WO2005002016A1

WO2005002016A1 - Box with knock-out openings for electricalapparatus

Info

Publication number: WO2005002016A1
Application number: PCT/GB2004/002776
Authority: WO
Inventors: Nigel Timothy Court
Original assignee: Pittway Systems Technology Group Europe Limited
Priority date: 2003-06-25
Filing date: 2004-06-25
Publication date: 2005-01-06
Also published as: GB0314809D0; WO2005002016B1; EP1642367A1

Abstract

Knockout (22) circular shape is defined in enclosure (21) by circular groove (23) and has a cavity (27) extending in a generally radial directions and with a base surface inclined to the plane of knockout (22). Knockout (22) is attached to enclosure (21) by shearable material (26). The attachment between enclosure (21) and material (26) features sharp edge (24), while the attachment between knockout (22) and material (26) features arcuate edge (25). An operator employs tool (30) (e.g. a screwdriver blade) located by cavity (27), to apply force to the knockout (22) in order to tear material (26), detaching the knockout (22) from enclosure (21) and creating an aperture of a precise, predetermined size, suitable for insertion of a cable. The tear initiation point (40) corresponds with the comer of tool (30). Arcuate edge (25) serves to increase substantially the likelihood of shearable material (26) tearing near to sharp edge (24) when force is applied to knockout (22), thereby providing greater consistency in the dimensions and finish of the resulting aperture, by ensuring that material (26) is more resistant to shearing at arcuate edge (25).

Description

BOX WITH KNOCK-OUT OPENINGS FOR ELECTRICAL APPARATUS

The present invention relates to enclosures with knockout portions.

Background of the Invention

Typically, plastics enclosures for electrical equipment have portions which have weakened regions enabling them to be readily "knocked out" to allow the passage of cables, wires and so on.

However, often the knocking out is done imprecisely causing a larger hole that needed or wanted, and/or an aperture with ragged edges which may cause damage to a cable passed therethrough.

If the aperture has to. be trimmed to remove sharp edges, inevitably the aperture is made larger thereby preventing any snug fit of cable and aperture.

If the enclosure is made with increased weakening at the knockout perimeter to make the knockout operation easier, the inherent strength of the enclosure when containing the knockout region is also made weaker.

Summary of the Invention

According to the present invention, there is provided a device comprising a sheet of material having a region of weakness to effect tearing in that region upon the application of a force and focussing means for the tear force to effect precise tear propagation of the material.

By tear propagation, there is understood initiation of tearing of the material, and/or further progressive tearing, as appropriate. Preferably, the focussing means comprises a recess in the sheet of material directed to a region of material weakness.

The recess may be sized to accommodate snugly a tool for the application of force, and may be inclined to the main plane of the surface .

The focussing means may effect tear propagation being tear initiation and further tearing.

The device may have a defined edge of material in a region of material weakness to promote tearing.

The material weakness may be promoted by one or more of the following (i) reduced thickness in that region; (ii) increased thickness in another region; (iii) scorings, perforations or other mechanical, thermal, chemical weakness in that region; (iv) use of different materials and/or density of materials.

Suitable materials for the invention may, for example, be acrylonitrile butadiene styrene (ABS), polycarbonate (PC), thermoplastics, hybrids of ABS and PC, and polyester dough moulding compounds (DMC).

The present invention enables a well-defined predetermined aperture to be readily and quickly provided in a low-cost enclosure, pipe, duct or tray, typically made of plastics material but other materials e.g. soft metals may be used. The knockout can be provided in any appropriate shape required and can have a complex (e.g. undulating) profile provided that particularly sharp changes in direction are avoided. General Description of the Present Invention

In order that the present invention may more readily be understood, a description is now given, by way of example only, reference being made to the accompanying drawings in which:- Figure 1 is a perspective view of a knockout detail as known from the prior art; Figure 2 is a section view of the knockout detail of Figure 1; Figure 3 is a perspective view of a knockout detail embodying the present invention; Figure 4 is a section view of the knockout detail of Figure 3; Figure 5 shows the knockout detail of Figure 3 in the process of being removed.

Detailed Implementations of the Present Invention

Figure 1 shows a conventional knockout detail 10 in an enclosure 11 having a knockout section 12 surrounded by a circular groove 13.

Figure 2 is a section view across line X-X of Figure 1. Knockout 12 is attached to enclosure 11 by shearable material 16. The attachment between enclosure 11, knockout 12 and material 16 features sharp edges 14 and 15.

Knockout 12 is removed from enclosure 11 by the application of force thereto, such that shearable material 16 tears and breaks. The applied force may cause tearing of material 16 at any point between sharp edges 14 and 15, resulting in an uneven, ragged hole, requiring trimming so that a cable gland or other cable termination device can be inserted through enclosure 11. These ragged edges may cause damage to cables, for example chaffing due to being drawn through the aperture, especially in the case of knockout areas used for multiple cable entry. The imprecise and uneven nature of the tearing, and the strength of the knockout material essential for safety requirements, may necessitate an inconveniently large force to be applied. The size of the aperture produced, whether or not trimming is required, is indeterminate and variable, and is most unlikely to result in a close fit with the intended cable gland, or whatever device is involved, precluding any effective sealing action, which in some instances may be an important factor.

Figure 3 shows a knockout detail 20 embodying the features of the present invention. Knockout 22 of thickness 3x10^" metres and circular shape with diameter 2x10^" metres is defined in enclosure 21 of thickness 3x10 metres by circular groove 23 and has a cavity 27 extending in a generally radial direction and with a base surface inclined to the plane of knockout 22.

Figure 4 shows a section across line Y-Y of Figure 3. Knockout 22 is attached to enclosure 21 by shearable material 26, of thickness 5x10^"4 metres. The attachment between enclosure 21 and material 26 features sharp edge 24, while the attachment between knockout 22 and material 26 features arcuate edge 25.

Figure 5 shows a stage of the tear operation for the knockout. An operator employs tool 30 (e.g. a screwdriver blade) located by cavity 27, to apply force to the knockout 22 in order to tear material 26, detaching the knockout 22 from enclosure 21 and creating an aperture of a precise, predetermined size, suitable for insertion of a cable. The tear initiation point 40 corresponds with the comer of tool 30.

Arcuate edge 25 serves to increase substantially the likelihood of shearable material 26 tearing near to sharp edge 24 when force is applied to knockout 22, thereby providing greater consistency in the dimensions and finish of the resulting aperture, by ensuring that material 26 is more resistant to shearing at arcuate edge 25.

Other features which enhance the tear propagation (whether initiation or progressive tearing) either separately or in some combination include:- (i) the increasing material thickness at arcuate edge 25; (ii) scorings, perforations or other mechanical, thermal, chemical weakness in the edge 24 areas; (iii) use of different materials and/or density of materials (e.g. using multiple shot moulding techniques).

Cavity 27, used to locate a screwdriver or other tool, causes the force applied using the tool to be focused in a small areas of shearable material 26, guaranteeing the location of tear initiation. This results in a gradual propagation of the tear from this location through and around material 26, and consequently minimises the force necessary to remove knockout 22 from enclosure 21. Cavity 27 at the same time provides a thicker section of material nearer the centre of knockout 22 for the tool to bear against.

In a variant, more than one cavity 27 may be provided to ensure effective tearing of the knockout, especially when a large or complex outline shape of the knockout is required. Thus, the present invention provides a knockout detail wherein the knockout force is reduced due to the focusing of force and gradual tear propagation, and the consistency of the resulting aperture is greatly improved, whilst maintaining the strength of current designs when intact.

Although the above embodiment of the present invention has been described using a circular knockout for cable entry as an example, clearly the invention is applicable to any appropriate geometric shape of knockout, for example a triangle, ellipse, square, rectangle, rhombus, parallelogram, polygon (given that sharp edges are softened slightly to enable the tear to continue to propagate).

Also, the invention is applicable to knockouts for any appropriate type of conduit, object, or other device requiring an aperture, for example pipes, trunking, duct trays and such like.

Materials suitable for use as knockouts in the present invention include ABS, polycarbonate (PC) and other thermoplastics with similar characteristics, hybrids of -ABS and PC, also thermoset plastics such as polyester DMC especially with short, low-content glass fibres.

Additionally, it will be apparent to a person skilled in the art, that method other than those described herein of apply force, and of focusing this force to a particular location, could be employed without departing from the scope of the invention as claimed.

Claims

1. A device comprising a sheet of material having a region of weakness to effect tearing in that region upon the application of force and focussing means for the tear force to effect precise tear propagation of material.

2. A device according to Claim 1 wherein the focussing means comprises a recess in the sheet of material directed to a region of material weakness.

3. A device according to Claim 2 wherein the recess is sized to accommodate snugly a tool for the application of force to effect tearing.

4. A device according to any preceding claim comprising a recess inclined to the mainplane of the surface.

5. A device according to any preceding claim comprising focussing means to effect tear propagation being tear initiation and progressive tearing.

6. A device according to any preceding claim wherein the sheet of material has a defined edge of material in a region of material weakness to promote tearing.

7. A device according to any preceding claim wherein sheet material weakness in the region is promoted by one or more of the following:- (i) reduced thickness in that region; (ii) increased thickness in another region; (iii) scorings, perforations or other mechanical, thermal, chemical weakness in that region; (iv) use of different materials and/or density of materials.

8. A device according to any preceding claim wherein the material comprises one of the following:- • ABS thermoplastics; • Polycarbonate (PC) thermoplastics; • Mixtures of ABS and PC thermoplastics; • Polyester DMC.

9. An enclosure having a device according to any of Claims 1 to 9.

10. A duct having a device according to any of Claims 1 to 9.