It is well known to use tamper resistant fastenings to secure the wheels of a motor vehicle. Typically, such tamper resistant fastenings include a non-circular profile which can be engaged by a key to facilitate rotation of the fastening, but which cannot be rotated by means of standard tools (for example standard bi-hexagonal sockets).

Typically, tamper resistant fastenings are used to retain alloy wheels which are chosen, at least in part, for aesthetic considerations. Accordingly, it is important that the tamper resistant fastenings used do not detract from the aesthetic appeal of the wheels they secure. This factor, combined with the desire to provide as much tamper resistance as possible within a robust structure imposes severe limitations on the designer of tamper-resistant screw-threaded fastenings.

The preferred embodiment of the present invention provides a tamper resistant fastening screw-threaded which is suitable for use in retaining vehicle wheels, which is robust, which has a pleasing appearance, and which in the preferred embodiment is assembled by means of a particularly simple and efficient fabrication technique.

According to a first aspect of the present invention there is provided a screw- threaded fastening comprising: a screw-threaded member ; and a cap rotatably mounted on the screw-threaded member ; the screw-threaded member comprising a screw-threaded portion for screw-threaded engagement with a complementary screw- threaded member on or of a base, a shoulder for forcing an article into engagement with the base as the screw-threaded fastening is tightened, and a non-circular profile for engagement by a key for rotating the screw-threaded member; and the cap comprising a skirt which telescopically receives an end portion of the screw-threaded member, the skirt including relieved portions in the periphery thereof which may be moved, by rotation of the cap, into register with the non-circular profile of the screw- threaded member to facilitate the application ofthe key to the screw-threaded member and land portions between the relieved portions which at least partially cover the non- circular profile of the screw-threaded member when the relieved portions are out of register with the non-circular profile.

Preferably, the screw-threaded fastening is used to secure an object, for example the wheel of a motor vehicle, which includes a recess which receives the non- circular portion of the screw-threaded member and at least part of the skirt of the cap.

Preferably, the recess is sized to receive the screw-threaded fastening with only a small radial clearance. Accordingly, when the screw-threaded fastening is in use the view which is presented to someone endeavouring the remove the fastening is of the cap which is partially received within the recess. Because the cap is rotatable relative to the screw threaded member, the application of a tool (for example a self-gripping wrench) to the cap is not effective to rotate the screw-threaded member. In order to rotate the screw-threaded member the cap must be rotated to bring the relieved portions of the cap skirt into register with the non-circular profile of the screw- threaded member and a key applied to the non-circular profile of the screw-threaded member to facilitate rotation of the screw-threaded member.

Preferably, the non-circular profile of the screw-threaded member is provided by means of relieved portions formed in the outer surface of a circular portion of the screw-threaded member. In the preferred embodiment of the invention the circular portion is a flange, one side of which defines the shoulder of the screw-threaded member. Preferably, the relieved portions of the screw-threaded member are part- cylindrical recesses in the periphery of the circular member.

Preferably, the recesses in the screw-threaded member and/or the skirt of the cap limit the zone with which the key engages screw-threaded member to a relatively shallow circumferential band. In other words, the components are preferably configured so that the key does not extend far inwardly from the overall circular profile of the portion of the screw-threaded member in which the relieved portions are formed. With such an arrangement, although a satisfactory force can be applied with the correct key, if attempts are made to fabricate a"master"key which engages only one recess portion of the screw-threaded member, these attempts will be frustrated because the master key will tend to cam out of the recess which it engages, before sufficient torque can be applied to the screw-threaded member to loosen it.

Attempts to form a"master"key by grinding off all but one of the projections of one regular key can further be frustrated if the regular key is shaped such that grinding off of a projection thereof of necessity breaks the skirt of the key at the point of the projection. This desirable characteristic can be obtained if each projection has associated therewith a relieved portion of the outer surface of the key. If the projection is ground off the relieved portion forms a break in the periphery of the key thereby substantially reducing the strength of the key and enhancing to the camming- out action referred to above.

In the particularly preferred embodiment of the invention the cap is retained on the screw-threaded member by means of an outwardly flared portion at the end of the screw-threaded member which is received in an under-cut groove within the cap.

Preferably, the under-cut groove is of frusto-conical form and the outwardly flared portion of the nut closely conforms to the part-conical surface of the under-cut groove.

Preferably, the components are assembled by forming a deformable end portion of the threaded member which has substantially the same diameter as the portion of the cap between the undercut groove and the open mouth thereof. The cap may accordingly be slid onto the screw-threaded member and the deformable region deformed outwardly into the undercut. Preferably, the components are shaped such that the required deformation of the end region ofthe screw-threaded member can be achieved simply by forcing the cap onto the screw-threaded member. Preferably, the force required is applied as a percussive force by means of a blow to the cap. It has been found that such an arrangement forms a reliable and high strength connection between the cap and the screw-threaded member, whilst at the same time provides a rotatable connection which is not capable of transferring substantial torque from the cap to the screw-threaded member.

The invention will be better understood from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawings, wherein: Figure 1 is a longitudinal cross-section of a preferred embodiment of the invention ; Figures 2 and 3 are respectively a longitudinal cross-sectional view and end view of the screw-threaded member of the fastening of Figure 1, prior to assembly with the cap ; Figures 4 and 5 are respectively a longitudinal cross-sectional view and end view of the cap of the screw-threaded fastening of Figure 1 ; and Figures 6 and 7 are respectively a longitudinal cross-section view and end view of a key for the screw-threaded fastening of Figure 1.

Referring firstly to Figure 1, the screw-threaded fastening 1 shown in the drawings is in the form of a wheel nut for a motor vehicle. The fastening comprises a screw-threaded member 2, a cap 3, and a captive washer 4. The screw-threaded member includes an axial bore 5 formed with screw-threads 6 for engaging the threads of a stud of the hub of a motor vehicle. Whilst the preferred embodiment of the invention is embodied as a nut, it will be appreciated that the invention can be embodied as a bolt which can be secured to a threaded aperture provided, for example, in the hub of a motor vehicle.

The screw-threaded member defines a shoulder 7 which, via the washer 4 is used to clamp a wheel against the hub of a motor vehicle. The shoulder 7 is formed by one side of a flange 8 which is an integral part of the screw-threaded member 2.

The cap 3 is retained on the screw-threaded member 2 by means of an outwardly flared portion 9 of the screw-threaded member which is received within an undercut groovelO of the cap. The undercut groovelO is substantially frusto-conical in shape and the outwardly flared portion 9 lies against the part-conical surface ofthe undercut groove. With such an arrangement, the distal end 11 ofthe cap is maintained in light rubbing engagement with the proximal surface 12 of the flange 8 and rattling of the cap 3 on the screw-threaded member 2 is prevented. However, the cap 3 is free to rotate relative to the screw-threaded member 2 with the result that any attempt to unscrew the screw-threaded member from its use position by rotating the cap is ineffective.

Referring now to Figures 2 and 3, it would be seen that the external profile 13 of the flange 8 is circular. In order to facilitate rotation of the screw-threaded member by means of a key a plurality of part-cylindrical recesses 14 are formed in the material of the flange. In order to rotate the screw-threaded member a key is required. A key suitable for rotating the screw-threaded member of Figures 2 and 3 is illustrated in Figures 6 and 7. It will be noted that the key includes a skirt 15 which has formed on the inner surface thereof a multiplicity of projections 16. The key is sized such that the projections 16 project a small distance into the recesses 14 so that torque can be applied by the key to the screw-threaded member. For this purpose, the key 17 includes a portion 18 formed with a conventional drive profile, for example a hexagonal profile. It will be noted that the profile of the projections 16 is not complementary to that of the recesses 14. The projections 16 are sized such that they project only a small distance into the recesses 14, for reasons which will be explained in more detail below.

It will be appreciated that the number and position of the recesses 14 may be varied in order to provide a large number of"differs". For example, the recesses can vary in number. For example, there may be three recesses, four recesses or five recesses. The angular position of the recesses relative to each other may also be varied. For example, looking at Figure 3 two of the recesses may remain opposite sides of a diameter and the other two recesses may be varied in position relative to that diameter in order to achieve the required number of differs. Each combination of recesses will have associated with it a unique key 17.

It will be noted from Figure 7 that each projection 16 has associated therewith a recess 19 in the outer surface of the key. It will further be noted that if an attempt is made to produce a"master key"having only one projection 16 by taking a standard key and grinding off all but one of the projections 16, the effect of grinding off the projections will be to break through the peripheral skirt 15 of the key at each point where a projection is ground off. The resultant gap in the peripheral skirt will substantially weaken the key and if an attempt is made to use such a key to remove a nut the key will deform allowing the one remaining projection 16 to cam out of the recess in which it is positioned. This feature substantially reduces the risk of unauthorised removal of the wheel nut.

Referring now to Figures 4 and 5, the cap 3 is shown. The cap 3 includes a skirt 20 in which recesses 21 are formed. The recesses 21 correspond to the recesses 14 of the screw-threaded member so that by rotating the cap 3 the recesses 21 may be brought into register with the recesses 14 to allow the key to be applied to the recesses 14. It will be noted that the portions 22 of the skirt 20 between the recesses 21 form lands which prevent general access to the flange 8. However, the recesses 21 do not extend to the full axial length of the skirt 20 with the result that the portions 23 of the skirt 20 which are located in line with the recesses 21 limit access to the recesses 21 and 14 to a relatively shallow circumferential band. This shallow circumferential band corresponds to the portions of the projections 16 which, in use, enter the recesses 14.

It will be noted from Figure 2 that, as manufactured, the proximal end of the threaded member includes a relatively thin-walled extension 24. In the assembled fastening of Figure 1 this portion 24 has been deformed outwardly to form the flared portion 9. It has been found that the required outward deflection of the extension portion 24 can be produced if the cap is slid onto the screw-threaded member until the end face 25 of the portion 24 engages a shoulder 26 provided at one end of the undercut groove 10. If the nut is then supported and a percussive blow is applied to the cap the portion 24 will deform outwardly to the configuration illustrated in Figure 1.

It will be noted that for most applications of the fastening illustrated in the drawings a recess will be provided in the member to be fastened which will receive a washer 4, flange 8 and at least part of the skirt 20. Such an arrangement not only limits access to the flange 8 but means that the only visible portion of the fastening is the cap 3. The cap 3 may be made of a pleasing appearance and may, for example, be chromium plated steel or of polished stainless steel.