Fig. 6.
Fig. 7.
Fig. 8.
The slide-valve is shown at [Fig. 5], where A is a view of the face. The centre is hollowed out, as shown at B of the section, to allow the steam to pass into the exhaust. The back is shown at C; the saw-cut receives the valve-rod, which is thinned down to fit it. The face of the valve, that is, all the outer part of A, is made perfectly flat, to fit steam-tight on the valve face of the cylinder. Contact is ensured by the pressure of the live steam in the steam-chest; this is always more than that of the exhaust.
The crank-shaft, marked E in [Fig. 1], is shown alone full size at [Fig. 6]. This is a rod of round steel 1⁄4 inch in diameter, the total length is 31⁄8 inches. At the right-hand end it is reduced in size a length of 7⁄8 inch, to receive the fly-wheel and the driving pulley. These are generally screwed on to a thread cut on the shaft, but wedging is a more workmanlike way of securing driving wheels and pulleys. The two journals are to rest in the bearings shown at [Fig. 7]; the neck at the left-hand end is to receive the crank-arm. The collars on the shaft outside of each journal are of the widths shown. One of the bearings for the crank-shaft is shown at [Fig. 7]. A is a side view, B an edge view, and C a view from the top; in this the dotted lines represent the screw-heads. These bearings are usually brass castings; they are fixed on the bed-plate by two screws each, and the cap is also held on by two other screws. Various designs may be obtained, but the one illustrated is as good as any. The thickness of the bearing is nearly 1⁄4 inch. The height must be precisely that which will bring the centre of the crank-shaft level with the centre of the piston-rod.
[Fig. 8] is the crank-arm, giving an end and side view. It should be made of steel and fixed on the shaft by keying, though more often it is screwed on. The thickness is shown about 3⁄16; the shape may be according to fancy. The hole at the bottom is for the crank-pin, which is riveted in. The ‘throw’ of the crank is an important point, and it must never be so much that the piston touches the ends of the cylinder. In the present case the ‘throw,’ that is, the distance from the centre of the crank-shaft to the centre of the crank-pin, is 5⁄8 inch. This gives 11⁄4-inch stroke; there is plenty of space in the cylinder for another 3⁄16 inch, and possibly the nominal stroke, 11⁄2 inches, could be managed by using a thin piston-head. The crank-pin is shown at the top of [Fig. 10].
Fig. 9.