BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a screw fastener and more particularly, but not exclusively, to a screw fastener having a low profile head as may be used for fixing steel frame members, windows and door lintels in building construction.

2. Description of the Prior Art

In building construction involving the use of the steel frames, particularly the construction of domestic housing, steel frame members are typically fastened together using self- tapping screws driven into holes pre-formed in the frame members. Usually, the hole is formed within a dimple or recess pressed into the frame member so that the head of the screw sits within the recess. Although the head sits within the recess, the head itself is also of relatively low profile or depth (usually less than about 2.5mm) so that it does not project beyond the surface of the frame member. The reason for this is to enable drywall cladding, plasterboard for example, to be fixed directly to the surface of the frame without the heads of the fasteners forming unsightly bulges and bumps in the cladding. Similar considerations apply to the fixing of other components in the structure such as window and door lintels and bracing straps. In existing screw fasteners for this purpose, the head is formed with an internal drive recess to receive a power driven driver tool. There are a number of commonly available internal drive systems such as Phillips, Torx and internal hex, and which system is selected is essentially a matter of design choice. With a low profile head, in practice the depth of the internal drive recess is restricted due to the reduced depth of the head. This has a weakening effect on the driver tool as the load bearing surfaces are reduced and this, in turn, increases the load per unit area resulting in an increase in driver wear and an increased tendency for driver breakage. Moreover, it can increase the tendency of the driver tool to "cam out" of the drive recess under the effect of driving torque when the torque exerts a greater force tending to dislodge the tool from the recess than that being applied to retain the tool in the recess. For most situations it is not practical to extend the depth of the drive recess so that it extends into the shank of the screw as this will result in a reduction in the cross sectional area of the screw shank in the zone at the base of the head leading to a reduction in the strength of the screw.

SUMMARY OF THE INVENTION

The present invention in its preferred embodiments seeks to provide a screw with a drive configuration which provides more reliable driving engagement between a driver tool and screw head even when the screw head is of low profile form. According to one aspect of the present invention there is provided a screw fastener having a head with a peripheral surface configured to apply a driving torque to the head by an external driver tool, the head including an axially-directed hole for receiving an axially- directed pin of the driver tool whereby engagement of the pin within the hole acts to maintain the driver tool in engagement with the screw head during driving of the screw.

In a preferred embodiment, the outer periphery of the head is formed with a hexagonal driving formation engageable within a correspondingly shaped drive socket of the drive tool, although other external driving configurations for use with a drive socket of corresponding shape may alternatively be used.

Advantageously the hole is tapered to receive a correspondingly tapered pin of the drive tool to provide a firm wedging fit of the pin into the hole.

According to a further aspect of the invention there is provided a driver tool for driving a screw fastener as specified above, the tool comprising a drive socket configured to a driving engagement with the screw head, and within the socket, an axially-directed pin for engagement in the hole in the screw head.

BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:

Figure 1 is an end view of the head of a screw in accordance with the preferred embodiment of the invention;

Figure 2 is a side view of the screw head - in this figure and in other subsequent figures, the threaded shank of the screw has been omitted;

Figure 3 is a view of the underside of the screw head;

Figure 4 is a perspective view of the screw head;

Figure 5 is a perspective view of the underside of the screw head;

Figure 6 is a perspective view of a driver tool for use with a screw head as shown in Figures 1 to 5;

Figure 7 is an enlarged view showing in greater detail the drive socket of the driver tool as indicated by A in Figure 6;

Figure 8 is a section showing the driver tool engaged with the screw head;

Figure 9 is an enlarged detail of Figure 8 as indicated by B in Figure 8;

Figure 10 is a perspective view of a screw head according to an alternative embodiment of the invention; and

Figure 11 is a perspective view showing the underside of the screw head of Figure 10. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The screw in accordance with the preferred embodiment of the invention has a low profile head 2 for use in applications such as those indicated above, but in contrast to existing screws of this type the head is configured for an external drive rather than an internal drive. In the embodiment shown, this is achieved by forming the outer periphery of the head with a hexagonal shape which cooperates with a correspondingly shaped drive socket 4 of a driver tool 6, although the external drive could be achieved by other shaping as will be subsequently described. While the external drive is able to apply a strong driving torque notwithstanding the reduced depth of the head, however the reduced depth of the head does mean that the driver tool 6 is able to be more easily dislodged from the head during the driving, in other words an effect similar to the cam out effect prevalent with an internal drive as described above. In the preferred embodiment in order to avoid this, the head of the screw is formed with an axially-directed hole 8 and the drive socket 4 is formed with an internal axial post or pin 10 which engages within the hole 8 when the socket is engaged with the head. The engagement of the pin 10 into the hole 8 is effective in preventing dislodgement of the drive socket 4 from the head 2 during driving.

In the preferred embodiment, the pin 10 is provided with a slight taper, preferably up to about 5 degrees, with the hole 8 in the head of the screw being correspondingly tapered so as to provide a tight fit by wedging of the pin 10 into the hole 8. The depth of the hole 8 is such as to provide a small clearance between the end of the pin 10 and base of the hole when the pin is firmly seated within the hole (see Figures 8 and 9). While the slight taper is preferred, both the pin and the hole could be cylindrical and sized to provide a close clearance fit of the pin into the hole; although this should be satisfactory, nevertheless the tight fit provided by the wedging action is believed to provide a better option.

In the preferred embodiment the pin and hole while extending in an axial direction are also positioned on the central axis of the screw and driver tool. While that is preferred, it is not essential and an arrangement in which the pin and hole are laterally offset from the central axis would be feasible.

The external hexagonal drive used in the preferred embodiment is advantageous in that it also enables the use of a conventional hexagonal drive socket if it is subsequently required to remove the screw in circumstances where the special drive socket with internal pin is unavailable. However, external drive shapes other than hexagonal can be provided, with the drive socket being correspondingly shaped. For example, the head could be of oval or ellipsoidal shape, or formed with a series of indents so as to form a series of angularly spaced peripheral teeth 12 as shown in Figures 10 and 11.

The screw of the preferred embodiment for use in connecting steel frame members and the like is a self tapping screw with a low profile head designed for the applications discussed earlier. As such, the head has a depth of less than about 2.5mm. Apart from the head, the screw is otherwise conventional and comprises a threaded shank which is not shown in the drawings. It is also preferred that the head is slightly domed so that the outer edge of the head is below a level at which it is likely to cause interference with wall board slid across the frame members. The underside of the screw head may optionally be formed with teeth 14 (see Figure 5) for locking the head against the steel frame member. It is however to be understood that the invention is not confined to a screw as described above and is applicable to a variety of other screws where a reliable engagement between the driver tool and screw head is required irrespective of whether or not the screw head is of low profile. Considering, for example, a typical roofing screw for fixing metal roofing sheet to an underlying timber or metal batten, in which the screw is a self-drilling screw having a head with an external hexagonal profile for application by a hexagonal drive socket of a driver tool. The head itself is of regular depth, rather than of low profile, and while the depth is such that in most situations reliable engagement between the drive socket and head is achieved, if the screw is being applied in a situation where the operator is not able to apply a firm force axially to the driver tool, for example when the screw is being applied at an angle at the edge of a roof, the driver tool can dislodge from the screw head, possibly quite suddenly, potentially causing injury to the operator. By forming the head of the roofing screw with an axially- directed hole whereby the head is configured for use with a hexagonal drive socket with an axial pin as described above, reliable engagement between the drive socket and screw head can be maintained even when the screw is driven at an awkward angle; it will of course be understood that screws with that capability can still be used with a conventional hexagonal drive socket.

The embodiments have been described by way of example only and modifications are possible within the scope of the invention.