Background to the Invention Several anti-theft devices for this purpose are already known, said devices being made as nuts or bolts as appropriate. In these known devices, the portion of the device which is accessible when the device is secured in place is typically encircled by a rotatable sleeve. This rotatable sleeve or shroud prevents the device from being turned by gripping it around its circumference using the conventional tools with which a motor vehicle is equipped. Rather a special key is required which is shaped in a complementary manner to a shape on the outward-facing end of the device. Examples of such known anti-theft devices are described in GB2298691 (GROPPO) and EP0253496 (ELLIS).

However, the known anti-theft devices suffer from a number of disadvantages. Firstly, there is inevitably a gap between the rotatable sleeve and the inner, shaped part of the device. This allows a would-be thief to either attack the sleeve or force an implement between the inner part of the device and the sleeve itself. In either case the device can then be undone relatively easy.

The second disadvantage relates to the number ofi possible key combinations. Where the combination resides in a curvilinear shape on the end portion then there are usually no more than thirty to fifty different keys. It is desirable to have a much larger number of combinations.

A third disadvantage relates to the method of manufacture of this type of product. Where these fixings are of one-piece construction, choosing the hardness of the metal is a compromise. A very hard material is preferred for the outer head region whereas a somewhat softer material is preferred for the screw threaded region.

It is an object of the present invention to overcome, or mitigate, some or all of the problems outlined above.

Summarv of the invention According to a first aspect of the present invention there is provided an anti- theft device for mounting an object to a base, said device comprising a nut or a bolt and a key, said nut or bolt comprising a rotatable member having a screw threaded portion inter-engagable with a threaded portion on the base and having a drive portion adapted to engage with the key, said device further comprising a rotatable sleeve extending substantially around the periphery of the drive portion, a clearance fit gap separating the sleeve and the drive portion, characterised in that the clearance fit gap has a stepped or non-linear configuration when viewed in cross- section, thus preventing a tool being driven directly into the gap between the drive portion and the sleeve.

This arrangement makes it virtually impossible to force a tool between the rotatable sleeve and the rotatable member in order to impart sufficient leverage to undo the anti-theft device without the proper key.

Preferably the drive portion overlaps the end of the rotatable sleeve. This has the advantage that the gap between sleeve and rotatable member is not visible from the end of the device. If it is not visible it does not present a target for a would- be thief.

In an alternative embodiment the sleeve is lipped over the perimeter of the outer end of the drive portion.

In a particularly preferred embodiment the rotatable member is formed in two parts, a first part incorporating a screw threaded portion and a second part incorporating a drive portion, thus enabling the two parts to be formed from metal of different hardness.

This eliminates the problem of finding a compromise hardness which is the case for all one-piece anti-theft devices of this type.

Preferably the two parts mate together to form an annular groove, said groove being adapted to accommodate an inwardly directed deformation on the rotatable sleeve. This form of construction assists greatly during assembly. There is a pre-formed channel which can accommodate a flange on the sleeve or which the sleeve can be punched into.

In a particularly preferred embodiment the first and second parts are held together by an interference fit spline.

Preferably the outer end of the drive portion comprises one or more depressions extending axially inwardly into the rotatable member.

It is further preferred that the outer surface of the depression comprises an endless curvilinear groove.

In a particularly preferred embodiment the depression further incorporates a central pin, the size, shape and location of the pin within the depression forming part of the security combination of the corresponding key.

This arrangement provides for a vastly increased number of combinations because the size, shape and profile of the central pin can be varied together with the shape of the outer surface of the depression.

Preferably the rotatable sleeve is frustoconical in shape, and in a particularly preferred embodiment the narrowest part of the frustoconical sleeve is located at the outer end of the drive portion. This makes it virtually impossible to grip the device around its perimeter with any form of gripping tool in order to undo the device without the proper key.

According to a second aspect of the present invention there is provided an anti-theft device for mounting an object to a base, said device comprising a nut or a bolt and a key, said nut or bolt comprising a rotatable member having a screw threaded portion inter-engagable with a threaded portion on the base and having a drive portion adapted to engage with the key, characterised in that the anti-theft device is formed in two parts, the first part incorporating the screw threaded portion and the second part incorporating the drive portion, thus enabling the two parts to be formed from metal of different hardness.

Preferably the first and second parts are held together by an interference fit spline.

In a further preferred embodiment the two parts mate together to form an annular groove adapted to accommodate a rotatable sleeve.

The present invention also encompasses methods of manufacturing anti- theft devices as described herein.

Brief description of the drawings Embodiments of the present invention will now be more particularly described by way of example only with reference to the accompanying drawings in which:-

Figure 1 illustrates plan, elevation with partial cross sectioning and underneath views of a first bolt according to the invention; Figure 2 illustrates plan, elevation with partial cross sectioning and underneath views of a key to fit the first bolt; Figure 3 illustrates the bolt and key of Figures 1 and 2 in combination; Figure 4 illustrates plan, elevation, underneath and perspective views of the first bolt illustrated in Figure 1; Figure 5 illustrates a nut corresponding to the first bolt of the invention; Figure 6 illustrates constructional details of a second bolt according to the present invention; Figure 7 illustrates constructural details of a third bolt according to the present invention.

Description of the preferred embodiments The present embodiments represent currently the best ways known to the applicant of putting the invention into practice. But they are not the only ways in which this can be achieved. They are illustrated and they will now be described by way of example only.

The invention is illustrated in relation to nuts and bolts for securing a wheel to a hub. However, it will be appreciated that the invention can be adapted to any anti-theft device for securing any object to a base. Generally it applies to any type of fixing where rotational movement is required to apply and remove the fixing.

Typically this involves a screw thread.

Referring to Figure 1, this shows a threaded bolt with a main body 1 having a threaded portion 1a, a head 2 and a rotatable sleeve 3. The head 2 contains a depression 6 which extends axially into the head along the longitudal axis of the bolt. The external perimeter or side surface of this depression is formed in a shape

which fits a complementary key 4 which is used to apply rotational force to the bolt in order to tighten or loosen it. In this example the outer perimeter of the depression comprises an endless, lobed, curvilinear surface. However, this is not the only type of shape that can be used. Straight-sided polygons of varying numbers of sides and side lengths would work equally well.

A pin 2b is provided in the centre of the depression 6. This pin extends axially out of the depression and away from the threaded end of the bolt. This pin is an important feature because not only does it help to locate the key but the pin itself can have a number of combination shapes. In this example the pin is shown as substantially circular but a wide variety of shapes are possible. For example, the pin could be elliptical or polygonal or curvilinear in profile. Not only that, but the size and orientation of the pin can be varied with respect to the perimeter surface of the depression. This provides for a virtually limitless number of combinations.

In a further possible modification the pin can be displaced away from the central axis of the bolt to increase still further the number of possible combinations and thus the security.

The main body of the bolt also includes a seating region 16 which generally corresponds to the shape of the object to be fixed around the fixing region. In this example the seating region is shown as a substantially 60° cone. However, any compatible shape can be used and substantially circular radiused-shapes and flats are commonplace.

For each bolt there is a key 4 which incorporates a mating version 4a of the combination shape 2a and central pin 2b in the head of the bolt or nut and which, when correctly aligned will engage in the bolt or nut head. The top part of the key 4b incorporates a conventional hexagon head for use with a standard wheel brace or socket. The size of this hexagon head can vary to suit the various sizes used by vehicle manufacturers but will typically be 17 mm A/F, 19 mm A/F or 21 mm A/F.

The purpose of the outer sleeve is to prevent a potential thief from gaining a grip on the main body of the bolt or nut, enabling him/her to remove the bolt or nut without the special key.

Turning now to the construction of the bolt, this may be formed from three components, a main body 1, a head or drive portion 2 and a rotatable sleeve 3. The head is fitted to the main body of the bolt or nut by an interference fit spline 2c. This type of construction is illustrated in Figures 1,6 and 7.

The rotatable sleeve may be held in place in a number of ways. In one such way the outer sleeve is retained by one or more small deformations in the sleeve which sit in a groove 3a created by the mating of the main body and the separate head. This is a particularly simple form of construction and one which ensures that the rotatable sleeve is free to rotate even after the device has been fitted and fully tightened.

A second method is illustrated in Figure 6. In this example the head 12 has a lip 17 which acts as a stop to prevent the sleeve passing over the head once assembled. The sleeve, which is held captive between a flange 18 on the main body and lip 17 on the head is free to rotate at all times.

This form of construction has particular advantages. It will be appreciated that the outer end of the clearance fit gap between the rotatable sleeve and the body of the anti-theft device is no longer visible or accessible from the outer face or head of the device. That is to say, when the device is used to fix a wheel to a vehicle, it is no longer possible to drive a pointed tool between the sleeve and the drive portion of the nut/bolt. This is a favourite method used by wheel thieves to steal locked wheels without using the proper key.

A further form of construction is illustrated in Figure 7. In this embodiment the end of the sleeve 23 furthest away from the threaded end of the main body 21 overlaps the exposed surface of the head 22. That it is to say, an inwardly directed

flange or lip 28 on the sleeve fits into a corresponding recess 29 in the head. Unlike a straight-sided sleeve where there is a possibility of forcing a tool into the gap between the sleeve and head, no such gap is accessible in this version. Attempts to drive a tool into the fine gap visible on the outer surface of the bolt/nut are immediately met with the resistance of the solid head component.

This form of construction and that illustrated in Figure 6 share a common feature. The gap between the rotatable sleeve and the rotatable member is stepped or non-liner when viewed in cross-section. In the prior art this gap was traditionally linear and open to attack from the outer face of the fixing. The stepped configuration provides considerable additional security.

If the two or three-part construction is not used then the random combination shape and central pin can be incorporated into the main body.

The two or three-part construction described above has a major advantage in that the materials and hardness of each component can be selected independently.

With a one piece construction, selecting the hardness has always been a compromise. A very hard material is preferred for the sleeve and the outer head to prevent unauthorised removal. However, very hard metal tends to become brittle and is unsuitable for the construction of screw threads, which are likely to fail in use.

Using this design allows the threaded portion and the head or drive portion component to both have the optimum hardness values to give the desired performance. For example, the head can be hardened to approximately 40 HRc to ensure that no deformation occurs when torque is applied.

In a further improvement, not illustrated, the frustoconical shape of the device is created by a frustoconical-shaped sleeve. That is to say, instead of being formed to a uniform thickness as illustrated in the figures, the sleeve is wider in cross-section at one end. The inner surface of the sleeve is straight-sided whereas the outer surface creates the frustoconical effect. This means that the various sides

of the main body can be straight and this simplifies the manufacturing process and reduces production costs.

Certain general features will have become apparent from the description above. This invention applies equally well to nuts and bolts. The terms should be considered interchangeable in the description.

The sleeve 3,13,23 is not an essential feature of the invention. Whilst it provides useful additional security it can be dispensed with, especially if the external surface of the visible part of the fixing is curved, frustoconical and/or highly polished.

This prevents the would-be thief from getting a firm grip on the head.

The central pin illustrated in certain embodiments is not essential and can be disposed with.

The optimum materials and material hardness will be selected by the materials specialist as will the various finishes.

Summarv The invention provides a novel bolt or nut for use on motor vehicles to prevent unauthorised removal of the road wheel (s). It is intended that the user will replace one (or more) of the standard wheel bolts per wheel with this security device.

Historically, many features have been incorporated into the design of this type of product to improve the level of security provided. One of the most effective features that is used is that of a freely rotating ring or sleeve, incorporated into the outer diameter of the product which remains freely rotatable even after the product has been fitted and fully tightened. This feature prevents a potential thief from gripping the outer diameter of the fastener sufficient enough to loosen and remove the part.

However, the main concern with this type of sleeve is that inevitably one of the edges of the sleeve is left exposed and by the very nature of a, sleeve, can be easily attacked. Therefore although the thief may be presented with this extra level of security to overcome, relatively quick removal of the sieeve can sometimes leave him with a simple task to remove the remaining part of the fastener.

The design of one embodiment of the invention provides a locking wheel bolt which is assembled in the following way:- A. The ring 3,13,23 is firstly placed over the main body 1,11,21 of the bolt, ensuring the narrow end of the ring is uppermost.

B. The separate head cap 2,12,22 is then located into the main body of the bolt and pressed fully into the position with the aid of a mechanical press (or similar).

C. The tolerances given by the design of the individual components should be such that once the head cap is fully pressed home, the ring is still free to rotate and is not pinched between the main body and the head cap. However, this feature must be checked after assembly has been completed.

Avantages The design of this product and the method of assembling the individual components offer the following significant advantages: 1. The lack of standardisation between the world's vehicle manufacturers in terms of wheel bolts (i. e. thread sizes, thread lengths, etc), combined with the need to offer a significant number of combinations (to obtain the level of security demanded) make it more economic to produce the combination containing part of the product (head cap) and the threaded portion (main body) as separate components.

2. The separate head cap and main body also offer the opportunity to heat treat the parts to different hardness values. This eliminates the compromise between the

hardness of threads (which if too hard become brittle and so likely to fail in use) and the combination part of the bolt head (which if too soft will deform, when torque is applied). Therefore this design allows the threads and the head to both have their optimum hardness value to give the best performance possible.

3. A step in the head cap, together with the assembly process described above, provides protection to both edges of the freely rotating sleeve, preventing the type of attack described above. Therefore the only way to remove the sleeve would be to firstly remove the head cap. However the interference fit between these two components would make this virtually impossible, especially when fitted to a vehicle's wheel.

4. The method of retaining the freely rotating sleeve in the manner described in 3 above, also provides the advantage of eliminating the need of either a staking operation or a retaining clip to assemble the sleeve into position.