GB2067699A

GB2067699A - Tamper-proof fastening

Info

Publication number: GB2067699A
Application number: GB8002279A
Authority: GB
Original assignee: Ferranti Measurements Ltd
Current assignee: Siemens Metering Ltd
Priority date: 1980-01-23
Filing date: 1980-01-23
Publication date: 1981-07-30
Also published as: GB2067699B

Abstract

A tamper-proof fastening 27 comprises screw having a head 28, threaded body, 29 and a frangible neck 30 designed to shear at a predetermined tightening torque. A head retaining member 32 of brittle plastics material surrounds the head and has flanges 38 and 37 to engage shoulders 33 and 34 on the head and body respectively. The retaining member prevents the severed head from being removed from the body and prevents access to a subsidiary head 31. If the head is pulled to gain access to the subsidiary head the retaining member is arranged to fracture, preferably at the root of flange 38 to leave evidence of the attempt. Where the head is recessed in a cavity the head retaining member may be formed of discrete moulded segments disposed around the head. <IMAGE>

Description

SPECIFICATION Tamper-proof fastenings This invention relates to tamper-proof fastenings and in particular to fastenings which are intended to provide evidence of an attempt, whether successfui or not, to gain access to a device secured by the fastening.

Many types of devices, such as electricity meters have enclosure covers required to be held by fastenings which are not readily removed, both to prevent fraudulent use of electricity by interference with the meter operation and to prevent danger to anyone so engaged. Electricity Boards have traditionally sealed the meter cover fastenings to convenient points on the structure by way of wire and lead seals. The wire, however, is readily cut and provides no physical impediment and subsequent joining of such wire is not always apparent without careful scrutiny.

Proposals have been made for alternative methods of indicating tampering or of physically preventing it.

One such arrangement proposed in British Patent Specification No. 1324973 employs a threaded fastener, such as a screw or nut having an axial aperture barbed by projections of the aperture wall. A plug of plastics material has a head and a dependent leg which is pushed into the aperture after the fastening is secured. The cap shields the fastening from removal and any attempt to remove it breaks the leg so that it can quickly be established that tampering has taken place. However once the cap is removed it becomes simple to remove the otherwise conventional fastening.

Another arrangement, aimed at preventing removal of the fastening, is disclosed in British patent specification No. 1,475,543. The fastening, for example, a screw, has a head joined to a shank by a frangible neck which is arranged to shear by the application of a predetermined torque in excess of the tightening torque. The shank carries a ratchet arrangement, such as a barbed spring washer, so that the torque required to unscrew the fastening is greater than the shearing torque of the neck. Any attempt to remove the screw results in the breaking-off of the head. This not only indicates that an attempt has been made to remove the screw but with the head sheared there is no way to remove the screw (from outside the case) other than by driliing it out. This makes unauthorised removal difficult but the difficulty is equally applicable to authorised removal.

It is an object of the present invention to provide a tamper-proof fastening which mitigates the above outlined problems.

According to the present invention a tamperproof fastening able to provide evidence of attempted unauthorised removal of the fastening from an operative tightened position comprises a head by which the fastening is rotated to tighten it, a threaded body, joined to the head by a frangible neck portion arranged to shear at a predetermined tightening torque, a subsidiary head formed on the body adjacent the neck portion and by which a tightened fastening can be loosened, and a head retaining member extending longitudinally of the fastening between the head and the body and having radially extending flanges located so that when the fastening is in an operative position the flanges abut shoulder members on the head and the body to permit relative rotation between the head and the body but to prevent the head and body from moving apart after shearing of the neck, the head retaining member having a portion of a frangible nature whereby force applied to the severed head to remove it from the body results in fracture of said portion.

The term "head" used in relation to the fastening refers to a portion adapted for engagement by a tool, such as a screwdriver, spanner or key, for turning the fastening.

The head retaining member may comprise a plurality of segments each having flanges for engaging the shoulders on both head and body, disposed around the periphery of the head, as the fastening is secured, in such a manner as to prevent subsequent disengagement of the flanges from the shoulder on the head.

In some constructions the head and retaining member may be recessed in a cavity in the operative position. The segments of the head retaining member may then be individual components retained from disengagement from the head and body by the walls of the cavity.

Alternatively, the segments may be secured to each other to prevent disengagement, whether the head is recessed or not.

A head retaining member formed from such segments may include an intermediate flange extending into the space surrounding the neck portion to overlie the subsidiary head.

Where the head and retaining member are recessed within a cavity in the operative position of the fastening the head retaining member may comprise a unitary moulding having an annular ring forming a flange arranged to locate adjacent one of the shoulders and a plurality of arms disposed about the ring and extending towards the other shoulder, each arm supporting a flange arranged to abut the other shoulder said arms being deformable to enable the flanges supported thereon to be moved into, and out of, engagement with said other shoulder during assembly but when in an operative position the arms are restrained from deformation by the walls of the cavity.

In another embodiment the head retaining member may be moulded onto the head and body.

In yet a further embodiment the head retaining member may comprise a pair of moulded sections each having an annular flange and arranged to be passed one each over the body portion and the head and be secured to each other with the flanges abutting their respective shoulders.

In all the above defined embodiments the shoulder on the head may be at the face of the head and in all embodiments but the last defined the shoulder may be formed by a recess in a peripheral wall of the head away from the face.

The portion of the head between the shoulders may then be of a reduced diameter.

The head retaining member may be formed of a plastics material such as an acetyl copolymer and where separate parts are to be secured to each other this may be by adhesives or by ultrasonic welding. Alternatively, the head retaining member may be die-cast from a metal alloy having suitable fracturing properties.

The head retaining member is preferably weakened at the roots of each flange abutting the shoulder on the head so as to frature preferentially when a suitable axial force is applied to the head.

A suitable force is one greater than can be applied accidentally but which can be applied conveniently by a hand-held tool.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional elevation through a conventional shear-head screw fastening, Figure 2 is a sectional elevation through a terminal block and a cover attached thereto by a fastening according to the present invention, Figure 3 is a perspective view of a retaining member employed in the fastening of Figure 2, Figure 4 is a perspective view of an alternative form of retaining member suitable for use when the head and neck portion are located in a cavity, Figure 5 is a sectional elevation through components of an alternative form of retaining member, Figure 6 is a sectional elevation through another form of fastening according to the present invention, and Figure 7 is a modified form of the head retaining member shown in Figures 2 and 3 including an intermediate flange.

Referring to Figure 1 a shear-head screw 10 comprises a head 11 having a screw driver slot 12 a threaded body 13, consisting of a plain shank threaded at 14, and a neck portion 1 5 of reduced diameter designed to shear when the screw is tightened to a predetermined torque. Such screws are employed where it is desired to tighten a member to be held by the screw to a predetermined torque to avoid damage by overtightening. The lower part of the neck 16, may be smooth to prevent subsequent removal or may be provided with a subsidiary head to facilitate this. Such a screw is employed in the fastening of the aforementioned patent specification No.

1,465,543.

Referring now to Figure 2 which illustrates an embodiment of the present invention there is shown a sectional elevation through a terminal block 20 of an electricity meter. The block comprises a base 21 of electrically insulating plastics material holding electrical connection terminals 22. The base has a raised portion 23 including an aperture 24 and threaded insert 25 to support a cover member 26. The cover member is secured to the base by a fastening 27. The fastening comprises a head 28, a threaded body 29, a frangible neck portion 30 joining the head to the body, a subsidiary head 31 formed on the body adjacent the neck portion and a head retaining member 32.

The head has a shoulder 33 formed around the periphery of the upper face of the head. Similarly the body has a shoulder 34 formed by a portion of the body of greater diameter than the shank.

An intermediate shoulder 34' abuts a portion of the cover 26 to hold it against the block 21 as the screw is tightened leaving the shoulder 34 spaced from the cover.

The retaining member 32, shown also in perspective view in Figure 3, comprises a pair of moulded segments 35, 36 of a rigid plastics material. Each segment is semi-cylindrical and has a lower (body) flange 37 and an upper (head) flange 38. The root of the flange 38 is weakened between it and the wall of the segment in relation to the link between flange 37 and the wall of the segment by forming the flange 38 more thinly than flange 37. The head of the fastening has a screw-driver slot 40 and a transverse through aperture 41 for a wire and lead seal. Each head retaining segment has a radial projection 42 adjacent the flange 38 adapted to co-operate with the screw-driver slot 40 to align an aperture 43 in the segment wall with the aperture 41 in the head.

The cover 26 has a thickened portion 44 in vicinity of the fastening location such that a substantial portion of the head and head retaining member are contained in a cavity 45 formed thereby. The thickened portion 24 has a through aperture 45' arranged to align with the aperture 41 in the head for securing it with the aforementioned wire and lead seal. In order to secure the cover by the fastening the fastening is assembled with the segments 35 and 36 against the head with the flanges abutting their respective shoulders. The shank of the assembled fastening is then passed through the cavity 45 which receives the head and the walls of which prevent radial movements of the segments relative to the head sufficient for the flanges to become disengaged from the shoulder.The threaded shank' 29 is engaged with the threaded insert 25 and a screw driver is engaged with the slot 40 to tighten the screw. At a predetermined tightening torque the neck 30 shears permitting the head and head retaining member segments (which are keyed to the slot 40 by projections 42) to rotate relative to the body. The engagement of flanges 37 with shoulder 34 prevents easy removal of the head and/or retaining member from the cavity thereby denying access to the subsidiary head 31 for removal of the fastening screw.

To remove the fastening, the head is gripped and moved axially away from the body. This may be achieved by means of a hand tool (not shown) having pegs which engage opposite ends of the aperture 41, and a fulcrum point adapted to bear against the cover such that a large axial force can be applied to the head. The head bears against the retaining member by way of shoulder 33 and flange 38. At a sufficiently large axial force the flange 38 tends to fracture in preference to any other part of the retaining member permitting the head to be removed from the retaining member. A suitable box spanner is applied to subsidiary head 31 to loosen and unscrew the fastening.

It will be appreciated that once the fastening has been tightened correctly it is not possible to remove it without causing irreparable damage to the retaining member. In the case of authorised removal this is of no consequence but in the case of an unauthorised attempt it provides evidence of a determined attempt to pull the head clear of the body even if the body is not subsequently removed. Frequently evidence of such an attempt is as important as actual removal for the purposes of prosecution but from a safety aspect it is also desirable to place some impediment in the way of removal of the fastening body in addition to the impediment of removing the head from the retaining member. In the simple form shown, removal requires the use of a tubular, or box spanner because of the recessed subsidiary head.

Such a tool is not commonly available but to make unauthorised removal more difficult the subsidiary head may have to be a non-standard size or consist of an unusual number of flats, say five.

Alternatively, the subsidiary head may have a conical surface drilled in a coded pattern to mate with corresponding pegs on a removal tool.

The above described embodiment is open to variations in its structure, some of which will be outlined. Referring also to Figure 4, the head retaining member may comprise a unitary structure 50 consisting of an annular ring 51 arranged to fit over the shank and intermediate shoulders 34' to abut shoulder 34 and at least two upwardly projecting arms 52, 53 supporting inwardly directed flanges 54, 55 respectively. The member is formed of a brittle plastics material such that the flanges 54 and 55 will break rather than bend relative to their supporting arms 52 and 53 but the arms have sufficient flexibility to permit small radial movement of the flanges 54 and 55 relative to the ring 5. The fastening may be preassembled, that is the shank inserted through the ring 51 and the flanges 54 and 55 prised apart to clip over the head.Conveniently the flanges are of such width as to locate in a screw-driver slot in the head.

In order to secure a cover by the fastening the pre-assembled unit is passed through an aperture in the cover such that a substantial portion of the head and arms 52, 53 of the head retaining member are contained within a cavity in the cover (similar to that shown in Figure 2) the walls of which prevent sufficient flexing of the arms 52, 53 flanges 54, 55 to move clear of the shoulder formed by the head.

Removal of the fastening is again by axial movement of the head after the flanges 54 and 55 have been broken off providing evidence of such removal.

Where two flanges 54 and 55 are to be employed and the head has a screw-driver slot then the shoulder 33 may be omitted from the head. However if different means of tightening the fastening is employed or if further arms, such as that shown ghosted at 57, are provided, then the shoulder 33 is retained.

One advantage to this form of retaining member is that prior assembly of the fastening may take place remote from the use of the fastening.

The retaining members described with reference to Figures 2 and 3 require the fastening to be recessed in a cavity. If it is not possible or desirable to recess the fastening then the retaining member shown in Figure 2 may be employed providing the segments are secured to each other around the head. Such securing may be achieved by adhesive or ultrasonic welding.

An alternative arrangement of head retaining member is shown in cross-section in Figure 5. The shape of the retaining member is similar to that shown in Figure 2 but instead of comprising longitudinally split segments it comprises radially split sections 58 and 59. The sections 58 and 59 have annular flanges 60 and 61 respectively. The fastening is assembled by passing the shank through the flange 61 which abuts the body shoulder 34. The segment 58 is then placed over the head such that the flange 60 rests on the shoulder 33. The two segments are then secured to each other about their periphery by adhesive or ultrasonic welding. The flange 60 is again made weaker than flange 61 to induce preferential fracture when the severed head is pulled.

A fastening employing this form of retaining member is suitable for mounting both recessed and standing proud of the surface.

In all of the above described embodiments the shoulder 33 is formed on the upper face of the head where fracture of the flange or flanges is readily detectable. It will be appreciated that the shoulder may be formed by a recess in the peripheral wall of the head away from the face. In such a case the fracture of the flanges would not be readily visible with the head in a recessed position, but a gentle pull on the head would quickly establish if the flange had been fractured and a close examination of the retaining member after removal of the head would establish if parts had been broken and glued together.Where it is appropriate to employ fastenings in which head retaining member is not recessed or the location of the fracture of a recessed retaining member does not have to be visible any part of the fastening, such as the other flanges or even the wall members may be made susceptible to fracture by an appropriate axial force applied to the head.

Instead of fabricating the head retaining member as a separate moulding or mouldings and attaching it to the screw to complete the fastening prior to, or during use, the head retaining member may be moulded onto the screw as shown in elevation of Figure 6.

The head 28' has a shoulder 33' defined by a recess in the wall of the head. The portion of the head below the recess is of smaller diameter than the upper part of the head but in all other respects the screw is as shown in Figure 2 and the same reference numerals have been used. The head retaining member 32' is moulded onto the screw from a molten state so that the retaining member forms a continuation of the head towards the body. Where the material abuts shoulders 33' and 34 flanges are effectively formed. Such a fastening may be employed with the head and retaining member recessed or not.

Figure 7 shows a modified form of the head retaining member shown in Figure 2. Each moulded segment, for example, 35, has an intermediate flange 62 located so that when the retaining member is in position on the fastening the flange extends into the neck region and shields the subsidiary head from access after the head 28 has been removed. Being of brittle material access to the subsidiary head is obtained by fracturing the flange with a blow from the tool used to engage the subsidiary head. The intermediate flange enables differentiation between mere removal of the head without further attempt to remove the screw and achievement of access to the subsidiary head. An intermediate flange as described is, of course, effective only where the head retaining member is reasonably certain to break initialiy above this flange.

All of the above embodiments have described and illustrated the fastening member as being a screw having an externally threaded body. It will be appreciated that with suitable changes to the detailed structure of the retaining member, the body of the fastening may be an internally threaded nut having a severable portion frangible on tightening.

The head retaining member may be of any material having the properties required, essentially brittleness of the flanges. It may be die-cast from a metal alloy having suitable fracturing properties, but preferably it is formed of plastics material such as acetyl copolymer.

In the embodiments described above with reference to Figures 2, 3 and 7 in which a plurality of segments are arranged around the head, it is convenient for two semi-cylindrical segments to be used but a larger number of shorter segments may also be used.

Claims

1. A tamper-proof fastening able to provide evidence of attempted unauthorised removal of the fastening from an operative, tightened, position comprising a head by which the fastening is rotated to tighten it, a threaded body joined to the head by a frangible neck portion arranged to shear at a predetermined tightening torque, a subsidiary head formed on the body adjacent the neck portion and by which a tightened fastening can be loosened and a head retaining member extending longitudinally of the fastening between the head and the body and having radially extending flanges located so that when the fastening is in its operative position the flanges abut shoulder members on the head and the body to permit relative rotation between the head and the body but to prevent the head and body from moving apart after severing of the head, the head retaining member having a portion of a frangible nature whereby a suitable force applied to the severed head to remove it from the body results in fracture of said portion.

2. A tamper-proof fastening as claimed in claim 1 in which the head retaining member comprises a plurality of segments, each having flanges for engaging the shoulders on both head and body, disposed around the periphery of the head, as the fastening is secured, in such a manner as to prevent subsequent disengagement of the flanges from the shoulder on the head.

3. A tamper-proof fastening as claimed in claim 2 designed for at least a part of the head retaining member to be recessed in a cavity in its operative position, in which fastening the segments of the head retaining member are restrained from disengagement from the head and body by the walls of the cavity.

4. A tamper-proof fastening as claimed in claim 2 in which the segments of the head retaining member are secured to each other to restrain from them disengagement from the head and body.

5. A tamper-proof fastening as claimed in any one of claims 2 to 4 including an intermediate flange extending into the space surrounding the neck portion to overlie the subsidiary head.

6. A tamper-proof fastening as claimed in claim 1 in which the head retaining member comprises a unitary moulding having an annular ring forming a flange arranged to locate adjacent one of the shoulders, and a plurality of arms disposed about the ring and extending towards the other shoulder, each arm, supporting a flange arranged to abut the other shoulder, said arms being deformable to enable the flanges supported thereon to be moved into, and out of, engagement with said other shoulder during assembly but when in an operative position in which at least part of the head retaining member is recessed within a cavity the arms are restrained from deformation by the walls of the cavity.

7. A tamper-proof fastening as claimed in claim 1 in which the head retaining member is moulded onto the head and the body.

8. A tamper-proof fastening as claimed in claim 1 in which the head retaining member comprises a pair of moulded sections each having an annular flange and arranged to be passed one each over the body portion and the head and be secured to each other with the flanges abutting their respective shoulders.

9. A tamper-proof fastening as claimed in any one of claims 2 to 8 in which the shoulder on the head is at the face of the head.

10. A tamper-proof fastening as claimed in any one of the claims 2 to 7 in which the shoulder on the head is formed by a recess in the peripheral wall of the head away from the face.

11. A tamper-proof fastening as claimed in claim 10 in which the portion of head between the shoulders is of reduced diameter.

12. A tamper-proof fastening as claimed in any one of the preceding claims in which the head retaining member is formed of a plastics material.

13. A tamper-proof fastening as claimed in claim 12 in which the plastics material is an acetyl copolymer.

14. A tamper-proof fastening as claimed in any one of claims 1 to 11 the head retaining member is die-cast metal alloy.

1 5. A tamper-proof fastening as claimed either in claim 12 or claim 13 when dependent from claim 4 or claim 8 in which the parts of the head retaining member are secured by ultrasonic welding.

1 6. A tamper-proof fastening as claimed in any one of the preceding claims in which the head retaining member is weakened at the roots of each flange abutting the shoulder on the head so as to fracture preferentially at the weakest point when said suitable axial force is applied to the severed head.

1 7. A tamper-proof fastening as claimed in claim 1 6 in which said suitable axial force is arranged to be within a range greater than can be applied accidentally to the fastening in an operative position but which can be applied by a hand-held tool.

1 8. A tamper-proof fastening as claimed in any one of the preceding claims in which the subsidiary head is arranged to be engageable, after removal of the head from the retaining member, by a tubular spanner.

1 9. A tamper-proof fastening as claimed in claim 1 8 in which the subsidiary head has five flats extending parallel to, and disposed around, the longitudinal axis of the fastening.

20. A tamper-proof fastening as claimed in any one of the preceding claims including apertures extending traversely to the longitudinal axis of the fastening through the head and the head retaining member and through which a seal wire may be threaded, the head and head retaining member being provided with co-operating projections and recesses to maintain alignment between the apertures.

21. A tamper-proof fastening substantially as herein described with reference to and as shown in Figures 2 and 3 or any one of Figures 4 to 7 of the accompanying drawings.