GB2175623A - Cable fairing support ring - Google Patents

Abstract

A support ring (21) for supporting streamline-fairing on a tow-cable, for towing an underwater device from a ship,comprises a sleeve (11) and two tapered collets (12,13). The collets are forced into the sleeve using only an axially-directed force. The radially inwardly directed grip from the collets is limited, by the end of the sleeve acting as a stop for the forcing device and by the sleeve being stretchable, to avoid damage to the cable. Plastic bushes act as bearings between the ring and the fairing, and act also to protect the ring from direct contact with a pulley while winching the cable. Adhesive may be applied between the sleeve and collets. <IMAGE>

Description

SPECIFICATION Fairing support ring This invention relates to tow-cables for towing un- der-water devices from ships.

Such cables are often provided with fairing for streamlining purposes. The fairing is divided into sections which are located by support rings clamped at regular intervals along the cable. The invention is in the support rings and in the manner of attaching them to the cable.

U.S. Patent No. 3,407,777 (ANASTASIO, 29 October 1968) shows the use of a fairing support ring which comprises a rubber bush that is bonded to the strands of the cable, and encircled by a steel sleeve.

Such a ring is compliant, and allows the cable to be wrapped around a pulley with little danger of the ring causing damage to the cable. On the other hand such a ring is not very strong and can be torn off by only slightly abusive forces.

Other proposals have been in the area of crimping the rings to the cable, a process which involves the use of a radial force. There is a difficulty with a crimped ring in that it is difficult to strike the right balance between the ring being tight enough not to work loose, yet not so tight that the cable is damaged.

In the invention, the ring is held onto the cable by means of tapered collets inside a sleeve.

The following advantages have been found.

1) The tightness of the grip of the ring onto the cable can be very high, yet since the grip is evenly spread, there is little danger of damage to the ca ble.

2) The tightness of the grip can be consistent ring-to-rfng and cable-to-cable. The consistency arises because the tightness can be made to de pend on the fit of the collets in the sleeve. These components may be easily manufactured to the ac curacy required, by simple lathe-turning.

3) No rubber or other stress-absorbing medium is required, which, if present, would render the ring liable to damage due to abuse and due to pro longed contact with sea-water.

4) To assemble the ring to the cable only a sim ple axial squeeze of the collets into the sleeve is required. It is difficult to damage the cable by acci dent in this circumstance. This inherent resistance to damage might be contrasted with the way a ca ble can be easily damaged by a mishandled radial crimping operation.

The tow-cables used for towing devices from ships are constructed typically in the following manner: on the inside are the electrical conductors for conveying signals from the instruments on the device up to the ship; these conductors are sur rounded by a protective insulative jacket of a ure thane plastic; and the conductors also are embedded in a filler material so as to leave no air spaces between the conductors.

The tensile strength of the cable comes from its outer layers which comprise strands of wire, which is usually a galvanized steel wire. The layers are wound cross-helically, so that the cable has no tendency to twist or untwist when it is put under tension.

The strands of wire are packed as tightly as possible during manufacture of the cable. Even so, the cable does deflect inwardly radially if crushed.

Sometimes a cable is pre-stretched axially, to compact the strands radially prior to use. Cables can undergo a 2% to 5% reduction in diameter due to this pre-tension. It is not just the reduction, but the variation in the magnitude of the reduction, cableto-cable, that is important from the point of view of ensuring a consistent grip of the rings. A preferred feature of the invention makes light of these variations in the magnitude of the reduction.

The preferred feature is the provision of a stopmeans. This prevents the collets from being inserted too far into the sleeve. The stop-means can comprise simply the end surface of the sleeve, such that the device for squeezing the collets axially into the sleeve can push the collets flush with the end of the sleeve but no further. The provision of the stop-means has the effect that a pre-determined magnitude of radial grip cannot be exceeded even if the collets are pressed too hard accidentally.

Using the stop-means, the only factor (besides the dimensions of the collets and sleeve) that determines the magnitude of the radial grip is the thickness of the tow-cable. If the tow-cable is at the thick end of its permitted tolerance band, then the grip of the ring will inevitably be tighter than when the cable is thinner, but, in the invention, even this variation can be catered for. The sleeve is made thin enough that it can stretch to some extent due to hoop-tension induced in the sleeve. Thus, it has some resilience, by which variations in cable diameter are prevented from causing large differences in the tightness of the grip.

The sleeve, it has been found, may be made to perform in the following manner: that the sleeve stretches resiliently over most of the range of permitted thicknesses of the cable, but yet when the cable is at its very thickest, the sleeve y:elds. This ability to yield if a very high hoop tension is induced means that a further limit is placed on the magnitude of the radial grip.

Hence, in the invention, a very tight grip may be called for as the normal condition, since little allowance need be left to cater for overgripping.

Both the stop-means and the stretchable sleeve act as limiters to prevent over-gripping.

Preferably in the invention, the collets are glued into the sleeve. This has the manifest benefit of permanence, but also has this further benefit, that the adhesive, in liquid form, acts as a lubricant while the collets are being inserted. Hence the axial force required to achieve a given magnitude of radial clamp is reduced, because the friction between the sleeve and the collets is reduced.

The magnitude of the radial gripping force is determined by and limited by the stop means. The presence of the glue reduces the axial force needed to press the collets as far as the stop means. The glue does not reduce the induced hoop tension, nor the radial gripping force: the lubricating effect of the glue means that the radial gripping force can be achieved with a small axial force.

Providing all these features of the invention in combination gives a manner of attaching a fairing support ring that, as compared with the rings known hitherto, has an extraordinarily reliable, consistent, and predictable grip onto the cable. The method is inexpensive, and can be put into effect without sophisticated tools and with little danger of damage from production-line accidents.

Further aspects of the invention will become apparent from the following.

In the accompanying drawings: Figure 1 is a view of a tow-cable, with a sleeve and collets; Figure 2 is a cross-section of the cable of Figure 1, after assembly of the collets and sleeve; Figure 3 is a view of the cable of Figure 1 to which fairing has been added, shown passing over a pulley; Figure 4 shows a sleeve that can be assembled to the cable in a radial direction.

Figure 1 shows a tow-cable 10 onto which has been threaded hollow sleeve 11. Two collets 12, 13 are positioned ready to be inserted into the annular space 14 between the inside surface 15 of the sleeve 11 and the outer surface of the tow cable 10.

In Figure 2, a squeezing means 16 is in position to squeeze the collets 12, 13 axially into the space 14. The end surface 19 of the sleeve 11 comprises a stop. When the squeezing means 16 contacts the stop, the collets cannot be inserted any further into the space 14.

An epoxy adhesive is spread onto the outside surfaces 20 of the collets 12, 13, and the squeezing means 16 is left in place as shown in Figure 2 while the adhesive cures.

When the collets and sleeve have been so assembled, they comprise a fairing support ring 21 on the tow-cable 10. As may be seen in Figure 2, the cable 10 becomes somewhat necked, as at 22, due to the presence of the ring 21.

Figure 3 shows the tow-cable 11 being wound around a pulley 23. Sections of fairing 24 are mounted on the cable 11. Also mounted on the cable are two plastic bushes 25, 26. These bushes are large in diameter than the ring 21, to the extent that the bushes 25, 26 hold the ring 21 out of contact with the pulley 23. This is very effective in protecting the ring from causing a concentration of stresses in the cable during winching over a pulley.

The bushes 25, 26 are soft enough that they cannot themselves damage the cable.

The necking of the cable, as shown at 22 in Figure 2, is important. Even though the cable is prestretched (and hence compressed radially) the necking caused by the ring is still quite noticeable.

However, the cable is not damaged by the ring, when the ring is applied in the manner of the invention. The necking provides some additional resistance to movement of the ring. The ends of the collets preferably should be smoothly radiused as at 27: a sharp edge might nick the strands, leading to early failure.

The reason the cable does neck to so noticeable a degree probably is due to the presence of the urethane jacket around the electrical conductors. The material of the jacket becomes extruded axially outwards from the ring region, to the extent necessary to absorb the induced stresses of compression in the urethane material.

The controlled, consistent, nature of the grip from the collets thus has the effect of compacting the jacket radially beneath the outer strands, and of providing a firm cushion to protect the strands.

The invention may be applied to rings that can be assembled radiay, such as the ring shown in Figure 4.

The sleeve 30 is in two components 31, 32.

These are hinged together at the hinge-pin 34, in such a way that when the components 31, 32 are spread apart they can be assembled over the cable. The parts are then snapped together and locked by means of the complementarily inter-acting shapes 35, 36. Collets may be fitted between the assembled sleeve 30 and the cable as previously described.

It should be noted that the step of locking the components together is done before the collets are inserted. If the inter-acting shapes 35, 36 had to be designed to snap together under a heavy force, then that would be quite a restriction. As it is, the sleeve 30 is loose on the cable during the locking operation. Consequently, the shapes 35, 36 only need resist heavy forces after locking has been completed, and not while locking is taking place.

Instead of the shapes 35, 36, a second hinge-pin could be used to lock the components together.

The shear and bending forces in the hinge-pin 34 are very large. The pin 34 cannot be allowed to break, so it must be of a large diameter, which adds to the radial bulk of the sleeve. An alternative to giving the pin a large diameter would be to form the hinge as a comb-hinge, as shown in Figure 4. Here, the many teeth 40, 41 of the hinge ensure that the shear forces are distributed over the length of the pin, and that the bending moments on the pin are kept to a minimum. Such a hinge is more likely to seize, but that is no great problem in the circumstances, since repeated assembly is not required.

The extra radial bulk of the hinge and of the shapes 35, 36 can be compensated to some extent by making the bushes 25, 26 also with an increased radial thickness. The bushes 25, 26 may be C-shaped, for example, to enable them to be assembled radially.

The components 31, 32 of the sleeve 30 can be manufactured by casting or sintering, for example.

It may be that in some cases the tapered inside surface cannot be made accurately enough by such methods, but if so it is an easy matter to ream the taper to the required accuracy Alterntively, the components might be made as extrusions; the hinge teeth formations and the taper could be put on by machining.

The main regions of the two components, i.e., the regions away from the long edges, preferably are slim in the radial direction to allow that region to stretch and if necessary to yield in the manner previously described. In Figure 4, the slimness in these regions has been achieved yet the circumference of the sleeve 30 is a right cylinder with no protrusions. In the figure, the radial bulk of the hinge teeth 40, 41 and of the shapes 35, 36 has been accommodated in the area circumferentially between the collets, where there is radial space available. Hence, the collets occupy much less of the circumference of the ring than was the case with the ring of Figures 1 and 2.

Claims (23)

1. Method of attaching a fairing support ring to a tow-cable, comprising the steps: of providing a hollow sleeve, which has a conically tapered inside surface; of providing two collets dimensioned to fit in the annular space between the tow-cable and the inside surface of the sleeve, each collet having an outside surface that is complementarily tapered; of assembling the collets radially to the cable at the required location, and of starting the collets into the said annular space; and of applying a solely axial force between the sleeve and the collets to squeeze the collets into the annular space.

2. Method of claim 1, wherein the tow-cable has an outer casing of metal strands wound cross helically, and where the outer casing is such that, when squeezed locally, the casing becomes necked.

3. Method of claim 1, where the inside surface of the collet is of a slightly smaller diameter than the nominal outside diameter of the tow-cable.

4. Method of claim 1 including the further steps: of providing an axial squeezing means for apply ing the said large axial force; of providing a stop means, which allows the squeezing means to squeeze the collets into the annular space up to a pre-determined point but which is effective to prevent the squeezing means from squeezing the collets axially any further into the annular space beyond that point.

5. Method of claim 4, where the stop means comprises an end surface of the hollow sleeve, in co-operation with which the squeezing means is so arranged that it can squeeze the collet into the an nular space until the collet is flush with the end surface of the sleeve, but no further.

6. Method of claim 1, where the sleeve is so di mensioned, in relation to the collets and the tow cable, that its circumference stretches to a substan tial degree when the collet is inserted as far as per mitted by the stop means.

7. Method of claim 6, where the sleeve is so di mensioned that the extent of the circumferential stretch is sufficient to cause the material of the sleeve to yield when the collet is inserted as far as permitted by the stop means, thereby limiting the magnitude of the radial grip of the collets onto the tow-cable.

8. Method of claim 1, including the further steps: of applying liquid adhesive between the outside surfaces of the collets and the inside surface of the sleeve; and of holding the collets within the sleeve until the adhesive is cured.

9. Method of claim 1 including the further steps of providing a loosely fitting bush around the tow cable, the bush being of a larger outside diameter than the ring.

10. Method of claim 9, where the bush is positioned in tandem with the support ring and acts as a bearing washer through which axial loads are transmitted between a section of fairing and the support ring.

11. Method of claim 9, where respective bushes are placed one either side of the ring, and are of sufficiently larger outside diameter than the ring that even when the cable is fully bent over a pulley to its minimum normal working radius the bushes hold the ring out of touching contact with the pulley.

12. Method of claim 1, including the further step, where the sleeve is made in one piece, of threading the sleeve axially along the cable to a required location.

13. Method of claim 1 including the further steps, where the sleeve is made in two components: of assembling the two components radially onto the cable; and of providing locking means for locking the two pieces together to form a complete annular sleeve; where the locking means is so arranged that the complete sleeve is capable of resisting hoop tension induced in the sleeve.

14. Method of claim 13, where the components are joined together at a hinge which is disposed with the axis of the hinge-pin parallel to the axis of the cable.

15. Method of claim 14, where the locking means includes respective complementarily interacting shapes formed on the components, so arranged that the shapes interact with each other after the two components have been assembled to the cable.

16. Method of claim 14, where the hinge has many teeth.

17. Method of attaching a fairing support ring to a tow-cable, comprising the steps: of providing a hollow sleeve, which has a conically tapered inside surface; of providing two collets dimensioned to fit in the annular space between the tow-cable and the inside surface of the sleeve, each collet having an outside surface that is complementarily tapered; of threading a sleeve axially along the cable to a required location; of assembling the collets radially to the cable at the required location, and of starting the collets into the said annular space; of providing a squeezing means to apply a large force axially between the sleeve and the collets to squeeze the collets into the annular space;; of providing an end surface of the hollow sleeve, in co-operation with which the squeezing means is so arranged that it can squeeze the collet into the annular space until the collet is flush with end surface of the sleeve, but no further; where the sleeve is so dimensioned that its circumference is stretchable sufficiently to cause the material of the sleeve to yield when the collet is inserted as far as permitted by the stop means, thereby limiting the magnitude of the radial grip of the collets onto the tow-cable; of applying liquid adhesive between the outside surfaces of the collets and the inside surface of the sleeve; of holding the collets within the sleeve until the adhesive is cured; and of providing a loosely fitting bush around the tow cable, the bush being of a larger outside diameter than the ring;; where the bush is positioned in tandem with the support ring and acts as a bearing washer through which axial loads are transmitted between a section of fairing and the support ring; and where respective such bushes are placed one either side of the ring, and are of sufficiently larger outside diameter than the ring that even when the cable is fully bent over a pulley to its minimum normal working radius the bushes hold the ring out of touching contact with the pulley.

18. Tow-cable having a support ring, where the ring comprises an outer sleeve having a tapered inner surface, and two complementarily tapered collets, assembled between the cable and the sleeve.

19. Cable of claim 18, where the collets are assembled flush with an end surface of the sleeve.

20. Cable of claim 18, where the collets are glued to the sleeve.

21. Cable of claim 18, further comprising a loosely fitting plastic bush positioned in tandem with the support ring, and serving as a bearing washer through which axial loads are transmitted to the support ring.

22. Cable of claim 21, where respective such bushes are placed one either side of the ring, and are of sufficiently larger outside diameter than the ring that even when the cable is fully bent over a pully to its minimum normal working radius the bushes hold the ring out of touching contact with the pulley.

23. Tow-cable arrangement substantially as described herein with reference to the accompanying drawings.