GB1600116A - Self-erecting arm asembly - Google Patents

Description

(54) SELF-ERECTING ARM ASSEMBLY (71) We, ETAT FRANCAIS, represented by the DÉLÉGUÉ MINISTARIEL POUR L ARMEMENT, of 14 rue Saint Dominique, 75997-Paris Armies, France, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a self-erecting arm assembly for use under water. The invention is for example applicable to the automatic erection of ahdrophone network, at considerable depths under water, from a compact assembly released from a ship or an aircraft with the aid of a parachute or any other device.

To carry out under-sea detection operations it has proved necessary to produce support arms several metres long which are capable of automatic deployment into a horizontal position, this being a way of achieving good reception directivity at low frequencies and, consequently, of achieving good sensitivity.

In Patent Specification No. 1,517,853, a mast assembly is described which is selferecting but remains in a vertical position.

Thus such an assembly does not provide an entirely satisfactory support arm for hydrophones.

It is accordingly desirable to provide a self-erecting arm assembly which automatically assumes a horizontal position on erection and is able to remain in that position for the period of operation of a hydrophonic unit which the assembly is intended to support.

According to the present invention, there is provided a self-erecting arm assembly for use under water, the assembly comprising a set of tubular arm elements which are adapted to fit together to form an arm and are threaded on a flexible band whose one end is attached to band-tensioning meansand whose other end carries a ballast weight, the elements being packed together side by side in a compact unit adapted to open out, when suspended under water, so that the band is drawn out downwards by the ballast weight and the arm elements become consecutively aligned on the drawn-out band and, through tensioning of the drawn-out band by the band-tensioning means, become fitted together to assemble the arm, there being means for releasing the ballast weight from the band on assembly of the arm, means for bringing the assembled arm into a horizontal position and means for maintaining it horizontal.

An assembly embodying the invention may include at least one further such set of tubular arm elements which are adapted to fit together to form an arm, are threaded on a flexible band whose one end is attached to the band-tensioning means and whose other end carries a ballast weight, and are packed together in the above-mentioned compact unit in like manner to the tubular arm elements of the first-mentioned set. In this case, there are means for releasing the ballast weight from each band on assembly of each arm, means for bringing each assembled arm into a horizontal position and means for maintaining each assembled arm horizontal.

For a two-or-more arm assembly, the compact unit may incorporate a central support to which the sets of arm elements are connected so that the arms assembled therefrom when the unit opens out extend radially from the support. This support may comprise an upper housing part and a lower housing part which are respectively attached to opposite ends of a telescopic rod which is upright in the working disposition of the support. The tubular arm elements of the assembly may then be packed, around the telescopic rod, between the upper and lower housing parts and within a casing constituted by casing shells which have axial ends that engage in corresponding grooves provided therefor in the upper and lower housing parts.The telescopic rod may consist of two component rods respectively attached to the upper and lower housing parts, these being biased apart by resilient means arranged between the free end of one component rod and the housing part to which the other component rod is attached. To lock the component rods against relative axial displacement under the action of the resilient means, a locking pin may be arranged to engage the rods, and a suspension cable for the compact unit may be coupled with the pin in such manner as to disengage the pin from the component rods when the compact unit is fully suspended, with the result that the telescopic rod expands under the action of the resilient means and the casing shell ends are disengaged from the corresponding grooves provided in the upper and lower housing parts.

By such means it is possible to ensure that the compact unit does not open out until fully suspended.

For a better understanding of the invention, and to show how it may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: Figure 1 is a side view of a one-arm selferecting arm assembly embodying the invention when in the erected condition but prior to assuming a horizontal position, Figure 2 is a sectional view of an upper housing part of the assembly, Figures 3, 4 and 5 are sectional views of a lower housing part of the assembly at three successive stages in the erection of the assembly, Figure 6 is a sectional view of a modified form of the lower housing part, Figure 7 is a side view of the erected assembly after assuming a horizontal position, Figure 8 is a perspective view of a threearm self-erecting arm assembly embodying the invention when in the erected condition with its arms in horizontal positions, Figure 9 is a sectional view of the threearm assembly prior to erection, Figure 10 is a cross-section taken along the line 10-10 of Figure 9, Figure 11 is a perspective view of the three-arm assembly in the course of erection, part of the assembly being omitted, Figure 12 is a sectional view in greater detail of part of what is shown in Figure 11, Figure 13 is a sectional view showing the elements of Figure 12 fitted together in a horizontal position, Figure 14 is a sectional view of a modified form of the three-arm assembly prior to erection, and Figure 15 is a sectional view, to an enlarged scale, of a detail from Figure 14.

The erected arm assembly shown in Figure 1 is in the form of an arm or mast 1 made up of a series of tubular arm elements 2 provided with conical end portions allowing the elements to fit into one another.

Attached to the arm element which is uppermost when the arm 1 is vertical, as shown in Figure 1, is an upper housing part accommodating a device 4 for tensioning a flexible band 5 on which the arm elements 2 are threaded. Any elongate connecting member of sufficient length and flexibility may be employed as the flexible band 5.

The device 4, one possible form of which is illustrated in the sectional view of Figure 2, also serves to tension stays 6 and to hold in position support yards 7 extending between the stays 6 and the arm 1.

At its end provided with the tensioning device 4, the arm 1 is supported from a cable 8, for example an electro-suspension cable.

In the form in which it is illustrated in Figure 2, the tensioning device 4 comprises a winch 9 arranged to be driven by an electric motor 10 supplied with a battery (not shown) adapted to be primed by sea water.

A pawl 11 cooperating with a ratchet wheel 12 connected to the winch 9 prevents any movement of the winch in the opposite direction to that which produces tensioning of the band 5.

Attached to that arm element to which is lowermost when the erected arm 1 is vertical, as shown in Figure 1, is a device 13 incorporating means for bringing the erected arm into a horizontal position and for keeping it horizontal. One possible form of this device is illustrated in Figures 3, 4 and 5.

In the form in which it is illustrated in Figures 3, 4 and 5, the device 13 comprises a housing 14 which is provided with apertures 15 by way of which the flexible band 5 is able to extend through the housing. In the stage of erection illustrated in Figure 3, the lower end of the flexible band 5 is attached to a ballast weight 16 beneath the housing.

To effect this attachment, a ball 17 on the band 5 is located in a cavity 18 provided in the ballast 16, the ball engaging a construction 19 provided in the cavity. The constriction 19 is deformable by the ball 17 when the latter is drawn under sufficient tension by the winch 9.

Provided in the housing 14 is a gas-filled chamber 20 serving as a float for the arm element 2 furthest from the tensioning device 4. The gas filling the chamber may for example be generated (in situ) from calcium hydride. Alternatively, the buoyancy of the float may be provided by the presence in the chamber 20 of an incompressible material lighter than water, for example mineral oil or a substance made up of amalgamated glass bubbles.

A rocker 21 is pivotally mounted in the housing 14. This rocker is in the form of a rigid pendulum having a bent flat pendulum rod 22 carrying a weight 23 at its free end. A line 25 (for example, a length of wire) is wound on a bobbin 26 rotatably mounted in the housing. This line extends outside the housing and at its free end carries a control weight 27 in the form of a plate-form aquatic parachute. The line 25 extends, within the housing 14, along a path between the bent rod 22 of the rocker 21 and a cooperating abutment 24 fixed within the housing.

As explained hereinafter, the rod 22 can cooperate with the abutment 24 to grip the line 25 temporarily and thus prevent unwinding of the line from the bobbin 26 under the action of the control weight 27.

The control weight 27 is held against a lateral wall of the housing 14 by a locking device 28. The locking device 8 is in the form of a bolt provided on the flexible band 5 and serves to lock the control weight 27 to the housing 14 until, on release of the ballast weight 16 from the band 5, the band is drawn through the housing by the tensioning device 4.

In the modified form of lower housing part illustrated in Figure 6, the device 13 is the same as the device 13 shown in Figure 3, with the exception that in place of the chamber 20 the float for the lowermost arm element 2 is provided by an chamber defined between the outer tubular portion of the element and a coaxial inner tubular portion, this chamber being axially bounded by opposite flanges 30 welded to the tubular portions.

The one-arm self-erecting arm assembly shown in Figures 1 to 7 operates as follows.

Initially, the tubular arm elements 2 are packed together side by side in a compact unit taking the form of a cartridge. When this is released into the sea from a ship or an aircraft, the unit opens out so that the flexible band 5 is drawn out downwards by the ballast weight 16 and the arm elements 2 become vertically aligned on the drawn-out band. The tensioning device 4 tensions the drawn-out band, with the result that the vertically aligned arm elements 2 fit together to assemble the arm 1. So far, the operation is similar to that of the mast assembly described and illustrated in Patent Specification No. 1,517,853.

Figure 1 shows the arm assembly at the stage where the arm elements have fitted together. Figure 3 shows the condition of the device 13 at this stage. In this condition the ballast weight 16 is still being held against the housing 14 of the device 13.

The winch 9 of the tensioning device 4 exerts an additional pull on the flexible band 5, with the result that the ball 17 deforms the constriction 19 in the cavity 18 in such manner that the ballast weight 16 is released from the band and falls away. Because of the float provided by the gas-filled chamber 20 (or, in the Figure 6 modification, the cylindrical air chamber incorporated in the lowermost arm element 2) the lower end of the assembled arm begins to rise so that the arm swings round towards a horizontal position as shown in Figure 7.

On release of the ballast weight 16, the accompanying displacement of the flexible band 5 moves the bolt 28 so as to unlock the control weight 27 from the side of the housing 14. This stage is illustrated in Figure 4, where the control weight 17 in the form of an aquatic parachute can be seen falling downwards away from the housing 14. As the control weight 27 begins to descend it draws the line 25 off the bobbin 26. The descent of the control weight 27 is fairly slow, and so the initial pay-out of the line 25 is mainly due to the movement of the assembled arm 1 towards the horizontal position.

When the assembled arm 1 reaches the horizontal position, the condition of the device 13 is as illustrated in Figure 5. In this condition of the device 13, the bent rod 22 of the pendulum-form rocker 21 presses the line 25 against the abutment 24, with the result that unwinding of the line 25 from the bobbin 26 is interrupted and the control weight 27 exerts a downward force on the arm 11. The arm 11 thus tends to swing downwards again until the bent rod 22 of the rocker 21 moves away from the abutment 24 sufficiently to free the line 25.

Thereupon, the control weight 27 starts to draw the line 25 off the bobbin 26 again, and no longer exerts any substantial downward force on the assembled arm 1. Accordingly, the arm begins to swing back towards the horizontal position. This series of actions may be repeated, with the result that the assembled arm 1 oscillates between two closely adjacent positions and remains virtually horizontal as long as there is line 25 remaining on the bobbin 26.

Hydrophones to be supported by the asembly may be mounted concentrically on the arm 1. They can be supplemented by further hydrophones 85 mounted on flexible wires attached to the arm 1 at regular intervals, as shown in Figure 7.

The three-arm self-erecting arm assembly illustrated in Figures 8 to 13 is also intended for supporting hydrophones under water. In Figure 9, this assembly is shown prior to erection. In this state, tubular arm elements which fit together to form the three arms 31, 32 and 33 of the erected assembly are packed together side by side in a compact unit. This unit is in the form of a cartridge comprising a central support and a casing.

The central support has an upper housing part 36 and a lower housing part 37 which are respectively attached to opposite ends of a central telescopic rod 40 which is upright in the working disposition of the central support. As shown in Figures 9 and 10, the tubular arm elements, which for each arm comprise a corresponding set of elements which consists of a first element 51, a second element 52 and a series of further elements 54, are packed together side by side around the central telescopic rod 40 and between the upper housing part 36 and the lower housing part 37. The casing of the cartridge encloses the packed elements, this casing comprising two casing shells 34 at least one of which is perforated by apertures 35 to allow water to penetrate into the interior of the cartridge.To keep the shells 34 in place, the housing parts 36 and 37 are respectively provided with grooves 38 and 39 in which the axial ends of the casing shells engage.

The telescopic rod 40 consists of a first component rod 41, which is attached to the upper housing part 36, and a second component rod 42, which is attached to the lower housing part 37 and mounted slidably within the first component rod 41. Resilient means in the form of a spring 43 arranged between the housing part 36 and the free end of the component rod 42 serve to bias the housing parts 36 and 37 apart from one another.

Prior to erection of the assembly, relative axial displacement between the component rods 41 and 42 of the telescopic rod 40 is prevented by means of a locking pin 44 arranged to engage the component rods.

The free end of the locking pin 44 is attached to a suspension cable 45 for the cartridge. This cable is attached to the lower end of the component rod 42 by means of a ring 46 loosely mounted on the rod and held in axial position by an abutment 47 provided on the rod.

The upper housing part 36 accommodates a tensioning device 48 of the same form as the tensioning device 4 described with reference to Figure 2. Above the tensioning device 48, the housing part 36 provides an open compartment 49 in which the suspension cable 45 is accommodated, in coiled form, prior to use of the assembly.

The lower housing part 37 contains a magnetic compass and has a compartment of electronic circuits for hydrophones (not shown) mounted on the tubular arm elements. Beneath the housing part 37 a ballast weight 50 of streamlined form, for example of hemispherical form as illustrated, is attached. This weights the lower housing part so that the central support, i.e. the housing parts 36 and 37 and the telescopic rod 40 interconnecting them, is of negative buoyancy and is in a condition of stable equilibrium when in its working disposition, i.e. with the rod 40 upright.

The streamlined shape of the ballast weight 50 allows the cartridge, when released from an aircraft, to touch the water at a relatively high speed without being damaged, with the result that the size of release parachutes employed can be reduced.

As shown in Figures 10 and 11, the first arm element 51 of each set is articulated to the lower housing part 37 of the central support, while the second arm element 52 is articulated to the first. The remaining arm elements 54 of each set, which are of smaller diameter, and therefore of smaller crosssection, than the first and second elements 51 and 52, are seven in number in this case.

The tubular arm elements 51, 52 and 54 of each set are threaded on a corresponding flexible band 56. One end of each band is attached to the tensioning device 48. At its other end, each flexible band 56 carries a ballast weight 55 on the end of the last element 54 of the associated set of arm elements 51, 52 and 54. The last element 54 of each set of elements 51, 52 and 54 may be provided with a float serving to assist in bringing the arm eventually assembled from the set into a horizontal position.

In the cartridge, the elements 51. 52 and 54 of each set are arranged head to tail and kept together by means of a paper strip 57 which disintegrates in sea water.

Each set of arm elements is identical, and the tensioning device 48 is common to the flexible bands 56 on which the sets of elements are threaded. Thus the winch of the tensioning device is able to tension the three flexible bands 56 simultaneously, thereby ensuring simultaneous assembly of the corresponding arms, denoted 31, 32 and 33 in Figure 8. Equally, the tensioning device 48 serves to tension stays provided for each arm.

Figure 12 shows part of the first element 51, the second element 52 and part of the following element 54. The first element 51 is articulated to the lower housing part 37 in such fashion that it is constrained to adopt a substantially horizontal position when the cartridge opens out with the central support in its upright working disposition. Remote from the lower housing part 37, this element has a frusto-conical female end portion into which which a cooperating frusto-conical male end portion of a tubular engagement member 59 slidingly mounted in the second arm element 52 is adapted to engage.

The first and second tubular arm elements 51 and 52 of each set are articulated together by means of hinges 53 made up of springs whose action is such that, on opening out of the cartridge with the central support thereof in its vertical working position, the second arm element 52 extends downwards from the first element 51 obliquely to the horizontal, and, in the case illustrated, at an angle of 120a to the first element (see Figure 12).

At this end remote from its frusto-conical end portion, the tubular engagement member 59 is provided with a base 70 in which is formed an aperture for the flexible band 56 to pass through. A knot 72 in the flexible band 56 is arranged to engage the base 70 so that the engagement member 59 is entrained by the band, when tensioned.

Fixed to the inside of the base 70 is a sleeve 71 of elastic material which extends in and between the first and second elements 51 and 52 and serves as a protective guide for the passage of the flexible band 56 between those elements.

At its end remote from the first element 51, the second element 52 has a frustoconical female end portion 73 intended to receive a corresponding frusto-conical male end portion 74 of the following arm element 54 of smaller diameter. In order to facilitate the engagement of this element 54 in the second element 52, the latter is provided with a nose 75 which, when the second element 52 is at an angle of 1200 to the first element 51, forms a guide wall which is substantially vertical.

The operation of the assembly illustrated in Figures 8 to 13 will now be described.

When, after its release from a ship or an aircraft, the unpacked assembly has fallen to the depth at which it is fully suspended from the suspension cable 45, i.e. when the cable 45 is completely uncoiled from the compartment 49, the cable pulls on the locking pin 44, thereby disengaging the pin from the component rods 41 and 42 of the telescopic rod 40. As a result, the telescopic rod 40 expands under the action of the spring 43 acting between the upper housing part 36 and the component rod 42, and the axial ends of the casing shells 34 are accordingly disengaged from the corresponding grooves 38 and 39 provided in the upper and lower housing parts 36 and 37. The casing shells 34 thus fall away, as illustrated in Figure 11.

The sets of packed arm elements 54 held in bundles by the paper strips 57 fall away from the vertical positions they occupy initially, the paper then disintegrates and, as the ballast weights 55 draw the flexible bands 56 downwards, the arm elements 54 become vertically aligned consecutivelv upon the corresponding flexible bands 5.

As each bundle of arm elements 54 falls away, the associated arm elements 51 and 52 take up the positions illustrated in Figures 11 and 12. Thus each element 51 takes up a horizontal position in abutment against the upper surface of the lower housing part 37, whereas each element 52 makes an angle of 1200 with the associated element 51, under the action of the spring hinges 53.

When each set of elements 5,52and54 is aligned as indicated in Figure 11, the tensioning device 48 comes into operation. To achieve this, the ull exerted bv each of the flexible bands 56 is arranged to trigger an interrupter (not shown) provided in the supply circuit of the motor of the tensioning device. In the present case, the device has three interrupters in series, corresponding to the three arms to be assembled.

When the motor of the tensioning device 48 is supplied with power, the motor drives the winch of the device, which simultaneously winds the three flexible bands 56 and causes the arm elements 54 of each set to fit together, the uppermost element 54 of each set to be held against the second element 52 of the set and the associated ballast weight 55 to be discarded in the manner described with reference to Figure 3.

With the ballast weights 55 released from the lowermost elements 54 of the arms thus formed, the arms have a slightly positive buoyancy at their lower ends, due to the provision of floats at the lower ends of the lowermost arm elements 54.

The combined effect of this buoyancy and the tensioning device 48 is to cause the uppermost arm element 54 of each arm to engage in the associated second element 52 by bearing on the nose 75 thereof. Furthermore, the arm elements 54 and the second element 52 of each set move into alignment with the first element 51 of the set by swinging around the hinges 53. This is accompanied by entrainment of each tubular engagement member 59 by the associated flexible band 56, with the result that the engagement member 59 slides in the associated arm element 52, entraining as it does so the associated flexible sleeve 71, and the frusto-conical male end portion of the engagement member is drawn into engagement with the frusto-conical female end portion of the associated arm element 51.In this way, each second arm element 52 is brought into alignment and connection with the associated first arm element 51. When the operation is complete, the various parts illustrated in Figure 12 have come to occupy the positions indicated in Figure 13.

The tensioning device 48 similarly produces the tension of the stays for the arms 31,32 and 33.

While the tensioning device 48, in combination with the first and second tubular arm elements 51 and 52 of each set of arm elements, serves as means for bringing the arms into horizontal positions, the central support consisting of the upper and lower housing parts 36 and 37 and the telescopic rod 40 serves as means for maintaining the assembled arms horizontal. To assure the stability of the erected assembly in the horizontal state indicated in Figure 8, in addition to weighting the lower housing part 37 with the ballast weight 50 the upper housing part 36 is designed to provide a degree of positive buoyancy.

Because the ring 46 by means of which the suspension cable 45 Is attached to the telescopic rod 40 in the vicinity of the lower housing part 37 is able to rotate freely on the rod, there is a surplus length of cable allowing the free rotation of the ring relative to the rest of the assembly. This prevents the latter from tilting in undesirable manner, for example under the effect of different forces exerted on the three arms 31, 32 and 33.

The modified form of three-arm selferecting arm assembly illustrated in Figures 14 and 15 is similar to that described with reference to the Figure 8 to 13 except insofar as a different form of tensioning device is provided.

In the modified assembly, in which parts similar to those already described are respectively denoted by the same reference numerals, the tensioning device comprises a piston 76 slidingly mounted in a pistonreceiving bore constituted in the present case by the interior of the component rod 42 of the telescopic rod 40. On one side of the piston 76, the lower side in the present case, the piston-receiving bore communicates with the exterior. For this purpose, the bore leads into a duct 78 extending through the lower housing part 37 and also through the ballast weight 50 attached to the lower housing part on the side thereof remote from the telescopic rod 40. The piston-receiving bore is closed at its upper end by a wall 77 extending across the component rod 42.

Prior to alignment of the tubular arm elements 54 on the corresponding flexible bands 56, displacement of the piston 76 in the bore of the component rod 42 is prevented by locking means which are best seen in Figure 15. The locking means comprise locking fingers 80 accommodated in respective sleeves 81 which are attached to the exterior of the component rod 42 and open into the bore thereof through lateral holes provided in the component rod 42. Through these holes, rounded inner ends of the locking fingers 80 are able to project into the bore so as to engage in a groove 79 of correspondingly rounded cross-section provided in the piston 76. Until the arm elements 54 are aligned, displacement of the locking fingers 80 in the sleeves 81 is prevented by removable locking pins 82 respectively inserted into the sleeves 81 accommodating the locking fingers 80.

The piston 76 is held in place by as many locking fingers 80 as the assembly has arms, and the locking pins 82 are respectively attached to the flexible bands 56 on which the sets of arm elements are threaded. In the present case, the attachment is effected by passage of the flexible bands 56 through corresponding eyelets 83 provided at the free ends of the locking pins 82. Moreover, to couple the piston 76 to the flexible bands 56 so that displacement of the piston along the bore of the component rod 42 (when the piston is released from its illustrated position) tensions the bands, the bands 56 extend into the bore through apertures 84 and are fixed to the lower face of the piston 76.

Since no tensioning device 48 is accommodated in the upper housing part 36 of the modified three-arm self-erecting arm assembly, this housing part can be used to accommodate electronic circuits.

When the sets of arm elements 51, 52 and 54 are aligned, the pull exerted on each of the flexible bands 56 causes the retraction of the corresponding locking pins 82. With the locking pins 82 thus withdrawn, the associated locking fingers 80 are free to move in the corresponding sleeves 81. The pressure exerted on the lower face of the piston 76 on account of the assembly's immersion is sufficient to expel the locking fingers 80 from the bore of the component rod 42 on account of the rounded shape of the inwardly projecting ends of the fingers and the complementary shape of the groove 79.

The piston 76 thus moves along the bore of the component rod 42 and entrains the flexible band 56 with it, thereby bringing the aligned arms 31, 32 and 33 into horizontal positions in the manner described with reference to Figures 11 to 13.

While the assemblies described with reference to Figures 8 to 15 provide, when erected, three horizontal arms at 1200 to each other, equivalent assemblies providing a greater number of arms may be constructed.

WHAT WE CLAIM IS: 1. A self-erecting arm assembly for use under water, the assembly comprising a set of tubular arm elements which are adapted to fit together to form an arm and are threaded on a flexible band whose one end is attached to band-tensioning means and whose other end carries a ballast weight, the arm elements being packed together side by side in a compact unit adapted to open out, when suspended under water, so that the band is drawn out downwards by the ballast weight and the arm elements become consecutively aligned on the drawn-out band and, through tensioning of the drawn-out band by the band-tensioning means, become fitted together to assemble the arm, there being means for releasing the ballast weight from the band on assembly of the arm, means for bringing the assembled arm into a horizontal position and means for maintaining it horizontal.

2. An assembly as claimed in claim 1, wherein the means for bringing the arm into a horizontal position comprise a float provided on the arm element which is furthest from the tensioning means when the arm is assembled.

3. An assembly as claimed in claim 2,

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (29)

**WARNING** start of CLMS field may overlap end of DESC **. telescopic rod 40 in the vicinity of the lower housing part 37 is able to rotate freely on the rod, there is a surplus length of cable allowing the free rotation of the ring relative to the rest of the assembly. This prevents the latter from tilting in undesirable manner, for example under the effect of different forces exerted on the three arms 31, 32 and 33. The modified form of three-arm selferecting arm assembly illustrated in Figures 14 and 15 is similar to that described with reference to the Figure 8 to 13 except insofar as a different form of tensioning device is provided. In the modified assembly, in which parts similar to those already described are respectively denoted by the same reference numerals, the tensioning device comprises a piston 76 slidingly mounted in a pistonreceiving bore constituted in the present case by the interior of the component rod 42 of the telescopic rod 40. On one side of the piston 76, the lower side in the present case, the piston-receiving bore communicates with the exterior. For this purpose, the bore leads into a duct 78 extending through the lower housing part 37 and also through the ballast weight 50 attached to the lower housing part on the side thereof remote from the telescopic rod 40. The piston-receiving bore is closed at its upper end by a wall 77 extending across the component rod 42. Prior to alignment of the tubular arm elements 54 on the corresponding flexible bands 56, displacement of the piston 76 in the bore of the component rod 42 is prevented by locking means which are best seen in Figure 15. The locking means comprise locking fingers 80 accommodated in respective sleeves 81 which are attached to the exterior of the component rod 42 and open into the bore thereof through lateral holes provided in the component rod 42. Through these holes, rounded inner ends of the locking fingers 80 are able to project into the bore so as to engage in a groove 79 of correspondingly rounded cross-section provided in the piston 76. Until the arm elements 54 are aligned, displacement of the locking fingers 80 in the sleeves 81 is prevented by removable locking pins 82 respectively inserted into the sleeves 81 accommodating the locking fingers 80. The piston 76 is held in place by as many locking fingers 80 as the assembly has arms, and the locking pins 82 are respectively attached to the flexible bands 56 on which the sets of arm elements are threaded. In the present case, the attachment is effected by passage of the flexible bands 56 through corresponding eyelets 83 provided at the free ends of the locking pins 82. Moreover, to couple the piston 76 to the flexible bands 56 so that displacement of the piston along the bore of the component rod 42 (when the piston is released from its illustrated position) tensions the bands, the bands 56 extend into the bore through apertures 84 and are fixed to the lower face of the piston 76. Since no tensioning device 48 is accommodated in the upper housing part 36 of the modified three-arm self-erecting arm assembly, this housing part can be used to accommodate electronic circuits. When the sets of arm elements 51, 52 and 54 are aligned, the pull exerted on each of the flexible bands 56 causes the retraction of the corresponding locking pins 82. With the locking pins 82 thus withdrawn, the associated locking fingers 80 are free to move in the corresponding sleeves 81. The pressure exerted on the lower face of the piston 76 on account of the assembly's immersion is sufficient to expel the locking fingers 80 from the bore of the component rod 42 on account of the rounded shape of the inwardly projecting ends of the fingers and the complementary shape of the groove 79. The piston 76 thus moves along the bore of the component rod 42 and entrains the flexible band 56 with it, thereby bringing the aligned arms 31, 32 and 33 into horizontal positions in the manner described with reference to Figures 11 to 13. While the assemblies described with reference to Figures 8 to 15 provide, when erected, three horizontal arms at 1200 to each other, equivalent assemblies providing a greater number of arms may be constructed. WHAT WE CLAIM IS:

1. A self-erecting arm assembly for use under water, the assembly comprising a set of tubular arm elements which are adapted to fit together to form an arm and are threaded on a flexible band whose one end is attached to band-tensioning means and whose other end carries a ballast weight, the arm elements being packed together side by side in a compact unit adapted to open out, when suspended under water, so that the band is drawn out downwards by the ballast weight and the arm elements become consecutively aligned on the drawn-out band and, through tensioning of the drawn-out band by the band-tensioning means, become fitted together to assemble the arm, there being means for releasing the ballast weight from the band on assembly of the arm, means for bringing the assembled arm into a horizontal position and means for maintaining it horizontal.

2. An assembly as claimed in claim 1, wherein the means for bringing the arm into a horizontal position comprise a float provided on the arm element which is furthest from the tensioning means when the arm is assembled.

3. An assembly as claimed in claim 2,

wherein the buoyancy of the float is provided by a gas-filled chamber.

4. An assembly as claimed in claim 2, wherein the byoyancy of the float is provided by a quantity of incompressible material lighter than water.

5. An assembly as claimed in any preceding claim, wherein the means for maintaining the assembled arm in a horizontal position comprise a rocker pivotally mounted within a housing attached to the arm element furthest from the tensioning means when the arm is assembled, and a control weight carried, outside the housing, on a line which is wound on a bobbin rotatably mounted in the said housing and extends along a path between the rocker and a cooperating abutment within the housing, the arrangement being such that the control weight is able to descend when the assembled arm is non-horizontal, thereby drawing the line off the bobbin, but is prevented from descending by gripping of the line between the rocker and the abutment when the assembled arm is horizontal.

6. An assembly as claimed in claim 5, wherein the control weight is constituted by a plate-form aquatic parachute.

7. An assembly as claimed in claim 5 or 6, wherein the rocker is in the form of a rigid pendulum having a bent pendulum rod carrying a weight at its free end.

8. An assembly as claimed in claim 5, 6 or 7, when appended to claim 2, 3 or 4, wherein the said float is provided in the said housing.

9. An assembly as claimed in claim 5, 6 or 7, when appended to claim 3, wherein the said gas-filled chamber is defined between coaxial inner and outer tubular portions of the arm element which is furthest from the tensioning means when the arm is assembled, the chamber being axially bounded by opposite flanges.

10. An assembly as claimed in any one of claims 5 to 9, wherein the ballast weight is located beneath the base of the said housing when the assembly is oriented ready for erection, and the flexible band is arranged to extend through the housing into attachment with the ballast weight, the band being provided in the said housing with a locking device which serves to lock the said control weight to the housing until, on release of the ballast weight from the band, the band is drawn through the said housing.

11. An assembly as claimed in claim 1, including at least one further such set of tubular arm elements which are adapted to fit together to form an arm, are threaded on a flexible band whose one end is attached to the band-tensioning means and whose other end carries a ballast weight, and are packed together in the said compact unit in like manner to the arm elements of the set specified in claim 1, there being means for releasing the ballast weight from each band on assembly of each arm, means for bringing each assembled arm into a horizontal position and means for maintaining each assembled arm horizontal.

12. An assembly as claimed in claim 11, wherein the said compact unit incorporates a central support to which the sets of arm elements are connected so that the arms assembled therefrom when the unit opens out extend radially from the support, the said support comprising an upper housing part and a lower housing part which are respectively attached to opposite ends of a telescopic rod which is upright in the working disposition of the support, the tubular arm elements being packed, around the telescopic rod, between the said upper and lower housing parts and within a casing composed of casing shells which have axial ends that engage in corresponding grooves provided therefor in the upper and lower housing parts.

13. An assembly as claimed in claim 12, wherein the telescopic rod is composed of two component rods respectively attached to the said upper and lower housing parts, there being resilient means arranged, between the free end of one component rod and the housing part to which the other component rod is attached, to bias the housing parts apart.

14. An assembly as claimed in claim 13, wherein a locking pin engages the said component rods so as to lock them against relative axial displacement under the action of the said resilient means, a suspension cable for the said compact unit being coupled with the pin in such manner as to disengage the pin from the said component rods when the compact unit is fully suspended, with the result that the telescopic rod expands under the action of the resilient means and the casing shell ends are disengaged from the corresponding grooves in the said upper and lower housing parts.

15. An assembly as claimed in claim 14, wherein the said upper housing part is provided with a compartment for accommodating the suspension cable prior to suspension of the said compact unit.

16. An assembly as claimed in claim 12, 13, 14 or 15, wherein each of the said sets comprises a first tubular arm element, which is articulated to the said lower housing part, a second tubular arm element whicis articulated to the first tubular arm element, and a series of further tubular arm elements of smaller cross-section than the first and second tubular arm elements.

17. An assembly as claimed in claim 16, wherein the first tubular arm element of each set is constrained so as to adopt a substantially horizontal position when the said compact unit opens out with the said support in its working disposition, the second tubular arm element of each set being articulated to the first tubular arm element thereof that, on opening out of the said compact unit with the said support in its working disposition, the second element extends downwards from the first obliquely to the horizontal.

18. An assembly as claimed in any one of claims 12 to 17, wherein the said bandtensioning means comprise a bandtensioning device which is common to all the flexible bands and serves, in combination with the first and second tubular arm elements of each set, as means for bringing the assembled arms into horizontal positions.

19. An assembly as claimed in claim 18 when appended to claim 16 or 17, wherein, at its end further from the band-tensioning means when the arm to which it belongs is assembled, that tubular arm element of each set which is furthest from the bandtensioning means when the said compact unit is opened out is provided with a float serving to assist in bringing the assembled arm into a horizontal position.

20. An assembly as claimed in any one of claims 12 to 19, wherein the said lower housing part is weighted so that the said support is of negative buoyancy and is in stable equilibrium in the said working disposition, thereby serving as means for maintaining the assembled arms horizontal.

21. An assembly as claimed in any one of claims 16 to 20, wherein the first tubular arm element of each set has, remote from the said lower housing part when the said compact unit is opened out, a frusto-conical female end portion, and there is slidingly mounted in the second tubular arm element of the set an engagement member with a cooperating frusto-conical male end portion adapted to engage in the female end portion, the engagement member being arranged to be entrained by the associated flexible band, when tensioned, so that the male end portion is drawn into engagement in the female end portion and the second tubular arm element is thereby brought into alignment and connection with the first tubular arm element, there being a flexible sleeve which extends into the first and second tubular arm elements and serves as a protective guide for the passage of the flexible band between those elements.

22. An assembly as claimed in any one of claims 12 to 21, wherein the bandtensioning means are accommodated in the said upper housing part.

23. An assembly as claimed in claim 22, wherein the band-tensioning means comprise a motor-driven winch.

24. An assembly as claimed in any one of claims 12 to 21, wherein the bandtensioning means comprise a piston slidingly mounted in a piston-receiving bore which to one side of the piston communicates with the exterior, the piston being coupled with the flexible bands so that displacement of the piston along the bore due to pressure acting on the piston when the assembly is suspended under water tensions the bands, there being locking means arranged to prevent such displacement of the piston prior to alignment of the tubular arm elements of each set on the respective flexible bands.

25. An assembly as claimed in claim 24, wherein the piston-receiving bore is constituted by the interior of a component rod of the said telescopic rod, the bore being arranged to communicate with the exterior via a duct which extends through the said lower housing part.

26. An assembly as claimed in claim 25, wherein the said duct also extends through a weight attached to the said lower housing part on that side of the latter remote from the telescopic rod.

27. An assembly as claimed in claim 25 or 26, wherein the said locking means comprise locking fingers which are accommodated in sleeves opening into the pistonreceiving bore and have rounded ends which project into the bore to engage in a groove of correspondingly rounded cross-section provided in the piston, displacement of the locking fingers in the associated sleeves being prevented by removable locking pins which are respectively inserted into the associated sleeves and are respectively attached to the flexible bands, the bands extending, through respective apertures provided therefor leading into the bore, and being fixed to the piston.

28. An assembly as claimed in any preceding claim, wherein hydrophones are attached to the tubular arm elements.

29. A self-erecting arm assembly for use under water, substantially as hereinbefore described with reference to Figures 1 to 5 and 7, or Figures 1, 2, 6 and 7, or Figures 8 to 13 or Figures 8 and 10 to 15, of the accompanying drawings.