The Boy's Own Book of Indoor Games and Recreations / A Popular Encyclopædia for Boys

by C. Stansfeld Hicks; John Nevil Maskelyne; Gordon Stables

Fig. 5.

The stuffing-box, 1, of the piston is made with a gland drawn down on the packing by two screws. This arrangement is shown in section at [Fig. 5]. The method of fitting the gland, whether by screwing direct into the boss of the cylinder cover, as shown at the right in [Fig. 5], or by screws tapped through the flange, as in the left-hand illustration, is quite optional. By referring to [Fig. 5], the construction of the two forms of stuffing-boxes will be understood. The gland belonging to each is shown separate immediately above the sections. The same lettering is used in both. A is the cylinder cover, with the projecting boss into which the gland B is fitted. The space for the stuffing or packing is marked C. This is filled with lamp cotton, and when the gland is screwed down the cotton is compressed, so that it makes a steam-tight fitting for the piston-rod to slide in. The hole for the piston-rod is shown through the centres of both sections. As before explained, the gland on the left is secured by two screws shown in the section; it is fitted into a plain cylindrical hole. The other gland is threaded to screw direct into the cylinder cover, which is tapped to receive it. The first method is the one always employed in large engines. The screwed gland has a milled edge, so that it may be turned with the thumb and finger.

The action of the valves in a double-action oscillating cylinder will be best explained by reference to [Fig. 3]. This shows the face of the standard and the section of the cylinder. There is a flat face to the cylinder, usually circular; this has the two steam-ways a, a, bored in it. These holes meet others, drilled from the ends of the cylinder, parallel with its bore, and conduct the steam to the ends of the cylinder through the passages at b, b. On the face of the standard are two holes a and b drilled from the back, one to receive the steam from the boiler, the other to take the exhaust pipe. These holes are not bored through, but communicate with the circular groove c, d. This groove is stopped at e and f. The cylinder is placed against the standard and held close to it, as shown in [Fig. 1], by means of the trunnion illustrated by [Fig. 4].

When the cylinder is vertical, as shown in [Fig. 2], the port-holes are opposite the solid part of the face e and f. Suppose live steam issues from a and fills c, directly the cylinder is moved on one side and one of the port-holes comes over the groove e, the steam enters the cylinder and, pressing against the piston, compels the crank to revolve. By the same motion the other port is uncovered into d, and the dead steam escapes. When the cylinder again reaches a vertical position the steam-ports are again closed, but the momentum of the fly-wheel carries it over the dead centre, and then the positions of the ports are reversed. The one formerly over the exhaust now opens to the live steam and vice versâ. Thus the steam is admitted alternately at both sides of the piston, and so the engine continues to work.

VI.—Model Boilers and their Construction.

A few words on model boilers and their construction will now be advisable. They have been mentioned several times incidentally in the course of these sections, but, with the exception of the small tin boiler for the oscillating engine first described, particulars of their construction have been omitted. It is not an easy task making a steam boiler, and in most cases it will be found cheapest in the end to purchase ready made.

The materials most generally used are brass and copper; sometimes iron, or what amounts to the same thing, tin-plate, is employed. Brass or copper, from the ease with which they can be manipulated, are the best for a beginner to work on.

Brass can be bought in the form of tube of sufficient size for small models, and strong enough to stand the steam pressure. The edges of the bought tube are brazed together, and thus the joint is made nearly as strong as the other part. The tube is afterwards drawn, and, except from a slight discoloration, the joint is not noticeable.

Brass tube, from two inches to six inches in diameter, cut in lengths suited for boilers, is sold by most of the model engineers. The price of the tube ranges from about 2s. per foot for the small to about 10s. per foot for the large size; the short length necessary for a boiler being charged at about the same proportion. This is merely for the tubular body part of the boiler, and it may be placed vertically or horizontally as required.

The ends or flanges which have to be fitted on are extra pieces. Sometimes a plain disc of metal is fixed by soldering with pewter; but this plan should be strenuously avoided. The ends should at least be brazed on. It is best also to use discs with a rim round them to fit over the boiler tube. This gives a much stronger hold than is possible with a plain disc of sheet metal.