BACKGROUND a. Field of the Invention

The present invention relates to an apparatus and method for processing the outer surface of cylindrical members, such as tubes and conduits, and in particular to the processing of such members to form a collapsible sub-duct for installation inside a larger main duct into which cable may be inserted. b. Related Art

There is often the need to install cables into existing main ducts, which may run for hundreds of metres between access points. Such main ducts will, in general, house previously installed cables and may only have limited room to accommodate any additional cables.

There are a number of different installation methods for running new cables through an existing duct network. Compressed gas can be use to blow a cable down a main duct. Usually, the cable is pushed by a traction apparatus at the same time as the compressed air is blown into the main duct. However, when space becomes limited this can become impractical. One solution to this problem is disclosed in patent document WO 03/092134, in which a sub-duct is formed using a highly flexible material which may be inflated using a compressed gas and which naturally deflates when the gas pressure is relieved. The sub-duct is first installed in a main duct while in a collapsed state and then when inflated the highly flexible material stretches to increase the cross- sectional area of the sub-duct. A cable, for example a fibre optic cable, may then be blown down the length of the sub-duct. When the inflation pressure is released, the sub-duct collapses, thereby increasing the space available inside the main duct for any future installation of additional sub-ducts.

A limitation of this system is that because the material is highly flexible, it is not possible to push the sub-duct down a main duct, even when the sub-duct is fully collapsed.

Therefore, this system requires that highly flexible material folded back on itself so the inner surface of the duct is outermost. Compressed gas is then blown in between the opposed outer surfaces of the sub-duct where this is folded back against itself, causing the sub-duct to unfurl inside the main duct as the fold point advances down the length of the main duct.

While this is an effective installation method, the highly flexible material is prone to damage either during installation or afterwards if this becomes caught on any sharp surfaces. Another limitation stems from the requirement to inflate the installed sub-duct prior to blowing a cable, for example a fibre optic cable, down the length of the sub-duct. Because the material of the sub-duct is flexible, the inflated sub-duct wall can be pinched together at points by any obstructions in the main duct, thereby hindering insertion of the cable down the length of the sub-duct.

It is an object of the present invention to provide a sub-duct that addresses these issues and also a method of manufacturing such a sub-duct. SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided a collapsible sub-duct for holding a cable within a main duct, comprising an elongate tubular member having an outer wall and within the outer wall an interior volume extending along the length of said tubular member for holding said cable, the wall being formed from a polymeric material and having in said material a plurality of longitudinally extending lines of flexibility, said lines defining a plurality of adjacent panels in said material, said panels extending longitudinally along the tubular member and said lines of flexibility allowing each of said panels to be folded and unfolded with respect to each other along said lines of flexibility such that the elongate tubular member is substantially rigid in a longitudinal direction, as compared with a radial direction in which the sub-duct is collapsible, wherein in use said panels can first be folded along said lines of flexibility towards or against one another, to collapse said panels in a radially inward direction to reduce said interior volume and thereby aid insertion of the sub-duct into a main duct and then unfolded along said lines of flexibility away from one another to expand said interior volume to aid insertion of a cable down the length of said interior volume, and then folded again along said lines of flexibility towards or against one another to reduce said interior volume and thereby reduce the volume occupied by the sub-duct in the main duct.

According to a second aspect of the invention, there is provided a method of forming a collapsible sub-duct for holding a cable within a main duct, comprising the steps of:

forming in a polymeric material an elongate tubular member having an outer wall and within the outer wall an interior volume extending along the length of said tubular member for holding said cable; and

- forming in said wall a plurality of longitudinally extending lines of flexibility, said lines defining a plurality of adjacent panels in said material, said panels extending longitudinally along the tubular member and said lines of flexibility allowing each of said panels to be folded and unfolded with respect to each other along said lines of flexibility such that the elongate tubular member is substantially rigid in a longitudinal direction, as compared with a radial direction in which the sub-duct is collapsible.

According to a third aspect of the invention, there is provided a method of installing a cable inside a main duct using a collapsible sub-duct according to the first aspect of the invention, comprising the steps of:

folding said panels to reduce the interior volume of the tubular member and then inserting the sub-duct into a main duct; unfolding said inserted sub-duct to expand said interior volume and then inserting a cable down the length of said expanded interior volume; and

after said insertion of the cable folding said panels again to reduce said interior volume and thereby reduce the volume occupied by the sub-duct in the main duct.

The material of the outer wall extends continuously around the circumference of the tubular member in both collapsed and expanded states so that the cable is always retained within the tubular member.

The panels are more rigid than the lines of flexibility such that most of the deformation shape of the in the outer wall during collapse and expansion of the sub-duct is provided by bending along the lines of weakness rather than by bending of the panels.

The sub-duct is preferably substantially air tight or gas tight so that this may readily be collapsed by applying a partial vacuum within the tubular member and/or by increasing the air pressure around the outside of the tubular member. The reverse pressure differential may then be used to expand the sub-duct.

The polymeric material will, in general, comprise polymer chains. In a preferred embodiment of the invention, the step of forming the lines of flexibility comprises a rearrangement of the polymer chains to increase the flexibility of the polymeric material along the lines of flexibility.

This rearrangement may be effected by a local thinning of said wall, for example, by a processing step in which the walls is compressed to inelastically deform the wall.

In a second embodiment of the invention the lines of flexibility are created by co-extruding stripes of a flexible thermoplastic elastomer along the length of the elongate tubular member so that they form the total wall thickness in those regions between the panels. The thermoplastic elastomer being a compatible material to the polymeric material of the panels and forming a homogeneous bond between adjacent panels. The lines of flexibility preferably permit the panels to be folded against an adjacent panel. li is preferred if the sub-duct has an even plurality of at least six of said panels, and a corresponding even plurality of lines of flexibility. The panels are preferably all have the same size in terms of width in the circumferential dimension. Therefore, when there are six panels, each panel occupies up to one-sixth the circumference of the tubular member. When the tubular member is unfolded and approximately circular in cross-section, each panel then extends across an angle of up to 60° as viewed from a central portion of the interior volume.

Also in a preferred embodiment of the invention, alternate ones of said lines of flexibility have a tendency or are adapted to fold in opposite directions inwards or outwards relative to said interior volume. The alternate panels may be oppositely curved in a circumferential direction when the panels are unfolded so that a pair of the alternate panels adjacent to each other both curve in the same direction when folded together. The folded arrangement is then star-shaped in cross-section with the points of the star being formed by the outwardly folding lines of flexibility, and the bases of each point being formed by the adjacent inwardly folding lines of flexibility. Preferably, each of said pairs of said panels when folded together curve around a central region of said interior volume when the panels are folded together. This helps to minimise the volume occupied by the sub-duct when the walls have been collapsed in this way. The lines of flexibility which are adapted to fold inwards each preferably extend to a central region of the interior volume when the panels are folded. This helps to close up the interior volume when the walls have been folded together. In one embodiment of the invention, there is an even number of longitudinally extending lines of flexibility, these lines defining the same even number of adjacent panels in the wall material. Then, adjacent pairs of panels can be folded towards or against one another.

The lines of flexibility may form live hinges (also called "living hinges"). In one embodiment of the invention, the hinges extend substantially parallel to one another in a longitudinal direction of the tubular member and being provided in the material of the outer wall proximate an inner surface of the outer wall. In an alternative embodiment of the invention, each of the hinges is provided alternately in the material of the outer wall proximate an inner surface of said outer wall and in the material of the outer wall proximate an outer surface of said outer wall. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example, with reference to the following Figures, in which: Figure 1 shows a collapsible sub-duct for holding a cable within a main duct according to a first preferred embodiment of the invention, having six similar longitudinal panels separate by six similar lines of flexibility;

Figure 2 shows a collapsible sub-duct for holding a cable within a main duct according to a second preferred embodiment of the invention, having six longitudinal panels separate by six lines of flexibility, alternating panels being oppositely curved and alternating lines of flexibility having an alternating tendency to fold either inwards or outwards; Figures 3 to 5 show in end on view the sub-duct of Figure 2 as this is progressively collapsed; Figure 6 shows schematically a first stage of manufacture of the sub-duct of Figures 1 or 2;

Figures 7 shows schematically a second stage of manufacture of the collapsible sub-duct; and

Figure 8 shows schematically a third stage of manufacture of the sub-duct.

DETAILED DESCRI PTION

Figure 1 shows a radially collapsible sub-duct 1 for holding a cable 2, which may be a fibre optic cable within a main duct 4, which may be part of the cabling routing infrastructure in a telecommunications network. There may be more than one cable 2 inside a sub-duct, but in general there will be a number of other similar sub-ducts inside the main duct 4.

The sub-duct 1 is in the form of an elongate tubular member having a thin outer wall 6 (typically between 0.5 and 4mm thick) and within the outer wall an interior volume 8 extending along the length of the tubular member for holding the cable 2. When expanded as drawn, the diameter of the sub-duct is about (typically between 12 and 25 mm outer diameter). The wall is formed from a polymeric material, for example a polypropylene compound and has six similar longitudinally extending lines of flexibility, which are here grooves 10 in an outer surface 12 of the wall 6. The lines of flexibility 10 form live hinges (also called living hinges) positioned symmetrically around the circumference of the tubular member.

The wall extends continuously around the circumference of the tubular member and without any breaks or holes along the length of the tubular member. Therefore, the lines of flexibility do not provide any passage through the material of the wall. The tubular member is therefore air tight or gas tight between the ends of the tubular member. This permits the tubular member to be expanded or collapsed by providing a differential air pressure across the wall of the tubular member.

The lines of flexibility define a six corresponding panels 14 in the material of the wall. These lines of flexibility 10 are grooves in the external surface 12 that allow each of the panels to be folded and unfolded with respect to each other along these lines.

The sub-duct is substantially rigid in the longitudinal direction, as compared with the radial direction in which the sub-duct is collapsible.

In use, the panels 14 are first folded together to reduce the interior volume 8 and thereby aid insertion of the sub-duct into a main duct 4. Because the sub-duct is substantially rigid in the longitudinal direction it may be inserted down the length of main duct 4 by pushing the sub-duct with a traction apparatus (not shown) into the main duct, aided if needed by blowing air down the main duct at the same time.

Once in place, the sub-duct 2 is expanded, for example by blowing air or another gas into the sub-duct, which expands the interior volume 8 to aid insertion of the cable 2 down the length of said interior volume. Compressed air or another gas may used to aid the insertion of the cable 2, or the cable may be pushed into place, or the cable may be pulled into place using a pre-installed pull cord.

After insertion of the cable, the sub-duct is allowed to collapse or actively made to collapse, for example by residual tension left in the hinges 10, 1 10, 1 10' or by sucking air or gas out of the interior volume 8 or by the application of compressed air to a second or subsequent sub-duct or the installation of a subsequent cable. This causes the panels 14 to fold back towards or against one another, to reduce the interior volume and thereby reduce the volume occupied by the sub-duct in the main duct 4. ure 2 shows a second embodiment of a sub-duct 101 similar to that of Figure 1 Features which correspond with those of Figure 1 are indicated by reference numerals incremented by 100. The main difference with the first embodiment 1 is that there are two types of lines of flexibility 1 10, 1 10' forming live hinges. One type 1 10' of hinge is a groove in an inner surface of the wall and therefore has a propensity to fold inwards, and the other type 1 10 is a groove in the outer wall and therefore has a propensity to fold outwards, whereas in the first embodiment, the lines of flexibility may fold either inwards or outwards. The other difference is that the panels 1 14, 1 14' are alternatingly oppositely curved, so that when the sub-duct collapses, as shown by Figures 3 to 5, the oppositely curved panels come together to minimise the space occupied by the sub-duct within the main duct 4.

In this embodiment, the two types of live hinge 1 10, 1 10'are provided alternately in the radially innermost portion and radially outermost portion of the material forming the wall. The "inner" hinges 1 10 then resists inward folding when the relatively rigid groove side walls come into contact, so that this hinge tends to folds radially outwards while the "outer" hinge 1 10' then resists outward folding when the relatively rigid groove side walls come into contact, so that this hinge tends to fold radially inwards. As shown in Figure 3, this arrangement allows the live hinges along the tubular member 101 to be creased or folded longitudinally into a non-circular format, in this example, a three pointed star. A greater number of hinges would permit a greater number of points to the star shape. Preferably, the arms of the star are such that they cooperatively wrap around each other as shown in Figures 4 and 5 to create a tight cylinder, thereby reducing to a minimum the interior volume 108 and the external circumference of the creased tubular member.

The sub-duct 102 is installed in the same manner as described above.

Figures 6 to 8 show how the tubular member 1 , 101 can be formed with lines of flexibility as described above by passing a tube 20 having a uniform wall thickness through suitable forming dies and rotating rollers 22, 22' to compress the wall 6, 106 at points on opposite side where the rollers have tips 24, 24' in the form of a rotating blunt knife.

The polymeric material undergoes molecular re-alignment during compression between the rollers in which the material is longitudinally embossed. It has been found that the molecular re-alignment of discrete longitudinal hinges within the wall of an extruded tube provides lines of flexibility which do not fracture or tear in normal use, and which retain some memory of a preferred collapsed orientation, so that after installation of the cable the sub-duct reverts naturally to a compact form.

The gap between the pair of rollers 22, 22' is such that the combined wall thickness of the flattened tubular member 20 is reduced to typically 50% by the embossing process. This causes the initial molecular re-alignment. Further molecular re-alignment is encouraged by flexing the embossed grooves and this occurs by passing the flattened tube through a die that re-shapes it back to a circular section, followed by flattening the tube 20 at 90 degrees to the embossed grooves as shown in Figure 7. As shown in Figure 8, secondary grooves are then embossed by a pair of rotating rollers 26, 26', 27' 27' provided with double blunt circular knife edges 28, 28'.

A further process of reforming and flattening the tube in several planes is enacted to complete the molecular realignment of each and all of the live hinges. Finally a process of forming the processed tube into the required star shape and subsequent collapsed format is achieved with suitable dies and longitudinal rollers.

As an alternative to the embossing process described above the tubular member 1 , 101 including the lines of flexibility can be formed by co-extruding stripes of a relatively flexible thermoplastic elastomer together with a relatively inflexible polymeric material along the length of the elongate tubular member. An advantage of this approach is that the total wall thickness in the hinge regions is the same as that of the panels. The thermoplastic elastomer is preferably a compatible material to the polymeric material of the panels and forms a homogeneous bond between adjacent panels. With such an arrangement it is possible to perform the extrusion with the tubular member being configured in a collapsed orientation so that the tubular member is naturally biased towards this orientation without the need for any post-processing.

The invention described above provides several benefits. The reduced volume of the collapsed sub-duct enables many more sub-ducts to be installed into a rigid main duct than can normally be accommodated when using conventional tubular rigid wall sub-ducts. The use of a resilient and durable material such as polypropylene, which is substantially rigid in the longitudinal direction, helps to eliminate the possibility of the wall being damaged or torn, as would be the case with a flexible material wall. This enables more cables to be accommodated in the rigid duct and enables a higher utilization factor and efficiency of use to be achieved.

The invention therefore provides a convenient sub-duct for holding a cable within a main duct and also a method of manufacturing such a sub-duct.