GB2218171A

GB2218171A - Externally threaded bush for electrical enclosures

Info

Publication number: GB2218171A
Application number: GB8908663A
Authority: GB
Inventors: Howard Arthur Finch
Original assignee: BARTON ENGINEERING Ltd
Current assignee: BARTON ENGINEERING Ltd
Priority date: 1988-05-07
Filing date: 1989-04-17
Publication date: 1989-11-08
Also published as: GB8908663D0; GB8810854D0

Abstract

A bush (10) for an electrical enclosure (15) has a head (11) and a threaded shank (12), separated by a thread undercut (14) having a frustoconical surface (21) joining the head at a radial distance from a central longitudinal axis (A) of the bush which is greater than the radius of the hole (16) in a wall of the enclosure through which the shank is received, in use, so as to align the head (11) in the hole (16) when the head (11) is received in the hole (11) and to deform the peripheral edge of the hole (11) to produce good electrical contact and a seal. <IMAGE>

Description

BUSH, PARTICULARLY FOR ELECTRICAL ENCLOSURES This invention relates to a bush, particularly for electrical enclosures.

It is well known to use an externally threaded, hexagon headed metallic bush for fixing conduit tube, rigid or flexible electrical wire or cable protection tube or fittings to an electrical enclosure, such as a conduit box. The bush is inserted through a hole provided in the electrical enclosure case so that an internally threaded coupling or protective tube can be connected to the electrical enclosure.

In some applications the dimensions of the bush and the hole in the enclosure case are precisely defined by relevant British Standards. Additionally in some cases the connecting tube is used as part of a path for electrically earthing the system, so that there must be a low resistance electrical contact between the threaded bush and the metal enclosure. In some applications it is also required that when tightened to secure a protective tube or fitting, the threaded bush must provide an adequate seal to prevent ingress o-f dust or water into the electrical enclosure. This seal has been provided by a washer, which may also have an attached earthing tag to provide the necessary electrical earth from the threaded coupling to the electrical enclosure.

Some British Standard Specifications require the threaded bush to have a thread undercut which is wider than the thickness of the metal electrical enclosure.

This is due to difficulty in threading accurately to the shoulder formed by the head of the bush. As a result considerable movement is possible between the bush and the hole in the enclosure case. For example if a threaded bush for an electrical conduit box has a 20mm nominal diameter and a 17.7mm to 18.0mum diameter undercut, and is to fit an enclosure hole of 20.25mm to 20.75mm diameter, the bush can be off-centre by between 1.125mm and 1.525mm, so that flats on the hexagonal head may no longer cover the hole.

The object of the invention is to provide an improved bush, particularly for electrical enclosures, which overcomes or reduces at least some of the aforementioned problems.

According to the invention there is provided a bush for an electrical enclosure, comprising a head, a hollow shank extending from the head and defining a central longitudinal axis, an external thread on the shank terminating at a position spaced from the head, and an undercut between said termination of the thread and said head, the undercut having a tapering surface which joins the head at a radial distance from said axis which is greater than the radius of a circular hole in a wall of the enclosure through which hole the bush is intended to be received, so that, in use, at least part of the peripheral edge of said hole and/or of said surface at or adjacent said join is deformed if said head is received against a surround at one side of the hole.

Preferably said surface of the undercut is frustoconical and conveniently it is at 450 to said axis of the bush.

According to a further aspect of the invention there is provided an electrical enclosure having a circular hole in a wall thereof, a bush received through said hole, the bush comprising a head, a hollow shank extending from the head and defining a central longitudinal axis, an external thread on the shank terminating at a position spaced from the head, and an undercut between said termination of the thread and said head, the undercut having a tapering surface which joins the head at a radial distance from said axis which is greater than the radius of the circular hole so that with said head received against a surround at one side of the hole at least part of the peripheral edge of said hole and/or of said surface at or adjacent said join is deformed.

The invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a part sectional side view showing one embodiment of a bush of the invention being used to secure a coupling tube for a conduit to an electrical enclosure, and Figure 2 is an enlarged scrap section of an undercut of an external thread of the bush.

Figure 1 shows a metallic bush 10 having a hexagonal head 11 and a hollow cylindrical shank 12 which has an external screw thread 13. The thread 13 extends from the free end of the shank towards the head but terminates a little way short thereof where, as shown best in Figure 2, there is a thread undercut 14 extending to the head 11.

An electrical enclosure 15, preferably of steel, is shown in Figure 1 having a circular hole 16 in a side wall thereof. The head 11 of the bush is received in the enclosure whilst its threaded shank 12 extends through the hole and has one end of a hollow, internally threaded coupling tube 17 engaged thereon. A conduit tube 18 is threadedly engaged in the other end of the coupling tube 17. A central longitudinal axis A of the bush 10 is shown in Figure 1.

Figure 2 shows the undercut in detail. At the end of the thread is an inwardly sloping surface 19 leading to a surface 20 parallel to the axis A. Extending from the surface 20 to an inner face Ila of the head 11 is a frusto-conical surface 21, which slopes at 450 to said axis A. The width of the undercut, namely the distance between the end of the screw thread 13 and the plane of the inner face lla of the head 11, is greater than the thickness of the enclosure side wall. Most importantly, however, the junction of the surface 21 and the head 11 is at a radial distance from axis A which is greater than the radius of the hole 16.

Thus when the bush is initially engaged in the hole 16, the inner peripheral edge of the hole will engage against the surface 21 with the inner face lla of the head slightly spaced from the inner surface of the enclosure side wall. The coupling tube 17 is then engaged on the shank 12 and screwed up towards the outer surface of the enclosure side wall until it engages it. The head is then pulled towards the coupling tube 17 so that its surface lla abuts the inner surface of the enclosure side wall and the coupling tube 17 can be screwed tightly against the outer surface of the enclosure side wall, as shown in Figure 1. This causes deformation of the peripheral edge of said hole 16 at the inner surface of the enclosure side wall as it rides up the surface 21, to produce a seal between the edge and the surface 21.

For example for a 20mm nominal diameter hole 16 (20.25 to 20.75mm) it has been found that if the junction of surface 21 at surface lla is between the limits 20.95mm diameter to 21.lmm diameter, the metal of the electrical enclosure side wall will deform so as to form a seal and an electrical contact area of approximately 10sq.mm.

Additionally the bush will of course self centre at the hole 16. The sizes given for this example have been found to work well and the tolerances can be held in mass production. The radial distance from axis A to the junction of surfaces 21 and lla should not be too large to prevent the surface lla seating against the enclosure since this might cause an unacceptable pivot point.

Thus the bush described will self centre on tightening, provide a dust and splashproof seal, provide good electrical earth contact, provide annular strengthening between the head and the undercut, and provide some measure of anti-vibration to prevent loosening of the bush.

The bush 10 and hole 16 can of course be of sizes different to those specified and the coupling tube 17 and tube 18 can also be of different form. Instead of sloping at 450, the surface 21 can slope at a suitable different angle. Moreover the surface 21 could be curved rather than straight as shown in Figure 2.

Instead of the metal of the enclosure deforming as the bush is tightened, the materials of the enclosure and the bush could be such that the bush surface 21 alone deforms or deforms in addition to the deformation of the peripheral edge of the hole in the enclosure. The bush has application to, among other things, electrical conduit boxes, electrical enclosures, electrical trunking, fluorescent light fittings, and switch and socket boxes. For convenience, all these are herein referred to as an 'electrical enclosure'.

Claims

1. A bush for an electrical enclosure, comprising a head, a hollow shank extending from the head and defining a central longitudinal axis, an external thread on the shank terminating at a position spaced from the head, and an undercut between said termination of the thread and said head, the undercut having a tapering surface which joins the head at a radial distance from said axis which is greater than the radius of a circular hole in a wall of the enclosure through which hole the bush is intended to be received, so that, in use, at least part of the peripheral edge of said hole and/or of said surface at or adjacent said join is deformed if said head is received against a surround at one side of the hole.

2. A bush as claimed in claim 1, wherein said tapering surface is a frustoconical surface.

3. A bush as claimed in claim 2, wherein said surface slopes at 450 to said longitudinal axis.

4. A bush as claimed in any one of claims 1 to 3, wherein the width of said undercut is greater than the thickness of said wall of the enclosure containing said circular hole.

5. A bush as claimed in any one of claims 1 to 4, wherein the bush is made of a material harder than that of the enclosure with which it is intended to be used, so that the peripheral edge of said hole is deformed if the bush head is received, in use, against an interior surface of the enclosure surrounding said hole, with said shank passed through said hole.

6. A bush as claimed in any one of the preceding claims, made of electrically conductive material.

7. An electrical enclosure assembly, comprising a circular hole in a wall thereof, a bush received through said hole, the bush comprising a head, a hollow shank extending from the head and defining a central longitudinal axis, an external thread on the shank terminating at a position spaced from the head, and an undercut between said termination of the thread and said head, the undercut having a tapering surface which joins the head at a radial distance from said axis which is greater than the radius of the circular hole so that with said head received against a surround at one side of the hole at least part of the peripheral edge of said hole and/or of said surface at or adjacent said join is deformed.

8. An electrical enclosure assembly as claimed in claim 7, wherein said tapering surface of the bush is frustoconical.

9. An electrical enclosure assembly as claimed in claim 8, wherein said surface of the bush slopes at 450 to said longitudinal axis.

10. An electrical enclosure assembly as claimed in any one of claims 7 to 9, wherein the width of said undercut is greater than the thickness of said wall of the enclosure.

11. An electrical enclosure assembly as claimed in any one of claims 7 to 10, wherein said bush is made of a material harder than that of the enclosure and said peripheral edge of the hole is deformed by said head being received against an interior surface of the enclosure surrounding said hole.

12. An electrical enclosure assembly as claimed in any one of the preceding claims, wherein the bush and/or the enclosure is made of electrically insulating material.

13. A bush for an electrical enclosure substantially as hereinbefore described with reference to and as shown in the accompanying drawings.

14. An electrical enclosure assembly substantially as hereinbefore described with reference to and as shown in Figure 1 of the accompanying drawings.