IMPROVEMENTS IN OR RELATING TO SCREWS

Abstract: A tamper resistant screw (30) comprises a shank ((31) and a head (30a) having a top surface (32) on the opposite side thereof to the shank (31) and there being at least five channels (18) in the top surface (32) extending radially from a central part (35) of the top surface (32) disposed transversely of the longitudinal axis (X-X) of the shank (31), the channels (18) each having side surfaces (20) at opposite sides of a bottom surface (21), the side surfaces (20) being disposed at equal distances from and on opposite sides of a radial plane (P) containing the longitudinal axis (X-X) of the shank (31) and being of increasing depth from a minimum depth where they are closest to the central part (35) to a maximum depth at the periphery of the head (30a) and no channel (18) being disposed diametrically opposite any other channel.

Description: Title: "Improvements in or relating to screws" This invention relates to screws and more particularly to tamper resistant screws. That is to say, screws which, subsequent to being driven,are difficult to unscrew, at least without a special tool. An object of the invention is to provide a new and improved tamper resistant screw. According to the invention, we provide a tamper resistant screw comprising a shank and a head having a top surface on the opposite side thereof to the shank and there being at least five channels in the top surface extending radially from a central part of the top surface disposed transversely of the longitudinal axis of the shank, the channels each having side surfaces at opposite sides of a bottom surface, the side surfaces being disposed at equal distances from and on opposite sides of a radial plane containing the longitudinal axis of the shank and being of increasing depth from a minimum depth where they are closest to said central part to a maximum depth at the periphery of the head and no channel being disposed diametrically opposite any other channel. The side surfaces are preferably parallel to said radial plane. The bottom surface of each channel is preferably planar. The maximum depth of said side walls may lie in the range 0.8 millimetres to 2.5 millimetres. Five channels may be provided angularly spaced apart at 720. The head may have an undersurface on the opposite side thereof to said top surface which is perpendicular to the longitudinal axis of the shank. Alternatively, the head may have an undersurface on the opposite side thereof to said top surface which is of conical configuration. The bottom surface may be inclined to a plane normal to said longitudinal axis at an angle of approximately 0 25 According to a further aspect of the invention, we provide a screw driver comprising a shank portion and a head having a recess in the surface thereof opposite to the shank portion and comprising a centrally disposed planar portion in said recess disposed transversely of a longitudinal axis of the shank and not less than five ribs extending radially and axially outwards from said planar portion and having side surfaces disposed at equal distances from and on opposite side of a top surface, said side surfaces being disposed at equal distances from and on opposite side of a radial plane containing the longitudinal axis of the shank and being of minimum depth where they are closest to said central part and of maximum depth at the periphery of the recess and no rib being disposed diametrically opposite any other rib. The invention will now be described in more detail by way of example, with reference to the accompanying drawings wherein: FIGURE 1 is a plan view of a screw embodying the invention, FIGURE 2 is a side elevation of part of the screw of Figure 1 and Figure 3 is a perspective view of a tool for driving the screw of Figures 1 and 2. Referring to the drawings, there is illustrated a screw 10 having a head 11 and a shank 12. The shank 12 is provided with a thread of any desired configuration, such as a machine thread, or a self-tapping thread for use in metal or a wood screw thread. On the opposite side of the head 11 to the shank 12, the head has a top surface 13 of "pan-head configuration, that is to say a generally flat portion 14 bounded by a frusto conical portion 15 having a small included angle of 100 and the portions 14 and 15 merging together in a rounded portion 16. On the opposite side of the head 11 to the upper surface 13 is an undersurface 17 of annular configuration. If desired, the undersurface 17 may be of other configuration, for example,it may be of generally conical shape. Five radially extending channels 18 are formed in the head 11. Each channel 18 extends from a pentagonal shaped central part 19 which extends transversely of and is centred on the longitudinal axis X-X of the screw shank. Each channel 18 comprises side surfaces 20 on opposite sides of a bottom surface 21 thereof. The surfaces 20 are disposed parallel to and at equal distances from a radial plane P which contains the axis X-X of the shank. The surface 21 is inclined to a plane normal to the axis X-X at an angle of 250. That is to say, the line of intersection between the plane P and the surface 21 is inclined to said normal plane at 250 If desired, the angle inclination of the surface 21 to said normal plane may be at an angle other than 250. As a result of the inclination of the surface 21, the walls 20 increase in depth from the end thereof closest to the axis X-X to a maximum depth at the periphery of the head. In the example illustrated, the walls 20 increase from zero depth, where they meet at the pentagonal central part l9,to a maximum depth lying in the range 1.00-1.75 millimetres depending upon the size of screw, as can be seen from the table below. 'A'* 'B1* 'C'* Dia Dim Width Depth M3 6.00 1.80 1.00 1.00 5.70 1.66 M3.5 7.00 2.10 1.15 1.15 6.64 1.96 8.00 2.40 1.20 1.25 7.64 2.26 M5 10.00 3.00 1.65 1.65 9.64 2.86 M6 12.00 3.60 1.75 1.75 11.57 3.42 *All dimensions are in millimetres. The channels 18 are equally angularly spaced by 720. Because of the channel configuration described above, it is difficult to drive or remove a screw embodying the invention without a tool adapted to engage with the driving surfaces of the head. That is to say, a tool having ribs of a configuration complementary to the channels of the head. Because the channels are asymmetric diagonally of the screw head, and because they merge, at the ends thereof closest to the central axis of the screw, with the central part 19 of the head, it is not possible to engage a conventional screwdriver blade therewith. Further, because the depth of each channel, even at its maximum depth, is relatively small, it is not possible to easily rotate such a screw by engaging a screwdriver blade or other tool with one channel alone since there is a relatively small surface available for engagement in this fashion. Therefore, whilst a screw embodying the invention can be easily driven to a desired torque by means of a complementary driving tool by an authorised user of the screw, any unauthorised user has the above described difficulty in driving the screw,or in removing the screw. By providing five channels,an adequate surface area is available for engagement by an authorised tool whilst at the same time, each driving surface is of relatively small size thereby making it difficult for the screw to be rotated by an unauthorised tool engaged only with a single driving surface. If desired, more than five channels may be provided, disposed so that no channel is diametrically opposite and in this case each driving surface can be made still smaller. However, less than five channels is unsuitable since each channel has to be adequately deep to provide the necessary area to permit driving to a high torque and consequently, each channel provides a relatively large surface for engagement by an unauthorised tool and such a screw would therefore be more prone to unauthorised rotation. Although in the above described example, the side surfaces of each channel have been described as being in a plane parallel to the plane P, they may, if desired, be slightly inclined to the said plane. Whilst this would still permit rotation with an especially adapted driver, it would make unauthorised rotation still more difficult as the tendency would be for any such unauthorised tool to "cam out" of the channel. Although the case of a pan head screw with a undersurface lying in a plane normal to the axis X-X has been described, if desired, the screw head may be of other configuration, for example the undersurface may be of conical configuration so as to be receivable ina countersunk hole, and/or the upper surface may be of shallow conical configuration with a central portion centred on and extending transversely to the central axis, similar to the central portion 19 of the screw hereinbefore described. If desired, other head configurations may be provided, for example, the upper surface may be of domed configuration and the undersurface similar to the surface 17 or the above described conical configuration. Referring now to Figure 3, there is shown a driving tool for use with the screw described hereinbefore. The tool has a head 30 and a shank 31 of any desired configuration. The shank may be provided with a handle for manual rotation of the tool, or may be adapted to be engaged in a power tool. The head 30 comprises a generally cylindrical outer surface 30a and is formed with a recess 32. Within the recess 32 are five ribs 33 of a configuration complementary to the channels 18 of the screw with which the driver is intended to be used. That is to say, the ribs have side surfaces which are spaced apart a distance slightly smaller than the surfaces 20 of the channels 18 permitting engagement of the ribs within the channels and the ribs have a top surface 34 which is intended, in use, to lie parallel to and slightly spaced from the surfaces 21 of the channels. At the centre, the recess has a pentagonal portion 35 of dimensions complementary to the pentagonal portion 19 of the screw, and again intended to lie closely adjacent and parallel to the surface 19 of the screw in use. Said side surfaces are preferably parallel to said radial plane. Said top surface is preferably planar. The maximum depth of said side walls may lie in the range 0.8 millimetres to 2.5 millimetres. Five ribs may be provided angularly spaced apart at 720.

Claims: CLAIMS: 1. A tamper resistant screw comprising a shank (12) and a head (11) characterised in that a top surface (13) on the opposite side of the head (11) to the shank (12) is provided with at least five channels (18) extending radially from a central part (19) of the top surface (13) disposed transversely of the longitudinal axis of the shank, the channels (18) each having side surfaces (20) at opposite sides of a bottom surface (21), the side surfaces (20) being disposed at equal distances from and on opposite sides of a radial plane containing the longitudinal axis (X-X) of the shank (12) and being of increasing depth from a minimum depth where they are closest to said central part (19) to a maximum depth at the periphery of the head (11) and no channel (18) being disposed diametrically opposite any other channel (18). 2. A screw according to Claim 1 characterised in that the side surfaces (20) are parallel to said radial plane. 3. A screw according to Claim 1 or Claim 2 characterised in that the bottom surface (21) of each channel (18) is planar. 4. A screw according to Claim 1 or Claim 2 characterised in that the bottom surface (21) of each channel (18) is inclined to a plane normal to said longitudinal axis ( X-X ) . 5. A screw according to Claim 4 characterised in that the bottom surface (21) is inclined at an angle of 0 approximately 25 6. A screw according to any one of Claims 1 to 5 characterised in that the maximum depth of said side walls (20) lies in the range 0.8 millimetres to 2.5 millimetres. 7. A screw according to any one of Claims 1 to 6 characterised in that the five channels (18) are provided angularly spaced apart at 720. 8. A screw according to any one of the preceding claims characterised in that the head (11) has an undersurface (17) on the opposite side thereof to said top surface (13) which is perpendicular to the longitudinal axis (X X) of the shank (12). 9. A screw according to any one of Claims 1 to 8 characterised in that the head (11) has an undersurface (17) on the opposite side thereof to said top surface (13) which is of conical configuration. 10. A screw driver comprising a shank portion (31) and a head (30) characterised in that the head (30) has a recess (32) in the surface thereof opposite to the shank portion (31) and comprising a centrally disposed planar portion (35) in said recess (32) disposed transversely of a longitudinal axis of the shank (31) and not less than five ribs (33) extending radially and axially outwards from said planar portion (35) and having side surfaces disposed at equal distances from and on opposite side of a top surface (34), said side surfaces being disposed at equal distances from and on opposite side of a radial plane containing the longitudinal axis of the shank (31) and being of minimum depth where they are closest to said central part (35) and of maximum depth at the periphery of the recess (32) and no rib (33) being disposed diametrically opposite any other rib.

Inventors: WARHAM, PETER GEOFFREY
Applicants: COLMAN FASTENERS CO. LIMITED
Application year: 1982
Main classification: F16B23
IPC codes: F16B23, B25B15