field gun used in the Italian army. The principle is applied also in the British 3-y-in. mountain howitzer. No German guns used in the war had split trails. Perhaps the best-known split-trail weapon is the French 155-mm. G.P.F. gun used also by the Americans.
The spade is secured to a sole-plate and the latter to the under-side of the trail at the rear; the spade prevents rearward movement of the carriage while the sole-plate prevents the trail from sinking into the ground. Spades are generally broad and shallow, for a deep- pointed spade is likely to be damaged when bedding and may not give sufficient clearance for travelling. With axle-traverse equip- ments the spade must be more pointed, as the trail pivots about it, and it must also prevent lateral movement of the trail.
The spike is a special form of detachable spade, which requires to be driven through a slot in the trail; the point of the trail is in fact nailed to the ground. Spikes differ from spades mainly in that there is no limitation within reason to their depth; their removal facilitates travelling; they have to be driven in in taking recoil, whereas the ordinary spades bed themselves; they prevent forward movement during recuperation, and braking is unnecessary.
The axletree, which must be made to withstand the downward blow due to firing, may be either straight or cranked.
The straight axletree is usually a steel tube having the ends tapered to form arms to receive the wheels; straight axletrees must of necessity be used with axle-traverse equipments. The cranked axletree is favoured by the modern 3-point support carriage. It consists in a hollow steel forging, the extremities of which are cranked upwards, aad to which are attached tapered arms for the wheels; the central or low portion supports the front of the trail. Cranked axles generally have to be made heavier than straight axles. Their great advantage is that they enable the firing height to be lowered and so increase both the travelling and firing stability. Thus, with a given wheel-diameter, a cranked axle admits of a shorter and lighter trail, and stability is maintained with a more compact carriage.
During firing, the function of a brake gear is to prevent forward movement of the carriage during recuperation if spades be used; with spikes brakes are not necessary. All brakes must be provided with a form of quick release for use when a new line of fire is to be taken up, and also, in the case of cross-axle traverse, when the traversing gear is operated.
The typical intermediate carriage consists in two bracket sides suitably connected; it is pivoted near the front to the trail, slides over the trail at the rear under the control of the traversing gear, and has trunnion bearings forward at the top. Holding-down clips are normally provided at front and rear to prevent lifting on firing ; and stops to limit the traverse to the designed amount each way. With a split trail a different pattern is necessary owing to the splay- ing of the legs ; it is therefore pivoted to the centre of the axle, the upper portion is forked and provided with trunnion bearings, while the lower portion in the form of a stem passes through the axle tree and fits into a ball-joint in the cross-bar beneath.
The traversing gear is generally of the nut-and-screw type; in the main it consists in a screw with hand-wheel supported in- a bearing pivoted to the trail and engaging with a nut pivoted to the rear of the intermediate carriage. With split-trail carriages, the screw is supported by the intermediate carriage and engages with a nut pivoted to the left of the axle. With axle-traverse equipments, either the central portion of the axle is threaded to form the screw and the nut carried within the front of the trail or a separate screw and nut parallel to the axle are employed ; in the latter case, rollers support the front of the trail on the axle, and ease in traverse is ob- tained by the incorporation of ball-bearings.
Cradles may be distinguished as of the ring and trough types. The ring cradle is in the form of a short hollow cylinder in which the gun is supported and through which it recoils ; chambers are formed on top or below to accommodate the recoil system ; and the cradle is pivoted transversely to the intermediate carriage by means of trun- nions. This type is used principally with heavy howitzers and fixed mountings in which relatively short recoil-length is given. It is not a good design for long-recoil field guns because of its shortness, and relatively bulky construction. The trough cradle is shaped like a trough, trunnions being formed on either side to pivot it to the inter- mediate carriage; the recoil system is housed within the trough, while the gun is supported on slides above. The trough forms a suitable casing for the recoil system and is less bulky than the ring type and assists rearward extension; on the other hand, it tends to bend during firing and necessitates the carrying of the recoil system below the gun.
Elevation is given by means of a screw and arc, or arcs, which causes the cradle to pivot about the trunnions. The screw may be single or telescopic to give aggregate motion; its lower end passes through a nut supported by the intermediate carriage and operated through a bevel drive. Generally, with light equipments the gear is designed to embody the principle of the " independent line of sight." Any elevating gear should support the cradle either centrally or equally on either side so as to reduce torque and wear. The modern method is to employ either a central arc, or one on either side, op- erated through a worm-and-arc-pinion drive. Owing to the high elevation for which equipments are now designed, with light and medium natures an arrangement to facilitate quick loading is neces- sary to permit of the cradle being depressed by hand from a high
elevation to a convenient position for loading without disturbing the elevating gear or sight; when this is embodied, the arcs are sleeved on the trunnions, while the arrangement provides the means for locking them to or unlocking them from the cradle; with heavier equipments quick and slow motion gears are installed.
Shields are provided for light horse-drawn equipments, and give protection against shrapnel and rifle bullets only. Such protection is not considered necessary with heavier equipments; and the weight of a shield to give protection against shell-fire would be prohibitive. The typical shield is supported on the axle and trail, and is provided with hinged flaps at top and bottom to give addi- tional protection ; these flaps are folded up for travelling.
Heavy mobile equipments which cannot be travelled as entities are split into loads generally three the gun, the cradle, and the firing platform. For the gun a special travelling carriage is provided ; for the cradle and platform axle-arms are attached upon which wheels can be placed, and a light limber keyed to the load.
The latest pattern of 6o-pdr. carriage embodies most of the modern ideas; it is a typical 3-point-support carriage, and its main features are: a travelling load of 6 tons drawn by 8 heavy-draught horses or by tractor as an alternative; wheels of 5 ft. diameter with 6-in. tires, and extension felloes fitted to increase the tread for travelling over soft ground ; a 79-in. track to give a large upsetting angle and the necessary traverse ; a distribution of the load in the approximate proportion of 5 to 3 on the carriage and limber wheels respectively, obtained by transferring the gun to a travelling position on the trail ; a special limber connector to preserve the balance of the pole ; a cranked axle to give greater lateral and firing stability; brake gear cross-connected and operated from either side ; a box trail to permit of 35 f elevation; a wide trail for 4 of traverse each way; a long trail for stability; a shallow and broad spade; an intermediate car- riage with holding-down clips; nut-and-screw traversing gear with spur drive and ball-thrust bearings; an extended trough cradle, mounted on roller-bearings, to support the centre of gravity of the recoiling parts even at full recoil ; a single centrally placed elevating- arc operated by worm-and-arc-pinion gearing; hydro-pneumatic recuperator and controlled recoil; a recoil of 54 in. at the horizontal, reduced to 24 in. at 35 of elevation; replaceable bushed bearings for all working parts. (F. M. R.)
(III.) WEAR AND LIFE OF GUNS
Erosion is a general term applied to the effects produced by the action of the powder gas on the bore of a gun. These effects are of two kinds: (i) the gradual enlargement of the bore and smooth wearing-away of the surface by the action of the gas behind the projectile; this is termed " wear," and is independent of the sealing of the bore by the projectile; (2) the irregular eating-away of the surface of the bore in holes and gutters; this is called " scoring-," and is mainly due to imperfect sealing of the bore. These two actions may occur separately or con- jointly, and with cordite charges the differences are marked.
In the latest high-velocity B.L. guns, in which the sealing is nearly perfect, wear occurs with very slight or no trace of scoring until towards the end of the life of a gun. When a new gun is taken into use with cordite charges, wear begins at once; the lands near the breech end of the bore are worn down rapidly, while the grooves are also worn, but not to the same extent as the lands. The wear is always more with cordite than with gunpowder and is much more rapid with Mark I. cordite than with M.D. cordite. The rate of enlargement increases greatly with the weight of charge and pressure, and a heavy gun wears much more quickly than a light gun; for example a o.-2-in. can fire only about 600 rounds with full charges, whereas an i8-pdr. Q.F. gun can fire 10,000 or more rounds during its life. The " wear " per round is greater at first than subsequently. 1
1 The following figures as to the " life " of German land service ordnance during the war have been published (Schwarte, Militdrische Lehren des grossen Krieges) :
Average
In some cases
7'7-cm. field gun lO'5-cm. field howitzer . . io-5-cm. gun 13-o-cm. gun 15-o-cm. heavy land howitzer . ijyO-cm. gun. model 1916 Ehrhardt
15,000 10,000 10,000 3,500 15,000 2,200
20,000 13,000 15,000 6,000 20,000 3,000
The table makes no distinction between the 96 n.A. and the 16 patterns of field gun, nor between the various patterns of 10-5- cm. field howitzer and gun.
