GB2367872A

GB2367872A - Device for restricting access to the head of a fastener

Info

Publication number: GB2367872A
Application number: GB0025280A
Authority: GB
Inventors: Philip Denman
Original assignee: PHILIP DENMAN C O DETROIT INTE
Current assignee: PHILIP DENMAN C O DETROIT INTE
Priority date: 2000-10-14
Filing date: 2000-10-14
Publication date: 2002-04-17
Also published as: GB0025280D0

Abstract

A device for restricting access to a fastener (12) having a head (16) and shank (14) includes a first part (18) adapted to be captive between the head (16) and a workpiece (22) in which the shank (14) is received, and a second part (20) dimensioned to accommodate the head (16) and to slidingly receive the first part (18). The first and second parts (18, 20) are adapted to cooperate (for instance, by an interference fit) such that withdrawal of the second part (20) from the first part (18) is prevented and thus access to the fastener head (16) is also prevented.

Description

DEVICE FOR RESTRICTING ACCESS TO THE HEAD OF A FASTENER AND ASSEMLY COMPRISING THE DEVICE AND A FASTENER This invention is concerned with a device for restricting access to the head of a fastener having a head element and a shank element and to an assembly comprising the device and such a fastener. Such fasteners include bolts (in which the head element is integral with the threaded shank element) and studs which are threaded and provide the shank element, the head element being provided by a nut threadingly engaged with the threaded stud. The fastener may also be provided by a screw, for example having a tapered threaded shank element and an integral head element.

In some circumstances it is desirable to prevent access to the head element of a fastener having a head element and a shank element, for example a rotatable bolt which may be received in a member to which the bolt is to be attached by threaded engagement with a threaded bore in the member or by other means. Various means have been proposed for restricting access to the head element of a fastener. Amongst the proposals which have been made are those described in GB-A-2082711 which describes a protective cap which is retained in place over the head of a bolt, to restrict access, by means of a locking member, WO-A-8502447 which describes a collar disposed around the fastener head and a snap-fit closure member which is a resilient snap-fit with a collar for shielding the fastener head and GB-A-2220723 which describes a security means comprising a component with a recess within which a nut (in its tightened position) is received and a closure component received as a non-removable force fit in the recess in the first component to prevent access to the nut.

None of these have been altogether satisfactory insofar as, although they restrict access to the head element of the fastener, they provide protrusions which can be readily attacked by a suitable instrument, for example cold chisels, to remove the protection. Furthermore, there are other disadvantages, for example the means by which the parts are retained together may require use of a number of separate components (GB-A2082711), or rely on means which does not provide a satisfactory arrangement to secure the parts together (WO-A-8502447).

A device is proposed in our patent GB-B-2315309 which overcomes many of the problems associated with the previously known devices but this device not only requires two parts but also a degree of complex machining: this is time consuming and relatively expensive.

One of the various objects of the present invention is to provide an improved device for restricting access to a head element of a fastener having a head element and a shank element.

Another of the various objects of the present invention is to provide an improved assembly comprising a device in accordance with the invention and a fastener having a head element and a shank element.

In one aspect the invention may be considered to provide a device for restricting access to the head of a fastener having a head element and a shank element, the device having a first part adapted to be captive between the head element and a member to which the shank of the fastener is secured and a second part having a recess dimensioned to accommodate the head element of the fastener and to slidingly receive the first part in the recess without engaging the head element, the first and second parts being adapted to cooperate when the first part is sufficiently inserted into the

recess to prevent withdrawal of the second part from the first part, whereby to prevent access to the head element of the fastener.

Preferably in a device in accordance with the invention, an outer surface of the second part remote from the recess has a shape which militates against gripping of the second part (for example using a gripping tool). The outer surface of the second part may be of any suitable shape which militates against, or more preferably completely prevents, gripping of the second part. Preferably in a device in accordance with the invention the outer surface of the second part comprises a frustrum of a cone having an apex angle greater than 50 degrees, preferably greater than 60 ; more preferably the apex angle is about 80 .

In a device in accordance with the invention, the first and second parts may be arranged to cooperate by any convenient means to prevent withdrawal of the second part from the first part. However, it is preferred that the first and second parts cooperate by an interference fit.

Conveniently the outer surface of the first part is substantially cylindrical and the recess in the second part is likewise cylindrical and of a dimension to receive the first part in a sliding interference fit when the second part is forced over the first part by any suitable means, for example by hitting the second part firmly with a hammer. Preferably the outer surface of the first part is knurled. In a preferred device in accordance with the invention the first part has a chamfered leading portion to facilitate introduction of the first part into the recess of the second part of the device.

In a preferred device in accordance with the invention the first and second parts are made of hardened steel. This militates against attack on the first and second parts. Because the outer surface of the second part of the

device is frustroconical, not only is it difficult to provide any form of grip on this outer surface but it is also difficult for a cold chisel or any form of mechanical cutting tool to gain a satisfactory purchase on the parts for cutting them away, thus providing additional security.

There now follows a detailed description to be read with reference to the accompanying drawings of a device for restricting access to the head element of a fastener, namely a bolt, and an assembly comprising the device and the bolt, embodying the invention. It will be realised that this device and assembly have been selected for description to illustrate the invention by way of example.

In the accompanying drawings: Figure 1 is a side view, partly in section, showing a device embodying the invention in the course of assembly to a fastener; Figure 2 is a front view showing the illustrative device as it appears when assembled with the fastener; and Figure 3 is a view in the direction of arrow A of Figure 2 with a member in which the fastener is secured, broken away.

The illustrative assembly 10 comprises a fastener namely a bolt 12 having a shank 14 and head 16 and a device embodying the invention. The device embodying the invention has a first part, namely an anchor ring 18 and a second part, namely a cap 20.

In the illustrative assembly the shank 14 is threadedly received in a member 22 with the anchor ring 18 of the device captive between the head 16 and the member 22.

The first part 18 of the illustrative device has a cylindrical outer surface 24 and a chamfered leading-end portion 26 adjacent the head 16 in the assembly. The cylindrical surface 24 of the first part 18 is knurled. The first part of the illustrative device has a central hole (not shown) through which the shank 14 freely passes but with the hole having a diameter only slightly greater than that of the shank 14 so that there is little play between the first part 18 and the shank 14 in a direction transverse to the axis of the shank 14. The first part may conveniently be made from a suitable cylindrical steel bar having a knurled surface and having a central hole, a leading end portion of the bar being provided with the chamfer and then the first part cut from the leading end. A plurality of similar first parts may be made from a single bar in this way.

The second part 20 of the illustrative device is machined from hardened steel and is provided with the recess 28. An outer surface of the second part 20 comprises a frustroconical portion 30 and a flat end 32.

The recess 28 of the second part is of sufficient depth to accommodate both the head 16 of the bolt 12 and the first part 18 of the illustrative device when the illustrative assembly is in place on a member 22 as shown in Figure 2 with a surface 34 of the second part 20 remote from the flat portion 32 of the outer surface in engagement with a surface 36 of the member 22 into which the shank 14 of the bolt 12 is threaded.

The recess 28 is of cylindrical shape and has a diameter which is substantially identical with the diameter of the cylindrical surface 24 of the first part 18.

In providing the illustrative assembly, the first part 18 of the illustrative device is slid onto the shank 14 of the bolt 12 and the shank 14

threaded into the member 22 with the first part of the device 18 disposed between the head 16 of the bolt 12 and the member 22. The bolt is tightened in position using the head 16 in normal fashion as shown in Figure 1. The second part 20 of the illustrative device is then place over the bolt head 16 with the leading end of the recess 28 remote from the flat surface 32 engaging the chamfered portion 26 of the first part 18 of the device. The flat surface 32 of the second part 20 is then struck firmly with a hammer a sufficient number of times to force the second part 20 of the illustrative device over the first part 18 until the surface 34 of the second part 20 engages the surface 36 of the member 22. The second part 20 of the illustrative device is a tight interference fit with the first part of the illustrative device and this is sufficient to prevent withdrawal of the second part 20 from the first part 18 even when attempts are made to remove the second part using a chisel or other mechanical means. This prevents access to the head 16 of the bolt 12, thereby preventing any unauthorised person from removing the bolt 12. The chamfered portion 26 of the anchor ring 18 provides a leading portion which guides the recess onto the cylindrical outer surface 24 of the anchor ring 18.

The surface 30 of the cap 20 is a frustrum of a cone having an apex angle X (see Figure 2) of at least 50 , preferably at least 600 and more preferably of about 80 . As can be seen, the outer surface 30 tapers in a direction away from the member 22 to which the device is secured in use.

Because the cap 20 of the illustrative device is made of hardened steel it is difficult to remove the illustrative device by any form of drilling or cutting, mechanically. The frustroconical surface 30 makes it very difficult for any gripping means or any cutting implement or chisel to gain any purchase on the illustrative device for the purpose of attempting to remove the cap 20.

The illustrative device is relatively inexpensive, requiring much less machining than previously known devices as discussed above ; furthermore, in contrast with the device described in our aforementioned patent there are no access restrictions to the head 16 when attempting to tighten the fastener to secure it in the member 22, prior to application of the cap 20.

Claims

1. A device for restricting access to the head element of a fastener having a head element and a shank element, the device having a first part adapted to be captive between the head element and a member in which the shank of the fastener is received and a second part having a recess dimensioned to accommodate the head element of the fastener and to slidingly receive the first part in the recess without engaging the head element, the first and second parts being adapted to cooperate when the first part is sufficiently inserted into the recess to prevent withdrawal of the second part from the first part, whereby to prevent access to the fastener.

2. A device according to Claim 1 wherein an outer surface of the second part remote from the recess has a shape which militates against gripping of the second part.

3. A device according to either one of Claims 1 and 2 wherein the outer surface of the second part comprises a frustrum of a cone having an apex angle greater than 50 , preferably greater than 60 .

4. A device according to Claim 3 wherein the apex angle is about 80 .

5. A device according to any one of the preceding claims wherein the first and second parts cooperate by an interference fit to prevent withdrawal of the second part from the first part.

6. A device according to any one of the preceding claims wherein the outer surface of the first part is substantially cylindrical and is knurled and the recess in the second part is cylindrical.

7. A device according to any one of the preceding claims wherein the first part has a chamfered portion to facilitate introduction of the first part into the recess of the second part of the device.

8. A device for restricting access to the head element of a fastener constructed arranged and adapted to operate substantially as hereinbefore described with reference to the accompanying drawings.

9. An assembly comprising a device according to any one of the preceding claims and a fastener having a head element and a shank element, wherein the shank element is received in a member and the first part of the device is captive between the head element and the member and wherein the second part of the device has the first part received in the recess and cooperates with the first part to prevent withdrawal of the second part from the first part, whereby to prevent access to the fastener.

10. A device generally as herein described, with reference to and/or as illustrated in the accompanying drawings.

11. An assembly generally as herein described, with reference to and/or as illustrated in the accompanying drawings.