CH624221A5 - Optical-fibre cable having a reinforcing element - Google Patents

Description

The present invention relates to a light guide cable with a reinforcing member.

The conventional structure of an optical fiber cable contains an axial reinforcing member which is surrounded by at least one layer of plastic-coated optical fibers; a polyester tape is then wrapped around this structure and the entire structure is provided with a protective plastic jacket. The reinforcing member located in the axis of the cable ensures the greatest possible flexibility of the cable and the lowest possible tensile stress on the optical fibers. Until recently, the main focus when designing an optical fiber cable was on a small diameter and possibly a small weight; such a cable, usually up to 100 m long, should have sufficient tensile strength to withstand the usual handling during installation without damage. Reinforcement links, which consisted of oriented single polyester threads, met these requirements. Today's longer cables, 500-1000 m long, however, require better reinforcing elements.

In the construction of fiber optic cables, the rule applies that the elongation of the cable under maximum tensile stress, which the cable may be subjected to during its use, may only move within certain limits. In the case of a helical arrangement of the fibers within the cable, the fiber elongation is smaller than the elongation of the cable itself, the difference in elongation depending on the pitch angle. For reasons of optical transmission, however, it is important to keep the slope large; in practice, the fiber elongation is the same as the cable elongation with pure tensile stress.

The elongation at break of glass and optical fibers made of silicon dioxide is small and ranges between 0.2% and slightly more than 5% depending on the manufacturing method. The choice of an elongation number is therefore somewhat arbitrary, but the value of 1% seems to be useful in determining the permissible tensile load. Regardless of this choice, the ratio of the tensile load to the elongation of the cable should be as high as possible for the given dimensions or for the given weight of a cable; this is achieved by using components with a high modulus of elasticity. On the other hand, good flexibility should also be guaranteed,

however, which presupposes the use of materials with a low modulus of elasticity. According to the invention, this problem is solved by the measures specified in claim 1.

Embodiments of the invention will now be explained in more detail with reference to the drawing. Figures 1 to 3 show 3 different forms of braided reinforcing members for fiber optic cables.

In one embodiment, an aromatic polyamide fiber material, known under the name KEVLAR, is braided together to form a rope shown in the figures. The rope, or braid, is preferably wrapped in plastic material to form a smooth surface, which is advantageous in the subsequent stranding of the individual plastic-coated optical fibers onto the reinforcing member. At least one layer of polyester tape is then wrapped around the optical fibers with a reinforcing member, and finally an outer plastic jacket is applied by extrusion. The reinforcing member advantageously consists of fibers made of KEVLAR-49, but fibers made of KEVLAR-29 can also be used. Table I below gives comparative values of the materials mentioned in thread form and a steel wire:

Table I

Spec. Weight

Modulus of elasticity 105N / mm2

Elongation at break

%

Steel wire

7.86

1.93

2nd

Kevlar 29 fiber

1.44

0.83

4th

Kevlar 49-

fiber

1.45

1.31

2.5

The effective tensile modulus of a KEVLAR braid is smaller than that of the individual threads; however, its size is sufficiently high.

The materials mentioned were generally used for high tensile cables and in particular for light guide cables, the high tensile modules being used without taking into account the flexibility of these cables. A sufficiently break-proof, single steel wire is usually unacceptably stiff, which is due to the fact that the strength is proportional to the square of the diameter, but the stiffness is the fourth power. The ratio of flexibility to strength is improved by using strands that have smaller individual diameters. KEVLAR single fibers have very small diameters (e.g. 12 (Am) and must therefore be used in a multi-fiber structure to form a sufficiently strong reinforcing member. The ratio of flexibility to strength is therefore very favorable in principle. A maximum tensile modulus is achieved when the Fibers lie parallel and axially, but in this case they do not form a coherent strand. A certain degree of coherence is achieved by twisting, especially when using a binding resin that glues the fibers together. This in turn deteriorates the flexibility, which leads to permanent kinking All these problems are practically eliminated by using braids, see Figures 1 to 3.

Another advantage of the KEVLAR braids results from their stress / strain characteristic curves, which basically have two independent regions: a region with a low modulus with small strains, which merges into a region with a high modulus with increasing strain. Depending on the structure of the braid, the elongation range at low modulus can be 0.05-0.4%, usually 0.2. The transition to the region with high modulus at

5

10th

15

20th

25th

30th

35

40

45

50

55

60

65

3rd

624 221

Greater stretch is quick, so the effective module is up to 1% high. The following are measured values for the 1% module:

Kevlar 29

0.33 X 105 N / mm2

Kevlar 49

0.94 X 105 N / mm2

The presence of the region with a low modulus indicates a high degree of flexibility with little or no tensile load. The flexibility of a braid is thus promoted by its characteristic structure, in which the strands pass between the inner and outer positions of a band, the pitch being only a few millimeters.

In order to emphasize the advantages of KEVLAR braids, a comparison should now be made with a conventional cable, consisting of a stranded, centrally located steel reinforcement element, further of eight coated fibers, and finally of layers of tape and sheathing. It is assumed that the packing density of the strands is constant along their entire length and that the steel reinforcement member takes on 80% of the strain on the cable. It is also assumed that the cable is subjected to tensile stress at an elongation of 1%. Specific weights: steel 7.86, KEVLAR 1.45, other parts 0.9. The tensile strength and the weight are related to the same sizes of the steel cable. The results are summarized in Table II below:

Reinforcement of the total tensile weight tensile strength / reinforcement diam. stability weight

Note: In group 2, the KEVLAR reinforcement links have the same diameter as the steel reinforcement link; in group 3 the KEVLAR links have the same tensile strength as the steel link.

5 Various braid shapes are shown in FIGS. 1 to 3, although other shapes could of course also be used. The table below deals with the physical properties of the KEVLAR braids.

10th

Table III

Fig.

Braid type

Total-

Slope weight

fiber

0

mm g / m across

15

mm

cut

mm2

1

2 pairs of S and Z intertwined

1.1

13

0.8

0.55

2nd

8 strands 4S + 4Z

1.6

19th

1.5

1.04

20 3

8 strands 4S + 4Z

0.7

7

0.38

0.27

25th

Note: «S» or «Z» indicate the twisting of helical strands clockwise or counterclockwise.

element

Limbs

1 steel

1.5mm

6.5mm

1.00

1.00

1.00

2 Kev. 49

1.5mm

6.5mm

0.62

0.75

0.83

Kev. 29

1.5mm

6.5mm

0.35

0.75

0.47

3 Kev. 49

2.1mm

7.1mm

1.00

0.88

1.11

Kev. 29

3.5mm

8.5mm

1.00

1.33

0.74

In a further embodiment, the KEVLAR braid with a plastic covering can be used as a reinforcing filling, which forms a cushion between the optical fibers 30 in the cable. The low initial modulus allows this material to be used in non-axial positions without losing flexibility and while maintaining high tensile strength.

1 sheet of drawings

Claims (5)

624 221 1. Optical cable with a reinforcing member, characterized in that this member extends in the axial direction in the cable and consists of plastic multiple fibers, that a number of optical fibers are arranged in the longitudinal direction of the cable along the reinforcing member, and that at least one protective sheath surrounds the optical fibers and the reinforcing member . 2. Cable according to claim 1, characterized in that the axial reinforcing member is formed from a rope consisting of a synthetic plastic braid, along which the optical fibers are arranged, running in the longitudinal direction of the cable, that at least one layer of polyester tape is wound around this structure, and that the whole thing is protected by an outer protective plastic sheath. 2nd PATENT CLAIMS 3. Cable according to claim 2, characterized in that the reinforcing member is provided with a plastic layer which ensures a smooth surface of the reinforcing member, mn which the optical fibers are wound. 4. Cable according to claim 2 or 3, characterized in that a reinforcing plastic fiber is present between the optical fibers of the cable. 5. Cable according to claim 1, characterized in that the reinforcing member is formed from an aromatic polyamide.