JP7495267B2 - Optical fiber cable and its manufacturing method - Google Patents

Description

The present invention relates to an optical fiber cable with excellent bendability.

Optical fiber cables are used that contain many optical fiber cores and tension members covered with an outer jacket.

One example of such an optical fiber cable is one in which tension members (tensile members) that have resistance to tension and compression are arranged on both sides of a core made of optical fiber wire, and the materials for the tension members can be metal wires (such as steel wires), tensile fibers (such as aramid fibers), FRP, etc. (for example, Patent Document 1).

JP 2020-042175 A

Optical fiber cables such as those in Patent Document 1 are usually branched as necessary, connected to drop cables, and pulled into buildings, etc. In the following, the slotted or slotless cable with an ultra-high number of cores (e.g., 200 cores or more) that is commonly used for high-density transmission of large amounts of information such as for long-distance transmission or between data centers, as in Patent Document 1, will simply be referred to as an optical fiber cable, and optical fiber cables for drop applications (e.g., optical fiber cables with a roughly rectangular cross section and a notch formed in the outer sheath) will be referred to as drop cables to distinguish them from these.

Unlike drop cables, these types of optical fiber cables are usually used outdoors and not for pull-in applications. Therefore, situations in which a surge current would flow from outdoors into a building due to a lightning strike are not anticipated, and the optical fiber cable itself does not need to be non-inductive. For this reason, steel wires have traditionally been applicable to tension members.

However, in recent years, the use of this cable in locations close to power lines has been considered, and the demand for non-inductive cable has increased, necessitating the use of glass FRP (Fiber-Reinforced Plastics) or aramid FRP for the cable's tension members.

As in Patent Document 1, in a slotless cable, at least two tension members are arranged away from the center of the cable core. Therefore, the optical fiber cable can only be bent in a direction that has the line connecting the two tension members as the center of bending. In contrast, if an attempt is made to bend the optical fiber cable in a direction other than the line connecting the two tension members, the optical fiber cable will rotate, and as a result, the optical fiber cable will bend in a direction that has the line connecting the two tension members as the center of bending.

However, when an optical fiber cable is laid in the field, the bending or rotation of the optical fiber cable may be hindered, and the optical fiber cable may be bent in a direction that it cannot bend. In this case, a strong compressive force may be applied to the tension member, but conventional steel wires can adequately withstand such a situation.

On the other hand, unlike steel wire, glass FRP and aramid FRP have a lower compressive modulus relative to the tensile modulus, and so have low strength when compressed. For this reason, as mentioned above, there is a problem that the tension member buckles and breaks when bent in a direction that it should not be bent in.

On the other hand, if strength is increased by making the tension member thicker, the diameter of the optical fiber cable will increase and the flexibility of the optical fiber cable will deteriorate.

The present invention was made in consideration of these problems, and aims to provide an optical fiber cable that can suppress buckling of tension members even when tension members other than steel wires are used.

In order to achieve the above-mentioned object, the first invention is an optical fiber cable comprising a core consisting of a plurality of optical fiber cores, tension members provided on both sides of the core in cross section, and a substantially circular outer sheath provided to cover the tension members and the optical fiber cores, wherein the core is formed by twisting together a plurality of optical fiber units and is housed within the sheath via a pressure winding, the compressive elastic modulus of the tension member is less than or equal to 1/2 of the tensile elastic modulus, the adhesion between the tension member and the sheath is 30 N/10 mm or more and 42 N/10 mm or less , and the tension member is made of glass FRP or aramid FRP.

It is desirable that the adhesion strength between the tension member and the outer sheath is 40N/10mm or more.

It is preferable that the tension member is coated in advance with the same resin as the outer covering.
It is desirable that the pressure winding be provided as a vertical splice winding.
The optical fiber unit is preferably formed by twisting together a plurality of optical fiber cores.
The optical fiber may be an intermittently bonded optical fiber ribbon that is bonded intermittently in the longitudinal direction.
The second invention is a method for manufacturing an optical fiber according to the first invention, which is a method for manufacturing an optical fiber cable, characterized in that a tension member and a core, which have been previously coated with resin, are extrusion-coated with an outer sheath made of the same resin as the resin .
The tension member, which has been previously coated with resin, may be preheated before being extruded.

According to the present invention, when a tension member having a compressive elastic modulus of less than half the tensile elastic modulus is used, the adhesion force between the tension member and the outer sheath is greater than or equal to a predetermined value, so that buckling during compression of the tension member can be suppressed.

The present invention is particularly effective when the tension member is made of glass FRP or aramid FRP.

The present invention provides an optical fiber cable that can suppress buckling of the tension member even when a tension member other than a steel wire is used.

FIG. 4A to 4C are diagrams showing a method for measuring the compressive strength of a tension member 9. 4 is a diagram showing a method for measuring the adhesion between the tension member 9 and the outer sheath 13. FIG. 4A is a diagram showing a bending test method for an optical fiber cable 1, and FIG. 4B is an enlarged cross-sectional view of part C in FIG.

The following describes an embodiment of the present invention with reference to the drawings. Fig. 1 is a cross-sectional view of an optical fiber cable 1. The optical fiber cable 1 is a slotless type cable that does not use slots, and is composed of a core 4, a tension member 9, an outer jacket 13, etc.

The core 4 is formed by twisting together a number of optical fiber units 5. The optical fiber unit 5 is formed by twisting together a number of optical fiber core wires 3. The optical fiber core wires 3 are, for example, intermittently bonded optical fiber ribbon core wires that are intermittently bonded in the longitudinal direction.

As shown in FIG. 1, a pressure wrap 7 is provided on the outer circumference of the multiple optical fiber units 5. The pressure wrap 7 is a tape-like member or nonwoven fabric, and is arranged, for example, by vertical wrapping to collectively cover the outer circumference of the multiple optical fiber units 5. That is, the pressure wrap 7 is vertically wrapped around the outer circumference of the multiple optical fiber units 5 so that the longitudinal direction of the pressure wrap 7 is approximately aligned with the axial direction of the optical fiber cable 1, and the width direction of the pressure wrap 7 is the circumferential direction of the optical fiber cable 1. Note that the pressure wrap 7 is not necessarily required, and the pressure wrap 7 may be referred to as the core 4.

In a cross-sectional view perpendicular to the longitudinal direction of the optical fiber cable 1, tension members 9 are provided on both sides of the core 4. That is, a pair of tension members 9 are provided at positions facing each other with the core 4 in between. Details of the tension members 9 will be described later. In addition, tear cords 11 are provided in a direction approximately perpendicular to the facing direction of the tension members 9, facing each other with the core 4 in between.

An outer jacket 13 is provided on the outer circumference of the core 4. The tension member 9 and the tear cord 11 are embedded in the outer jacket 13. In other words, the outer jacket 13 is provided so as to cover the core 4 (multiple optical fiber cores 3) and the tension member 9, etc. The outer shape of the outer jacket 13 is approximately circular. The outer jacket 13 is, for example, a polyolefin-based resin.

Next, the tension member 9 will be described. The tension member 9 bears the tension of the optical fiber cable 1. For example, fiber reinforced plastic (FRP) made of aramid fiber, glass fiber, etc. can be used for the tension member 9. Such FRP has a small compressive elastic modulus compared to its tensile elastic modulus. For example, the compressive elastic modulus of the tension member 9 is 1/2 or less of its tensile elastic modulus.

The tensile modulus of the tension member 9 can be measured according to JIS K7161. Meanwhile, the compressive modulus of the tension member 9 is measured by the method shown in Figure 2. First, 20 pieces of tension member 9 are cut and bundled together to create a sample, with the top and bottom ends solidified with epoxy resin 15. The distance between the resins 15 is 15 mm (for example, approximately the same as the outer diameter of the bundle of tension members 9). The compressive modulus can be measured from the load and displacement when this sample is compressed (A in the figure) using an Instron testing machine.

The adhesion between the tension member 9 and the outer sheath 13 is preferably 30 N/10 mm or more, and more preferably 40 N/10 mm or more. The adhesion between the tension member 9 and the outer sheath 13 can be measured by the method shown in Figure 3. First, the outer sheath 13 is removed from the outer periphery of the tension member 9, leaving a portion of the outer sheath 13. The length of the remaining outer sheath 13 (the length in the longitudinal direction of the tension member 9) is set to 10 mm.

Then, the tension member 9 is inserted into a die 17 having a hole. The size of the hole in the die 17 is slightly larger than the outer diameter of the tension member 9 so that the tension member 9 can pass through without resistance, and is smaller than the size of the outer sheath 13 left on the outer periphery of the tension member 9. For example, for a tension member 9 with a diameter of 2 mm, the inner diameter of the hole is set to about 2.3 mm.

After the tension member 9 with the outer sheath 13 is passed through the die 17, the tension member 9 is pulled out at a speed of 50 mm/min (arrow B in the figure). This allows the outer sheath 13 to be peeled off from the tension member 9 by the die 17. The maximum stress at this time is the adhesion force between the tension member 9 and the outer sheath 13.

Here, if the adhesion between the tension member 9 and the outer jacket 13 is low, when the optical fiber cable 1 is bent, if the internal tension member 9 bends, there is a risk that the tension member 9 and the outer jacket 13 will peel off. In this case, if the optical fiber cable 1 is bent so that a compressive force is applied to one of the tension members 9, the tension member 9 and the outer jacket 13 will have peeled off, and the restraining force of the outer jacket 13 from the outer periphery of the tension member 9 will be reduced. This means that there is a risk that the tension member 9 will buckle.

However, if the adhesion between the tension member 9 and the outer sheath 13 is strong and the tension member 9 and the outer sheath 13 are in close contact with each other, even when a bending force is applied to the tension member 9, the restraining force from the outer periphery of the tension member 9 by the outer sheath 13 is strong, which suppresses buckling of the tension member 9. This is particularly effective when a material such as FRP, which has a low compressive elastic modulus relative to the tensile elastic modulus, is used for the tension member 9, as in this embodiment.

Next, we will explain the manufacturing method of the optical fiber cable 1. The optical fiber cable 1 can be manufactured using a manufacturing method similar to that of a normal optical fiber cable, but as mentioned above, it is necessary to increase the adhesion between the tension member 9 and the outer sheath 13. In particular, it is difficult to increase the adhesion between FRP, which is made by solidifying glass or aramid fibers with a thermosetting resin, and the outer sheath 13, which is made of low-density polyethylene (LDPE) or the like.

For this reason, it is desirable to first coat the outer circumference of the FRP wire used in the tension member with the same resin as the sheath 13. By extruding the sheath 13 using the tension member 9 coated with this resin, it is possible to increase the adhesion between the tension member 9 and the sheath 13. In this case, it is desirable to raise the extrusion temperature higher than usual. For example, when extruding the sheath 13 of linear low density polyethylene (LLDPE), the extrusion temperature is usually around 180 degrees, but it is desirable to set it to around 200 degrees.

Furthermore, in order to increase the adhesion between the tension member 9 and the jacket 13, it is desirable to preheat the tension member 9 before extrusion. For example, normally, the tension member 9 is used at room temperature, but by preheating it to about 60°C (for example, between 40°C and the extrusion temperature), the adhesion during jacket extrusion can be increased.

Furthermore, it is desirable to lengthen the time until the jacket 13 is cooled after extrusion coating. For example, after normal extrusion coating, the distance to the cooling water tank is about 100 mm, but by lengthening this distance to about 500 mm, the fusion time between the tension member 9 and the jacket 13 can be secured, and the adhesion between the tension member 9 and the jacket 13 can be increased. By combining these, it is possible to obtain an optical fiber cable 1 with a stronger adhesion between the tension member 9 and the jacket 13.

As described above, according to this embodiment, the adhesion between the tension member 9 and the outer sheath 13 is strong, so that buckling of the tension member 9 when bent can be suppressed even if the compressive elastic modulus of the tension member 9 is smaller than the tensile elastic modulus of the steel wire.

Bending tests were conducted on optical fiber cables by changing the configuration of the tension member. The optical fiber cable used for evaluation had a cross-sectional shape roughly as shown in Figure 1. First, eight optical fibers with a diameter of 250 μm were intermittently bonded together to create an eight-core optical fiber ribbon. Ten of these were then twisted together and wrapped with a 2 mm-wide plastic tape to create an 80-core optical fiber unit.

25 of these 80-core optical fiber units were supplied and twisted together, then absorbent nonwoven fabric was attached lengthwise, rolled with a forming jig, and nylon pressure thread was wrapped around them to create a 2,000-core core. The core thus created, a tension member made of FRP with a diameter of 2.0 mm, and a ripping string for ripping the outer jacket were sheathed into a cylindrical shape with the outer jacket material to create a cable. The outer jacket material was LLDPE. The outer jacket thickness was 3.0 mm.

The adhesion was adjusted as follows. For Examples 1 to 4, the outer circumference of the FRP wire was covered with 0.15 mm thick LDPE before the outer jacket was extruded, so that the thermosetting resin of the FRP and the polyethylene were preliminarily adhered to each other. Samples with different adhesion were also created by adjusting the presence or absence of preheating of the tension member, the extrusion temperature, and the time until cooling after extrusion. On the other hand, Comparative Examples 1 and 2 were manufactured under conventional conditions.

For each optical fiber cable, the tensile modulus of the tension member was measured according to JIS K 7161, and the compressive modulus was measured according to the method shown in Figure 2. The ratio of the obtained tensile modulus to the compressive modulus was calculated. The adhesion between the tension member and the jacket was measured according to the method shown in Figure 3.

Two types of bending tests were performed on the optical fiber cable: (1) a bending test without twisting, and (2) a bending test with twisting. Figure 4 (a) shows the bending test method.

The optical fiber cable 1 was placed on a φ500 mm mandrel 19 and bent at an angle of 90 degrees. At this time, the optical fiber cable 1 was held at positions 30 cm on either side of the point where it was placed on the mandrel 19, and was supported by hand to prevent the optical fiber cable 1 from rotating.

Figure 4(b) is an enlarged cross-sectional view of part C in Figure 4(a). The tension member 9 is positioned on the mandrel 19 side, and the bending direction of the optical fiber cable 1 is the direction in which the tension member 9 is arranged. In other words, the bending direction is perpendicular to the normal bending direction of the optical fiber cable 1.

After bending in this manner, the presence or absence of buckling or damage to the tension member 9 was judged based on the presence or absence of damage to the outer sheath 13. In other words, when buckling or the like occurs in the tension member 9, the outer sheath 13 surrounding the tension member 9 is also damaged, and this was used to judge the presence or absence of buckling of the tension member 9. Regarding the outer sheath 13, cracks were confirmed in the outer sheath 13 on the inside of the bend (D in the figure, the outer sheath 13 on the core 4 side of the tension member on the inside of the bend) and cracks in the outer sheath 13 on the outside of the bend (E in the figure, the outer sheath 13 on the outer periphery of the tension member on the outside of the bend).

The above test was evaluated (1) when it was performed without twisting, and (2) when it was performed with a twist of 180 degrees/m. When twisting was performed, the bending test was performed with the tension member 9 positioned exactly on the mandrel 19. Twisting the tension member 9 applies a twist, so the evaluation is performed under more severe conditions. The results are shown in Table 1.

As shown in Table 1, when the tension member 9 was made of glass FRP, the compressive elastic modulus/tensile elastic modulus was 0.48, and when it was made of aramid FRP, the compressive elastic modulus/tensile elastic modulus was 0.43.

In Example 1, the adhesion between the tension member 9 and the outer sheath was 30 N/10 m. In Example 1, no cracks were found in the outer sheath 13 in the bending test (1). In Example 2, the extrusion temperature was increased compared to Example 1, and the time until cooling was extended, resulting in an adhesion between the tension member 9 and the outer sheath 13 of 41 N/10 m. In Example 2, no cracks were found in the outer sheath in the bending test (1) or in the bending test (2).

Example 3 was manufactured under substantially the same conditions as Example 1, but the tension member was changed to aramid FRP, and the results were similar to those of Example 1. Similarly, Example 4 was manufactured under substantially the same conditions as Example 4, but the tension member was changed to aramid FRP, and the results were similar to those of Example 2.

On the other hand, in Comparative Examples 1 and 2, which were manufactured using the conventional manufacturing method, the adhesion between the tension member 9 and the outer sheath was less than 30 N/10 m, and cracks in the outer sheath were confirmed in both bending tests (1) and (2).

From the above, if the adhesion force between the tension member 9 and the outer sheath is 30N/10m or more, it is possible to suppress the occurrence of cracks in the bending test (1), and if the adhesion force between the tension member 9 and the outer sheath is 40N/10m or more, it is possible to suppress the occurrence of cracks in the bending test (2) as well as in the bending test (1).

Although the embodiment of the present invention has been described above with reference to the attached drawings, the technical scope of the present invention is not limited to the above-mentioned embodiment. It is clear that a person skilled in the art can come up with various modified or revised examples within the scope of the technical ideas described in the claims, and it is understood that these also naturally fall within the technical scope of the present invention.

1... Optical fiber cable 3... Optical fiber core 4... Core 5... Optical fiber unit 7... Pressure winding 9... Tension member 11... Tear string 13... Outer jacket 15... Resin 17... Die 19... Mandrel

Claims (8)

A core made up of a plurality of optical fiber cores;
A tension member is provided on both sides of the core in a cross section;
a substantially circular outer jacket provided to cover the tension member and the optical fiber;
Equipped with
The core is formed by twisting together a plurality of optical fiber units, and is housed in the jacket via a pressure winding,
The compressive elastic modulus of the tension member is 1/2 or less of the tensile elastic modulus,
The adhesion strength between the tension member and the outer covering is 30 N/10 mm or more and 42 N/10 mm or less ,
13. An optical fiber cable, wherein the tension member is made of glass FRP or aramid FRP. The optical fiber cable according to claim 1, characterized in that the adhesion between the tension member and the jacket is 40 N/10 mm or more. The optical fiber cable according to claim 1 or 2, characterized in that the tension member is pre-coated with the same resin as the outer sheath. The optical fiber cable according to any one of claims 1 to 3, characterized in that the pressure winding is provided as a vertical overlap winding. The optical fiber cable according to any one of claims 1 to 4, characterized in that the optical fiber unit is formed by twisting together a plurality of optical fiber cores. The optical fiber cable according to claim 5, characterized in that the optical fiber core is an intermittently bonded optical fiber ribbon core that is bonded intermittently in the longitudinal direction. 4. The method for producing an optical fiber cable according to claim 3,
A method of manufacturing an optical fiber cable, comprising extrusion-coating a strength member and a core, each of which is pre-coated with resin, with an outer sheath made of the same resin as the resin pre-coated. The method for manufacturing an optical fiber cable according to claim 7, characterized in that the tension member, which has been coated with resin in advance, is preheated before being extruded.

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