Theory of Gun Construction

THEORY OF GUN CONSTRUCTION The important effect already noted is that the shrinkage pres sure produces circumferential compression on the inside tube at the expense of circumferential tension in the outside tube. This opposes the radial pressure and circumferential tension which re sult from radial powder pressure in the bore of the gun.

Circumferential Strength.

A theory of gun construction, which is based on Barlow's law and has stood the test of experience, is known as Lame's theory. It relates the radial pow der and shrinkage pressures to the induced tensions and com pressions with respect to the internal and external radii of the tubes concerned. This relation is known as the gunmaker's for mula, and it enables the designer to calculate pressures, tensions or compressions in any position in the walls of the gun both when the gun is in repose, and only affected by shrinkages, and also when it is firing and affected by both shrinkages and powder bore pres sures. A design can thus be produced giving the necessary thick ness of tubes and degree of shrinkage estimated as of suitable circumferential strength not only to resist fracture longitudinally, but to avoid straining any of the material beyond the elastic limit. Several theories of construction are followed, but in all certain assumptions are made, particularly in regard to the mutual effects of stresses in different directions. To cover any inaccuracies in calculation due to these and to ensure a margin of security in the use of the gun, a suitable safety factor is allowed. The gun is generally designed to stand pressures ranging from twice the normal pressure experienced at the muzzle end to one and a half times at the breech end.

Longitudinal Strength.

Provision must also be made for suitable longitudinal strength in the gun to meet the stresses pro duced by the pressure of gases on the face of the breech block and on the base of the projectile, forces which stress the material in a direction tending to produce circumferential rupture. Minor longitudinal stresses are also brought into play by the resisting action of the recoil brake. The maximum bore pressure is con sidered fully effective on the breech surface exposed and it is necessary to distribute the stresses as far as possible to the other members of the gun. This is done in an all-steel gun with a screw breech mechanism through the medium of a screwed breech bush in which the threads of the breech screw engage, while in a gun with a sliding breech mechanism the direct thrust brought on the surface of the sliding block is transmitted to the breech ring and so to the other hoops or tubes which are connected longitudinally by means of shoulders. In a wired gun, the arrangement at the breech end must be such as to connect the tubes inside the wire with those outside, since the wire itself is incapable of contribut ing in any way to the longitudinal strength of the gun.

Girder Strength.

In addition to the longitudinal and cir cumferential strength, the gun is required to have a certain de gree of girder strength so that the gravitational deflection is kept as small as possible. The gun resting in its mounting may be

regarded as a beam supported at a certain distance from one end and its girder strength depends on the rigidity with which the various parts are connected. In a wired gun the wire makes no contribution to this strength, which is therefore dependent upon the tubes and the means taken to connect them rigidly in place.

The design of a post-World War all-steel British gun is shown in fig. 9. This gun is fitted with an inner "A" tube or liner so that when the rifling is considerably worn the gun may be eco nomically repaired by introducing a new liner in place of the old one. (This arrangement is very general but not universal as some guns, usually those of small calibre, have no liners and are repaired by replacing the entire "A" tube.) The liners are usually tapered on the exterior and are driven in to the tapered bore of the outer "A" tube by hydraulic pressure or other suitable means so that when in place there is a certain shrinkage pressure operat ing. In medium calibre guns, since the World War, "loose" liners have also been tried, i.e., liners with a small clearance or air space. Such liners are necessarily of high quality steel with a large elastic range so that when they temporarily expand on firing they receive support from the walls of the gun before they have been strained beyond their elastic limit. The material of the liner contracts to its original dimensions after firing and is easily removable. The liner is keyed at the ends to prevent rotation under the action of the projectile and is shouldered at the rear end to prevent for ward movement. Such a system of ready repair has obvious ad vantages in connection with guns which wear out rapidly or which, like anti-aircraft guns, are subject to very considerable use in a short space of time. The repair is of such a simple nature that it can be effected without dismounting the gun. The "B" tube, the jacket and the breech ring are built by shrinkage, which is effected by machining the internal diameter of the tube to be built to a smaller diameter than the surface on which it is built. The differ ence in diameters must be such as will give the required corn pression and must permit of sufficient expansion for building at a temperature below that which could affect the temper of the material. Sudden changes in the section of tubes are avoided to obviate longitudinal weakness and to prevent difficulties in build ing through the thinner section of the tube cooling more rapidly than the thicker and possibly seizing before properly in place. In all guns the aim is to relieve from all longitudinal stress the metal which bears the brunt of the circumferential stress. Con sequently, the breech bush is free from the liner and attached to the "A" tube.