A METHOD OF AND AN APPARATUS FOR PRODUCING AN OPTICAL MULTI-FIBRE CABLE ELEMENT

Abstract: A method of producing an optical multi-fibre conductor element (11) by extruding (5) a secondary sheathing (6) loosely around a fibre bundle of several optical fibres (1); providing the fibres with a desired extra length (8) with respect to the secondary sheathing; and reeling the conductor element onto a receiving reel (12). In order to provide an extra length to compensate for differences occurring in the tension of the fibres and to avoid permanent variation in the length of the fibres in the secondary-sheathed fibre bundle, the fibres are spread (14, 15) side by side before the extrusion of the secondary sheathing, and passed side by side around an equalizer wheel (13), and further to the extruder while still positioned side by side. An apparatus for realizing the invention comprises means (14, 15) for spreading the fibres before the extruder (5); and an equalizer wheel (13) around which the fibres are arranged to be passed side by side.

Description: A method of and an apparatus for producing an optical multi-fibre cable element This invention relates to a method of producing an optical multi-fibre conductor element provided with a secondary sheathing, comprising extruding a secondary sheathing loosely around a fibre bundle of several optical fibres; - providing the fibres with a desired extra length with respect to the secondary sheathing; and reeling a secondary-sheathed conductor element so obtained onto a receiving reel. In the production of a conductor element com¬ prising optical fibres, it is important that the length of the fibres is made greater than that of the sheathing in the secondary sheathing step in order that the fibres would not be exposed to dis¬ advantageous strains when the conductor element is bent on reeling or installation. Finnish Patent Specification 75 9.39 (NOKIA) discloses one simple way of providing an extra length by stretching the secondary sheathing with a stretch¬ ing device, whereby the optical fibres remain longer when the sheathing returns to its original length after the stretching. The stretching device comprises a drawing means which draws the sheathing loosely surrounding the fibres from an extruder, and a locking wheel around which the sheathing together with the fibres is passed at a speed slightly greater than at the drawing means. On the locking wheel the fibres are positioned against the inside of the sheathing, so that the sheathing is stretched between the drawing means and the locking wheel while the fibres slide axially with respect to the sheathing. In the production of optical multi-fibre con- ductor elements, the locking wheel, however, causes variation in the length of the fibres as the turning radius of the fibres around the wheel varies. This is because the fibres which are passed through the extruder in parallel with each other are always passed onto the periphery of the idler wheel in the same order with respect to each other, that is, the same fibres are always positioned against the peri¬ pheral surface of the wheel, whereas the other fibres are always positioned upon the first-mentioned fibres, and as a consequence have a greater turning radius. As is well-known, such variation in the fibre length causes various disadvantages in the conductor elemen . U.S. Patent 4,458,476 (SIEMENS) teaches one way of avoiding the problem caused by the variation in the fibre length in multi-fibre conductor elements. In the U.S. Patent, fibres in the fibre bundle are SZ twisted before the extrusion of the loose secondary sheathing around the fibres and before the sheathing is filled with grease. In the secondary-sheathed conductor element the fibres remain twisted to some extent, which ensures that the fibres have a greater length than the sheathing. However, the twisting de- vice required for realizing this kind of method makes the secondary sheathing device complicated, long and slow. In addition, it is known that a few major dis¬ advantages are caused by the twist remaining in the finished conductor element. The SZ twisting creates a torque in the fibres, which results in the formation of static strains. The middle rod around which the fibres are twisted brakes the sliding of the fibres, thus increasing friction and strains, whereby the differences between the strains of different fibres are increased with resultant variation in the fibre lengths and impairment of the technical properties of the conductor element. Finnish Patent Specification 75 940 (NOKIA) teaches that the fibres in the fibre bundle will not be passed onto the locking wheel of the stretching device always in the same order with respect to each other when the order of the individual fibres in the fibre bundle which is passed into the extruder for providing it with a secondary sheathing is varied re- peatedly in the step of forming the fibre bundle. In this way it is ensured that the fibres in the sheathed fibre bundle are passed onto the wheel in a continuously changing order, which compensates for the variation in the fibre length and improves the properties of the conductor element. However, the device required for realizing the method, i.e. for varying the order of the fibres, makes the sheathing equipment relatively complicated and expensive. The object of the present invention is to pro- vide a method for the production of an optical multi- fibre conductor element, which avoids the above- mentioned disadvantages and enables the fibres in the fibre bundle to be passed in parallel with each other into the extruder without any risk of the stretching device causing variation in the length of the fibres in the secondary-sheathed fibre bundle. This object is achieved by means of the method of the invention which is characterized in that - the fibres in the fibre bundle are spread side by side before the extrusion of the secondary sheathing; - the spread fibres are passed side by side around an equalizer wheel; and - the fibres are removed from the equalizer wheel while still positioned side by side and passed to the extrusion step. The invention is based on the idea that individual fibres are locked on the equalizer wheel before the extrusion of the secondary sheathing to avoid variation in the length of the fibres in the sheathed conductor element. On leaving the starting reels, the fibres have approximately the same tension and pass over the equalizer wheel with the same- tension over a short period of time,, so that it is obvious that the fibres around which the secondary sheathing is extruded are of equal length. Between the equalizer wheel . and the locking wheel of the stretching device there occurs variation in the tension of individual fibres as the fibres are posi- tioned one upon another on the periphery of the lock¬ ing wheel, so that the outer fibres stretch more than the inner fibres. This, however, does not cause variation in the length of the fibres, because the variation in the tension of the fibres disappears when the fibres are released from the locking wheel so that the fibres are of the same length as when they are passed on the equalizer wheel. The locking of the fibres according to the invention before the extrusion step causes the fibres to return to sub- stantially equal length in spite of the momentary unequal stretching. Variation in the fibre length is thus restricted between the equalizer wheel and the locking wheel. The method of the invention enables the fibres to be fed in parallel with each other inside the secondary sheathing, which is of great importance when aiming at a parallel spiral fibre configuration in so-called maxi tubes. It has proved that it is very difficult to attain a spiral configuration in a SZ twisted fibre bundle, and the firmness of the obtained fibre bundle is not such as desired. In the method of the invention, the initial tension of the fibres is not critical. With higher production rates and stiffer greases, ever increasing initial tensions are required in accurate control of the extra fibre length and variation in the fibre length. The invention is also concerned with an apparatus for realizing the method, which apparatus is characterized by what is disclosed in claim 3. The apparatus is very simple in structure, and does not require any external control or operation. It does not restrict the initial tension of the fibres as compared with central tube or secondary tube twisting machines. In the following the invention will be described in greater detail with reference to the attached drawing, in which Figure 1 is a schematic side view of the production of a secondary sheathing for a fibre bundle of an optical cable; and Figure 2 is an enlarged, more detailed view of an equalizing device of one embodiment of the apparatus according to the invention, In Figure 1 of the drawings, a number of primary-sheathed fibres 1 are passed from fibre reels 2 through a collector 3 into an equalizing device 4 and further into an extruder 5. The secondary- sheathed fibre bundle 6 is then passed through a cooling chute 7 into a stretching device 8 to provide the fibres with an extra length with respect to the secondary sheathing. The stretching device comprises a braking means 9 and a locking wheel 10. Finally, a finished cable element 11 is wound on a reel 12. The equalizing device 4 comprises an equalizer wheel 13 with a cylindrical peripheral surface, an inlet comb 14, and an outlet comb 15. The equalizer wheel is mounted rotatably on a support not shown. The fibre collector 3, the slits of the combs, the highest point of the equalizer wheel and the guide means before the extrusion head of the extruder are positioned substantially on the same line. The fibres are wound once around the equalizer wheel, and the combs spread the fibres in a plane apart from each other, so that each fibre is positioned against the peripheral surface of the equalizer wheel. In this way the fibres are locked on the equalizer wheel so that they are not able to move in their axial direction with respect to each other on the wheel. On leaving the braked reels 2, the fibres have approximately the same tension, so that they are of substantially equal length when they are locked on the equalizer wheel, and are passed in parallel with each other inside the secondary sheathing. In the stretching device 8 the fibres are locked axially immovably to each other and to the secondary sheathing when they are passed around the locking wheel 10. As the fibres in the fibre bundle are posi¬ tioned side by side and one upon another on the periphery of the locking wheel, variation will occur in the tension of the fibres between the equalizer wheel and the locking wheel. The fibres are, however, locked to the equalizer wheel, so that the tension differences will not go beyond the equalizer wheel, but the differences are levelled out and the length variation caused by these differences disappears when the fibres are released after the locking wheel. In Figure 1, the tensions of the fibres advancing from the braked reels are indicated with the reference tl; these tensions are substantially equal before the equalizer wheel. Fibre tensions be- tween the equalizer wheel and the locking wheel are indicated t2, t3, t4 and t5; these tensions may differ from each other. Fibre tensions after the locking wheel are indicated with the reference t6, and they are substantially equal. It is noted that the apparatus of the invention compensates for variation in the tension of the fibres caused in the secondary sheathing step, so that a permanent variation in length is avoided in the secondary-sheathed fibre bundle. The drawing and the description related to it are only intended to illustrate the idea of the invention. In their details, the method and the apparatus according to the invention may vary within the scope of the invention. As used in this con¬ nection, the word "fibre bundle" does not only refer to two or more fibres positioned separately in the same sheathing but also to a fibre bunch of two or more fibres bound together e.g. by gluing, by a bind- ing thread or a binding hose. The secondary-sheathed optical conductor element may form a part of a multi¬ element cable (minitube structure), or it may be the basic element of a single-element cable (maxitube). In the multi-fibre cable element, the fibres may form a bundle, a bunch or several bunches.

Claims: Claims : 1. A method of producing an optical multi-fibre conductor element (11) provided with a secondary sheathing, comprising extruding (5) a secondary sheathing (6 ) loosely around a fibre bundle of several optical fibres (1); - providing the fibres with a desired extra length (8) with respect to the secondary sheathing; and reeling a secondary-sheathed conductor element so obtained onto a receiving reel (12) , c h a r a c t e r i z e d in that - the fibres (1) in the fibre bundle are spread (14, 15) side by side before the extrusion (5) of the secondary sheathing (6) ; - the spread fibres are passed side by side around an equalizer wheel (13); and - the fibres are removed from the equalizer wheel while still positioned side by side, and passed to the extrusion step (5 ) . 2. A method according to claim 1, c h a r a c ¬ t e r i z e d in that the fibres (1) in the fibre bundle are spread (14, 15) in the same plane. 3. An apparatus for producing an optical secondary-sheathed conductor element (11), comprising an extruder (5) for extruding a secondary sheathing (6) loosely around a fibre bundle of several optical fibres (1) ; - a stretching device (8) for providing the fibres with a desired extra length with respect to the secondary sheathing ( 6) ; and means (12) for receiving the secondary- sheathed conductor element (11), c h a r a c t e r i z e d in that - the apparatus comprises means (14, 15) for spreading the fibres (1) in the fibre bundle so as to be positioned side by side, said means being posi- tioned before the extruder (5); - and an equalizer wheel (13) around which the fibres are arranged to be passed side by side. 4. An apparatus according to claim 3, c h a r ¬ a c t e r i z e d in that the spreading means (14, 15) are positioned on the path of the fibres (1) before the equalizer wheel (13) and after it before the extruder (5). 5. An apparatus according to claim 3 or 4, c h a r a c t e r i z e d in that the spreading means (14, 15) are comb-like and spread the fibres (1) in one common plane. 6. An apparatus according to any of claims 3 to 5, c h a r a c t e r i z e d in that the equalizer wheel (13) comprises a cylindrical peripheral surface for the fibres (1). 7. An apparatus according to any of the pre¬ ceding claims, wherein the device (8) providing the extra length comprises a transfer means (9) for braking the secondary sheathing (6) and a locking wheel (10) around which the secondary sheathing (6) and the fibres are passed to lock the sheathing (6) and the fibre bundle axially immovably to each other, c h a r a c t e r i z e d in that the equalizer wheel (13) is arranged to receive the fibres (1) rom braked fibre reels (2).

Inventors: PAEIVINEN TEUVO, PAEIVINEN, TEUVO
Applicants: NOKIA KAAPELI OY
Application year: 1990
Main classification: G02B6
IPC codes: G02B6