« PreviousContinue »
Section at AA
the hinged door, with the breech plug resting against its front Up to about 1850 the dimensions of canon had been propor, face, was swung into the breech opening, and the plug was tioned by means of empirical rules, as the real principles underpushed forward to its position in the chamber of the gun; the lying the construction of ordnance had been little
understood. It was known of course that a gun was
Built-up subjected to two fundamental stresses--a circumferential tension tending to split the gun open longitudinally, and a longitudinal tension tending to pull the gun apart lengthwise; the longitudinal strength of a gun is usually greatly in excess of any requirements. It is easy to demonstrate that any so-called homogeneous gun, i.e. a gun made of solid material and not built up, soon reaches a limit of thickness beyond which additional thickness is practically useless in giving strength to resist circumferential stress. This is due to the fact that the stress on the metal near the bore is far higher than that on the outer portion and soon reaches its maximum resistance which additional thickness of metal does not materially increase. The gun can, however, be arranged to withstand a considerably higher working pressure by building it up on the principle of
initial tensions. The inner layers of the metal are thereby Fig. 9.-Wahrendorff Gun, 1846.
compressed so that the gas pressure has first to reverse this back, and, finally, a nut screwed to the rear end of the plug to endure a gas pressure which can be expressed as being prowedge was then pushed across to prevent the plug being blown compression and then to extend the metal. The gun barrel
supported by the contraction of the outer hoops will then be able rod was given a couple of turns so that all was made tight and secure. After firing, the breech was opened by reversing these old type solid gun it was proportional to the extension only. The
portionalto the initial compression plus the extension,wḥereasinthe operations. The Armstrong system of breech-loading introduced in 1854 of a gun was Lord Armstrong (9.v.), who in 1855–1856 produced
first to employ successfully this important principle for all parts was the first to give satisfactory results; its simple design and few parts produced a favourable effect in the minds of artillerists, which was increased by the excellent accuracy obtained in shooting. The gun (fig. 10) had a removable breech block having
ki de la
FIG. 11.--Armstrong B.L. Construction. a breech-loading field gun with a steel barrel strengthened by wrought iron hoops. In this system (fig. 11) wrought iron coils
were shrunk over one another so that the inner tube, or barrel, 123
was placed in a state of compression and the outer portions in a state of tension--the parts so proportioned that each performs
its maximum duty in resisting the pressure from within. Further, Fig. 10.-Armstrong B.L. Arrangement.
by forming the outer parts of wrought iron bar coiled round a
mandril and then welding the coil into a solid hoop, the fibre on its front face a coned copper ring which fitted into a coned of the iron was arranged circumferentially and was thus in seating at the breech end of the powder chamber. The breech the best position to resist this stress. These outer coils were block was secured by means of a powerful breech.screw; a hole shrunk over a hollow breech-piece of forged iron, having the fibre was made through the screw so that, in loading, the shell and running lengthwise to resist the longitudinal stress. The several cartridge could be passed through it after the breech block had cylinders were shrunk over the steel inner tube or barrel. To been removed. After loading, the block was dropped into its obtain the necessary compression the exterior diameter of the place and the breech screw turned rapidly so that it might jam inner portion is turned in a lathe slightly greater than the interior the block against its seating, and so prevent the escape of powder diameter of the outer coil. The outer coil is heated and expands; gas when the gun was fired. This gun was most successful, and it is then slipped over the inner portion and contracts on cooling. a great number of guns of this type were soon introduced into If the strength of the two parts has been properly adjusted the the British army and navy.
outer will remain in a state of tension and the inner in a state They were employed in the China campaign of 1860, and of compression. satisfactory reports were made as to their serviceableness; but Every nation has adopted this fundamental principle which while the breech-loading system had obtained a firm footing on the governs all systems of modern gun construction. The winding, Continent of Europe, there was a strong prejudice against it in at a high tension, of thin wire or ribbon on the barrel or on one of England, and about 1864 M.L.R. guns were adopted. Breech- the outer coils may be considered as having an exactly similar loaders did not again find favour until about 1882, when a demand effect to the shrinking of thin hoops over one another. The was made for more powerful guns than the M.L.R. In conse- American, Dr Woodbridge, claims to have originated the system quence, M.L. guns having enlarged chambers for burning large of strengthening guns by wire in 1850; Brunel, the great railway charges of prismatic powder were experimented with by the engineer, also had similar plans; to Longridge; however, belongs Elswick Ordnance Co. and subsequently by the War Office. the credit of pointing out the proper mode of winding on the The results were so promising that means were sought for further wire with initial tension so adjusted as to make the firing tension improvements, and breech-loading guns, having the Elswick (i.e. the tension which exists when the gun is fired) of the cup obturation, were reintroduced.
wire uniform for the maximum proof powder pressure. Great
success attended the early introduction of the coil system. A stronger material than ordinary carbon gun steel was conse-
In the Fraser system an attempt was made to obtain rigidity of construction and additional longitudinal strength by interlocking the various coils from breech to muzzle; this feature still exists in all designs adopted by the English
FIG. 14.- Modern B.L. Construction. government, but foreign designers do not favour it altogether, and toughness and endurance under a suitable oil hardening and annealing many of their guns of the latest type have a number of short process, the yielding stress being about 26, tons to 28 tons and the independent hoops shrunk on, especially over the
chase. Their view breaking stress from 45 tons to 55 tons per square inch, with an
practically all barrels and hoops (except jackets) of modern guns
The gun steel, whether of the carbon or nickel quality, used in
forgings. and from this it is allowed to run into a cast iron ingot mould of from 10 to 12 ft. high and 2 ft. or more in diameter. With very large ingots two furnaces may have to be employed. The external
shape of these ingots varies in different steel works, but they are so 1160
arranged that, as the ingot slowly cools, the contraction of the metal FIG. 12.-M.L. Gun Construction.
shall not set up dangerous internal stresses. The top of the ingot
is generally porous, and consequently, after cooling, it is usual conducive to strength and durability, but it is essential that massive for about one-third of the length of the ingot to be cut from the top tubes of the highest quality of steel should be employed.
and remelted; a small part of the bottom is also often discarded. The actual building up of a gun entails operations which are
The centre of the larger ingots is also inclined to be unsound, and exactly similar, whether it be of the M.L. or B.L. system; and Whitworth and Harmet methods of Auid compressed steel, this
a hole is therefore bored through them to remove this part. In the the hardening treatment of the steel is also the same-the coiled porosity at the top and centre of the ingot does not occur to iron hoops when welded, of course, received no such treatment. the same extent, and a much greater portion can therefore be
The sound portion of the ingot is now heated in a reheating gas furnace, which is usually built in close proximity to a hydraulic forging press (fig. 15, Plate I.). This press is now almost exclusively used for forging the steel in place of the steam hammers which were formerly an important feature in all large works. The largest of these steam hammers could not deliver a blow of much more than some 500 ft. tons of energy; with the hydraulic press, however, the pressure amounts to, for ordinary purposes, from 1000 tons to 5000 tons, while for the manufacture of armour plates it may amount
to as much as 10,000 or 12,000 tons. 148
For forgings of 8-in. internal diameter and upwards, the bored out
ingot, just mentioned, is forged hollow on a tubular mandril, kept FIG. 13.-M.L. Gun Construction (Fraser).
cool by water running through the centre; from two to four hours Fig. 14 shows the various stages of building up a B.L. gun and forging work can be performed before the metal has cooled down illustrates at the same time the principle of the interlocking too much.. Generally one end of the ingot is forged down to the
proper size; it is then reheated and the other end similarly treated. system.
The forging of the steel and the subsequent operations have a very The steel barrels of the M.L. guns were forged solid; the material marked influence on the structure of the metal, as will be seen from was then tested so as to determine the most suitable temperature, at the micro-photographs shown in the article ALLOYS, where (a) and which the oil hardening treatment should be carried out after the (b) show the structure of the cast steel of the actual ingot; from barrel had been bored. The bored barrel was simply heated to the this it will be noticed that the crystals are very large and prominent, required temperature and plunged vertically into a tank of oil. but, as the metal passes through the various operations, these The subsequent annealing process was not introduced until some crystals become smaller and less pronounced. Thus (c) and (d) years after; it is therefore not to be wondered at that steel proved show the metal after forging; (e) shows the pearlite structure with untrustworthy and so was used with reluctance.
a magnification of 1000 diameters, which disappears on the steel Since 1880 the steel industry has made so much progress that this being oil hardened, and () shows the oil hardened and annealed material is now regarded as the metal most to be relied on. The long crystals. At the Bofors Works in Sweden, gun barrels up to 24 cm. high-power guns, however, require to be worked at a greater chamber 9-5 in.) calibre have been formed of an unforged cast steel tube; pressure than the older B.L. guns, with which 15 tons or 16 tons per but this practice, although allowing of the production of an in. square inch was considered the maximum. With the designs now expensive gun, is not followed by other nations. produced 18-5 tons to 20 tons per square inch working pressure in the After the forging is completed, it is annealed by reheating and chamber is the general rule. Trasla
cooling slowly, and test pieces are cut from each end tangentially
to the circumference of the bore; these are tested to ascertain the interior of the covering tube or hoop finished to suit. The covering quality of the steel in the soft state.
hoop is allowed usually only a small shrinkage, or sometimes none, It is found that the quality of the steel is greatly improved by as it is simply intended as a protection to the wire and to give forging, so long as this is not carried so far as to set up a laminar longitudinal strength; but in order to place it over the wire it must structure in the metal, which is thereby rendered less suitable for be heated and thus some little contraction always does take place gun construction-being weaker across the laminae than in the on cooling. The heat to which these hoops are brought for shrinking other directions. It is then termed over-forged.
never exceeds that used in annealing, otherwise the modifying If the tests are satisfactory the forging is rough-turned and bored, effects of this process would be interfered with. then reheated to a temperature of about 1600 ° F., and hardened by In the earliest modern type B.L. guns, the breech screw engaged plunging it into a vertical tank of rape oil. This process is a some directly with a serew thread cut in the barrel, which thus had to what critical one and great care is observed in uniformly heating, resist a large portion, if not all, of the longitudinal stress. This was to the required temperature, the whole of the forging in a furnace also the system first adopted in France, but there are certain in close proximity to the oil tank, into which plunged and objections to it, the principal being that the barrel must be made of completely submerged as rapidly as possible. In some cases the large diameter to meet the longitudinal stress, and this in consequence oil in the tank is circulated by pumping, so that uniformity of cooling reduces the circumferential strength of the gun. Again, the diameter is ensured; and, in addition, the oil tank is surrounded by a water of the screw is always considerably larger than the breech opening, jacket which also helps to keep it át a uniform heat. The forging is and so an abrupt change of section takes place,
which it is always subsequently again heated to about 1200° F. and allowed to cool best to avoid in structures liable to sudden shocks. The thick slowly by being placed in warm sand, &c. This last operation is barrel, however, gives stiffness against bending and, moreover, does termed annealing, and is intended to dissipate any internal stress not materially lengthen with firing; thin barrels on the other hand which may have been induced in the forging by any of the previous are gradually extended by the
drawing out action of the shot as it processes, especially that of oil-hardening. After this annealing is forced through the gun. In some large guns with excessively thin process a second set of test pieces, two for tensile and two for bending barrels this action was so pronounced as to entail considerable iest, are cut from each end of the forging in the positions above inconvenience. In the English system the breech screw is engaged mentioned; for guns of less than 3-in. calibre only half this number either in the breech piece, i.e. the hoop which is shrunk on over of test pieces is taken; and with hoops of less than 48 in. in length the breech end of the barrel, or in a special bush screwed into the the test pieces are taken only from the end which formed the upper breech piece. This latter method suits the latest system of conpart of the cast ingot.
struction in which the breech piece is put on the barrel from the In all cases of annealed steel the test pieces of 2 in. length and muzzle, while with the earlier type it was put on from the breech end. 0.533 in. diameter must ive the stipu
ccording to the With the earlier modern guns short hoops were used wh never character of the steel. For breech screws the steel is made of a possible, as, for instance, over the chase, principally because the harder quality, as it has
to resist a crushing stress. These are the steel in short lengths was less likely to contain flaws, but as the tests required in England, but they differ in different countries; for metallurgical processes of steel making developed the necessity for instance in France a harder class of carbon steel is employed for this disappeared, and the hoops became gradually longer. This has hoops, in which the tensile strength must not be less than 44.5 tons, however, increased correspondingly the difficulties in boring and nor the elastic limit less than 28.5 tons per square inch, neither must turning, and, to a much greater extent, those encountered in building the elongation fall below 12%.
up the gun. In this operation the greatest care has to be taken, or Assuming that the tests of the annealed forging are satisfactory, warping will occur during heating. The tubes are heated in a vertical the forging, which we will suppose to be a barrel, is tested for straight cylindrical furnace, gas jets playing both on the exterior and interior ness and if necessary rectified. It is then rough-turned in a lathe of the tube. When sufficiently hot, known by the diameter of the (fig. 16) “ to break the skin " (as it is termed technically) and so tube expanding to equal previously prepared gauges, the tube is
FIG, 16.—Lathe used in Gun Construction, prevent warping during the subsequent operations. It is then | raised out of the furnace and dropped vertically over the barrel or bored out to nearly the finished dimension and afterwards fine other portion of the gun (fig. 18, Plate II.). In cooling it shrinks turned on the exterior. In the meantime the other portions of the longitudinally as well as circumferentially, and in order to avoid gun are in progress, and as it is far easier to turn down the
outside gaps between adjoining tubes the tube is, after being placed in of a tube than to bore out the interior of the superimposed one to position, cooled at one end by a ring, of water jets to make it grip. the exact measurements required to allow for shrinkage, the interior while the other portions are kept hot by rings of burning gas of the jacket and other hoops are bored out and finished before the flames, which are successively extinguished to allow the hoop to exterior of the internal tubes or of the barrel is fine turned. The shorten gradually and thus prevent internal longitudinal stress. A process of boring is illustrated in fig. 17. The barrel or hoop A, to stream of water is also directed along the interior of the gun be bored, is passed through the revolving headstock B and firmly during the building up process, in order to ensure the hoop cooling held by jaws C, the other end being supported on rollers D. Å from the interior. After the building up has been completed, the head E, mounted on the end of a boring bar F, is drawn gradually barrel is fine-bored, then chambered and rifled. The breech is then through the barrel, as it revolves, by the leading screw K actuated screwed either for the bush or breech screw and the breech by the gear G. The boring head is provided with two or more mechanism fitted to the gun.
FIG. 17.-Boring. cutting tools, and also with a number of brass pins or pieces of In order to obtain additional longitudinal strength the outer hard wood to act as guides, in order to keep the boring head central tubes are so arranged that each hooks on to its neighbour from after it has entered the barrel. The revolving headstock B is muzzle to breech. Thus, the chase hoop hooks on to the barrel by driven by a belt and suitable gearing
step, and the succeeding hoops hook on to each other until the With wire guns the procedure is somewhat different. The wire jacket is reached which is then secured to the breech piece by a & round on to its tube, which has been previously fine turned, the strong, screwed ring. In all the latest patterns of English guns exterior diameter of the wire is then carefully measured and the there is a single chase hoop covering the forward portion of the