Page:EB1922 - Volume 31.djvu/1239

The diameter of the piston-rod must withstand, with a safety margin, the tensile and compressive stresses to which it is subjected. The rod must be capable of being effectively packed and easily handled in manufacture. With light equipments the diameter as calculated is usually too small for effective packing, for which about I in. is the minimum diameter; with heavier equipments the cal- culated diameter increases and is used in the design.

The diameter of the cylinder is determined, within limits, by the design of the carriage. The larger the cylinder, the less the pressure required to act on the face of the piston and the sooner will the con- trol of recuperation become operative ; but a large cylinder has to be

Recoil FIG. 3. Resistance-space diagram. Controlled recoil.

made proportionately stronger, and therefore heavier to withstand the bursting stress to which it is subjected. The ratio of the diam- eters of cylinder and rod usually adopted is about 2 to I. The necessary thickness of the cylinder is then determined from the pressure it must stand.

The ratio of the areas of the piston and rod is 4 to I ; consequently the control to recuperation becomes operative at J of the total run- up. This must be in accordance with the stability requirements, and the sooner it becomes operative the greater stability-margin will be preserved. With some equipments the ratio is less, so that the control becomes operative at a slightly later stage; but the ratio should be such as to give a stability-margin.

Flow-Space. The area of flow-space can be calculated from a knowledge of the resistance-space curve, the velocity-space curve during recoil, and the piston area; the calculated area requires to be corrected experimentally.

Typical velocity-space and flow-space curves are shown in fig. 5.

FIG. 4. Resistance-space diagram.

Recoil Equalized recoil.

The experimental correction of the flow-space is carried out in two ways: (i.j by taking practically a velocity-space curve for the recoil and altering the apertures until this curve corresponds with that ob- tained theoretically ; (ii.) by taking a pressure-space curve by means of an instrument known as the buffer-gauge. To obtain good pressure curves the maximum flow-space should correspond with the maxi- mum velocity of recoil.

'/ The modern methods of varying the flow-space are as follows: / (a) A circular piston and longitudinal grooves, usually of constant width and varying depth, cut internally in the cylinder. The piston

tends to become fluted with wear, for which no ready means of com- pensation exist. Some pre-war field carriages use this system.

(ft) Rectangular slots or ports in the piston, sliding over one or two longitudinal valve-keys, metal bars of such contour as to leave the required flow-space during recoil fixed to the cylinder walls. This system is simple, but, owing to the difficulties of accurately cutting the valve-keys, it is suitable for only relatively short recoils.

(c) A hollow piston-rod with circular piston sliding over a tapered rod, holes being cut in the piston-rod near the piston for the passage of the liquid. This system is more suitable for short than long re- coils owing to the difficulty in accurately tapering a long rod ; with it control to recuperation becomes operative from the first instant. It is used with the systems of equalized and graduated resistance.

(d) A rotary piston, with two or more ports cut in it, fitting loosely on the rod, rotation being governed by ribs fitting into grooves in the cylinder, and a valve with corresponding ports, against which the piston rotates, fixed to the piston-rod. This method is used with modern field carriages with combined systems of graduated and con- trolled resistance to recoil; it is the method of variation which best lends itself to controlled recoil. It has advantages, in that a slight turn of the piston-rod regulates the flow-space, and that wear of the piston or valve can be compensated for in a smaller manner.

(e) A modification of (d), in which grooves form the flow-space, is found in the latest German equipments. This consists in a hollow piston and rod with an extension tube carrying intermediate and slop rings; corresponding ports are cut radially in the piston, inter- mediate ring, and extension tube, while ribs on the intermediate and stop rings engage respectively with straight and rifled grooves in the cylinder. When elevating, the piston and tube are rotated to reduce the flow-space, eventually cutting off the intermediate ports; the variation in flow-space is obtained by the rifled grooves passing gradually off the ports.

Recoil FIG. 5. Typical velocity-space and flow-space curves.

Controlled recoil necessitates cut-off gear to rotate the piston-rod automatically with the elevation. This gear consists in link or cam- gearing placed between the cradle and the intermediate carriage, and rotates the rod through a bevel drive as the cradle is elevated. This rotation causes an overlap of the piston and valve ports as the piston is held stationary by the grooves, thus reducing the flow- space, so that the cut-off may take place earlier in the recoil stroke.

Control of Recuperation. As the buffer flow-space is greatest at the termination of recuperation, some check is required to prevent a nietal-to-metal blow. To produce this final control, the main de- vices in use are :

(a) The control plunger and chamber. The plunger may be an extension of the piston-rod and the chamber a recess in the cylinder into^ which it fits; or the plunger may be fixed to the cylinder and the chamber a recess in the piston-rod. The flow-space is formed by cutting a tapered flat on the plunger, and in modern designs the construction provides for a further adjustable exit for the liquid so that the control may be regulated according as the elevation is low, medium or high. The recess becomes filled with liquid during recoil, and, before the gun can run up and the plunger return to the recess, the liquid must be forced through a small flow-space by the entry of the plunger. With this device recuperation takes place in three stages first, unresisted except for friction, secondly, resisted by the buffer and friction; thirdly, resisted by the buffer, control plunger and friction.

(ft) Separate recoil and running-up valves as used with the rotary piston buffer. The recoil and running-up valves which are on opposite sides of the piston are permitted slight longitudinal move- ment but prevented from rotating on the rod; the running-up valve is pressed against the piston during recuperation and a clearance ob- tained between the piston and recoil valve which renders the latter inoperative. The rotation of the piston decreases the flow-space