GB2138220A - Cable glands - Google Patents

Abstract

A gland for a coaxial high-frequency signalling cable, such as for CATV, comprises an annular fluid-tight insulating body 9, Fig. 1, or 29, Fig. 3, which surrounds the inner conductor 1 of the cable or an extension 6 thereof and is itself surrounded by an annular conductive body 12. A sleeve 7 surrounds the conductive body at one end and the cable sheath 5 at its other end. Four resilient sealing rings form respective gas-tight seals between (i) the inner conductor or extension and the insulating body (ring 11), (ii) the insulating body and the conductive body (ring 13), (iii) the conductive body and the sleeve (ring 14), and (iv) the sleeve and the cable sheath (ring 18, Fig. 1, or ring 22, Fig. 3). In this way, a fluid-tight seal can be formed even if no part of the cable except its sheath is fluid-tight. <IMAGE>

Description

SPECIFICATION Cable glands This invention relates to glands for terminating coaxial high-frequency electric signalling cables especially (but not exclusively) for current communal-aerial television (CATV) systems and projected cable television and/or data link systems.

At least in urban areas such cables will be buried and/or drawn into ducts beneath the pavements and will have many joints in underground chambers (manholes) that are close to other services, and in particular to gas mains that are liable in many cases to be old and corroded and besides delivering natural gas at a pressure several times greater than the pressure of coal gas for which the mains network was originally designed. Especially after the surrounding soil has been disturbed in a cable-laying operation, there is an appreciable risk of leaks leading to a high concentration of methane and other combustible gases in the soil and in underground chambers therein. This is relatively innocuous of itself, but it is imperative that such gases are not permitted to flow in any appreciable quantity into adjacent buidlings, where an explosive mixture might form.From this point of view, the highfrequency cables entering subscribers' premises are potentially hazardous, because electrical requirements dictate the use of cables with a dielectric consisting predominantly of air and the best designs (from the point of view of transmission characteristics) have longitudinally-continuous air spaces occupying a large fraction of the cable cross-section.

These air-spaces must be sealed at the cable terminations to prevent the flow of gas through them, and this has hitherto been difficult to achieve without interference with electrical characteristics.

The present invention provides a gland for sealing such cables that provides effective gas-tight sealing when used with a wide range of cable designs and without unacceptable effects on electrical transmission characteristics.

In accordance with the invention, a gland for a coaxial high-frequency electric signalling cable comprises an annularfluid-tight insulating body for surrounding the inner conductor of the cable or an extension thereof; an annular conductive body for surrounding the insulating body and means for connecting the outer conductor of the cable thereto; a sleeve for surrounding the conductive body and the sheath of the cable at respective ends of the sleeve; and four resilient sealing rings forming gas-tight seals between (i) the inner conductor or extension thereof and the insulating body, (ii) the insulating body and the conductive body, (iii) the conductive body and the sleeve, and (iv) the sleeve and the cable sheath.

In this way, a fluid-tight seal is ensured even though the cable dielectric and/or the outer conductor may offer no useful resistance to gas flow, or when an inner conductor extension is used even though the inner conductor of the cable may offer no useful resistance to gas flow either; and the material(s) shape and dimensions ofthe insulating body may be chosen freely to secure the required characteristic impedance and other electrical properties.

The means for connecting the outer conductor electrically to the conductive body will usually also provide the main mechanical grip between the gland and the cable; it may comprise a cone and gripping ring in accordance with conventional gland practice.

Preferably the cone is not formed on the conductive body itself but on an auxiliary conductive member in contact with it but relatively rotatable; the conductive member can then be threaded for screwing directly into a connecting box or other apparatus without requiring to be so screwed before the outer conductor is connected. The cone will usually be of brass, aluminium or some other suitable metal, but it could be of a hard plastics material plated or otherwise covered with metal plate over at least part of its area.

The ring is preferably of a non-conducting, moderately hard, plastics material, such as nylon, acetal copolymer, or polypropylene.

It is desirable for glands to offer resistance to relative movement between the inner conductor of the cable and the remaining parts thereof, at least in the direction tending to withdraw the inner conductor from its termination.

Preferably, this facility is provided by a washer of solid insolating material bearing on the end of the cable dielectric and secured by a retainer fixed to the inner conductor (which may be the extension of that conductor, when present).

The sleeve is preferably metallic, but (unless it provides part of the electrical connection for the outer conductor) could be of a suitable insulating material.

Except for the seal to the cable sheath, which must accomodate tolerances in cable dimensions, the sealing rings are preferably simple interference-fit rings, desirably made captive, or bonded, to one of the parts to which they seal in order to avoid risk of being omitted in assembly. The seal to the cable sheath is preferably of the kind in which the sealing ring is compressed by a gland nut.

One of the preferred forms of gland in accordance with the invention is designated primarily for use with cables having an outer conductor or screen of low mechanical strength enclosed by a relatively strong sheath of non-metallic material.

Presently available fittings for cables of this kind require the exercise of considerable skill and patience in the preparation of the cable end, especially the end of the outer conductor (or "screen" as it is usually called when of low tensile strength), to enable it to transfer the required tensile loading to the fitting. This form of the invention provides a fitting that is easier and quicker to apply and which allows at least a substantial part of the tensile loading to be transmitted by the sheath of the cable.

In this form of the invention, the gripping ring is made of hard resilient material and comprises a first part with bore dimensions so related to those of the cone that it will surround the outer conductor of the cable where it is supported by the cone and cooperate with the cone to grip it and a second part with a bore of larger cross-section than the first part; and the gland further includes means for urging the cone and the ring axially together and for urging the second part of the ring radially inwards.

The first part may be of such thickness that it is practically rigid, or it may be given a degree of flexibility by limiting its radial thickness and/or by forming one (or possibly more than one) line of weakness, for example a slot extending wholly or partly through its wall thickness.

The second part of the ring must be flexible enough to be deformed into an effective gripping engagement with the cable sheath, and this is preferably achieved by providing a number (for instance three) of radial slots spaced round the circumference and dividing this part into a number of separate gripping jaws interconnected only by the first part. Preferably the bore of the second part is ribbed or serrated to provide enhanced grip on the cable sheath.

The preferred urging means is a gland nut, threaded in the cone or on a separate member bearing directly or indirectly on the cone and having a suitably tapered internal surface bearing on the free end of the second part of the ring to urge it both axially and radially as required; preferably that free end has a correspondingly tapered bearing surface, our a group of such surfaces.

The invention includes cable terminations including the glands described.

The invention will be further described by way of example with reference to the accompanying drawings in which: Figure 1 is one half of a longitudinal cross-section through a cable termination comprising a gland in accordance with the invention; Figure 2 is a half-sectional view of a modified form of gland, shown partly disassembled; and Figure 3 is anew, similar to Figure 1, showing an example of a gland in accordance with the invention for use with cable having an outer conductor of low tensile strength.

The cable terminated in the example of Figure 1 comprises a solid or stranded inner conductor 1, dielectric 2 (which may be of the "cartwheel" type, the "thread and tube" type or any other suitable type), a composite outer conductor comprising a longitudinally applied copper tape 3 and a copper wire braid 4, and a fluid-tight outer sheath 5. In making the termination shown, the cable end is cut back to expose appropiate lengths of each layer as shown.

A conductor extension 6 of solid metal is first threaded on the inner conductor 1 and secured by soldering or compression jointing, in the latter case preferably of a kind that preserves a high degree of symmetry. An outer sleeve assembly 7 and clamping ring 8 are pushed back over the cable end, initially beyond the positions shown in the drawing. Next a sub-assembly comprising insulating body 9 and metallic armour cone 10, fixed together by a snap-fit (as shown), screwing, or any other appropriate manner is slid over the inner conductor extension 6, to which it is sealed by a rubber O-ring 11 preferably bonded to the conductor extension, over the dielectric 2 and underneath the copper tape 3 of the outer conductor.A conductive body member 12 is now slid over the insulating body 9 to which it is sealed by a second rubber O-ring 13 that is held captive in a groove in the insulating body and abutted with the metallic armour cone 10. The gripping ring 8 is now pushed towards the cone and the assembly secured by screwing the sleeve assembly 7 onto the conductive body member 12, members 7 and 8 moving further left than the position shown in the drawings to clamp the outer conductor firmly and at the same time to form a fluid-tight seal between the conductive body 12 and the sleeve assembly 7 at a third 0ring 14, which is captive in a groove in the conductive body member 12.If required, sleeve assembly 7 can immediately or at any later stage be unscrewed to allow the conductive body member 12 free rotation for screwing the thread 15 into an appropriate entry thread; an auxiliary sealing ring 16 is provided for sealing round the entry thread if required. After screwing the sleeve assembly 7 onto the conductive body member 12 again if appropriate, the termination is completed by tightening gland nut 17 to compress sealing ring 18 onto the cable sheath 5.

The gland of Figure 2 is similar to and that of Figure 1, except as hereinafter described, reference numerals common to the two Figures have the same significance. This gland is drawn as it would appear if after completion of assembly the sleeve assembly 7 were unscrewed and withdrawn and body member 12 also withdrawn to allow inspection of the outerconductor termination end/or screwing of the body member 12 into a joint box or other apparatus. In this example the insulating member 9 and cone 10 differ in shape but not in principle from the arrangement of Figure 1; and to ensure that the inner conductor 1 cannot be pulled back, with its extension 6, into the cable dielectric 2, a washer 20, suitably of PTFE, bears on the end of the dielectric and is retained by the conductor extension 6.

In using the fitting shown in Figure 3, the cable end is cut back as in the other examples, the outer conductor 21 (which may be of various constructions some of which are rather flimsy) being simply trimmed approximately to length. This sleeve assembly 7 is pushed back over the cable sheath and a nylon ring 22 pushed into engagement with the end of the cable sheath 5.

The ring 22 may be of two forms, identical in the cross-section of the drawing. The first form has a single narrow radial slit extending from end to end and through the whole thickness of the ring; the second has three equally spaced radial slots extending through the whole wall thickness but only from the end 23 to the narrow bridge 24 in the middle of the ring so that the first (left-hand) part 25 of the ring is solid and substantially rigid while the second (right-hand) part 26 is divided into three gripping jaws connected only by the first part. The cone 27 is now inserted under the end of the outer conductor 21 and the washer 28 (similar in function to 20 in Figure 2) pushed into place and secured by fixing the extension 6 to the inner conductor.The insulating body 29 (which is not in this case connected with the cone 27 but simply abuts it) is slid into place with the conductive body member 12 around it; the sleeve assembly 7 is tightened onto the conductive body member 12 to complete the termination as before. The outer conductor 21 is thus wedged between the cone 27 and the first part 25 of the ring, forming an adequate electrical connection between the outer conductor and the cone but not necessarily providing any useful mechanical grip. At the same time, the second part 26 of the ring is radially compressed by the engagement of its tapered surface 30 with the mating conical surface 31 of the sleeve member, causing the ribs 32 to bite into the sheath of the cable and grip it firmly.

Other features of the glands shown in the drawings are described in another application filed on the same day as this one and claiming priority from British Application No. 8309498 and others.

Claims (15)

1. A gland for a coaxial high-frequency electric signalling cable comprising an annular fluid-tight insulating body for surrounding the inner conductor of the cable or an extension thereof; an annular conductive body for surrounding the insulating body and means for connecting the outer conductor of the cable thereto; a sleeve for surrounding the conductive body and the sheath of the cable at respective ends of the sleeve; and four resilient sealing rings forming gas-tight seals between (i) the inner conductor or extension thereof and the insulating body, (ii) the insulating body and the conductive body, (iii) the conductive body and the sleeve, and (iv) the sleeve and the cable sheath.

2. A gland as claimed in Claim 1 in which the means for connecting the outer conductor comprises a cone formed on an auxiliary conductive member in contact with the conductive body but relatively rotatable.

3. A gland as claimed in Claim 1 or Claim 2 in which the sealing ring that forms the seal between the sleeve and the cable sheath is of the kind in which the sealing ring is compressed by a gland nut and the other sealing rings are simple interferencefit rings.

4. A gland as claimed in any one of Claims 1 to 3 in which a washer of solid insulating material bears on the end of the cable dielectric and is secured by a retainer fixed to the inner conductor.

5. A gland as claimed in any one of the preceding claims which is suitable for electric cables having an outer conductor or screen of low mechanical strength enclosed by a relatively strong sheath of non-metallic material in which gland the gripping ring is made of hard resilient material and comprises a first part with bore dimensions so related to those of the cone that it will surround the outer conductor of the cable where it is supported by the cone and co-operate with the cone to grip it and a second part with a bore of larger cross-section than the first part; and the gland further including means for urging the cone and the ring axially together and for urging the second part of the ring radially inwards.

6. Afitting as claimed in Claim 5 in which the ring is of a non-conducting, moderately hard, plastics material.

7. Afitting as claimed in Claim 5 and Claim 6 in which the first part of the ring is practically rigid.

8. A fitting as claimed in Claim 7 in which the second part of the ring is made flexible by a number of radial slots spaced round the circumference and dividing the said second part to a number of separate gripping jaws interconnected only by the first part of the ring.

9. A fitting as claimed in Claim 5 or Claim 6 in which the ring is formed with slot or other line of weakness.

10. A cable gland substantially as described with reference to and as shown in Figure 1.

11. A cable gland substantially as described with reference to and as shown in Figure 2.

12. A cable gland substantially as described with reference to and as shown in Figure 3.

13. A cable termination including a gland claimed in any one of Claims 1 to 3 or in Claim 10.

14. A cable termination including a gland claimed in Claim 4 or Claim 11.

15. Acabletermination including a gland claimed in any one of Claims 5 to 9 or Claim 12.