Page:Model Steam Locomotives by Greenly Henry.pdf/203

CHAPTER XII

Boiler Construction Simple Boilers.—The type of boiler fitted to toy models in which the flame of the lamp impinges on the lower outside surface of the barrel is usually made of extremely thin brass with a lapped bent-in seam along the bottom. The ends are flanged over the barrel and the whole is secured with a good grade of soft solder. Such boilers are quite safe up to a pressure of about two atmospheres (30 lb.), and, as already stated, owe their steaming power to the very thin metal (No. 24 or 26 s.w.G.), of which they are made and to the fact that a spirit lamp burns best in the open air. Details of this method of construction are shown at a in Fig. 236. There are limits to their usefulness, and, therefore, for all home-made models using direct-acting slide-valve cylinders made up from castings, other systems of boiler construction providing greater heating surface are essential. With the type of model illustrated in Fig. 151, Chapter VIII, which has a small cylinder fixed on or quite near to the boiler and connected to a shaft geared to the driving axle, it is possible to rely on a plain outside-fired boiler, although one or two water tubes might be introduced into simple designs of this character, especially in the larger gauges.

Water-tube Boilers.—The water-tube boiler is one which fulfils the requirements of the average model locomotive builder, and while it can be made up without brazing or silver-soldering this is undoubtedly the simplest 194