GB2248729A - Flexible reinforced tubes for cable bend strain relief - Google Patents

Abstract

A bend strain reliever or tube comprises a flexible matrix material (7) in which are embedded a plurality of reinforcing fibres (6) which extend in at least two directions. As embodied, a bend limiter is progressively more flexible away from a rigid end piece (2) and the reinforcement is a helically-wound fabric tape. The rigid end piece (2) may have added reinforcement in the form of steel wires (16) attached thereto. The wires may be replaced by a slit tube. <IMAGE>

Description

FLEXIBLE REINFORCED STRUCTURES The present invention relates to flexible reinforced structures and especially those suitable for use as bend strain relievers for protecting parts of cables adjacent to fixed attachment points.

Such cable parts are subject to frequent bending forces which can lead to breakage of the cable as a result of metal fatigue. For this reason so-called bend strain relievers in the form of integrally or separately formed sleeves with progressively increasing (in the direction away from the fixed attachment point) flexibility are provided to reduce bending adjacent the fixed attachment point. One known bend strain reliever (conveniently referred to as a BSR) comprises a tapering sleeve of flexible material. This requires however a large diameter at the fixed attachment end to provide adequate protection to the cable. In order to avoid this problem a limited number of more or less helical steel rods or wires as reinforcing elements is embedded in a flexible matrix such as rubber or polyurethane.Whilst this does provide increased performance and protection it is difficult to control the stiffness profile i.e. the progessive increase in flexibility along its length, including rate of change, absolute value etc., of the BSR.

It is an object of the present invention to avoid or minimise one or more of the above disadvantages.

The present invention provides a bend strain reliever having a sleeve of progessively increasing flexibility away from a fixed attachment end portion, which sleeve comprises a plurality of elongate reinforcement elements embedded in a flexible matrix characterised in that said reinforcement elements are in the form of a large plurality of fibres of at least one length of a fabric tape in which said fibres extend in at least two different directions.

By means of the present invention it is possible to provide BSRs with a wide range of more or less accurately controlled strain relief profiles in a simple and economic manner. In more detail the profiles may be readily varied by using tapes with different types of construction e.g. woven or knitted, different relationships between the pitch, strength, flexibility of the fibres laid in the different directions (e.g.

warp and weft yarn fibres in a woven tape), by using such tapes in different arrangements in the matrix e.g.

different widths, different angle(s) of winding around the sleeve, etc. Once the desired tape or tape combination has been selected it (they) can be laid in the sleeve in a particularly simple and convenient manner. With such a multi-directional fibre tape reinforcement the reinforcement elements can be easily handled during manufacture of the reinforced structures and the disposition of the fibre elements is substantially maintained during such handling i.e. the fibres do not lose their substantially parallel alignment in a woven tape thereby avoiding substantial deviations in stiffness from the desired stiffness profile of the structure.

The matrix material can be incorporated in the sleeve in various different ways including, for example, using pre-calendered rubber strip lengths incorporating the tape, winding these onto a steel mandrel, and then vulcanising the sleeve-form assembly made up of the rubber strip to form a more or less integral body. In another method successive layers of the tape are wound onto a rotating steel or pre-formed flexible matrix, e.g. polyurethane, tube, and successive layers of the tape impregnated with liquid polyurethane which then cures to form a flexible semi-rigid composite structure.

Various fibres may be used in the tapes including natural fibres such as cotton and synthetic ones such as polyamide and polyester, which are usually employed in the form of yarn or thread.

Various configurations of tapes and windings may be used to suit various specifications but a preferred winding configuration is made by laying a first layer at +400 to +500 to the cable axis, preferably +45 , a second layer at -400 to -50 , preferably -45 , a third slightly shorter layer at +200 and a fourth layer of similar length to the third, at 200 to the cable axis. Further layers which may be shorter or longer than some or all of the previous ones, are applied at gradually reducing angles. Some layers may be applied parallel to the cable axis if desired. It has been found that a multiplicity of layers, advantageously from 30 to 100 layers, is generally preferred, depending on the specific requirements of the BSR.

Individual layer thickness depends to some extent on the materials used but generally a layer of tape and rubber matrix is conveniently in the region of 1 to 2 mm thick. A layer of tape and polyurathane matrix may be in the region of 2 to 3 mm thick. Thus a bend strain reliever may be several hundred millimetres thick at the fixed attachment end portion for a cable diameter of the order of 100 to 200 mm. The layers may be laid to form a generally rectilinear profile frustoconical shape or the outer profile may be slightly convex or concave, or more or less substantially stepped depending on the required bending characteristics and/or manufacturing convenience etc. Preferably the diameter of the BSR proximal the fixed attachment end portion is minimised so as to minimise the risk of fracture thereat.

The fixed attachment end portion is generally provided with a generally rigid connector means, conveniently in the form of an annular flange, formed and arranged for compling of the BSR, e.g. with the aid of bolts or other fixing means, to a rigid body from which the cable is to extend.

The fixed attachment end portion is advantageously further reinforced with a generally rigid, usually metal e.g. steel, ferrule having a multiplicity of generally helical fingers formed and arranged to support the fibres and matrix. The helical nature of the fingers permits a degree of bending in the BSR by partial unwinding of the fingers to permit a degree of axial extension in one side of the BSR. The cross-sectional configuration and the dimensions, especially diameter and length, of the fingers may be varied according to the specific requirements of a BSR but are generally in the region of 1 to 10 mm in diameter preferably 4 to 6 mm and some 50 to 150 mm long for use in a 100 mm diameter cable.

Most desirably, it has been found that by adding one or two wraps of fibres at 900 to the BSR axis proximal the most flexible end of the BSR and the opening exiting therefrom, that said opening remains more or less circular and any possible contact load between the BSR and a tube or cable contained therein, resulting from a tendency to flattening of the circular profile upon bending of the BSR, is substantially reduced.

It will be appreciated that various forms of reinforced structure other than BSRs may be made in accordance with the present invention. Thus the present invention extends to a reinforced tubular structure comprising a plurality of elongate reinforcement elements embedded in a matrix material characterised in that said reinforcement elements are in the form of a large plurality of fibres of at least one length of a fabric tape in which said fibres extend in at least two different directions. Such tubular structures may be used for various purposes including for example enclosing cable joints.

Further preferred features of the invention will appear from the following detailed description given by way of example of a preferred embodiment illustrated with reference to the accompanying drawings in which: Fig 1 is a partial longitudinal cross-section of a bend strain reliever of the invention; Fig. 2 is a schematic perspective view illustrating the mode of manufacture of the BSR of Fig. 1; Fig. 3 is a perspective view of a first ferrule end piece for use in a bend strain reliever of the invention; and Fig. 4 is a perspective view of a second ferrule end piece.

Fig. 1 shows a bend strain reliever (BSR) 1 comprising a substantially rigid ferrule end piece 2 with a flexible generally frusto-conical sleeve 3 extending therefrom.

The sleeve 3 is made up of inner and outer layers 4, 5 of flexible polymeric material comprising polyurethane, with several layers of reinforcing elements in the form of woven polyester tape 6 which are impregnated with polyurethane so as to form a layered polyurethane matrix 7 in which the tape reinforcing elements 6 are embedded.

As indicated schematically in Fig. 2 the BSR 1 is made by supporting the inner layer 4 on a cylindrical support 8, helically winding a first tape reinforcing element 6a around the inner layer in one direction, impregnating it with liquid polyurethane and allowing the polyurethane to cure to form a respective matrix layer 7. A second tape reinforcing element 6b is then wound helically around the first tape 6a (now embedded in polyurethane) in the opposite direction but is stopped short 9 of the free end 10 of the BSR and the end 11 of the first tape 6a as shown in dashed outline, and impregnated with liquid polyurethane which is allowed to cure.

Successive layers of tape reinforcing elements 6 are built up similarly and then encased in an outer layer 5 (see Fig. 1) to build up a progressively flexible frusto-conical sleeve 3 as shown in fig. 1.

It will be appreciated that by using different tapes and winding configurations the stiffness profile of the BSR may be readily varied to meet a wide range of different specifications providing optimum protection for various different cables.

In more detail, the BSR shown in Fig. 2 is made by winding a first layer 6a at 9450 to the cable axis, a second layer 6 at 450 to the cable axis, a shorter third layer at +200 and a shorter fourth at 200 (not shown). Fifth and sixth layers (not shown) are laid at +150 and 150 respectively and further layers of gradually reducing length at gradually reducing angles to the cable axis.

Figs. 3 and 4 show two different embodiments of ferrule 2. The ferrule 2 shown in Fig. 3 has a generally rigid connection means in the form of an annular flange 12 formed and arranged to be secured via bolt holes 14 to a fixed body such as a structure or an item of equipment (not shown). The fingers 16 of the ferrule 2 are helical metal pins of high tensile spring steel wire of a number of different lengths 16a, 16, 16c. The fingers 16 provide a more progessive change in stiffness between the rigid flange 12 and the flexible distal end of the BSR 1.

The fingers 16 are mounted into a collar 18 axially spaced from the flange 12 of the ferrule 2 which has a maximum diameter slightly greater than that of the pitch centre diameter mounting of the fingers 16. The localised increase in diameter behind the fingers 16 provides an undercut edge 20 for the matrix material 7 to engage thereby holding it more securely against axial loadings. As the matrix material 7 has a load applied to it, it tries to pull away from the base 12 and expand over the edge 20. Hoop stresses are generated in the maxtrix 7 around the undercut edge 20 thereby further locking the matrix 7 to the ferrule 2. The locking is further backed-up by chemical bonding between the sleeve 3 and the fingers 16 and the matrix 7.

Fig. 4 shows a second type of ferrule and like parts therein corresponding to those in the embodiment of Fig.

3 are indicated by like reference numbers.

The ferrule 2 and the fingers 16 are in this case formed in one piece. Generally helically extending slots 22 are machined along the sleeve 3 of the ferrule 2 to form the helical fingers 16.

In a further aspect the present invention includes a method of manufacture of a reinforced tubular structure of the invention comprising the steps of: providing at least one length of a fabric tape comprised of a large plurality of fibres extending in at least two different directions, winding said at least one length of tape around a tubular support, and impregnating said tape with a liquid matrix material which can be cured into a substantially solid form and bringing it into said cured form so as to form a tubular body of said matrix material in which said tape is embedded.

Claims (13)

1. A bend strain reliever having a sleeve of progessively increasing flexibility away from a fixed attachment end portion, which sleeve comprises a plurality of elongate reinforcement elements embedded in a flexible matrix characterised in that said reinforcement elements are in the form of a large plurality of fibres of at least one length of a fabric tape in which said fibres extend in at least two different directions.

2. A bend strain reliever as claimed in claim 1 wherein said fibre tape is of a woven construction.

3. A bend strain reliever as claimed in claim 2 wherein said fibre tape comprises a plurality of substantially adjacent warp threads and spaced apart weft thread means.

4. A bend strain reliever as claimed in claim 1 wherein said fibre tape is of a knitted construction.

5. A bend strain reliever as claimed in any one of claims 1 to 3 wherein said fibres are natural or synthetic fibres.

6. A bend strain reliever as claimed in any one of claims 1 to 5 wherein said flexible matrix is a vulcanised rubber.

7. A bend strain reliever as claimed in any one of claims 1 to 5 wherein said flexible matrix is a cured polyurethane.

8. A bend strain reliever as claimed in any one of claims 1 to 7 which includes a substantially rigid ferrule at the fixed attachment end portion, which ferrule has extending therefrom a plurality of resiliently deformable fingers, disposed generally helically of the sleeve, thereby to provide a more progressive decrease in stiffness away from the fixed attachment end portion of the bend strain reliever.

9. A bend strain reliever as claimed in claim 8 wherein said fingers are of spring steel.

10. A reinforced tubular structure comprising a plurality of elongate reinforcement elements embedded in a matrix material characterised in that said reinforcement elements are in the form of a large plurality of fibres of at least one length of a fabric tape in which said fibres extend in at least two different directions.

11. A method of manufacture of a bend strain reliever of the invention comprising the steps of: providing at least one length of a fabric tape comprised of a large plurality of fibres extending in at least two different directions, winding said at least one length of tape around a tubular support, impregnating said tape with a liquid matrix material which can be cured into a substantially solid form and bringing it into said cured form so as to form a said sleeve of said matrix material in which said tape is embedded.

12. A method of manufacture of a reinforced tubular structure of the invention comprising the steps of: providing at least one length of a fabric tape comprised of a large plurality of fibres extending in at least two different directions, winding said at least one length of tape around a tubular support, and impregnating said tape with a liquid matrix material which can be cured into a substantially solid form and bringing it into said cured form so as to form a tubular body of said matrix material in which said tape is embedded.

13. A bend strain reliever substantially as described hereinbefore with particular reference to Figs. 1 and 2, Figs. 1 and 3 and Figs. 1 and 4.