gun and a jacket covering the breech portion, an arrangement which simplifies the design but increases the difficulties of manufacture.
Fig. 19.—Wire Fastening.
Wire guns are now made of almost all calibres, ranging from 3 in. to 12 in. Many authorities objected to guns of less calibre than 4·7 in. being wound with wire, as they considered that on diameters so small the interior surface of each layer of wire is over-compressed, while the exterior is too much extended; but by proportioning the thickness of the wire to the diameter of the tube on which it is wound there is no reason for this to be so.
The wire is wound on the barrel at a certain tension, ascertained by calculation, and varying from about 50 tons per square inch for the layers first wound on the gun, to about 35 or 40 for the outer layers. To fasten the wire at the beginning and end several methods are adopted. In the Woolwich system a narrow annular ring (fig. 19), with slots cut into one of its faces, is shrunk on to the gun; into these slots one end of the wire is inserted and secured in position by a steel screwed plug. The wire is wound on for the distance desired and then back again to the ring, where the end is fastened off in the same way. At Elswick the wire is fastened by bending it into a shunt cut groove in a similar annular ring, but the wire is only fastened off in the same way after several layers have been wound.
With each succeeding layer of wire the interior layers are compressed, and these in turn compress the barrel. It is therefore necessary, in order to prevent the fatigue of the material, to make the barrel comparatively thick, or, better still, to have an outer barrel superimposed on the inner one. This latter arrangement is now used in all guns of 4 in. calibre and upwards. It is not so important with smaller guns as the barrel is always relatively thick, and therefore meets the conditions.
With many modern guns the interior of the outer barrel, termed the “A” tube, is taper bored, the larger end being towards the breech; and the exterior of the inner barrel or liner, called the “inner A tube,” is made tapered to correspond. The latter is, after careful fitting, inserted in the outer barrel while both are cold, and forced into position by hydraulic pressure or other mechanical means.
The details of the machines for winding on the wire (see fig. 20) differ somewhat in different works, but all are arranged so that any desired tension can be given to the wire as it is being wound on to the gun. The wire is manufactured in much the same way as ordinary wire. A red-hot bar of steel, gradually rolled down between rollers to a section about double that which it is finally intended to have, is annealed and carefully pickled in an acid bath to detach any scale. It is then wound on a drum, ready for the next process, which consists in drawing it through graduated holes made in a hardened steel draw-plate, the wire being often annealed and pickled during this process. The drawplate holes vary in size from slightly smaller than the rolled bar section to the finished size of the wire, and, as a rule, the sharp corners of the wire are only given by the last draw. It is found that considerable wear takes place in the holes of the draw-plate, and a new plate may be required for each hank of 500 or 600 yds. of wire. Great importance is attached to the absence of scale from the wire when it is being drawn, and, after pickling, the rolled bar and wire are treated with lime or some similar substance to facilitate the drawing. The tests for the finished wire are as follows: it has to stand a tensile stress of from 90 to 110 tons per square inch of section, and a test for ductility in which a short length of wire is twisted a considerable number of turns in one direction, then unwound and re-twisted in the opposite direction, without showing signs of fracture. It will be seen that the wire is extremely strong and the moderate stress of from 35 to 50 tons per square inch, which at most it is called upon to withstand in a gun, is far less than what it could endure with perfect safety.
Fig. 20.—Wire-winding Machine.
The wire after being manufactured is made up into hanks for storage purposes; but when required for gun construction it is thoroughly cleaned and wound on a drum R about 3 ft. 6 in. in diameter, which is placed in one portion of the machine in connexion, with a powerful band friction brake M. The wire is then led to the gun A placed between centres or on rollers B.B. parallel to the axis of the wire drum. By rotating the gun the wire is drawn off from the drum against the resistance of the band brake, which is so designed that, by adjusting the weight S suspended from the brake strap, any desired resistance can be given in order to produce the necessary tension in the wire as it is being wound on the gun. The stress on
