AU729539B2

AU729539B2 - Fibre-optic cable having enhanced crushing strength

Info

Publication number: AU729539B2
Application number: AU16440/97A
Authority: AU
Inventors: Herve Heude; Pierre Laroche; Bernard Michel Missout
Original assignee: Pirelli Cables y Sistemas SA; Cables Pirelli SA
Current assignee: Cables Pirelli SA
Priority date: 1996-02-29
Filing date: 1997-03-20
Publication date: 2001-02-01
Anticipated expiration: 2017-03-20
Also published as: FR2745642B1; DE69737577T2; NZ314463A; CA2200283C; CA2200283A1; EP0793127A1; AU1644097A; EP0793127B1; ES2285724T3; DE69737577D1; FR2745642A1

Description

'6 Our Ref: 633128 P/00/011 Regulation 3:2

AUSTRALIA

Patents Act 1990

ORIGINAL

COMPLETE SPECIFICATION STANDARD PATENT Applicant(s): Cables Pirelli S.A.

1, rue des Usines 94410 Saint Maurice

FRANCE

DAVIES COLLISON CAVE Patent Trade Mark Attorneys Level 10, 10 Barrack Street SYDNEY NSW 2000 Address for Service: Invention Title: Fibre-optic cable having enhanced crushing strength The following statement is a full description of this invention, including the best method of performing it known to me:- 5020 P:\WPOCS\AMD\SPECI\633128.PIR 20/3/97 -1- FIBRE-OPTIC CABLE HAVING ENHANCED CRUSHING STRENGTH The invention relates to fibre-optic cables and, more specifically, to cables the structure and shape of which are such that their crushing strength is quite markedly improved, in particular by comparison with cables having a circular cross-section of a convention design.

Fibre-optic cables are well known and have developed rapidly over the last few years, particularly in the field of telecommunications, owing to the considerable transmission 1 0 potential of optical fibres of very small dimensions. These fibres nonetheless have to be mechanically protected, by the structure of the cable, from the stresses associated with the radial forces applied to the cable, such as crushing, impact, etc. Cable structures enclosing these fibres have thus been developed to protect them mechanically according to the environments in which they are located.

Known fibre-optic cable have variety of structures. There is known, for instance, a type of tube-equipped cable comprising a central reinforcement, surrounded by a plurality of helically or pseudo-helically wound tubes enclosing a bundle of optical fibres capable of moving within the tubes. This ability of the fibres to move within the tubes compensates for the variations in length of the cable due to thermal or tensile stresses. This structure is, however, too voluminous by comparison with the real space taken up by the fibres it contains and it is unsuitable, for example, for distribution cables in an urban network in which the object is to obtain an optimal ratio of the numbers of fibres to the diameter of the cable.

It is thus preferred to use cables with a low expansion coefficient, wherein the fibres are, for example, disposed in bundles (round or flat ribbon micro-modules), the bundles being disposed practically without clearance between them in the inner cylindrical cavity of the cables.

A good ratio of the number of fibres to the outside diameter of the cable can thus be obtained. The cross-section of the cables is circular and the sheathes of the cables may P:\WPDOCS\AMD\SPEC\633128.PIR- 20/3/97 -2possibly incorporate metallic or non-metallic reinforcements, which may be two in number, and which extend along two diametrically opposed generating lines. By reason of the very small radial clearance that remains between the fibres and the inner cavity of the cable, when crushing forces are applied or in the event of impacts, the deformation of the inner cavity containing the fibres that is induced exerts inadmissible compressive stress on the said fibres, leading to additional weakening, or even failure.

The sheath therefore has to be reinforced to limit the deformation as far as possible when the aforementioned stresses are applied.

0000 0gg o The solutions adopted to reduce these deformations, such as the use of metallic reinforcements, materials having a high modulus of elasticity, wall thickening, etc., have drawbacks, however.

i'0. 015 The Applicant has thus applied itself to finding a solution to this problem to enable an optical cable, whatever the environment in which it is located, to possess increased resistance to the stresses to which it might be subjected and particularly capable of withstanding any localised crushing or impacts that it might be exposed to, for instance, at the time of cable laying operations.

0. *One main object of the present invention thus resides in an optical cable possessing crushing strength, comprising a plurality of optical fibres located inside a sheath, an optical cable in which the external part of the sheath possesses four bosses moulded integrally therewith, which extend over the entire length of the cable, and form along the generating lines of the cable four diametrically opposed ribs.

Other characteristics and advantages of the invention will emerge from the following description of a non-limitative example of a form of embodiment, with references to the figures, which represent cross-sectional views of optical cables, Figures 1 and 3 are crosssectional views of the cable according to the invention, Figure 2 illustrates the application of the stresses to a cylindrical cable of conventional design, Figure 4 diagrammatically P:\WPDOCS\AMD\SPECI633128.PIR 20/3/97 -3represents an alternative form of embodiment.

Cable 1, shown in Figure 1, comprises in its centre a group of eight optical fibre micro-modules 2. The micro-modules are covered by an outer sheath 4.

The outer portion of sheath 4 is not circular but has, along diametrically opposed generating lines, four bosses 5 which extend over the entire length of the cable. The bosses, which are integrally moulded into the body of the sheath, form, as it were, external ribs which project in relation to a tangent to the circular portion of sheath, and which have a rounded external profile. If necessary, traction reinforcements 6 can be provided inside one or more of these bosses, the reinforcements being metallic or non-metallic, for example, made of aramid resin, and being embedded in the sheath in the area of the bosses when the cable is extruded. Alternatively, traction elements or copper wires can also be housed in the bosses. These elements can serve as leads or limit the minimum radius of curvature of the 15 cable.

This arrangement thus makes it possible to avoid making the sheath 4, one of the functions of which is to withstand crushing, very thick. Thanks to the bosses, it is thus possible to reduce the thickness of the sheath, which reduces the weight of the cable by a corresponding amount and lowers the thermal expansion coefficient.

When a compressive stress, represented in Figure 1 by the presence of a plate 3 pushed by a force F, is applied to the cable, the stable position of the cable is as shown in Figure 1. If the stress is applied thereto along another generating line, it will automatically tip over to resume the position represented.

For the same sheath thickness, the case in which the stress is applied illustrated in Figure 3 is clearly more favourable, among other positive effects, than the case shown in Figure 2, as it produces a smaller bending moment at the inferior point. The result is, for the same stress, a deflection under load that is smaller in the case shown in Figure 3 than in that of Figure 2.

1.

P:\WPDOCS\AMD\SPECI\633128.PIR 20/39 -4- According to the same principle, -the external shape could be practically square, as illustrated in Figure 4.

For maximum efficiency, distance X must be positive, or at the very least nil, as shown in Figure 1.

S. S SS S

Claims

1. An optical fiber cable having improved crushing strength, said cable comprising: a tubular plastic sheath having a length and having a wall of a predetermined inner diameter which encircles a plurality of optical fibers; said wall having a radial thickness at four portions of the wall spaced substantially equally from each other in the circumferential direction of the wall which is greater than the thickness of the wall intermediate said portions to provide two pairs of ribs extending outwardly from said wall and lengthwise of said sheath, one pair of said ribs being diametrically opposed with respect to each other and the other pair of said ribs being diametrically opposed with respect to each other.

2. Optical cable according to claim 1, characterized in that two of the diametrically opposed ribs enclose metallic or non-metallic reinforcements.

3. Optical cable according to claim 1, characterized in that the optical fibres are grouped together in micro-modules.

4. Optical cable according to claim 1, characterized in that the optical fibres are grouped together in ribbons. Optical cable according to claim 1, characterized in that at least two of the diametrically opposed ribs enclose metallic or non-metallic reinforcements which can serve as leads or limit the minimum radius of curvature of the cable. Dated this 2 6 th day of April 2000 CABLES PIRELLI SA By Their Patent Attorneys DAVIES COLLISON CAVE