GB2115849A - Compliant coupling - Google Patents

Abstract

A compliant coupling for insertion between a towing cable and a towed sonar array to isolate the array from mechanically induced noise comprises an elastic core (2) encompassing the cable wires (1) helically wound to accommodate stretching of the elastic core with at least one high tensile strength wire (4-6) wound in a contra-helical manner around the elastic core and connected between the cable and the towed array. The coupling provides a low modulus of elasticity under normal working loads but at high applied loads provides a high modulus of elasticity. <IMAGE>

Description

SPECIFICATION Compliant coupling The invention relates to couplings used in towing and in particular though not exclusively to couplings for attaching apparatus to a towing ship to enable observations or measurements to be made remote from the ship.

One example of remote measuring apparatus is a sonar array which may be towed behind a surface ship or a submarine. Towed sonar arrays should be operated to minimise the effects of noise generated within the towing vessel and also noise induced by virtue of the inertia of the array. A linear sonar array comprises a number of spaced hydrophones encapsulated within a cylindrical sheath. The array sheath also contains electrical leads to the hydrophones and these leads are connected to the towing vessel via a cable connected to the array. Thus by towing the array behind a vessel noise produced within the vessel has small effect upon the array measurements. Surge fluctuations in speed of the towing vessel relative to the sonar array are a principal source of induced noise contributing to a reduction in the efficiency of the sonar.

The requirement to reduce surge induced noise would be met by providing an extensible compliant coupling device having a low elastic modulus (or stiffness) for the initial part of its extension and a high modulus towards the end of its travel. Thus in normal operating conditions speed surges would be countered by low modulus restoring forces. At present a coupling device which is used between the towing cable and the array includes two sets of ropes within the device. One set is very elastic and is generally in operation while the other set is stiff and is long enough to be slack when the device is unloaded. At higher speeds as the drag on the array increases the stiffer set of ropes becomes operative to provide the higher strength required.The greater resolution currently required of sonar arrays places demands for towed arrays which are much longer and consequently much heavier than arrays in current use. This places more stringent requirements on the compliant coupling used. Typically the coupling may need to operate at a breaking load of about 60 tons, working usefully at about 10-12 tons with spurts up to 20 tons.

The object of the present invention is to provide an improved compliant coupling.

The invention consists of a compliant coupling comprising a cylindrical elastic centre core and at least one load-carrying wire helically circumscribing the centre core. The compliant coupling may be provided with suitable terminations and connectable in appropriate incremental lengths (typically about 100 feet) between a tow cable and a sonar array in order to match the particular array require ments. Alternatively the coupling may be inte grally formed between two portions of cable.

By making the elastic core of low elastic modulus material such as natural rubber the compliant coupling exhibits low stiffness for the initial part of its extension as the centre core deforms to allow the angle between the helical turns of wire to increase as the radius of the coupling decreases. Advantageously there may be provided a plurality of load carrying wires of metal or plastics material which are wound in contra-helical layers around the centre core. The wires may be coated or sheathed with a low friction material to facilitate movement of the wires and reduce the stiffness of the coupling. The wires may also be wound on the centre core leaving spaces between consecutive turns to further reduce the stiffness of the coupling.

A signal wire or core may be located within the centre core of the coupling in helices of sufficiently short lay to avoid large angular deflection of the signal wire helices as the coupling expands and contracts. Alternatively a load carrying wire may comprise a central signal wire or core. The signal wire or core may be an electrical wire or an optical fibre.

Thus by preventing large deflection move ments of the signal wires the effects of non elastic strain with consequent fatigue and fail ure are reduced. Advantageously the load carrying wires may be surrounded by an outer elastic sheath.

The invention will now be described by way of example only with reference to the follow ing figures of which: Figure 1 is a cut-away view through an unloaded compliant coupling according to the invention; Figure 2 shows schematically the effect of the coupling; and Figure 3 illustrates the variation of exten sion of the coupling with increasing load.

The main components of the compliant de vice for isolating vibrations between a towing ship and a towed sonar array are shown in Fig. 1. Through the centre of the coupling a curly-cord 1 of electrical wires (conductors and signal wires) is wound up as a helix of sufficiently short lay to protect them from non-elastic strain during extension and con traction of the coupling. The curly-cord is located within a thick-walled elastic rubber tube 2. Wound around the tube 2 are high tensile strength wires wound in contra-helical manner to form four layers 3-6. An outer elastic sheath 7 is provided around the layers of wires. The device as shown is made up into incremental lengths, typically about 100 ft.

Suitable terminations are provided at each end of the coupling so that several coupling lengths can be joined together to match the isolation requirements for a particular towed array. Fig. 2 illustrates the effect of loading on the compliant coupling. When unloaded, the elastic tube 2 has a diameter D, and the wires 8 forming a layer 9 around the tube make an angle a1 with a plane drawn through the axis of the coupling. The axial length of each turn of wire is L. On loading the coupling, the elastic tube 2 and the elastic outer sheath stretch with a consequent diminution of diameter of the tube to D2. When loaded the axial length of each turn of wire 10 in the sheath 11 is L2 involving an angle a2 to an axial plane.

In order to improve the low elastic modulus in the useful working range of the coupling the wires can be coated or sheathed with a low friction material. In addition it is possible to wind the wires to leave gaps so as to minimise any stiffness in the coupling due to the wires themselves.

Fig. 3 illustrates the variation of extension of the coupling with applied load. The length of the coupling and the elasticity of the materials are selected such that in normal operating conditions of the towed array the working region of the load/extension graph is indicated by 12, where the modulus of elasticity is low. When the load on the towes array increases to the point 13, the extension of the coupling is such that the layers of helically wound wires resist further extension. The load /extension characteristic can be altered by an appropriate choice of metal or plastic material for the helical wires.

In an alternative arrangement to that shown in Fig. 1 signal and/or power cores, eg electrical conductors or optical fibres, may be formed as part of the wires in a helical strength layer towards the outside of the assembly. The signal and/or power cores would form the centre of a specially manufactured.

high strength wire. Thus the high strength material would form a protective sheath which would be used in addition to, or in place of, conventional insulation.

Further modififications of the invention will be apparent to those skilled in the art.

Claims (10)

1. A compliant coupling comprising a cylindrical elastic centre core and at least one load-carrying wire helically circumscribing the centre corn.

2. A compliant coupling according to claim 1 wherein there are provided a plurality of load-carrying wires of metal or plastics material which are wound in contra-helical layers around the centre core.

3. A compliant coupling according to any one preceding claim wherein the core is made of a low elastic modulus material such that the compliant coupling exhibits low stiffness for the initial part of its extension as the centre core deforms to allow the angle between the helical turns of wire to increase as the radius of the coupling decreases.

4. A compliant coupling according to any one preceding claim wherein the load-carrying wires are surrounded by an outer elastic sheath.

5. A compliant coupling according any one preceding claim wherein each wire is coated or sheathed with a low friction material to facilitate movement of the wire and reduce the stiffness of the coupling.

6. A compliant coupling according to any one preceding claim wherein the wire is wound on the centre core leaving spaces between consecutive turns to reduce the stiffness of the coupling.

7. A compliant coupling according to any one preceding claim and further including at least one signal wire or core located within the centre core of the coupling in a helix of sufficiently short lay to avoid large angular deflection of the signal wire helix as the coupling expands and contracts.

8. A compliant coupling according to any one of claims 1 to 6 wherein at least one of the load carrying wires comprises a central signal wire or core with an outer sheath of high strength material.

9. A compliant coupling according to claim 7 to 8 wherein the signal wire or core is an electrical wire or an optical fibre.

10. A compliant coupling substantially as described with reference to Figs. 1 to 3 of the accompanying Drawings.