Chapter III: Elements of Gun Construction and Design

“A gun is a machine by which the force of expanding gas is utilized for the purpose of propelling a projectile in a definite direction.” It is essentially a metal tube closed at one end, of sufficient strength to resist the pressure of the gases caused by the combustion of the powder charge in the confined space at the closed end of the tube behind the projectile. The rapid combustion of the powder, which produces a high temperature, gives rise to a pressure uniformly exerted in all directions within the confined space. The energy exerted is used in forcing the projectile from the tube.

TUBES

Due to the effort of the large amount of superheated gas generated, which tends to expand in all directions, tremendous rending stresses are set up in the tube. Formerly these stresses were met by a sheer mass of metal, but, as the size of the projectiles increased and the necessary pressure to give them muzzle velocity increased, the size of the guns increased beyond the practical limits of mobility. This was at first offset by forgings of refined alloyed steels, but even these failed to keep pace with the increasing pressure desired. The new condition was met by the introduction of the “built-up” and the “wire-wrapped” guns. The modern built-up gun is made by assembling one or more superimposed cylinders around a central tube. The superimposed cylinders, whose inside dimensions are slightly smaller than the outside dimensions of those on which they are to be assembled, are expanded by heat sufficiently to allow them to be assembled over the tube. The subsequent contraction on cooling causes each of them to exert a uniform pressure on the cylinder immediately underneath. This method of assembling is called “shrinkage.” This gives a compression to the inner tube and a slight tension to the outer one. The compression is so much additional strength to the tube because it must first be overcome before the powder gases can exert a tension on the inner tube fibers. The exact amount of the compression and tension for all parts of a gun at rest or resisting an explosion is a matter of mathematical calculation. The built-up construction has been used in practically all our present day types of field artillery.

THE WIRE-WRAPPED GUN.

Wire-wrapped guns consist of:

(a) An inner steel tube which forms a support on which the wire is wrapped and in which the rifling grooves are cut.

(b) Layers of wire wrapped upon the tube to increase its resistance by the application of an exterior pressure as well as to add to the strength of the structure by their own resistance to extension under fire.

(c) One or more layers consisting of a steel jacket and hoops placed over the wire with or without shrinkage. The jacket generally furnishes longitudinal strength to the guns, and the breech block is screwed into the jacket, or into a breech bushing, which is screwed into the jacket.

The principal advantages of this type of gun over the built-up is economy of manufacture and greater facilities for inspection of materiel in the layers over the tube. The wire wrapping has itself a large reserve of strength due to the high elastic limits that may be given it. Two methods are used to wrap the wire: (a) at constant tension (b) at varying tension so that when the gun is fired with the prescribed pressure, all layers of wire shall be subjected to the same tangential stress. The latter method is theoretically better, but because of the ease of manufacture, together with the large factor of safety possible, the wire is usually wrapped at a constant pressure.

THE BUILT-UP GUN.

All army guns except small howitzers or mortars are of the built-up or wire-wrapped type. Built up guns of less than 5” caliber, or howitzers up to 8” caliber consist of an inner tube and a jacket shrunk onto this tube. The jacket covers the breech end of the gun and extends forward to the center of gravity. Built-up guns of larger caliber have one more layer of hoops in addition to the jacket, one layer of hoops usually extending to the muzzle.

The bore of the tube forms the powder chamber, the seat for the projectile and the rifled bore. Rifling consists of a number of helical grooves cut in the surface of the bore. The soft metal of the rotating band of the projectile is forced into these grooves causing the projectile to take up a rotary motion as it passes through the bore. This is necessary in order to keep the projectile stable in its flight.

TWIST.

By twist of rifling is meant the inclination of one of the grooves to the element of the bore at any point. Rifling is of two kinds: (a) Uniform twist, or that in which the twist is constant throughout the bore, (b) Increasing twist or that in which the twist increases from the breech towards the muzzle.

The twist of rifling is usually expressed in the number of calibers length of bore in which it makes one complete turn. The twist actually required at the muzzle to maintain the stability of the projectile varies with the kind of projectile and the muzzle velocity. If a uniform twist be used, the driving force on the rotating band will be at a maximum when the pressure in the guns is at a maximum—or near the origin of rifling (seat of the projectile). The increasing twist serves to reduce the maximum driving force on the band thus lessening the danger of stripping the band. This is its principal advantage over the uniform twist, though it also reduces slightly the maximum pressure in the gun. The principal disadvantage of the increasing twist is the continued change in form of the grooves pressed in the rotating band, as the projectile passes through the bore. This results in increased friction and a higher value for the passive resistance than with a uniform twist. (Note: greater ranges obtained by cutting grooves in projectile, principal used on the long range gun by the Germans.) If the twist increases from zero at the breech uniformly to the muzzle, the rate of change in the tangent to the groove is constant. A twist in this form offers less resistance than the uniform twist to the initial rotation of the projectile. To still further diminish this resistance a twist that is at first less rapid than the uniformly increasing twist and later more rapid has been generally adopted for rifled guns.

Formerly in our service the twist was uniform; one turn in 25 calibres for guns and one turn in 20 calibres for howitzers. All the latest model army guns, however, have an increasing twist of one turn in 50 calibres at the breech to one turn in 25 calibres at a point from 2 to 4 calibres from the muzzle. In howitzers and mortars the twist is sometimes one turn in 40 calibres at the breech to one turn in 20 calibres at a point several calibres from the muzzle. Some mortars are rifled with a uniform twist and some guns have a rifling which begins with a zero twist. (The 1905 3” gun, 0 to 1 in 25.)

OUTER CYLINDERS.

Outside of the tube is the jacket. It extends to the rear of the tube a sufficient distance to allow of seating the breech block. In this manner the longitudinal stress due to the pressure of the powder gases on the face of the breech block is transmitted to the jacket thus relieving the metal of the tube from this stress. In all built-up guns there is some method devised for locking the tube to the jacket so as to prevent relative movement of these parts.

Considering the gun alone the greatest range is obtained at an angle of about 43 degrees from that gun which fires the heaviest projectile with the greatest velocity. The caliber being limited to from 2.95 inch to 3.3 inch, the projectile is limited in weight to from 12 to 18 pounds. The weight of the gun is limited to between 700 and 1000 pounds and in length to between 27 and 36 calibers. The longer the gun, the greater the weight and velocity from the same charge of powder. A pressure of 33,000 pounds per square inch with a corresponding velocity of 1700 f. s. has been found to be as high a pressure and velocity as are desirable for a reasonable length of life for a field gun, the average life of which is 10,000 accurate rounds.

Under the French school of artillery, which dominates our service at present, our bore is 75-mm, the weight of our shell 12 pounds, our shrapnel 16 pounds, the velocity for the one about 1,750 f. s. and for the other about 1,680 f. s.

BREECHBLOCKS.

The breechblock appears in four distinct types. Our own service has for years used the swinging interrupted screw breechblock which in the 1905 model is the equal of any of that type in existence. The swinging block has serious disadvantages for high angle fire in that it requires an excessive amount of room to operate and is difficult to load at high elevations.

The Italians have introduced a new breechblock in one of their recent guns, consisting of a half cylinder with superimposed spherical face on its cylindrical surface rotating vertically about a horizontal axis perpendicular to the axis of the bore. The gun is loaded through a groove in the breechblock when the latter is in its horizontal position. The block, which is semi-automatic, is very satisfactory. It is adapted to high angle fire.

The French in their “75” have used the rotating eccentric screw type, which is rapid in movement and lends itself fairly well to high angle fire. It is completely enclosed and of rugged construction.

The Germans have used the sliding wedge type of block, moving in a horizontal direction, which does not lend itself to high angle fire.

The United States in its recent field carriage adopted the sliding wedge type in a vertical plane on account of its manifest superiority in fire at high angles. This block is rather difficult to manufacture and the type has a tendency to stick. The automatic closing necessitates a strong closing spring which fatigues the block operator, No. 1 in the gun squad. It is interesting to note that in a prospective new design for the 1916 gun carriage the American Ordnance Department adopted the French breechblock; and the St. Chamond Company, designing for the American Expeditionary Forces, adopted the American drop block.

Requirements for a breech mechanism:

The following may be said to be the principal requirements for a successful breech mechanism.

1. _Safety._ To be safe: (a) the gas must be restrained from escaping to the rear; this sealing or obturation must be automatic, greater pressure insuring better obturation. (b) The breech of the gun must not be weakened by the fitting of the breech mechanism. (c) The parts must have ample strength to prevent any portion from being blown to the rear. (d) The danger of premature discharge must be minimized. (e) The breechblock must be securely locked to prevent opening on firing.

2. _Ease and Rapidity of Working._ Otherwise, rapid and continuous fire cannot be maintained. Hence this would include facility in loading and certainty of extraction for rapid fire guns.

3. _Not Easily Put Out of Order._ In other words it must be able to meet service conditions and hard usage. Parts should have a reserve strength.

4. _Ease of Repair._ Parts most exposed to wear should be so designed as to permit being replaced. This will also include accessibility to parts, so that breakage of a part will not disable the mechanism for a long time.

5. _Interchangeability._ Not only should individual parts be made interchangeable by accurate workmanship, but the whole mechanism should be capable of being mounted on similar guns. This is to meet service conditions.

GUN CARRIAGES.

A modern gun carriage is expected to stand steady on firing, so that in the first place it requires no running up, and in the second place it maintains the direction of the gun so that only a slight correction in elevation and direction is required after each round. The carriage is maintained in position by the spade, which sinks into the ground, and by the friction of the wheels upon the ground. If the force of the recoiling gun were communicated directly to the anchored carriage the effect would be to make it jump violently, which would not only disturb the lay, but would prevent the cannoneers from maintaining their position. The hydraulic recoil brake is therefore interposed between gun and carriage. If the guns were rigidly attached to the carriage the latter would be forced back a short distance at each round, and the whole of the recoil energy would have to be absorbed in that short motion. Instead of this the gun alone is allowed to recoil several feet and although the recoil energy is in this case greater than it would be if gun and carriage recoiled together yet it is so gradually communicated to the carriage that instead of a violent jerk we have a steady, uniform pull, the only effect of which is to slightly compress the earth behind the spade. In a well designed carriage the amount of this pull is always less than that required to lift the wheels off the ground by rotating the carriage about the spade.

The only motion of the carriage which takes place is that due to the elastic bending and rebound of its parts under the cross strains set up on discharge. These strains are inevitable since the direction of recoil cannot be always exactly in the line of the resistance of the earth behind the spade. This movement of the axis is known as jump and must be determined by experiment for the individual piece in its particular mounting.

The principal parts of the typical gun carriage are the cradle, a device for mounting the cradle—called in the different models rocker, pintle yoke, and top carriage, the trail, the wheels and axle. The gun slides in recoil on the upper surface of the cradle and the cradle contains the recoil controlling parts.

In the design of the carriage the constructional difficulty lies not so much in preventing the carriage from recoiling but in preventing the wheels from rising off the ground on the shock of discharge. The force of the recoil of the gun, acting in the line of motion of the center of gravity of the recoiling parts, tends to turn the carriage over backwards about the point of the trail or center of the spade. This force is resisted by the weight of the gun and carriage, which tends to keep the wheels on the ground. The leverage with which the overturning force acts is due to the distance of its line of action above the center of the spade; the leverage with which the overturning force acts is that due to the horizontal distance of the center of gravity of the gun and carriage from the center of the spade.

It follows that the steadiness of the carriage for a given muzzle energy may be promoted by four factors.

(a) Increasing the weight of the gun and recoiling parts. This reduces the recoil energy.

(b) Increasing the length of recoil allowed. This reduces the overturning pull.

(c) Keeping this gun as low as possible either by reducing the height of the wheels, or by cranking the axle downwards. This reduces the leverage of the overturning force.

(d) Increasing the length of the trail. This increases the leverage of the steadying force.

The well designed gun carriage is one that combines these factors in a practical way so as to give the greatest possible steadiness to the carriage at the same time keeping within the limits of weight imposed by the necessity of mobility.

Gun carriages are constructed so as to permit movement of the tube in the vertical and generally in the horizontal plane. These two motions may be made simultaneous if so desired by proper combination of the two motions and the axis of a gun aligned in any desired direction within the limits of motion of its mount. The two motions referred to are designated as follows: (a) Rotation of the piece about a vertical axis, its inclination with the horizontal remaining unchanged is called “traverse.” (b) Movement of the piece in a vertical plane, the direction of the horizontal projection of the axis of the bore remaining unchanged is called “elevation.”

In order to permit of the two motions mentioned, gun carriages are provided with mechanisms for giving the piece accurately controlled motion in both elevation and traverse. The elevating gear of most American guns is an application of the Chinese or Telescopic screw. This gives a short assembled length with the necessary extended length required for modern ranges. It also gives the necessary rapidity for action. An entirely different mechanism is used in our howitzers and American 75s. In this case motion is communicated to the rockers, which carry the gun and cradle, through the engagement of worms with teeth cut on the lower circular edge of the rockers, the latter being pivoted on the cradle trunnions.

TRAVERSING MECHANISMS.

Movement of the gun in traverse is accomplished in two general ways.

(=a=) Pivot on the gun carriage axle.

1. The gun and cradle move around a vertical pintle or pivot placed in a saddle or rocker which itself pivots on horizontal trunnions or bearings usually attached to the axle. The rear end of the rocker forms a seat or bearing on which the cradle and gun move in traverse. The upper end of the elevating screw is also attached to the rear end of the rocker. This allows the rocker and cradle to be moved together in elevation and the cradle to move on the rocker in traverse.

2. The gun and cradle are mounted by means of horizontal cradle trunnions on a top carriage. The top carriage moves in traverse around a vertical pintle or bearing attached over the axle and between the front end of the trail flasks. The rear end of the top carriage moves on bearings attached to the top of each side of the trail flasks. This allows the gun, cradle and rocker to move in elevation with respect to the top carriage and the top carriage bearing the gun, cradle rocker and elevating mechanism to move in traverse with respect to the bottom carriage.

(=b=) The gun, cradle and trail move in traverse by sliding along the axle of the carriage on bearings provided for that purpose. In this case the trail spade is the point of pivot.

The first system is the one generally used on all American gun carriages except the 155-mm Howitzer. It gives ease and speed in the manipulation of the piece in traverse, but has the disadvantage of rendering the carriage less stable in firing due to the fact that the direction of recoil of the gun is in the direction of the line joining the trail spade and the central pivot, _only_ when the gun is absolutely in the center of its traverse. In all other positions, which will be the usual case, there is a component at right angles to the line of trail spade-central pivot which tends to throw the gun off from its proper direction making it necessary to relay each time the gun is fired. The second method is that in use on the French 75-mm gun and the French and American 155 howitzer. While not so easy of manipulation and giving less freedom of movement in traverse (about 106 mils) it still has the advantage in that it gives greater stability in firing due to the fact that the gun always recoils along the line through the trail spade, perpendicular to the axle.

ELEVATING MECHANISMS.

There are two general classifications of gun carriages according to the manner in which the laying for elevation is effected. The 3” gun is an example of one type in which the total quadrant angle of departure is laid off as one angle necessitating at each resetting of a range a new resetting of the range bubble. It is evident that this militates against both speed and accuracy in laying for elevation. The other type is illustrated by the American 75 and by the French and British 75-mm gun carriages which have what is called the “independent line of site.” It differs from the other type in that there is placed between the top carriage and the cradle an intermediate carriage or rocker arm pivoting on the gun trunnions at one end, and carrying at the other the support for the elevating device which gives to the cradle and the gun the proper elevation for the range. This intermediate carriage or rocker arm has a toothed edge geared with a pinion fastened to the trail flask, by means of which, gun cradle and elevating device may be moved in elevation without disturbing the relation between itself and the gun, this being done by gears independent of the elevating mechanism. An angle of site may be set off for the intermediate carriage and after the bubble is centered this actuating pinion may be blocked and no further concern be paid to the angle of site. A cannoneer therefore has only to set his range accurately on the index to give the changes in range ordered.

Elevation is accomplished by telescoping screws, by circular racks and pinions, and by worms and arcs. Telescoping screws are good for from 15 to 20 degrees in elevation only, as for greater angles they become rather too large and cumbersome. The elevating arc is attached on the top, on the bottom or on the side of the cradle and, if possible, at its center. When the latter is not possible, two arcs are generally used to prevent torsional strains developing. Top racks are exposed to fire, side circular racks or arcs interfere with the traverse, with the placing of the sights, and with the general handling of the carriage. The bottom of the cradle is perhaps the best location for the rack. As in the traversing mechanism, a train of bevelled gears transmits the power from the hand wheel to the elevating mechanism.

In both the traversing and elevating mechanism, strength, simplicity, power, accessibility, non-interference and absence of lost motion are the features sought. To secure these is one of the most difficult things in gun-carriage design; for, since the traversing and elevating are the last two elements considered, they must, therefore, be the ground for modification and the means of adapting the great main elements—namely, wheels, axle, trail, recoil mechanism and gun—into a unit.

RECOIL MECHANISM.

The recoil system of the gun carriage consists of a recoil brake, a counter recoil mechanism and a counter recoil brake. The function of each part is apparent from its name. Various systems of checking recoil on field guns have been used, among them may be mentioned; friction devices including brakes on the wheel, inclined planes, pneumatic and hydraulic brakes. All have either been superseded by the last named or are used in connection with hydraulic brakes. The power of the brake lies in the pressure produced in the cylinder through the resistance to motion offered by the liquid flowing through apertures. By varying the size of these apertures the braking effect may be controlled so as to fulfill the conditions demanded by the service. In designing the brake, the effect of the counter recoil system, angle of fire, length of recoil, friction and pressure within the cylinder must all be considered. In howitzers which are designed to be fired at high angles of elevation, the recoil must be shortened to prevent the breech striking the ground, a condition successfully met. Since the pressure due to recoil is ultimately led into the ground through the trail and spade, consideration must be given to the problem of the moment of inertia about the trailspade. The tendency to turn over backward about the spade as the center is offset by the amount of the weight of the carriage with respect to the same point. This raises the question as to how much pressure may be allowed to act to the rear; all of which must be considered in designing the carriage. The counter recoil systems in general use are two: spring or pneumatic. The former is illustrated in our 3” and 75-mm field guns, the latter by the French 75-mm gun and 155-mm howitzer. Their purpose, of course, is to return the gun “into battery” after it has recoiled on the carriage.

The recoil mechanism is a study in itself, of which there are two schools—the advocates of the spring and of the air recuperators. Great Britain, Germany and the United States have been the advocates of spring recuperation and France of air recuperation. Great Britain and the United States were of the spring school, undoubtedly, because of the lack of a satisfactory air recuperating system; which is rather strikingly proven by the fact that both countries have adopted air recuperation since they have procured or developed satisfactory types.

Both schools have grounds for their position, however. The spring school has in its favor simplicity of design and manufacture and ease of replacement, which can be done in the field. On the other hand, spring recoils have many breakages and greater weight combined with a high replacement of weakened springs, the life varying from 3000 to 8000 rounds.

The air school has in its favor a high order of efficiency—smoother action, general all around efficiency and less weight. But the air recuperator is difficult to manufacture, costly, and when damaged must go to the rear to be repaired; which, however, it seldom needs.

In mounting the spring recuperator, the most recent practice has separated the recoil mechanism from the springs in order to distribute the piston rod pull, thus preventing whip and allowing easy access to the various parts for replacement, refilling and repairs. In order to lower the center of gravity, the gun is slung under the recoil cylinder with the two spring recuperators below and on either side.

Air recuperators are invariably located below the gun for protection and because of their large size and shape, which adapt them for attaching the elevating mechanism.

IN-BATTERY OR FIRING POSITION

HYDRO-SPRING RECOIL SYSTEM

HYDRO-PNEUMATIC RECOIL SYSTEM WITH FLUID IN DIRECT CONTACT WITH THE AIR]

In either system, the recoil is taken up by means of oil or glycerine and water passing through an orifice created by a slotted piston passing over ribs of varying height, or through a valve on the pressure side of the piston, or by a solid or perforated piston passing through a perforated intermediate cylinder.

The latter type is particularly adapted to variable recoil guns as the intermediate cylinder can be rotated, thus throwing varying orifices into position for the flow of oil.

Counter recoil is accomplished by the springs or by the air pressure in the hydro-pneumatic system, in which the air pressure is sufficient to hold the gun in battery at all elevations and is built up at recoil.

In all counter recoil systems, it is necessary to insert a buffer to take up the remaining energy of the springs or air pressure so as to bring the gun into battery without appreciable jar. Numerous types have been developed and perfected.

The counter recoil brake or buffer in our 3”-gun is a slightly tapered bronze rod, tightly fitting in the cylindrical bore of the piston rod. The retardation caused by forcing the oil in the piston rod out through the small clearance between the buffer and bore of the piston rod eases the return to battery without jar to the gun, which has been forced back by the counter recoil springs.

The physical law that action and reaction are equal has a peculiar emphasis when applied to the firing of a piece of high powered artillery. The force exerted to throw a heavy projectile 7 miles or more from the muzzle of the gun is toward the breech of the weapon in its recoil. How some of these forces are handled safely and easily by mechanical means are almost beyond the mind’s grasp. Not long ago a touring car, weighing two tons, traveled at the rate of 210 miles an hour along a Florida beach. Conceive of such a car going 337 miles an hour—which is much faster than any man ever traveled; then conceive of a mechanism which would stop this car, going nearly six miles a minute, stop it in 45 inches of space and one-half a second of time without the slightest damage to the car. This is precisely the equivalent of the feat performed by the recuperator of a heavy howitzer after a shot.

SHIELDS.

Although cover for the cannoneers had been used off and on since the invention of guns, it had fallen into disuse until it was firmly established as an essential feature by the French on their 75-mm in 1897. All modern field guns have such protection both for the cannoneers and for the delicate parts of the material which would be damaged by shrapnel balls or shell fragments. The shield is made of hardened steel capable of withstanding the impact of a bullet of a service rifle at a 100 yds. range at a standard velocity. For convenience the shield is divided into three parts known as the top shield, main shield and apron, with suitable ports equipped with shutters for the line of vision from sights. The main is fastened to the axle and is rigid. The apron is hinged to the main shield or the axle-swinging forward for the traveling position. The top shield is fastened to the main shield by hinges and swings forward and downward for traveling position.

SIGHTS.

The sights serve three important functions. They improve the vision of the gunner and lay the gun in elevation and direction. The simplest sight is the one over the line of metal which lays for direction only, the second is the tangent sight mounted on a range arc centered on the axis of rotation in elevation usually having a deflection scale to correct for drift and to lead the moving target. This unit lays for range and direction. The last sight is the telescopic or panoramic sight which is mounted on a range arc and lays for direction only, it is the unit for indirect fire, laying for direction, and markedly improving the vision of the gunner. The latest model of the American panoramic sight is superior to any in existence.

For precision in indirect fire, practically all carriages are equipped with some form of range quadrant, containing a means of setting off the angle of site and the range angle. All instruments are equipped to compensate for difference in wheel level. The British carriage automatically corrects for deviation, simplifying the firing date by that element.

WHEELS.

The height of the wheels affects the draft, weight, clearance, and stability of the carriage both as to road stability—i. e., low center of gravity; and firing stability—i. e., the overturning movement about the end of the trail. It is now believed that entirely too much stress has been laid on road clearance. Reducing the height of wheel reduces the weight and road clearance, lowers the center of gravity and increases the firing stability; but it also reduces the angle of gun elevation unless the trail is shortened. Firing stability in general is increased by adding to the weight of the gun, lengthening the recoil, slinging the gun as low as possible and lengthening the trail. The less the height of the wheel exceeds four feet the better, despite the poor draft feature, which is compensated for to some extent by a reduction in weight and turning radius.

AXLES.

Axles are straight or of the offset type. The straight axle is stronger for its weight. The drop axle allows the center of gravity to be lowered.

TRAILS.

Most modern trails are of the sectional built-up type. Some, however, are of tubular and telescopic. The most variable portion of the trail is the spade. It consists of two parts, the spade proper and the float. The former prevents recoil, the latter the burying of the trail. The spades proper are of three types: the fixed, as in the French 75-mm; semi-fixed, as in the 155-mm howitzer; and driven, as in the Deport and American 1916 75-mm. Each has advantages and disadvantages. The driven spade is considered essential for the split trail carriage, as the latter has no means of seating itself; and should one spade take, and not the other the carriage might be damaged when the gun is fired at an extreme traverse.

Split trails introduced a novelty in field gun carriages, in that a compensating device became necessary to adjust for the difference in ground level of the two spades.