WO1995024667A1

WO1995024667A1 - Arrangement for gripping a strength member of an optical fibre cable

Info

Publication number: WO1995024667A1
Application number: PCT/GB1995/000471
Authority: WO
Inventors: Roger Alaerts; Patrick Clits
Original assignee: Nv Raychem S.A.; Raychem Limited
Priority date: 1994-03-07
Filing date: 1995-03-06
Publication date: 1995-09-14
Also published as: GB9404397D0; AU1853295A

Abstract

An arrangement for gripping the strength member of an optical fibre cable comprises: (a) a hollow body, and (b) a longitudinally divided gripping member arranged to be received within the hollow body. The gripping member being movable relative to the hollow member to urge the gripping member into gripping engagement with the strength member when, in use, the strength member is positioned within the longitudinal division of the gripping member.

Description

Arrangement for gripping a strength member of an optical fibre cable.

The present invention relates to an arrangement for gripping a strength member of an optical fibre cable.

Optical fibre cables normally include a strength member for taking the loads applied to the cable so that the optical fibres .and the polymeric jacketing are not damaged by such loads. Such elongate strength members are commonly formed from metal, e.g. steel, or other materials, e.g. composite materials, especially glass fibre reinforced composite materials, and they often extend along a central region of the cable. In addition, many optical fibre cables include a strengthening jacket, for example formed from a weave of Kevlar fibres (Kevlar is a trade mark of Du Pont De Nemours Co.).

When optical fibre cables are spliced or otherwise terminated it is common to terTninate the strength members of the cables and to anchor them to the splice case or other appropriate article. A variety of different methods of anchoring optical fibre cable strength members are known. For example, one method comprises gripping an exposed portion of a strength member in an anchoring device by means of a screw tightened against the strength member. However, that method does not provide a sufficiently strong means of .anchoring the strength member for many applications, for example where extreme temperatures are experienced by the cable, leading to additional strain on the strength member.

Another method of anchoring a strength member of an optical fibre cable is disclosed in United States patent No. 4795230. This patent discloses a device for spreading optical fibres emerging from an end of an optical fibre cable, the device including means for securing it to the strength member of the cable. The means includes a hollow gripping member which is slid onto the strength member, the gripping member having four longitudinal jaws separated by slots extending along part of the length of the member. The jaws have rear truncated tapering portions which are pushed against a shoulder inside the device, by means of a clamping screw which is screwed into the device against the front portion of the gripping member, forcing the ends of the jaws together against the strength member of the cable, thereby gripping it.

According to the present invention there is provided an arrangement for gripping the strength member of an optical fibre cable, comprising:

(a) a hollow body, and

(b) a longitudinally divided gripping member arranged to be received within the hollow body,

the gripping member being movable relative to the hollow member to urge the gripping member into gripping engagement with the strength member when, in use, the strength member is positioned within the longitudinal division of the gripping member.

The gripping member is said to be longitudinally divided. By this we mean that the gripping member comprises an element that is open in cross-section. For example, the longitudinal division of the gripping member may comprise a slot (or slit) into which the strength member may be positioned. The invention has the advantage that one size of arrangement can normally grip a range of sizes of strength member. The gripping member described in US patent no. 4795230 is limited in the range of sizes of strength member which it can accommodate and grip. The upper limit is determined by the internal diameter of the gripping member because the strength member has to extend through the gripping member, and the lower limit is determined by the amount by which the jaws of the gripping member may bend inwards towards the strength member in order to grip it. In contrast, the present invention has neither of these limitations: the longitudinally divided gripping member may normally be opened sufficiently to accommodate the largest, and closed to grip the smallest, commonly used strength members. Preferably the gripping member is provided with opposing surfaces which come together into gripping engagement with the strength member when the gripping member is moved relative to the hollow member.

According to a preferred embodiment of the invention, the longitudinally divided gripping member comprises at least two discrete portions. This embodiment is particularly advantageous for accommodating a range of sizes of strength member, because the discrete portions may normally be separated sufficiently to accommodate the largest, and brought together to grip the smallest, commonly used strength members. For example, one embodiment of the invention may grip strength members having diameters in the range 1.0-3.5mm. Other embodiments may accommodate wider ranges of strength member diameter.

According to a further preferred embodiment of the invention, the gripping member is arranged to be in gripping engagement with the strength member along substantially the entire length of the longitudinal division of the gripping member. This has the advantage of spreading the force applied to the strength member so that localised excessive pressures are substantially avoided. The avoidance of localised pressure is particularly important for strength members formed from materials which may be relatively easily damaged by applied pressure, e.g. composite materials, including glass fibre reinforced composite materials. In addition, gripping the strength member along substantially the entire length of the longitudin.al division of the gripping member normally enhances the grip on the strength member compared to methods in which the strength member is gripped only over a relatively small surface area, for example when the grip is applied at a localized area of the strength member as is the case with the gripping member described in US 4795230. It has been found, for example, that at least one embodiment of the invention may grip a glass-fibre reinforced composite strength member substantially without slippage when a force of 800N is applied to the strength member, and it is possible that even greater forces may be resisted. Furthermore, gripping the strength member along the length of the gripping member often provides enhanced support for the member, thereby substantially preventing kinking of the strength member or providing strain relief against bending, for example. The length of the longitudinal division of the gripping member, which is preferably also the length of the gripping member itself, is preferably in the range 10 to 30 mm, more preferably between 15 and 25 mm, a particularly preferred length being 18 mm.

Preferably, the hollow body has an arrangement according to the invention, wherein the hollow body has a tapered internal surface and the gripping member has an external surface shaped to cooperate with the tapered internal surface of the hollow body to urge the gripping member into gripping engagement with the strength member when, in use, the gripping member is moved towards the narrower end of the internal surface of the hollow body. More preferably, the internal surface of the hollow body is substantially frusto-conical. The external surface of the gripping member is preferably tapered, more preferably substantially frusto-conical. A particularly preferred embodiment is one in which the gripping member comprises two discrete portions, especially when each discrete portion is generally semi-circular in cross section.

A particularly preferred arrangement is one in which the portions of the gripping member are urged together such that opposing gripping surfaces of the member come together in a substantially parallel fashion. This may, for example, be achieved by providing the internal surface of the hollow body and the external surface of the gripping member with substantially the same taper, e.g. when both are substantially frusto-conical.

The gripping member may advantageously be provided with at least one channel that is shaped to receive the strength member of the optical fibre cable. This has the advantage of allowing an increased surface area of the gripping member to be in contact with the strength member compared to, for example, the use of flat surfaces for gripping the strength member. In addition, the increased gripping surface provided by the channel(s) can often prevent the strength member being broken under torsional forces. The channel also has the advantage of providing accurate location and alignment of the strength member in the gripping member, thereby normally avoiding unnecessary strain on the strength member due to mis- alignment. In a preferred embodiment, the channel or at least one of the channels is provided with one or more teeth or other gripping means that are arranged to enhance the grip of the gripping member on the strength member.

In some embodiments of the invention, the gripping member may be permanently retained in the hollow body. For example, the gripping member may be retained in a chamber in the hollow body in which the gripping member is movable relative to the hollow body. Preferably, however, the hollow body is provided with an opening, at the wider end of the tapering internal surface, arranged to allow the gripping member to be inserted into the hollow body. This opening may additionally, for example, be used to allow the strength member to extend into the hollow body, preferably however, the hollow body is provided with an opening, at the narrower end of the internal surface, arranged to allow the strength member to extend therethrough into the hollow body.

Preferably, the arrangement according to the invention further comprises a retaining member arranged both to effect the relative movement between the gripping member and the hollow body and to retain the gripping member in the hollow body to maintain the gripping engagement of the gripping member on the strength member. More preferably, the retaining member is arranged to be inserted into a hollow end region of the hollow body which extends beyond the opening at the wider end of the tapering internal surface of the hollow body. Even more preferably, the retaining member is provided with a screw thread and is arranged to be screwed into the hollow end region of the hollow body, for example the retaining member may comprise a threaded bolt.

A particularly preferred embodiment of the invention is an arrangement comprising a hollow body, a longitudinally divided gripping member arranged to be received within the hollow body, and a retaining member. The hollow body preferably has a frusto-conical internal surface and the gripping member preferably comprises at least two, most preferably two, discrete portions and is generally frusto-conical in shape. In use, the strength member of an optical fibre cable may be placed between the two discrete portions of the gripping member, preferably in channels provided in the discrete portions, and the gripping member then moved into the hollow body with the aid of the retaining member in the form of a threaded bolt. The bolt is then preferably tightened into a threaded portion of the hollow body, thereby tightening the frusto-conical hollow body and causing the gripping member to be tightened into gripping engagement with the strength member of the cable.

As mentioned earlier, it is common for optical fibre cables to include a strengthening jacket, e.g. of Kevlar fibres (Kevlar is a trade mark of Du Pont De Nemours Co.) located around the slotted core, in addition to a central strength member, extending through the slotted core, for example formed from steel or a composite material. According to a preferred embodiment of the invention, the hollow body is provided with means for allowing attachment of a strengthening jacket of an optical fibre cable thereto. More preferably, the means for allowing attachment of the strengthening jacket comprises an orifice communicating between the exterior of the hollow body and the hollow end region, wherein the orifice is arranged to receive a portion of the jacket and to allow it to be gripped between the retaining member and the hollow body. Particularly preferred is for the arrangement further to comprise a channel located in the hollow end region of the hollow body, which channel is arranged to receive the portion of the strengthening jacket.

The arrangement according to the invention preferably also includes means for attaching the arrangement to an optical fibre cable splice case, e.g. a bracket or other attachment member. Accordingly, the invention may comprise an arrangement as described herein which is attached to an optical fibre cable splice case. When the arrangement is attached to a cable splice case, the retaining member, for example in the form of a threaded bolt, may advantageously prevent the strength member being pushed into the splice case due to forces experienced by the cable, thereby substantially preventing damage to the fibres or the splices which might otherwise occur.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows in isometric projection the hollow body, gripping member and retaining member of one embodiment of the arrangement;

Figure 2 shows the arrangement of Figure 1 from below;

Figure 3 shows the arrangement of Figure 1 with an optical fibre cable inserted into the hollow body;

Figure 4 shows the arrangement of Figure 3 from below;

Figure 5 shows in cross-section the arrangement of Figure 1 gripping the strength member of an optical fibre cable; and

Figure 6 shows an external view of the arrangement and cable of Figure 5.

Figure 1 shows the hollow body 1, gripping member 3 and retaining member 5 of an arrangement according to the invention. The hollow body includes a tapering portion 7 which is frusto-conical, inside which is the tapering, in this case frusto-conical, internal surface. At the wider end of the frusto-conical portion is an opening arranged to allow the gripping member to be inserted into the hollow body. A hollow end portion 9 located beyond the frusto-conical portion of the hollow body is provided with an internal screw thread so that the retaining member, in the form of a threaded bolt, may be screwed into the hollow body to retain the gripping member therein. The gripping member comprises a conical frustum having two substantially identical discrete portions 3a and 3b, each being a semi- cylindrical conical frustum. The gripping member is arranged to grip the strength member of an optical fibre cable between the two portions 3a and 3b, and to enhance their grip on the strength member the portions are provided with cooperating channels 11 containing a series of teeth, in which the strength member may be located. A channel 13 is provided in the hollow end portion 9 for trapping and gripping a portion of a strengthening jacket of the cable, e.g. Kevlar (trade mark of Du Pont De Nemours Co.) fibre between the hollow end portion 9 and the retaming member 5. An orifice (hidden from view in Figure 1) communicating between the inside of the hollow end portion and the exterior of the hollow body permits insertion of the portion of Kevlar fibre into the channel.

The orifice hidden from view in Figure 1 is shown in Figure 2 and indicated by the numeral 15. Figure 2 shows the arrangement of Figure 1 from below, and and this view shows an opening 17 beyond the narrower end of the frusto-conical portion 7 of the hollow body, for allowing the optical fibre cable to extend into the hollow body.

Figure 3 shows a view of the .arrangement similar to the view of Figure 1, with part of an optical fibre cable 19 extending through the opening 17 into the hollow body 1. Portions of the cable have been stripped back from its end to expose lengths of the strength member 21 and the slotted core (sometimes referred to as a 'jonc') 23. The jonc extends into a narrow end region 25 of the hollow body, thereby providing additional support for the cable and minimizing the length of strength member which needs to be exposed. The exposed length of the strength member preferably extends through the frusto-conical portion 7 of the hollow body and through the hollow end portion 9, to ensure that the full length of the gripping member grips the strength member, thereby maximizing the grip on the strength member. Figure 4 shows the arrangement of Figure 3, including the fibre optic cable, from below.

Figure 5 shows, in cross-section, the arrangement once the gripping member 3 has been inserted into the hollow body, gripping the exposed strength member 21 between its two frusto-conical portions 3a and 3b in the cooperating channels 1 1. Insertion of the gripping member into the hollow body has caused the frusto-concial internal surface of the hollow body to cooperate with the correspondingly frusto-conical external surface of the gripping member to urge the two portions of the gripping member together to grip the strength member substantially uniformly along the length of the strength member located between the two portions. The retaining member 5 has been screwed into the hollow end portion 9 of the hollow body to retain the gripping member inside the hollow body and to maintain the grip on the strength member. In addition, the Kevlar (trade mark) fibres of the strengthening jacket have been twisted into a braid 25 and a length of the braid has been inserted into the orifice 15 of the hollow body and located in the channel 13 provided in the hollow end portion of the hollow body. Upon screwing the retaining member into the hollow end portion, the Kevlar braid has been trapped and gripped by the retaining member, thereby providing additional securement for the optical fibre cable.

Figure 6 shows an external view of the arrangement gripping the optical fibre cable 19. Also shown in figures 1 to 5 is a bracket 27 for attaching the arrangement to a suitable object, e.g. an optical fibre splice case.

Claims

Claims:

1. An arrangement for gripping the strength member of an optical fibre cable, comprising:

(a) a hollow body, and

2. An arrangement according to claim 1, wherein the longitudinally divided gripping member comprises at least two discrete portions.

3. An arrangement according to claim 1 or claim 2, wherein the hollow body has a tapered internal surface and the gripping member has an external surface shaped to cooperate with the tapered internal surface of the hollow body to urge the gripping member into gripping engagement with the strength member when, in use, the gripping member is moved towards the narrower end of the internal surface of the hollow body.

4. An arrangement according to claim 3, wherein the internal surface of the hollow body is substantially frusto-conical.

5. An arrangement according to claim 3 or claim 4, wherein the external surface of the gripping member is tapered.

6. An arrangement according to claim 5, wherein the external surface of the gripping member is substantially frusto-conical.

7. An arrangement according to any preceding claim, wherein the gripping member is arranged to be in gripping engagement with the strength member along substantially the entire length of the longitudinal division of the gripping member.

8. An arrangement according to claim 7, wherein the length of the longitudinal division of the gripping member is in the range 10 to 30 mm, preferably between 15 and 25 mm.

9. An arrangement according to any one of claims 2 to 8, wherein the gripping member comprises two discrete portions.

10. An arrangement according to claim 9, wherein each discrete portion is generally semi-circular in cross-section.

11. An arrangement according to any preceding claim, further comprising a retaining member arranged both to effect the said relative movement between the gripping member and the hollow body and to retain the gripping member in the hollow body to maintain the said gripping engagement of the gripping member on the strength member.

12. An .arrangement according to any one of claims 3 to 11, wherein the hollow body is provided with an opening, at the wider end of the tapering internal surface, arranged to allow the gripping member to be inserted into the hollow body.

13. An arrangement according to any one of claims 3 to 12, wherein the hollow body is provided with an opening, at the narrower end of the internal surface, arranged to allow the strength member to extend therethrough into the hollow body.

14. An arrangement according to any preceding claim, wherein the gripping member is provided with at least one channel that is shaped to receive the strength member.

15. An arrangement according to claim 14, wherein the channel or at least one of the channels is provided with one or more teeth that are arranged to enhance the grip of the gripping member on the strength member.

16. An arrangement according to any one of claims 11 to 15, wherein the retaining member is arranged to be inserted into a hollow end region of the hollow body which extends beyond the opening at the wider end of the tapering internal surface of the hollow body.

17. An arrangement according to claim 16, wherein the retaining member is provided with a screw thread and is arranged to be screwed into the hollow end region of the hollow body.

18. An arrangement according to any preceding claim, wherein the hollow body is provided with means for allowing attachment of a strengthening jacket of the optical fibre cable thereto.

19. An arrangement according to claim 18, wherein the means for allowing attachment of a strengthening jacket of the cable comprises an orifice communicating between the exterior of the hollow body and the hollow end region, wherein the orifice is arranged to receive a portion of the jacket and to allow it to be gripped between the retaining member and the hollow body.

20. An arrangement according to claim 19, which further comprises a channel located in the hollow end region which is arranged to receive the portion of the jacket.

21. An arrangement according to any preceding claim, further comprising means for attaching the arrangement to an optical fibre cable splice case.

22. An arrangement according to any preceding claim, which is attached to an optical fibre cable splice case.