Page:EB1922 - Volume 31.djvu/1240

between it and the running-up valve and provides the necessary control. Recuperation takes place in two stages: first, unresisted except by friction, and secondly, resisted by the running-up valve of the buffer and friction. This method was largely used in America.

(c) A rod working inside a hollow piston-rod used with the tapered rod buffer. The end of the tapered rod is fitted with a slide- valve which permits liquid to pass over it and fill the piston-rod dur- ing recoil ; on recuperation the valve closes, and before the tapered rod can enter the piston-rod it must displace the liquid over the valve and through a groove, or grooves, of decreasing depth cut internally in the piston-rod. As the piston-rod is designed to fill during recoil, the control becomes operative from the commence- ment of recuperation.

(d) A modification of (c) is the principal German method. The piston-rod is designed to fill during recoil, the liquid having to pass out through a shallowing groove cut in a cylindrical control rod. A spring-loaded valve, which permits the piston-rod to fill during recoil, has its periphery shaped further to regulate control.

General Arrangement. With field equipments, the buffer and recuperator cylinders are bored in a steel ingot which is supported in a trough cradle and is lugged to and recoils with the gun; the piston-rods are secured to the cradle cap (fig. 6). The main features secured to the cradle. On recoil, the system works telescopically, the total deflection, equal to the recoil, being divided between the two banks; it has the advantage that the deflection per column is reduced, and stiffer springs of shorter free length can be used.

An air recuperator consists in a quantity of air under compression in a container and in contact, either directly or through liquid, with a movable piston. On recoil relative movement is given to the piston and container and the air further compressed. The air subsequently reacts on the piston and recuperates the gun. The difficulty of seal- ing compressed air accounts principally for the use of liquid with field-carriage recuperators, for the liquid seals the air and is itself sealed by packings. Thus air-recuperators may be classed as pneumatic, in use with field carriages ; and hydro-pneumatic, in use with fixed mountings. Hydro-pneumatic recuperators may be further subdivided according as the liquid is in direct contact with the air, or is separated from it by a floating piston.

With hydro-pneumatic recuperators one cylinder contains liquid which acts on the face of a packed piston connected by its piston- rod to the cap of the cradle, and communicates through a throttling valve, port and pipe with the other cylinder containing liquid and compressed air; the throttling valve permits a free passage for the liquid during recoil, but closes to control the rate of recuperation at

(a) Tail Rod (Leakage); (b) Cut-off Gear Segments; (c) Buffer-Cylinder; (d) Cut-off Valve ; (e) Buffer-Piston; (f) Control- Plunger; (g) Air-Filling Plug; (h) Air Valve; (I) Dust Cover; (j) Recuperator Cylinder; (k) Air Reservoir; (I) Floating Piston.

FIG. 6. General arrangement of buffer and recuperator.

of this type are a rotary piston for variation of flow-space, a control chamber with an adjustable liquid exit to provide recuperative con- trol according to the elevation, a tank for reserve oil supply, a cut-off gear for controlled recoil, and a valve to release any air that may be sucked into the cylinder during recoil. The cylinder at the point where the piston-rod enters is closed by a stuffing-box, soft packing, and gland.

With fixed mountings the buffer cylinder is secured to the under- side of a ring cradle, the piston-rod being lugged to the gun. The features are a valve-key, a control-plunger protruding inwards from the closed end of the cylinder and working inside a hollow in the piston-rod, and a tank for oil supply. The open end is closed by a self-tightening ring of L-section, stuffing-box, packing and gland.

Recuperation. Recuperation may be by means of steel springs or compressed air. Whichever the method, the recuperator must possess sufficient initial pressure to return the gun to, and hold it in, the firing position at the greatest elevation.

The life of springs is limited ; according to the calibre and weight of gun with which they are used so their weight increases and they suffer from fatigue. In single columns they are unsuitable for long- recoil guns, due to the large deflection necessary. The springs used are cylindrical spiral springs of round or rectangular-section wire; wire of flat rectangular section permits of greater deflection before the spring is crushed metal to metal and hence is that usually employed; but circular-section wire gives a spring of minimum weight. To obtain the best results, the length of initial compression should be equal to the deflection producea by recoil.

Spring design consists in determining, consistent with the limita- tions of the carriage, the section of the wire, the mean radius of the coils, the number of coils, the free length, and the initially compressed length of the spring; this can be done from known formulae, and as a spring when compressed is subjected almost to pure torsion the deflection is proportional to the load producing it.

Spring systems may be either single or telescopic columns; in Great Britain the former arrangement is used with short-recoil and the latter with long-recoil guns.

A typical single column is mounted on a compressor rod with end plate secured to the gun, and is carried in a spring-box secured to the cradle; it is subdivided into springs, placed end to end and separated from one another by parting plates. On recoil, the rod is withdrawn from the spring-box, between which and the end plate the springs are compressed; subsequently the springs extend and recuperate the gun.

In the telescopic system, the springs are divided into inner and outer banks and arranged to surround the buffer-cylinder which re- coils with the gun; they are subdivided for convenience. The two banks are separated by a concentric tube, flanged to connect with the inner bank at the rear and with the outer bank at the front; the buffer-cylinder is flanged to bear against the inner bank at the front, whilst the outer bank bears at the rear against the casing

the commencement of run-up ; the outlet of the pipe is designed to be below the liquid level at all elevations and depressions to prevent air from passing into the lower cylinder.

It is necessary that the correct quantity of liquid be in the system, so that the designed ratio of compression and terminal pressure of the air may not be disturbed. The initial air-pressure varies from 500 to 700 Ib. per sq. in. according to the equipment.

A disadvantage with those systems in which the air is in direct contact with the liquid is aeration (i.e. frothing or churning). Thus in a later type the upper cylinder is fitted with a packed floating piston to separate the air from the liquid to prevent aeration. The floating piston has a tail-rod projecting into the cradle-cap, which has the effect of increasing the pressure on the oil in front above the air-pressure in rear so that there is no tendency for air to leak past the piston; whilst the position of the tail-rod visible from outside acts as a simple tell-tale to indicate correct filling, and consequently any leakage of liquid. Leakage may be replaced by a screw-actuated force-pump until the tail-rod is correctly positioned.

The working length of the recuperator is the length of recoil, and the total initial pressure required is determined as previously indicated. The diameter of the piston-rod is obtainable from the maximum stress it has to withstand, but must not be less than I in. for convenience of packing.

The ratio of compression of the air i.e. the initially compressed volume divided by the finally compressed volumes-should be low, to reduce the terminal pressure and heating effect ; little is gained by exceeding 2, and it is best taken between 1-5 and 2.

The larger the area of piston head, the less the initial pressure required, and it is desirable to keep this low for convenience in sealing and charging ; but, relatively to the air-cylinder, the piston area must be small to give a low ratio of compression. Consideration of piston packings leads to the decision that they must be capable of withstanding a steady pressure up to the maximum initial pressure and a momentary pressure as high as the maximum terminal pressure.

From a knowledge of the ratio of compression, the piston area, and the length of recoil, an estimate of the volume of the initially compressed air can be arrived at; this_ decides relatively the sec- tional area of the air-cylinder, and the initial pressure when a floating piston with tail-rod is used.

The whole design, as regards bulk, is limited by the constructive arrangement of the cradle; but the desiderata are a low initial pressure and a low rate of compression, both of which entail bulky construction, more particularly of the air-cylinder. With field equipments the initial pressure is generally higher than with fixed mountings owing to the greater limitation imposed as regards bulk.

General Remarks. The advantages of air recuperators when compared with springs are, first, that an increase in power can be readily obtained without increasing materially the weight of the equipment; secondly, the recuperator in addition to the buffer acts to control run-up; thirdly, the troubles consequent on springs break-