I did mention that, at this time, both sides were casting solid and machining to remove the material, as this was prior to WPF going over to the Rodman, WATER CORE, hollow casting process which they did at around # 225 or 250 of about 550 parrott 100 pdrs. made.
Cast iron Parrotts were notorious for bursting in front of the wrought iron reinforcing band. Is there any documentation that the change to the wet core process made a difference in the tendency to burst?
I think in Gibbon's (?) book on artillery, he says that most barrel failures involved cracks between the chamber and trunnions. This seems to be an area that undergoes triaxial stress during firing, which is always a bad thing. Getting rid of trunnions was a step in the right direction.
George, Although I would have preferred a large body of evidence to prove beyond any doubt that Rodman's process made tubes of greater endurance, this statement from
A Treatise on Ordnance and Naval Gunnerythat Boom J recently discovered, indicates clearly that ordnance experts of that day were convinced that his methods worked.
The information on trunnions is very interesting also and is aligned with the info you wrote about.
Regards, Tracy and Mike
144. Rodman's Plan of Hollow-Casting. His manner of casting may be briefly described, as follows: a core is formed on a water-tight cast-iron tube, closed at the lower end. By means of an interior tube in the centre of the other, and open at the lower end, a stream of water is conducted to the bottom of the larger tube, and rising through the circular space between the two, flows out at the top. A fire is built around the jacket at the bottom of the casting pit, and the mould kept at nearly red heat. In casting an eight-inch columbiad, twenty-five hours after casting, the core was withdrawn, and the flow of water continued through the space left by it for forty hours longer. The amount of water used was about fifty times the weight of the casting, and the heat imparted to the water, and carried off by it, was equal to 60° on the whole quantity used. For larger pieces, the amount of water and time of cooling are greater.
All the guns cast in this way present a marked superiority in endurance over those cast solid in the ordinary way; and the eight-inch columbiad, mentioned above, sustained fifteen hundred rounds, including proof charges, without bursting, whilst another of the same calibre, cast solid, from the same metal, at the same time, and under precisely the same circumstances, failed at the seventy-third round.
It is evident how the metal of a piece cast in this manner avoids the defects pointed out as existing in a piece cast solid and allowed to cool in the old way; the merit of Captain Rodman's invention evidently consists in retarding the cooling of the outside by the application of heat, and hastening that of the inside by the application of cold, thus rendering the cooling of the whole mass more uniform, and preventing the formation of successive layers of different temperatures; or, if they are formed at all, making them commence on the interior, by which the strain is reversed, and actually made to add to the strength of the piece, the inner layer setting first, and each successive outer layer shrinking, in cooling, and setting around the layer inside of it.
145. Effect of the Trunnions. Another great cause of the want of strength in guns as at present formed, is the position of the trunnions, as regards the point to which the recoil of the gun is transmitted. The existence of these trunnions, by forming great re-entering angles on the surface of the gun, is of itself a great cause of weakness, as has already been explained; but their position on all pieces except mortars, has a weakening effect in a different way, which will now be explained. The force exerted to produce recoil, acting as a pressure against the interior of the breech, is propagated as a force tending to stretch the metal of the gun from the section y (fig. 29), in line of the axis toward the rig. 29. muzzle z; the rate of propagation being extremely rapid, so much so that, to the senses, the whole gun recoils together and as one mass, and at the same instant; yet in reality the first effect of the recoil is to elongate the gun, pushing out the breech part like one end of a spiral spring; the elongation traversing the whole length of the gun, and arriving at the muzzle, leaves it at its original length, assuming the elongation to have been far within the elastic limits of the metal. In its rapid progress, however, it has produced a strain in succession in the line of the axis upon every part of the gun.
146. If the gun have no trunnions, but, resting without friction, abut firmly against a fixed obstacle against the breech at x, then the segment in rear of the cartridge will be compressed by a force equal to the whole recoil in the direction y x, while the remaining parts of the gun will be extended by a force in the direction y z. which, at the transverse section y is equal to the recoil, and at the muzzle is equal to zero.
If the gun be fixed rigidly on trunnions placed in the usual position at t, the strain tending to tear or break them off is equal to the whole work done by the recoil could, therefore, a convenient method be adopted for supporting guns upon their carriages without the use of trunnions, the recoil being received by a support entirely in rear of the piece, the trunnions could be dispensed with, and the gun very much strengthened in two ways. Muskets and other small arms are therefore arranged in the way best suited to their strength, and they can therefore be made much lighter than would otherwise be safe. The cannon of the ancients possessed this element of strength, for they were without trunnions, and transmitted the recoil to supports placed behind. In some of the new rifled guns, supplied to the navy, an attempt has been made by Captain Dahlgren to apply this principle by strapping on the trunnions.
