CARRIAGES. Gun carriages are either station ary (as seacoast) or mobile (siege and field) ; as to mode of action, barbette, disappearing, tur ret or casemate, or motor carriages, or rapid fire mounts (recoil and non-recoil). The main function of a gun carriage is to support the gun during firing: secondly, means must be pro vided for pointing the glum. Mobile carriages must also possess the attributes of vehicles.
Modern seacoast carriages have a cast-iron base ring, solidly set in concrete, an iron or steel turntable or racer upon this, and the chassis assembled thereon. Generally a top carriage of some sort runs back on rails on the chassis, its motion in recoil controlled by a brake, generally hydraulic. Such a brake consists of a cylinder fastened to the top carriage, which contains a piston fastened to the chassis. Oil in the cylinder can pass the piston. as the cylinder moves to the rear, only through small orifices in the piston varying in size during recoil.
The first step in design is to construct a curve showing the velocity the gun would have if left free to recoil with no resistance but its own in ertia. A certain constant resistance is now fixed upon for the brake, and the resulting ve locity of retarded recoil is plotted. From this the opening in the piston. varying with the dis tance recoiled, to give the desired constant re sistance, is caleulatcd—resistance to flow through an orifice being known to vary inversely as the square of the velocity. Thus the constant pull transmitted to the chassis by the pi-ton rods, and hence the stresses borne by all parts, are deter mined and used to calculate their dimensions.
Prior to the adoption of the hydraulic brake various means of checking recoil were used. Up to about 1S50 the friction of wooden wheels and axles was used with a rope for final stop. About Ism naval carriages were made of iron and had no rear wheel, but a shoe (for friction). The front wheels helped in returning the gun to clearance. Sir XV. Siemens introduced holes in the piston head. So far. the orifice being con stant, the pressure varied, running up to high. values, then decreasing. To obtain constant pressure (to do the same work with less maxi mum strain) the orifices came to be varied, by taper rods passing through them, by rotating disks partially closing them, by by-pass pipes with spring valves and by other methods. among which are the 'throttling bars' used in the United States carriages. Here the piston has notches in its cireumference partly closed by bars of vary ing depth screwed to the cylinder. As the pis ton travels in the cylinder the orifices change with the depth of the bars.
Top carriages now recoil on rollers to elimi nate as much as possible the uncertain effect of sliding friction.
Return to firing position is accompli-hed by gravity, springs, or pneumatic or air power.
For giving elevation, guns are generally ro tated about their trunnions (at or near the cen tre of gravity) by a screw, rack-and-pinion, or similar device at the breech. In rapid-fire mounts the principle is the same although ele vation is generally given to a cradle or slide in which the gun's axis is fixed. In field guns, to economize space, the motion is doubled by having one screw work inside of another.
For traversing, the early carriages were pried bodily sidewise by levers; then the ehassis was pivoted and rotated by ropes and pulleys or later by gearing of one kind or another. Still later the chassis was fixed upon a turntable rotated battery. un land heavy carriages had 1-beam chassis, and top carriages like the above. Prie s tion of top carriage on cha--is rails as well as the inclination checked recoil. Eccentric axles allowed recoil on friction and return on rollers. As the power of guns increased, recoil had to be further controlled. Between 1ST() and 1880 various frictional devices were used. tine or more bands of iron fastened to chassis were gripped by plates on top carriages pressed tightly on them.
The earliest hydraulic cylinders (about 1876) had pistons either pulled out or pushed in by re coil, the oil flowing around the piston in the by a pinion working in a circular rack in the foundation and operated by gearing.
In barbette carriages the gun is generally mounted on trunnions directly in the top car riage, which slides to the rear (restrained by a hydraulic brake) on the chassis •rails. These incline upward slightly, to make the gun run into battery again. This type is mostly used in land forts for heavy guns. See illustration of S-inch Breech-loading Rifle on Plate of COAST ARTIL LERY.
The essential feature of carriages for use in casements or turrets is that the opening in the wall or armor necessary for them to fire through with the desired horizontal and vertical angles of train shall be as small as possible. The former was provided for up to very recent time by placing the pintle or point of revolution at the front end of the carriage, or even in the wall, and connected with the carriage by a metal rod called the tongue. In turrets, land or naval, it is customary to ro tate the turret, gun and all, and thus eliminate all horizontal movement of the gun with respect to the opening or To reduce the em brasure vertically the gun is lifted or lowered by two hydraulic presses, or other power, the muzzle being stationary. The necessity for such carriages for land service has decreased with the abandonment of masonry forts, but exists in naval mounts, whether in turrets or not, and in land turrets (as yet few in use).