BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to screw fasteners and more particularly to screws for use building construction such as for fixing composite panels, roofing sheets and cladding.

2. Description of the Prior Art

Screws for fixing panels, roofing sheets, and cladding in building construction are typically self-drilling screws having with a hexagonal drive head with integral washer. The screw is driven by engagement within a hexagonal socket of a power tool. While screws having a shank length of about 95mm or less are relatively easy to drive, as the shank length increases above 95mm, as may arise when the shank has to traverse thick insulation or a thermal break, the screw becomes increasingly difficult to install as the screw becomes less stable and tends to wobble. While the wobbling effect can be mitigated by the operator holding the shank of the screw in one hand, this leads to friction and heating and can cause substantial discomfort to the operator. Another effect arising from the combination of wobble and the length of the screw is the tendency for the head of the screw to break out of the drive socket when end load is applied and which is necessary to install the screw. In order to overcome the problem arising from wobbling of longer length screws, applicators are available which hold the screw shank but these tend to be quite expensive, heavy and not easy to use.

SUMMARY OF THE INVENTION

The present invention relates to a design of screw head and associated drive socket which provides a locking interface between the two to prevent wobble.

According to one aspect of the present invention there is provided a screw having a driving head of polygonal form, the head being tapered so as to mate with a wedging fit with a drive socket of corresponding form.

The present invention also provides a drive socket of tapered internal configuration for engagement with a wedging fit with the head of a screw as specified above.

In a preferred embodiment, the head is of hexagonal form and the screw is a self-drilling screw.

BRIEF DESCRIPTION OF THE DRAWINGS

An embodiment of the invention will now be described by way of example only with reference to the accompanying drawings in which:

Figure 1 is a side view of the head end of a screw in accordance with the preferred embodiment, the shank and remainder of the screw being omitted from that figure;

Figure 2 is an end view corresponding to Figure 1 ;

Figure 3 is a side view in section showing a drive bit engaged with the head of the screw; and

Figure 4 is an enlarged detail showing a wedging engagement between a socket of the drive bit and the head of the screw.

DESCRIPTION OF THE PREFERRED EMBODIMENT

A screw in accordance with the preferred embodiment of the invention has a hexagonal drive head 2 and integral washer 4. In contrast to conventional drive heads of this form, the hexagonal head is tapered towards its outer end. The taper angle is not required to be particularly large, a taper angle of only a few degrees per face, such as 5° equivalent to an included angle a of 10°, being sufficient. However a wedge fit could be provided with a taper angle of as little as 3° or as high as 25° although taper angles towards the lower end of that range are considered to provide the better wedging effect, particularly an angle of about 5° and a little above that, specifically up to about 8°. The head 2 is designed for use with a drive socket 6 of correspondingly tapered form of a drive bit 8, whereby when the socket 6 is engaged over the head there will be a tight locking fit between the head and the socket, effectively a wedging fit as is clearly shown in Figure 4. As a consequence, in the condition of driving engagement between the socket 6 and head 2 there is no clearance between the two and wobble between the socket and head is eliminated. In contrast, in a conventional hexagonal head with parallel sides there is necessarily a clearance between the socket and the head and it is that clearance which gives rise to the potential for wobble between the two; that clearance cannot be eliminated due to the need for the socket to fit the socket easily over the head, and due to manufacturing tolerances that clearance cannot be reduced to any practical extent. In the embodiment shown the socket 6 includes a magnetic pin 10 to retain the screw head 2, although that is not essential as the wedging fit between the screw head will itself provide a retention effect.

The remainder of the design of the screw in terms of its shank, thread, and tip is substantially conventional and where the screw is a self-drilling screw, the tip will include a drilling portion of conventional form. Although the primary utility of the preferred embodiment is in relation to self-drilling screws, it is to be understood that the invention is applicable also to screws without that capability. While the primary utility of the invention is in relation to screws having a length of greater than 95mm as it is with screws of that length where the major wobble problems arise, nevertheless it could also be used with screws of shorter length.

Although in the preferred embodiment the drive head is of hexagonal form it could alternatively be of other polygonal shape having a series of angularly spaced driving faces which are tapered, in other words inclined, relative to the longitudinal axis of the screw for co-operation with a drive socket of corresponding form.

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