Page:EB1911 - Volume 20.djvu/248

Rh independent line of sight is an arrangement of sights and elevating gear found in many modern field guns, which divides between two gunners the work of aiming (called laying) the gun, and of giving it the elevation required to hit the target. In fig. 59 the gun is shown mounted on an intermediate carriage elevated and depressed by the screw A. The telescopic or ordinary sight is fixed to this carriage. The gun, in its cradle, is elevated and depressed by the screw B. To lay the gun, the layer works the laying screw A till the telescope points at the target; the gun also, if no elevation has been given, is then pointing straight at the target. To give the gun the elevation necessary for the range, the elevating number on the right of the gun now works the elevating screw B till the gun is sufficiently elevated, the amount given being shown in yards of range on a drum. The motion given to the gun does not disturb the intermediate carriage with the telescope attached to it, and the telescope still remains layed on the target. Once the sights are layed on the target, the elevation of the gun may be changed in a moment by a turn of the elevating wheel, without disturbing the laying. The layer does not have to concern himself about the elevation; he has only to keep his sights on the target while the other numbers continue the service of the gun. This device is especially valuable when firing at moving targets, when the range and the laying have to be altered simultaneously.

The same result may also be obtained by other mechanical devices without the use of the intermediate carriage. Thus the British field guns have a long elevating screw with the sight connected to its centre, the lower end passing through a nut at the side of the upper carriage, the upper end through a nut at the side of the cradle. Then, if the lower nut be turned by the laying wheel, the screw, the sight and the gun will go up or down together; if the upper nut be turned by the elevating wheel, the gun will go up or down the screw without moving the sights. Colonel Scott’s “automatic” line of sight is an improvement on the ordinary gear in that the sight can be cross-levelled to eliminate the error due to difference of level of wheels. Krupp has a similar device in which the sight-socket is on the cradle so that it can be cross-levelled. The sight itself is connected to the elevating gear, and is screwed out of its socket as the breech of the gun is depressed, so that the sight remains in the same place.

Construction of the Gun.—Field guns are made of steel, usually containing a small percentage of nickel or chromium, or both, and having a tensile breaking strain of about 50 tons per square inch. In Austria, for facility of local manufacture, hard-drawn bronze is still used, although this is considerably heavier than steel.

The Carriage (see, Plate I.).—The first field guns used in war were supported by crossed stakes under the muzzle and anchored by a spike on the breech which penetrated into the ground. The next improvement was to mount the gun on a sleigh. This method is still used in Norway and in Canada. The next step was to mount the gun on a two-wheeled carriage, connected to a second two-wheeled carriage (the limber) by a flexible coupling. For centuries the gun-carriage was a rigid construction, recoiling on firing, and having to be run up by hand after each round. In 1895 spring-spade equipments were introduced. In these a spade attached to a helical spring was set under the carriage; on discharge the spade dug into the ground, compressing the spring as the carriage recoiled. The extension of the spring ran the gun up again without assistance from the gunners.

The British 15 pr. used in the South African War (1899–1902) had a spring spade carriage designed by Sir George Clarke. Similar equipments were introduced by several continental powers. The Japanese gun used in Manchuria (1904) had dragshoes attached by wire ropes passing round drums on the wheels to a strong spring in the trail. On recoil the wheels revolved backwards, compressing the spring; after recoil the pull of the spring on the wire ropes revolved the wheels forward and returned the gun to its former position. The Italian 1902 semi-Q.F. carriage was constructed on a very similar principle. All these semi-Q.F. equipments were open to the objection that the gunners had to stand clear of the shield every time the gun was fired. They have since been superseded by Q.F. gun-recoil equipments.

The gun-carriage must be strong enough to carry the gun across country, and it must be so constructed as not to move when the gun is fired. If the gun-carriage were allowed to recoil to the rear on discharge, the gunners would have to stand clear on firing, abandoning the protection of the shield, and, moreover, the loss of time entailed by running up and relaying the gun would render the fire slow. The requirement of steadiness of the carriage is met by allowing the gun itself to recoil on its carriage. Its motion is gradually checked by the hydraulic buffer (see below) and the gun is returned to the firing position by helical springs, or, in the French, Spanish and Portuguese equipments, by compressed air. The carriage is held from recoiling by a spade fixed to the point of the trail, which digs into the earth on discharge, and (usually) by brakes on the wheels. This is known as the gun-recoil system, and is now universally adopted. Field guns constructed on this principle are styled Q.F., or quick-firing, guns.

Steadiness of Carriage.—In the gun-recoil equipment the constructional difficulty lies not in preventing the carriage from recoiling but in preventing the wheels from rising off the ground on the shock of discharge. The force of recoil of the gun, acting in the line of motion of the centre of gravity of the recoiling parts, tends to turn the carriage over backwards about the point of the trail, or, more correctly, about the centre 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 that due to the distance of its line of motion above the centre of the spade; the leverage with which the steadying force acts is that due to the horizontal distance of the centre of gravity of the gun and carriage from the centre of the spade. If the force of recoil be 6 ft.-tons, and if it be absorbed during a recoil of 3 ft., the average overturning force is 2 tons; since the weight of the gun in action may not greatly exceed 1 ton, the trail must be so long as to give a leverage of at least two to one in favour of the steadying force. It follows from the above 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 the gun as low as possible, either by reducing the height of the wheels, or by cranking the axletree downwards. This reduces the leverage of the overturning force. (d) Increasing the length of the trail. This increases the leverage of the steadying force.

It will be seen from Table A that the condition of steadiness is satisfied in the various Q.F. equipments by not very dissimilar combinations of the above factors.

The cradle is the portion of the carriage upon which the gun slides when it recoils. It also contains the buffer and running-up springs, which are fixed either above or below the gun. The latter method gives the stronger and simpler construction, and is favoured by all nations except Great Britain. By putting the buffer on top the gun can be set lower on the carriage, which is an advantage as regards steadiness. A top-buffer cradle is of ring section, surrounding the gun; the gun is formed with ribs or guides extending for its whole length, which, on recoil, slide in grooves in the cradle. The cradle is pivoted on horizontal trunnions to the intermediate carriage and carries the buffer and springs on top. This construction is shown in the illustration of the 18 pr. Q.F. gun (fig. 60, Plate III.).

In carriages having the buffer under the gun the cradle is a trough of steel plate, usually closed in at the top. It has guides formed on the upper edges fitted to take guide-blocks on the gun. The cradle contains the buffer-cylinder, which is fixed to a horn projecting downwards from the breech of the gun, and recoils with it; the piston-rod is fixed to the front of the cradle. The running-up springs are usually coiled round the buffer-cylinder, and, on recoil, are compressed between a shoulder on the front end of the cylinder and the rear plate of the cradle.

The cradle is mounted on a vertical pivot set in a saddle pivoted on horizontal trunnions between the sides of the trail (Krupp); or, as in the earlier Ehrhardt equipments, the vertical pivot is set in the axletree itself, which has then to turn when the gun is elevated or depressed. The Krupp cradle is shown in the drawing of the German gun.

The buffer consists of a steel cylinder nearly filled with oil or glycerine. In this cylinder works a piston with piston-rod attached to the carriage; the cylinder is attached to the gun. On recoil the piston is drawn from one end of the cylinder to the other, so that the liquid is forced to flow past the piston. The friction thus caused gradually absorbs the recoil of the gun and brings it gently to a standstill. As the gun recoils the centre of gravity of the gun and carriage shifts to the rear, reducing the stability. The buffer-resistance has to be gradually reduced proportionately to the reduced