Page:Model Steam Locomotives by Greenly Henry.pdf/233

Model Steam Locomotives seating, and (4) that the spring spindle is quite separate and not a part of the valve except where the valve is of spherical shape. A direct-loaded twin valve for a large model is shown in Fig. 273. Bronze balls may be used as safety valves. Two methods are illustrated on page 225. In the first (Fig. 274) the ball is screwed and soldered on to a spindle, the bronze wire spring being inside the boiler. This is per- haps the only satisfactory type of internal spring safety valve, a variety which is sometimes necessary in a smallscale model. In the other valve (Fig. 275) the ball is pressed down on to a knife-edged seating by a spring pillar having a rounded undersurface. This makes a very good valve. In both cases the seating is trued up by hammer- ing on it a steel ball of the same diameter as the bronze ball. Grinding in cannot be resorted to. To obtain steam tightness with the ordinary V-seated valve grinding in must be carefully done. Emery should not be used with brass valves; grindstone-mud is much to be preferred. Further, the part of the seating in con- tact with the valve should never be much more than .01 in. wide in model locomotives of small size. Domes.— Domes are used in actual engines to provide for the extraction of the steam from a point high up above water level. While their efficacy in real prac- tice is doubted by many engineers, model-makers will find them very useful in preventing “priming.” As a rule, in large engines the regulator is fitted inside the dome. This is not practicable in a small model, and therefore the dome simply houses the end of the pipe leading to the regulator valve, and further provides a ready means of filling the boiler. Domes should always 224