in the drawing, projects slightly, and carries a small disc with two pins, which engage in a fork on the end of the propeller shaft and so drive it, and the screw is attached to its end.
Fig. 3.
The valve-face of the standard, B, [Fig. 1]. must be made perfectly flat and at right angles to the boring for the crank-shaft C. [Fig. 3] shows the face of this standard as it would be seen in [Fig. 2] if the cylinder was removed. It is convenient to turn the valve-face in the lathe, and at the same time cut the circular groove, which, after being stopped by plugging at both top and bottom, forms the supply and exhaust ports respectively. The face may be made flat by filing when a lathe is not available, and the groove cut by means of a circular cutter. This is an annular bit with teeth on its edge, which cut a channel but do not touch the inner part.
Fig. 4.
Through the centre of the valve-face a hole is bored to receive the cylinder trunnion. This trunnion, [Fig. 4], is a steel pin screwed into the valve-face of the cylinder (see [Fig. 3], section of cylinder showing valve-face and steam-ways). The outer end is threaded for a nut, D; this has a washer beneath it, and keeps the cylinder close against the standard, with the faces of the valves held together steam-tight, yet so that the cylinder may oscillate freely. A spiral spring beneath the nut is sometimes used, but in good work the adjustment is made with a pair of lock-nuts. The hole through the standard must be perfectly at right angles to the face, and the trunnion in the cylinder must also be perpendicular to the valve-face, or the two faces cannot come together steam-tight.
