Page:Model Steam Locomotives by Greenly Henry.pdf/199

Model Steam Locomotives outside-cylindGr locomotives with a leading coupled wheel must be provided with a specially thin retaining nut, one form being illustrated in Fig. 89, Chapter VI. For tiny models a countersunk-headed crank pin is sufficient, or the device shown in Fig. 227. For a large locomotive the author uses the design shown by Fig. 228. The arrangement for securing the bush in an ordinary pattern of coupling rod end, shown in Fig. 229, is also a good one. Coupling rods should be measured off from tlie centres of the wheels after the latter iuive been fitted to the frames. This will provide for any inaccuracies which may occur in making the frames and fitting up the axle boxes.

Where the rods are jointed, as in the case of a six- or eight-coupled engine running on springs, all the joints should be made and fitted up before the holes for the crank pins are marked out and drilled. When this is done one pair of wheels (the drivers for preference) should be secured .to their axle at the orthodox 90^ angle. The coupling rods should then be fitted on the crank pins and the other wheels secured in their positions without removing the rods. The latter act as a jig, and if the crank pins are all at the correct throw and are fitted quite squarely into their wheels the rods should rotate quite freely without binding in the “dead-centre” positions.

Eccentrics and Straps.—Where the eccentrics are not turned up solid with the shaft and, as arranged in Fig. 235, have to take up a position between the adjacent crank webs of the axle they may have to be cast in pairs and split, as shown in Fig. 230. The screws are fitted after splitting and the sheaves are then turned on a suitably centred mandrel. Cast iron or mild steel should always be used for the sheaves. Straps are best made of bar brass or cast gunmetal. The lugs should be long, so that the bolts 190