This invention relates to tamper-resistant fasteners such as screw, bolts, heads for spindles of door catches and other devices where a rotary fastening movement or unfastening movement is required.

It is generally known to provide tamper-resistant screws and other fasteners with specially shaped heads that require special tools for their fastening and unfastening but which are comparatively inaccessible or difficult to operate using ordinary tools. Most of the proposals in the prior art have flats or other angled recesses which can, albeit with difficulty in some cases, be gripped other than by the special tool for which the screw is designed.

The object of the invention is to provide an improved tamper-resistant fastener and a corresponding tool. The invention is based on the use, in the head of the fastener of a double cylindrical recess of which the parts are eccentric with respect to each other.

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According to the invention, a tamper-resistant fastener which is rotatable about a defined axis comprises a head having a circular recess and, at the bottom of the said recess, a further circular recess which is of lesser diameter than the first recess and is eccentric relative thereto, one of the reces ^' ses being coaxial with the said axis.

Preferably, the further recess has a common tangent with the first recess.

The diameter of the further recess may be less than half that of the first recess and in one embodiment substantially less than half the first recess. However, a preferred embodiment of the invention provides for the further recess a diameter which is substantially more than half that of the -first recess.

The invention also includes within its scope a tool for use with a fastener according to the invention, the tool being characterised by a cylindrical shank portion having at its end a cylindrical extension which is of lesser diameter than the shank portion and is eccentric relative thereto. A further aspect of the invention comprises a combination of a fastener and a tool according to the invention.

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BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 of the accompanying drawings shows in section one embodiment of a screw according to the invention.

Figure 2 illustrates in section a screwdriver for use with a screw according to Figure 1.

Figure 3 illustrates in plan form a preferred configuration of the recesses which are characteristic of a screw according to the invention.

DETAILED DESCRIPTION

Figure 1 of the drawings illustrates one form of a screw according to the invention. The application of the invention to a screw is merely by way of example and the invention may be used as rotary fasteners generally.

The screw 1 has a head 2 which may be in any suitable form and a shank 3 which in this embodiment of the invention has a terminal threaded portion 4. The screw may be a self-tapping screw but other forms of screw or bolt may embody the invention.

Formed in the head of the screw is a circular recess 5 which is coaxial with the shank and the threaded portion 4. At the bottom of the recess 5 is a further recess 6 which is likewise circular in section and is eccentric relative to the recess 5. Preferably the recess 6 has a common tangent with the recess 5 ; in other words the circles defining the cross sections of the recesses 5 and 6 touch at one point.

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The screw head 2 is preferably countersunk. Thus, when it is fully screwed home by means of a tool as described in Figure 2, there is no flat or other angular portion by means of which some other tool can be used to obtain a purchase on the screw.

The tool shown in Figure 2 is formed in complementary manner to the recess in the head 2 of the screw 1. The tool or screwdriver 7 preferably has a handle 8, a shank or shaft 9 which is cylindrical at least in a terminal portion, the terminal part 11 of the shank 9 being formed with a cylindrical extension 10 which is eccentric relative to the shank 9 in a manner similar to the disposition of the recess 6 relative to the recess 5.

In use, the tool 7 is simply inserted into the recess 5, the extension 10 being located in the recess 6, the eccentricity of the recesses 5 and 6 enabling the tool to obtain purchase on the screw.

In the embodiment shown in Figure 1 the recess 6 has a very much smaller diameter than the recess 5. However, it is generally preferable to make the diameter of the recess 6 a substantial fraction of the diameter of the recess 5 and it is probably preferable to make the diameter of the recess 6 more than half that of the recess 5. Figure 3 shows, not to the same scale as Figures 1 and 2, the recesses 5 and 6 in plan, the recess 6 being slightly more than half the diameter of the recess 5.

In the foregoing description the upper recess is coaxial with the axis defined in the embodiment by the shank ^" but which is in any case the axis about which the head is to be

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turned. However it is feasible to make the second or lower recess coaxial with that defined axis i.e., in this form of the invention, coaxial with the shank. The cooperating tool would then have its extension 10 coaxial with the shank, the part 11 being eccentric relative thereto.

It is envisaged that tool 7 would be supplied in restricted quantities to authorised users so as to obtain some control over the use of the combination of fasteners and tools and thereby enhance the security provided by the construction of the fastener.

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