THE OTO MELARA 76/62 COMPACT GUN MOUNT COMMANDER M. MARTIN, C.ENG., F.I.MAR.E., R.N. (RET)

Transcription

1 THE OTO MELARA 76/62 COMPACT GUN MOUNT COMMANDER M. MARTIN, C.ENG., F.I.MAR.E., R.N. (RET) Introduction The Royal Navy has recently decided to purchase the OTO Melara 76/62 (3" calibre) Compact Gun Mount for installation in the new Hong Kong Defence Force ships and consideration is being given to fitting the same gun later in certain ships of the Royal Navy. British Naval policy over the last twenty years or so has rejected the gun in favour of the missile for air defence; and thus no modern AA guns have been seen in the Royal Navy. In other countries, however, the value of the small/medium calibre gun has always been accepted. Evidence of this can be seen in the adoption of the OTO 76/62 mount by six other NATO navies as well as by another twenty navies throughout the western world. The employment of the gun for close range AA defence and for surface and limited shore bombardment provides a cost effective and versatile solution particularly in ships of limited tonnage and light naval craft. This is especially true now that such guns are controlled by short reaction time, rapid settling, computerized fire-control systems and highly accurate and responsive servo systems. FIG. 1-OTO 76/62 INSTALLED IN ITALIANAVY DDG 'ARDITO'

2 Mounting Characteristics The mount has the following salient characteristics: (a) It is of compact configuration and of light weight in that, complete with 80 rounds of ready-use ammunition, it weighs only 8.35 tons. (6) It has a maximum rate of fire of 85 rounds per minute and the large readyuse stowage and the water-cooled barrel allow frequent bursts at a high rate of fire. (c) The mount is completely automatic and remotely operated and has a very short reaction time of 3 seconds. (d) The mount is totally enclosed in an effectively sealed gun house. (e) It has an advanced design of training and elevating servo system. (f) The hydraulically-operated ammunition feed system makes extensive use of proximity switches and conservatively stressed components and has a high reliability. (g) VT, DA, and AP ammunition are available. The light weight is obtained by the extensive use of aluminium alloys which, when welded up, form a very stiff and light structure, and by a GRP gun house shield. A local-control console, mounted in or adjacent to the gun bay, allows the Captain of Turret to bring the mount into operation and also houses the controls and monitoring devices for operation and maintenance. It also contains the main transformer, the SCRs for the servo systems and the electronic logic circuits. FIG. 1 shows the 76/62 installed in the Italian Navy DDG Audito, and FIG. 2 shows a cutaway view of the mount. The technical data is set out in TABLE I. TABLE I-Technical data of the OTO Melara 76/62 gun mount Mounting: Total weight without ammunition tons Rate of fire roundslmin Ready to fire rounds (optionally 115) Sweep radius of gun barrel mm Sweep radius of turret mm Diameter of deck opening for gun installation mm Recoiling Mass: Barrel length calibre = 4700 mm Muzzle velocity m/s Maximum recoil length mm Recoil force... 7 tons Training :.. Arc unlimited Maximum speed /s Maximum acceleration "/s2 Elevation: Arc " +85" Maximum speed... 35'1s Maximum acceleration... 72"/s2 Anznzunition: Cartridge weight... Cartridge length kg mm Circuits : Power... three-phase 440 V-60 Hz Signals... biphase 115 V-60 Hz Synchro V-400Hz

3 FIG. 2-CUT-AWAY DRAWING OF OTO 76/62 Key: 1. Muzzle brake 7. Elevation servo system 2. Bore evacuator 8. Screw feeder hoist 3. Cradle 9. Two-row evolving feed magazine 4. Left radial arm 10. Access door 5. Shield 11. Slip ring 6. Training servo system Ammunition Feed Arrangements The ammunition feed system is hydraulically operated. A single electricallydriven pump supplies pressure to operate a hydraulic motor which drives the revolving feed magazine and the screw feeder. It also provides pressure to the actuators which drive the rocking arms and loader drum. A nitrogen filled accumulator serves to eliminate pressure fluctuations during operation. The energy required for loading, breech operation, and spent cartridge ejection is obtained from gun recoil and run out. Ammunition is manually loaded into the double ring revolving feed magazine, the loading numbers taking rounds either from the top of the fixed structure hoists, where a deep magazine is en~ployed, or from ready use stowages around the gun bay. Topping-up of the revolving feed magazine may be carried out during firing.

4 FIG. 3-DOUBLE-RING MAGAZINE AND SCREW FEEDER FIG. 4-CUT-AWAY DRAWING SHOWING METHOD OF TRANSFER OF ROUNDS FROM FIXED TO OSCILLATING MASS The revolving feed magazine rotates in a clockwise direction (viewed from above) and, as it passes a transfer position and if spaces are available, rounds are transferred from the outer ring to the inner one and from the inner one to the screw feeder. FIG. 3 shows the double ring magazine and the screw feeder. The screw feeder consists of nine vertical semi-tubular guides arranged in a circle. The guides are shaped like half tubes and have a slot in the back of the guide. A lifting lug, driven by an endless chain hoist and synchronized with the screw feeder, slides up the slot as the assembly rotates. The lug bears on the base of a round so that. in describing a half helix, a round is lifted ;p and around to a position known as the 'Round waiting to be taken station' located just inside the left trunnion support and immediately below the left trunnion. Twin rocking arms, pivoted on the trunnion axis and located between the left trunnion support and the oscillating mass, are used to transfer the rounds from the fixed to the oscillating mass (see FIG. 4). The rocking arms move in synchronism, but out of phase with one another, so that when one arm is carrying a round from the waiting station alongside the top of the screw feeder to the oscillating mass, the other is descending empty from the oscillating mass to the waiting station. The round is held in the rocking arm by two clamps, one at the base of the cartridge and the other at the nose of the shell. A hydraulic actuator operates the clamps and another the rocking arm. Naturally the angular excursion of the rocking arm varies with the elevation of the oscillating mass (see FIG. 5). The loader drum, which receives the rounds from the rocking arms, rotates step by step so that it carries the rounds from the left side of the cradle to the loading tray. The loader drum is also hydraulically operated and moves in synchronism with the two rocking arms. The last position of the loader drum, above the loading tray is fitted with a VT fuse paralysing device. The loading tray is arranged inside the cradle and is linked to both the cradle and the recoiling mass by an articulated parallelogram linkage system. This linkage gives the loading tray an up-and-down motion as the gun recoils and runs out. When the gun is at 'Hooks', that is when it is held back by two release J.N.E., Vol.26,No. 1

5 FIG. 5-DIAGRAM SHOWING MOVEMENT OF ROCKING ARMS hooks in the fully recoiled position, the tray is in the upper position. When the firing push is operated the hooks release the gun. As the gun runs out the tray, together with the round on it, drops so that it comes into alignment with the barrel. At this juncture the rammer is tripped and the round is loaded. The cartridge lip seats the extractors, the breech closes and the round is fired. The ranliner has two lugs: the upper lug loads the new round, whilst the lower lug ejects the spent cartridge from the movii~g ejector tube, which is located under the loading tray and inoves with it. When the gun recoils the loading tray moves up- wards so that the moving ejector tube is aligned with the barrel ready to receive the spent cartridge. The fixed ejector tcbe is fitted inside the cradle and under the breech ring. As the next round is rammed, with the tray and hence the moving ejector tube in the lower position, the lower rammer lug throws the spent cartridge from the moving to the fixed ejector tube and on, out of the gunhouse. It passes through a guide in the lnantlet plate beneath the barrel and is then deflected downwards so that it falls onto the weather deck. Elevation and Train Power Drives In order to obtain the desired precision, bandwidth, and low resonant peak, a special transfer function was studied and adopted. This transfer function ensures very small errors during the typical motions experienced in stabilization and aiming. At the same time it has a good stability margin and also minimizes the consequences of mechanical resonance over the frequency range of 10 to 30 hertz. The special transfer function is obtained by using a system of three control loops: a current loop, a tachometric loop, and a position loop. The system BASE COVER l PERMANENT MAGNETS l POWER 1 SHAFT MOTOR CASE COVER FIG. 6-EXPLODED VIEW OF ELECTRIC SERVO SYSTEM DRIVING MOTOR

6 eniploys a high tachometric gain to provide imn~ediate response to any torque disturbance. The switching system between coarse and fine channels is designed to reduce the resetting time. The tachogenerator is directly coupled to the servo motor shaft to minimize the effects of backlash and the synchro boxes are directly connected to the load so as to improve precision of static transmission. Semiconductor technology is employed so as to maximize life and reliability of components and to minimize weights and dimensions. Silicon-controlled rectifiers are used to feed the motors, and linear integrated circuits, micrologics, transistors, and metal oxide on silicon are used in the amplifiers. Noteworthy are the servo motors themselves (FIG. 6). These direct current motors have printed circuit armatures of a disc type and permanent magnets. The motors have the advantages that they are sniall and of light weight; they have a high specific power and very low inertia; they have the capacity to supply a torque up to five times the nominal, depending upon the duty cycle; and they require very little maintenance, needing only a periodic check of the brushes. The type of power system employed-that is to say d.c. motors supplied by SCRs-normally suffers from the disadvantage of network distortion. In the case of the OTO Melara design, however, the distortion has been minimized by the use of a special six-phase bridge. The servo system employs the modularity concept and has been studied for ease of maintenance and access. In this context all servo motors are identical. One is used for elevation and two, mechanically coupled together, drive the train system. In the case of amplifiers and SCRs the design is aimed at reducing tuning and adjustments. To this end fixed value resistors, mounted exterior to the amplifiers, are employed. The number of adjustments required are few and normally the tuning carried out prior to acceptance does not need to be repeated. Special circuits prevent power being supplied to the servo motors unless locking bolts are removed, and other constraints ensure full safety. Other special devices prevent switching on of the servo systems should connectors, modules, or other parts not be correctly inserted. Special attention has been paid to the design of the 'Obstacle Contouring Device' since its correct functioning is vital to safety of personnel, the gun, and ship's structure. In this case the circuit is duplicated by the addition of an electromechanical safety circuit providing three different braking systems to back up the normal electronic control circuit. Amongst the most interesting components are the proxin~ity switches which are used in place of microswitches. The advantages obtained are threefold. No adjustments are required, they are highly reliable, and they have a very long life-a characteristic of semiconductors. The proximity switches are used to generate the signals required for the loading cycle, for safety interlocks, and to indicate the status of the gun and mount. A special circuit is used in conjunction with the proximity switches which guarantees a clean threshold and constant rating over a wide temperature range. Additionally, an advantage of this type of proximity switch is its ability to supply signals to micrologic circuits without the use of matching devices thus improving the system immunity to noise. The circuitry for loading and firing and for 'Status' signalling is hybrid. Relays and micrologics are both used since this solution gives the highest reliability. Other Electrical Equipment The safety firing gear is of electromechanical design and enlploys a number of flat cams mounted on the synchro boxes. Low temperature conditions are dealt with by a series of heating elements on the training base and adjacent to the Inantlet plate. These are thermostatically controlled and are fed by the ship's 440 volt network.

7 FIG. 7-MAIN DISTRIBUTION Box Apart from the electrical items mounted on the oscillating mass and the training base and shank, there is a main distribution box, or console, located on the fixed structure and normally found in or adjacent to the gunbay (FIG. 7). This box contains the main transformer, all the electronic modules for both the servo systems, and the logic circuits and other auxiliary devices. The front panel carries the main breaker knobs, all push buttons and toggle switches for local control, and indicator lamps and fuses. The console has been designed to permit easy operation of all maintenance manoeuvres or ~re~aration for remote control.?h; mechanical structure is watertight, being able to withstand a pressure equivalent to 3 metres of water. Electrical connection between the fixed structure and the mount is either by slip ring containing 31 power rings and 132 signal rings, or by flexible cables where a limitation in training is acceptable. Wherever possible connections are made by connectors so that stripping, replacement, and checks may be easily and quickly carried out. The majority of cables are screened. General protection against electromagnetic interference is obtained by the use of conductive paint inside the gun shield and by earthing the gun barrel. Fixed Structure Hoist. The mount can be provided with fixed structure hoists where deep magazines are employed. These consist of electrically-driven totally-enclosed endlesschain hoists for carrying the fixed ammunition from the magazine to the gun bay. Loading numbers are then used to transfer the rounds from the hoists to the revolving feed magazine on the mount shank. The hoists can also be employed for striking down ammunition. Integrated Local Control System The mount can be supplied with an integrated local-control system for surface fire. This consists of a closed-circuit TV camera, mounted in its own elevation and train drives in a cupola on the top of the gun shield. A local-control operator is seated at the control console, situated below decks and near to the gun bay. The console carries the TV monitor with crosswires for aiming, and the whole unit is self-contained, having its own joystick, stabilization system, and predictor and spotting correction devices. Range estimation must be made by the operator unless radar range can be provided from an exterior source.