The invention relates to a cable, comprising (i) at least one package of fibre ribbons, which package consists of at least one fibre ribbon com¬ prising several parallel optical fibres and a cover¬ ing joining them together, and (ii) at least one re¬ inforcing member extending in the longitudinal direc¬ tion of the cable. It is prior art to use in a cable optical fibre ribbons, comprising typically 4 to 12 parallel opti¬ cal fibres and a covering manufactured of plastic ma¬ terial joining them together.

In a known cable construction, fibre ribbons are normally packaged tightly inside a plastic tube and the plastic tube is surrounded by a protective structure comprising reinforcing members extending in the longitudinal direction of the cable, which mem¬ bers are placed at regular intervals in the peripher- al direction of the cable.

Another known cable construction is a so-called V-groove cable, in which fibre ribbons are placed in spirally or oscillatingly winding grooves positioned in a common centre element. A drawback of these known cable constructions is that, due to the complicated structures thereof, the production costs are high. In the abovementioned constructions, it is, furthermore, not possible to provide in a simple manner a proper play for the fi- bres against changing dimensions caused by mechanical forces directed to protective sheath or against changing thermic dimensions.

The object of the present invention is thus to avoid the drawbacks described above. This is achieved by means of a cable construction according to the

invention, which is characterized by a combination in which (i) said at least one package of fibre ribbons is arranged in a cavity with an elongated cross-sec¬ tion in the direction of the cable diameter in such a way that the natural bending plane of the fibre rib¬ bon package is substantially perpendicular with re¬ spect to the longitudinal plane of the cavity in the direction of the cable diameter, and (ii) said at least one reinforcing member is arranged in the cable construction in such a way that the bending plane of the whole cable defined thereby also is substantially perpendicular with respect to the longitudinal plane of the cavity in the direction of the cable diameter.

The basic idea according to the invention is to arrange the fibre ribbons and the reinforcing members with respect to the cavity formed in the cable and elongated in the direction of its diameter in such a way that the fibre ribbons are allowed to move inside the cavity, while extending, compressing or bending the cable, in such a manner that they bend with re¬ spect to their natural bending plane.

Thanks to the construction of the invention, the fibres can in an easy way be provided with a large play. Due to its simple structure, the cable of the invention also keeps the production costs low, and by means of that, a cable network can be con¬ structed more economically than before. The solution according to the invention improves the profitability especially at the lowest level of an optical cable network, i.e. within the area of a local network, where the number of fibres is highest and where the need to achieve cheaper solutions than the present ones is greatest.

In the construction of the invention, the cavi- ty, the fibre ribbons and the reinforcing members are

mutually arranged in such a way that a manufacture by means of a stationary (not turning) tool is made pos¬ sible and the fibres can be provided with the extra length they need with respect to the protective sheath simply by allowing them to bend with respect to their natural bending plane. Moreover, the cable geometry of the invention allows a minimization of the cross-section of the cable, and consequently, an increase in the packing density of the cable. According to one preferred embodiment of the invention, the reinforcing members consist of a pair of metallic conductors, whereby they can be used also for data transmission or e.g. supply of power.

According to a second preferred embodiment of the invention, the reinforcing members confine the cavity space within themselves and protect it. Thanks to such a construction, it is easy to branch the cable without damaging the fibres.

According to a third preferred embodiment of the invention, the cable core itself or a part of it constitutes a reinforcing member. No separate rein¬ forcing members integrated inside the construction are then needed, but the cable core or a part of it determines the properties of the cable alone. The cable geometry according to the invention makes it also possible to provide the protective structure with sliding/wear surfaces working as wear surfaces and reducing friction when the cable is drawn. The invention is explained more accurately in the following with reference to the examples accord¬ ing to the enclosed drawings, in which

Figure 1 shows a cross-section of a cable ac¬ cording to a first embodiment of the invention, Figure 2 shows a cross-section of a cable ac-

cording to a second embodiment of the invention,

Figure 3 shows a cross-section of a cable ac¬ cording to a third embodiment of the invention, and

Figures 4a to 4c show cross-sections of dif- ferent alternatives of a cable according to a fourth embodiment of the invention.

Figure 1 shows a cross-section of a cable con¬ struction according to a first embodiment of the in¬ vention, in which construction in a protective sheath 1 made of cable plastic, e.g. polymer, is formed a cavity 2 extending in the longitudinal direction of the cable, in which cavity is arranged a package 3 of fibre ribbons, in this case consisting of three fibre ribbons 4 one upon another and possibly tied togeth- er. Each fibre ribbon comprises four optical fibres 5 placed in parallel with each other and a covering 6 joining them together, which covering is typically made of acrylate plastic. Inside the protective sheath 1 are also arranged two threadlike reinforcing members 7a extending in the longitudinal direction of the cable.

The cavity 2 is situated centrally with respect to the longitudinal centre axis of the cable and it has a cross-section with an elongated form in the di- rection of the cable diameter. The fibre ribbon pack¬ age 3 is arranged inside the cavity in such a manner that its natural bending plane, indicated by the ref¬ erence mark A, is perpendicular with respect to the longitudinal plane B in the direction of the diame- ter of the hollow cable. The longitudinal plane B is a plane perpendicular to the cross-sectional plane of the cable and parallel with the longitudinal axis of the cavity in the direction of the cable diameter (a symmetry axis parallel with a long side 2b of the cavity at the cross-sectional level of the cable).

Inside the cavity 2, the fibre ribbons extend accord¬ ing to the invention along a path reminding that of a sine curve, oscillating with respect to their natural bending plane. On account of the oscillating path, the fibre ribbons have a specific excess length and thus also an allowance for strain with respect to the protective sheath. The natural bending plane of a fibre ribbon package means in this connection a plane on both sides of which the fibre ribbons extend and which is perpendicular to the cross-sectional plane of the cable and at the same time parallel with the transverse direction defined by one fibre ribbon 3.

The width of the cavity in the direction of the cable diameter is such that the fibre ribbon package is not able to move substantially in said direction, while the length L of the cavity in the direction of the cable diameter is such that the fi¬ bre package has play in this direction. In practice, the length L of the cavity is typically three- to fivefold compared with its width .

The reinforcing members 7a are arranged on op¬ posite sides of the cavity in such a way that the bending plane D of the whole cable defined by them is also perpendicular with respect to the longitudinal plane B of the cavity in the direction of the cable diameter. In this case, with the fibre ribbon package located centrally in the cavity 2, the bending plane D joins the bending plane A of the fibre ribbon pack¬ age. The bending plane of the cable means in this example a level which is perpendicular to the cross- sectional plane of the cable and extends via the centre axes of the reinforcing members 7a.

Because the reinforcing members 7a are arranged at that main axis of the cavity 2, which is parallel with the cable diameter, at which the cavity is nar-

rowest (at the transverse axis), the cross-section of the cable can be made as small as possible, and consequently, the packing density high.

Moreover, because the reinforcing members 7a are arranged symmetrically on opposite sides of the cavity, they can be made into a pair of conductors, which can be used for data transmission, e.g. for transmission of service signals, or for supply of power. In this case, it is preferable to use as rein- forcing members copper or aluminium coated steel wires, which make both reinforcing and data transmis¬ sion functions possible. In case the reinforcing mem¬ bers 7a are not used for data transmission, they can be e.g. mere steel wires or composite plastic bars (FRP = Fiber Reinforced Plastics).

Thanks to the location of the reinforcing mem¬ bers, the cable can also be branched easily, because the sheath 1 of an undamaged cable can be stripped in the direction of the reinforcing members, whereby the fibre ribbons are easily accessible without breaking them. An especially preferable embodiment in this respect is the construction shown in Figure 2, which will be described later on.

From the cable geometry according to the inven- tion follows also that at bends of an channel instal¬ lation only those areas of the outer surface of the protective sheath 1 of the cable touch the wall of the channel which are closest to the end parts 2a of the cavity 2. Due to this, it is preferable to pro- vide the protective sheath with sliding/wear surfaces functioning as wear surfaces and reducing friction when the cable is drawn. Such sliding surfaces, indi¬ cated by the reference numeral 8, can be realized e.g. by means of a reinforcement made on the surface of the protective sheath, which reinforcement is pro-

vided with grooves 8a in the longitudinal direction of the cable.

Figure 2 shows a cross-section of a cable ac¬ cording to a second embodiment of the invention. In this case, strength members 7b are semicircular in cross-section and they are in the direction of the longitudinal axis of the cavity at least as long as the cavity, preferably a little longer than the cavi¬ ty, however, whereby they thus confine the cavity space and protect it. Then the fibre ribbons are easily accessible by stripping the protective sheath 1 along a plane defined by the reinforcing members 7b, i.e. along the plane C indicated in Figure 2 by a broken line. Figure 3 shows a third embodiment of the cable construction of the invention, in which the reinforc¬ ing member consists of one metallic band 7c, the cross-sectional form of which is bent in such a way that four mutually parallel walls form three parallel substantially U-shaped spaces. At the manufacturing stage of the cable, tape 9 has been wound around the reinforcing member 7c, whereby the U-shaped spaces get closed and three parallel cavities 2c are formed for fibre packages 3. In this case, the cross-sectio- nal form of the metallic band 7c defines the location of the bending plane D of the cable.

The reinforcing member can also be realized in the manner shown in Figures 4a to 4c, by forming the protective sheath 1 in two parts in such a way that a cable core 7d (or at least a substantial part of it) is manufactured of a firm plastic or composite mate¬ rial and the surrounding protective sheath 1 of a softer and cheaper plastic material. Consequently, the cable core 7d (or the substantial part of it) acts as a reinforcing member defining the bending

plane D of the whole cable.

In the case of Figure 4a, the cable core 7d acting as a reinforcing member has an ellipse-shaped cross-section and encloses one cavity 3, in which the fibre ribbons can move freely.

In the case of Figure 4b, the cable core 7d is rectangular in cross-section and encloses three par¬ allel cavities 2d. The whole cable core with parti¬ tion walls 10a constitutes in this case a reinforcing member.

The construction of Figure 4c corresponds to the construction of Figure 4b- in other respects, ex¬ cept that partition walls 10b between the cavities are not reinforcing members there, but the reinforc- ing member consists of a peripheral part 10c of the cable core 7d.

In the constructions of the Figures 4a to 4c, the cable core 7d (or a part of it, such as the peri¬ pheral part 10c of the cable core in Figure 4c) de- fines alone the properties of the cable, whereby the surrounding protective sheath serves only as protec¬ tion against wear and humidity. Consequently, the protective sheath is not always even necessary.

One advantage of the present invention is also that, by means of a suitable dimensioning of the con¬ struction, the difference between the moments of in¬ ertia of the cable perpendicular to each other can be made as big as possible (the cable bends more easily in the direction in which the moment of inertia is smaller). The location of the bending plane of the cable can not always be defined directly by means of the centre axes of the reinforcing members or by means of other such features relating expressly to the structure of the cable, but the location of the bending plane also depends e.g. on the reinforcing

members chosen and the dimensions and shapes thereof, on the materials chosen or other such magnitudes de¬ fining the intensities of the moments of inertia.

Though the invention has above been described referring to the example of the enclosed drawing, it is clear that the invention is not restricted there¬ to, but it can be varied in many ways within the scope of the inventive idea described above and in the enclosed claims. For instance, the bending planes of the cable and the fibre ribbon package do not have to join in the manner presented in the examples above, because the location of the fibre ribbon pack¬ age in the cavity and the location of the cavity it¬ self in the cable can vary. The same cavity can even include two parallel fibre ribbon packages loosely joined together or the cable can comprise two or even several parallel cavities, each of which can include one fibre ribbon package or several parallel fibre ribbon packages tied together. In principle, the cable could also have e.g. two cavities on each other in the direction of its diameter, each of which in¬ cludes one fibre ribbon package or several packages. The cavity can also be filled with fat or pressur¬ ized, and to the whole construction can be added 1am- inated protective covers or other known protective or reinforcing constructions.