BACKGROUND OF THE INVENTION

1. Field of the Invention The present invention relates to a drive system for threaded fasteners and more particularly to a threaded fastener having a head with an internal drive recess, and a driver bit for engagement in the drive recess.

2. Description of the Prior Art A drive system in widespread use is the Phillips cross drive having an internal recess of cruciform section within the head of a screw or other threaded fastener, and a driver bit of complimentary shape. The detailed design of the recess and complimentary bit is such that the driver bit tends to "cam out" of the recess and may then break or wear at an unacceptable rate. "Cam out" occurs when the bit lifts out of the drive recess in the fastener under the effect of the torque being transmitted. As the bit "cams out", less surface area becomes available to transmit the torque which results in an increased load on the reduced available surface area. As the material of the bit and fastener can only withstand a certain maximum load, once past this limit, deformation of the available surfaces starts and continues until the surfaces are deformed to such an extent that driving can no longer occur, with the bit being thereafter unusable. Also, depending on the depth of the drive recess, a wobbling effect may be experienced when using the drive, the shallower the depth the greater will be the wobbling effect.

The wobbling effect and "cam out" is increased in the Phillips drive when thick corrosion resistant coatings are applied to the fastener as these have the effect of reducing the depth of the drive and which results in increased wobble and "cam out". All of these effects tend to be exacerbated in high torque applications and in screws having a low profile head such as are used in steel frame applications where the protrusion of the head has to be minimised to allow for the fixing of cladding to the frame. Although "cam out" can be avoided by using an internal straight-sided drive such as a hexagon drive or a six lobed drive such as the Torx drive, a straight-sided drive is still prone to a wobbling effect which is a function of the clearance between the recess and the complimentary drive bit.

SUMMARY OF THE INVENTION

The present invention in its preferred embodiments relates to a drive system in which "cam out" and wobble are substantially eliminated and which is capable of use even with screws having a low profile head.

According to one aspect of the present invention there is provided a threaded fastener having a head with a driving recess, the recess being of generally cruciform shape in cross-section transverse to the axis of the fastener to provide four driving segments uniformly spaced around the axis of the fastener, each segment having a driven face extending axially for engagement by a corresponding driving face of a driver of complimentary form engageable within the recess, a trailing face opposite the driven face, and a peripheral surface between the driven and trailing faces, wherein the peripheral surfaces of pairs of segments which are diametrically opposed converge towards the inner end of the recess at an included angle of between about 3° and 30°, the driven face extends parallel to a plane through the axis of the fastener, and the trailing face is inclined to the driven face such that the space defined between the driven and trailing faces is wedge-shaped tapering from the outer end of the recess to the inner end of the recess at a taper angle of from about 2 to 10° whereby when the driver of complimentary form is engaged within the recess with driving ribs of the driver located in the segments, the ribs of the driver will be wedged between the peripheral surfaces of the diametrically opposed segments and the opposed driven and trailing faces of each segment.

Advantageously the included angle between the peripheral surfaces of the pairs of opposed segments is between about 3° and 15°, with approximately 7 to 8° being especially preferred. Advantageously the taper angle of the trailing face to the driven face of each segment is about 5°.

Preferably the respective driving faces of each pair of driven faces are parallel to a common plane through the axis of the fastener and the two common planes associated with four segments intersect at right angles one to the other.

Preferably each driven face lies in close proximity to the plane and its associated trailing face is further to the rear of the plane.

According to another aspect of the invention there is provided a driver having a driving portion engageable in the recess of a fastener as defined above, wherein the driving portion is of complimentary form to that of the recess and is of cruciform cross-section to provide four driving ribs uniformly spaced around the axis of the driver, each driving rib being engageable within a respective one of the driving segments of the recess and having a peripheral surface engageable with the peripheral surface of the segment, a driving face engageable with the driven face of the segment, and a trailing face engageable with the trailing face of the segment whereby the opposed peripheral surfaces of each pair of diametrically opposed driving ribs will be wedged between the opposed peripheral edges of each pair of diametrically opposed segments, with the driving and trailing faces of each rib wedged between the driven and trailing faces of the segment.

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 perspective view of a driver bit of a drive system in accordance with one embodiment of the invention;

Figure 2 is a perspective view to an enlarged scale showing the driving portion of the bit;

Figure 3 is an end view of the driving portion;

Figure 3 A is an end view corresponding to Figure 3, but marked to show cross-sectional references for other Figures;

Figure 4 is a side view of the driving portion;

Figure 5 is a transverse section taken on line V-V of Figure 4;

Figure 6 is a transverse section taken on line VI-VI of Figure 3 A; Figure 7 is an axial section of a driving rib taken on line VII-VII of Figure

3A;

Figure 8 is a side view of a threaded fastener;

Figure 8A is a side view to an enlarged scale of the head portion of the fastener; Figure 9 is a plan view of the fastener from the head end;

Figure 10 is an axial section of the head portion taken on line X-X of Figure

9;

Figure 11 is a transverse section of the head portion taken on line XI-XI of

Figure 8A; and Figure 12 is an axial section of the head portion taken on line XII-XII of

Figure 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The drive system of the preferred embodiment will initially be described with reference to the design of a driver bit 2 and it is to be understood that the design of the recess in the head of the screw or other threaded fastener will be of complimentary form to that of the bit for co-operation with the bit.

With reference to Figures 1 to 7, the basic form of the driving portion of the bit 2 is of a cruciform cross-section to provide four driving ribs 4 equi-angularly spaced around the longitudinal axis 0-0 of the bit. The peripheral surfaces 6 of the four driving ribs 4 lie on an imaginary frustoconical surface which tapers towards the outer end of the bit. Accordingly, the opposed peripheral surfaces 6 of each pair of opposed driving ribs 4 will incline in a longitudinal direction towards the outer end of the bit. When the bit is engaged in a cruciform recess of complimentary form in the fastener, the engagement of the longitudinally inclined peripheral surfaces of the bit 2, with correspondingly inclined peripheral surfaces of the cruciform recess in the fastener will provide a wedging action of the bit within the recess.

The taper angle is important in providing an effective wedging action which assists in preventing wobble and also to prevent the "cam out" effect discussed earlier. The included angle α (see Figure 6) between the peripheral surfaces 6 of each pair of opposed driving ribs 4 (thereby corresponding to the included angle of the imaginary frustoconical surface on which they lie) is between about 3° and 15°. An included angle α of less than about 3° will provide an insufficient wedging effect for the prevention of wobble. The size of the included taper angle α is also a factor in avoiding "cam out", as large taper angles are a factor inducing "cam out". Although we have determined that "cam out" is unlikely to occur even with an included taper angle of up to about 30°, a taper angle of that order could be difficult to accommodate within some existing screw heads while retaining sufficient volume and thickness of metal within the head to properly withstand the driving torque. As such, we have determined that an upper limit of the included taper angle α of around 15° should be suitable for most existing sizes of screw head. In a particularly preferred form, the included taper angle is around 7-8°. An angle of this order provides an effective wedging action and is able readily to be accommodated even in screws with low profile heads.

With reference now to Figures 3 and 5, each driving rib 4 has a leading face 8 in the direction of normal driving rotation of the bit (and which thereby forms the driving face) and a trailing face 10. In relation to two mutually perpendicular axial reference planes A-A, B-B which intersect at the longitudinal axis 0-0 of the bit, the driving face 8 of each of the ribs 4 is flat and parallel to one or other of these planes. The driving face 8 of the rib will engage a corresponding driven face in the recess and which is likewise flat and parallel to one or other of two corresponding reference planes which interest at the longitudinal axis of the fastener.

It will also be seen from Figures 3 and 5 that in relation to its associated plane of reference (A-A or B-B), the rib 4 is not generally symmetrical about that plane but, rather, the driving face 8 is slightly forwardly of the plane whereas the trailing face 10 has a greater projection to the rear of the reference plane. The significance of this is that in relation to the recess in the head of the fastener, the proximity of the corresponding driven face in the recess to its associated axial reference plane allows for adequate thickness of material in the head forwardly of that plane for effective torque transmission, even in screws with low profile heads. The more substantial offset of the trailing face of the rib 4 to the rear of the axial reference plane ensures that the rib has sufficient strength for effective torque transmission. In contrast, with a driving rib of the same width but with its driving and trailing faces arranged approximately symmetrically in relation to the axial reference plane, the increased forwards projection of the driving face relative to the reference plane which would occur in that situation would result in a reduction in the material of the fastener head forwardly of its associated driven face and in the case of a low profile screw for example, the reduction in the volume of material in advance of the driven face could be such that under high torque conditions the head could shear from the shank of the fastener.

When considered in transverse section (Figure 5), the trailing face 10 of the rib 4 inclines away from the reference plane at the root or inner end of the rib and then merges with the driving face 8 of the next rib via a curvature of large radius. This ensures that the root portion of the rib has good torsional strength for the required torque transmission.

When considered in axial cross-section (see Figure 7), the trailing face 10 of the rib 4 inclines towards the associated reference plane, and thus towards its associated driving face 8, in an axial direction towards the outer end of the bit. Accordingly, when considered in axial section each driving rib 4 is wedge-shaped tapering towards its outer end as is clearly shown in Figure 7. The corresponding recess in the fastener will be similarly wedge-shaped provided by a corresponding inclination in a longitudinal direction of a trailing face relative to the driven face. Accordingly, the driving rib 4 becomes wedged within the corresponding recess and the effect of the wedging action is to lock the axial driving face 8 of the rib 4 firmly against the axial driven face of the recess to ensure that no wobble occurs. The taper angle β required to achieve this wedging action is not significant and is in the range of from about 2° to 10° with around 5° being especially preferred. A taper of more than about 10° is likely to be difficult to accommodate in a fastener having a conventional head size.

A threaded fastener for driving by the driver bit 2 is shown in Figures 8 to 12. Although the particular fastener shown is in the form of a self-drilling screw with a head of relatively low profile, this is given only by way of example only and it is to be understood that the drive system of the invention is applicable to a broad range of threaded fasteners. It will be seen the that head 20 of the fastener has a recess 22 of complimentary form to that of the driving portion of the bit 2 and thereby of a cruciform cross-section to provide two pairs of diametrically opposed segments 24 equi-angularly spaced around the longitudinal axis 0-0 of the fastener. The opposed peripheral surfaces 26 of each pair of opposed segments 24 are engaged by those 6 of the driving ribs 4 and incline at a taper angle α (see Figure 10) equivalent to that of the bit 2 to provide the wedging action for the bit previously discussed. Each segment 24 has a leading face 28 in the normal direction of driving rotation which thereby forms the driven face of the segment, and a trailing face 30. In relation to two mutually perpendicular axial reference planes A-A, B-B which intersect at the longitudinal axis of the fastener, the driven face 28 of each of the segments 24 is flat and parallel to one or other of these planes. As described in relation to the design of the bit, the driven face 28 is slightly forwardly of the plane whereas the trailing face 30 has a greater projection to the rear of the reference plane whereby the driven and trailing faces 28, 30 of the segment 24 are engaged respectively by the driving and trailing faces 8, 10 of the rib 4 of the bit.

When considered in axial section (Figure 12) the trailing face 30 of the segment 24 inclines towards the driven face 28 whereby the segment 24 is wedge-shaped tapering towards the inner end of the fastener for co-operation with the wedge- shaped rib 4, the taper angle β of the recess corresponding to that of the ribs of the drive bit. The overall taper of the driving portion of the bit (the frustoconical taper) and the corresponding taper of the recess 22 of the fastener, and the taper of each of the ribs

4 and the corresponding taper of each of segments 24 are at relatively shallow angles which provide for an effective wedging action which eliminates wobble and "cam out". The shallow taper angles also allow for a greater thickness of corrosion resistant coatings on the fastener without reducing the depth of the drive to any practical extent. The wedging action provided by the tapers also remove the tendency of the bit to slip out of the recess if the load during driving is not applied directly along the axis of the screw.

As "cam out" is avoided, the load per available surface area on the driving faces of the driver bit does not change and therefore deformation of the driving surfaces is significantly reduced.

The cross-sectional area of the driving portion of the bit is relatively large in relation to the required depth of penetration into a corresponding recess in the fastener head. As a result the recess does not need to be especially deep and a recess of adequate depth can be accommodated even in low profile heads to provide a high torque drive without being of a depth which might cause the head to shear. Likewise, the relatively large cross-sectional area of the driving portion ensures that it will not shear at the junction with the fastener head.

It is envisaged that the drive system will be produced in a range of different drive sizes (four, for example). The basic geometry for each size will be. as described herein with the size variations occurring to increase the strength of the drive and fastener to withstand a required torque.