GB2076492A

GB2076492A - Externally Threaded Conduit Bush

Info

Publication number: GB2076492A
Application number: GB8106692A
Authority: GB
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-03-07
Filing date: 1981-03-03
Publication date: 1981-12-02
Also published as: GB2076492B

Abstract

An externally threaded conduit bush 10 connecting a conduit 11 to an electrical box 12 by way of a coupling piece 17, has a circular flanged head 15 instead of the conventional hexagonal head, and a non-circular internal driving formation 18. The contact area between the head 15 and box 12 is thus increased improving earthing. The threaded tubular portion of the bush has an internal cross-section defined by two superimposed congruent hexagons which are mutually angularly displaced by 30 DEG . Tightening of the bush is performed by a tool T of hexagonal cross-section which engages grooves 18 in the internal surface of the portion 13 defined by a respective one of the hexagons, there being twelve possible angular positions in which the tool T can be engaged with the bush 10. The internal driving formation may be formed by twelve equispaced shallow triangular grooves disposed about a circular bore, the driving tool being of hexagonal cross-section with small V- shaped ridges at the apices for engagement with the grooves. <IMAGE>

Description

SPECIFICATION Externally Threaded Conduit Bush This invention relates to bushes for electrical trunking and the like.

British Standard 4568 part 2, 1970, as amended March 1973, Standard Sheet 8 prescribes an externally threaded bush for use in conjunction with electrical trunking or the like.

The bush comprises a tubular portion having screw-threading on its external surface and an integral head portion defined by an hexagonal flange extending outwardly from one end of the tubular portion. Typically, the bush is used to connect a length of conduit to an electrical box, being inserted through a hole in a side wall of the box from the interior of the latter and screwed into a coupling piece which is in turn screwed onto an end of the conduit. The hexagonal head enables the bush to be tightened by means of a spanner or similar tool.

This type of bush is, however, disadvantageous in two main respects. Firstly, it can be rather difficult to tighten the bush with a spanner as aforesaid due to the limited thickness of the head on which purchase can be gained, and also due to the fact that there are only six angular positions in which the spanner can be engaged with the head.

Secondly, where the electrical box, bush and conduit form an earth return the electrical connection between these components must be sufficient to satisfy the relevant regulations for the electrical equipment of buildings published by IEE: this is sometimes difficult to achieve due to the limited area of contact between the hexagonal head and the side wall of the electrical box.

It is an object of the present invention to obviate or mitigate the above disadvantage.

Accordingly, the present invention provides a bush for electrical trunking or the like comprising a tubular portion having screw-threading on its external surface, and an integral head portion defined by a generally circular flange extending outwardly from the tubular portion at or adjacent one end thereof, the internal surface of the tubular portion at least at said one end thereof having a transverse cross-section which is non-circular to permit a tool of corresponding cross-section to be inserted therein, whereby the bush can be tightened in use.

Preferably, said transverse cross-section is defined by a plurality of superimposed congruent shapes each of which has rotational symmetry about an axis, said shapes having their axes coincident and being anguiarly spaced about said axes. In this case, the tool can have a cross section the same as one of said shapes, so that the number of angular positions in which it can be inserted into the bush is determined not only by the rotational symmetry of the shape itself, but also the number of such shapes which make up said transverse cross-section.

Most preferably, said transverse cross-section is defined by a plurality of superimposed congruent regular polygons whose centres are coincident and which are mutually angularly spaced about said centre. In the most general case, where there are n superimposed polygons each having m sides, there are (m x n) possible angular positions in which the tool can be inserted in the bush, greatly facilitating tightening of the latter. Desirably, the polygons are mutually angularly spaced by 360 1 (x) m- n degrees. For example, where the cross-section is defined by two superimposed congruent hexagons, the hexagons are mutually spaced apart by 30 .

Although the bush differs in shape from that prescribed by the above-mentioned British Standard, the equivalent dimensions thereof (particularly thread form and length) conform to the Standard. In particular, the bush is applicable to both long and short threads. In addition, as prescribed by the Standard the internal surface of the tubular portion can be radiused at both ends of the latter, and the screw-threading can terminate at a point which is spaced from the head portion.

In order to facilitate grasping of the head portion during initial hand-tightening of the bush, the outer periphery of the generally circular flange can be milled, knurled or similarly roughened.

The invention will now be further described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a side view, partly in section, of a bush according to the present invention; Figure 2 is a front view of the bush shown in Figure 1; Figures 3 and 4 are respectively similar views to Figures 1 and 2, but showing an alternative form of bush; and Figure 5 is a section through a tool for use with the bush shown in Figures 3 and 4.

Referring first to Figures 1 and 2, the bush shown therein and referenced 10 is intended primarily to connect a length of electrical conduit 11 to a non-spouted electrical box 12. The bush 10 comprises generally a tubular portion 13 having screw-threading 14 (shown schematically) on its external surface, and an integral head portion 1 5 defined by a flange which extends radially outwardly from the tubular portion 13 at one end thereof. The bush is fitted by inserting the tubular portion 13 through an aperture 16 in the box 1 2 from the interior of the latter, and engaging the screw-threading 14 with complementary threading 17' on the internal surface of a coupling piece 17, the coupling piece 1 7 in turn being screwed onto an end of the conduit 11.The bush is then tightened so that the side wall of the box 12 is held between the end of the coupling piece 1 7 and the head portion 15, as shown in Figure 1. Electrical cables (not shown) pass along the conduit 11 and through the bush 10 into the box 12, with the components 10, 11, 1 2 and 17 forming an earth return.

Thus far described, the bush is similar to that prescribed by the above-mentioned British Standard. However, whereas in the prescribed bush the head portion 1 5 has an hexagonal outer periphery to enable a spanner or the like to be engaged therewith for tightening purposes, in the present invention the head portion 1 5 has a circular outer periphery and the bush 10 is instead tightened by means of a tool which is inserted into the tubular portion 1 3 from the end thereof adjacent the head portion 1 5. To this end, the internal surface of the tubular portion 1 3 has a transverse cross-section defined by two congruent hexagons whose centres are coincident both with each other and with the axis of the portion 13, and which are mutually angularly spaced around said centres by 300.Thus a series of triangular grooves 18 are provided in the internal surface of the tubular portion 13, with alternate grooves being defined by respective ones of the hexagons. Tightening of the bush is performed by means of an Allen key or similar tool T (indicated in broken lines in Figure 2) of hexagonal cross-section which engages alternate ones of the grooves 18. The tool T may be of solid or hollow construction.

The configuration of the internal surface of the tubular portion 13 enables the tool T to be engaged with the bush in twelve different angular positions, i.e. six different positions for each of the two hexagons which define the cross-section, thereby greatly facilitating tightening of the bush in the restricted space within the electrical box 1 2. Moreover, because the outer periphery of the head portion 1 5 is circular rather than hexagonal, the area over which the head portion contacts the electrical box is greater than in the bush prescribed by the above-mentioned British Standard, thereby providing greater mechanical contact between these parts. This can be appreciated from Figure 2, wherein an hexagonal head portion of the same maximum diameter is indicated by chain-dotted lines.

Although not illustrated, the outer periphery of the head portion 15 can be milled, knurled or otherwise roughened to assist manual gripping thereof during initial hand-tightening of the bush 10. Additionally, in common with the bush prescribed by the above-mentioned British Standard, the internal surface of the tubular portion 13 is radiused at both ends of the latter (as indicated respectively at 1 9 and 20 in Figure 1), and the screw-threading 14 terminates at a point which is spaced from the head portion 1 5 so that an annular recess 21 is defined therebetween.

The internal cross-section of the tubular portion 13 does not have to be defined by hexagons, and can instead be defined by other regular polygons or any other non-circular congruent shapes which have rotational symmetry. Moreover, more than two such shapes can be superimposed to form the cross-section.

Figures 3 and 4 show an alternative bush in which the internal cross-section of the tubular portion 1 3 is circular but with twelve shallow trianguiar grooves 22 therein spaced at 300 intervals.

Figure 5 illustrates a suitable tool 23 for this bush, which is generally of hexagonal cross-section but which has small V-shaped ridges 24 at the apices for engagement in respective ones of the grooves 22.

The bushes shown in Figures 1 to 4 can be made from any of the materials prescribed by the abovementioned British Standard: however, it is preferred that they are made of bright zinc-plated steel in order to give adequate strength to the tubular portion where its thickness is reduced by the grooves 18 or 22. The bushes can be manufactured by broaching, stamping or pressing.

Claims

1. A bush for electrical trunking or the like, comprising a tubular portion having screwthreading on its external surface, and an integral head portion defined by a generally circular flange extending outwardly from the tubular portion at or adjacent one end thereof, the internal surface of the tubular portion at least at said one end thereof having a transverse cross-section which is noncircular to permit a tool of corresponding crosssection to be inserted therein, whereby the bush can be tightened in use.

2. A bush as claimed in Claim 1, wherein said transverse cross-section is defined by a plurality of superimposed congruent shapes each of which has rotational symmetry about an axis, said shapes having their axes coincident and being angularly spaced about said axes.

3. A bush as claimed in Claim 1, wherein said transverse cross-section is defined by a plurality of superimposed congruent regular polygons whose centres are coincident and which are mutually angularly spaced about said centres.

4. A bush as claimed in Claim 3, wherein said transverse cross-section is defined by n superimposed congruent regular polygons each of which has m sides, the polygons being mutually angularly displaced by 360 1 x-) m n degrees.

5. A bush as claimed in Claim 1, wherein said transverse cross-section is defined by a plurality of superimposed congruent hexagaons whose centres are coincident and which are mutually angularly displaced about said centres.

6. A bush as claimed in Claim 5, wherein said transverse cross-section is defined by two such superimposed hexagons.

7. A bush as claimed in Claim 6, wherein the two hexagons are mutually angularly spaced by 300.

8. A bush as claimed in any preceding Claim, wherein the internal surface of the tubular portion is radiussed at both ends of the latter.

9. A bush as claimed in any preceding Claim, wherein the outer periphery of the generally circular flange is provided with milling, knurling or the like to facilitate manual grasping thereof.

10. A bush as claimed in any preceding Claim, wherein the screw-threading terminates at a point which is spaced from the head portion.

11. A bush for electrical trunking or the like, substantially as hereinbefore described with reference to Figures 1 and 2 or Figures 3 and 4 of the accompanying drawings.