Technical Field of the Invention

The invention relates to a system for driving screws, and is concerned with a screw and with a tool first to hold and then to drive a screw into a backing material.

The invention is applicable to any screws having screw heads with straight paraxial sidewall recesses as well as to screws having near straight axial sidewall recesses. In particular (but without limitation) the invention is concerned with a screw head having a hexalobular slot in its upper surface, and with a tool for use with this screw head. The tool and the screw head together comprise a system for fixing screws.

Background to the Invention

As an improvement over the straight slot on conventional screw heads, the Torx (TM) slot was developed. This had the general shape of a star and extended downwardly into the screw head, giving better engagement characteristics than the straight slot. Other screws had shaped recesses in their screw heads and had straight or near straight sidewalls. Examples are Phillips, Hex or Spline headed screws. A tool (screwdriver) of complementary shape was used to drive the screw into whatever backing material was appropriate. If the shaped slot extended too far downwardly into the head of the screw, the lowest parts of the extremities of the slot could approach the undersurface of the screw head. For instance the shaped slot might approach the external conical undersurface of a countersunk screw, so as to weaken the screw head. Several systems have been developed to overcome this and other disadvantages.

Examples of two such systems are shown in two earlier patents with which the present applicant was associated. These are US 6,951 ,158 and US 7,730,812. US 6,951,158 gives a detailed review of the prior art which will not be repeated.

In specification US 7,730,812 there is a screw head having a hexalobular slot in its upper surface. The curved wall of the slot is approximately parallel to the axis of the screw throughout its length, and may be tapered downwardly inward at an angle which may typically be l .Sdeg. At the base of the hexalobular slot there is a downwardly and inwardly tapered transition surface, leading to further a central recess of smaller diameter. This smaller further recess has tapered walls. Practical experience has led to the use of more broadly tapered walls, which may be tapered downwardly at between 6deg and 12deg, and preferably at 10.8333deg to the axis of the screw.

The intention of the further recess was to promote the accurate insertion of an appropriately shaped tool (screwdriver) within the hexalobular slot of the screw head. Engagement of a central end point of tapered circular cross section on the tip of the tool into the further recess allowed the screw to be retained on the tool while a craftsman manoeuvred the screw into position.

Retention of the screw on the tool by press engagement is particularly useful on one-handed power tools and for robotic tools.

Other screw and tool systems are known to have been manufactured and marketed. Examples are the 'anti cam out' ACR (TM) system and the Ribbed Phillips (TM) 'anti-cam out' rib system. These systems have a drive system with ribs, but in these examples the ribs extend down into the screw heads in one unbroken length, and have no transition slopes within the screw head recesses.

The present invention is intended to make a significant improvement inter alia on the system disclosed in US 7,730,812.

Summary of the Invention

The invention provides a screw in which the screw head has a slot in the upper surface of the screw head, an upper peripheral wall of regular planform extending down from the rim of the slot into the screwhead, the upper peripheral wall being generally aligned with the axis of the screw to form a recess, a sloping transition surface extending downwardly and inwardly with respect to the lower edge of the upper peripheral wall, and a further recess extending downwardly from the lower edge of the sloping transition surface, in which the dimensions of the screw head are such that when the screwhead is engaged by a tool having upper and lower engagement sections, and in which the upper engagement section extends downwardly with respect to the axis of the screw to which the tool is to engage and the lower engagement section extends further downwardly, and in which there is at least one protrusion, for example a rib, on the inside wall of the further recess, so that there is a stick fit due to friction between the protrusion on the inside wall of the further recess and the lower engagement section of the tool. That there is a stick fit between the tool and the screw entails that the protrusion on the inside wall of the further recess will cause sufficient friction between the tool and the screw such that the screw does not fall off the tool.

In one form the protrusion is helical, and is disposed on the inside wall of the further recess.

In another form the protrusion is straight, and is disposed on the inside wall of the further recess and is aligned with the axis of the screw. In this other form, it is preferred that there are three protrusions arranged with 120deg radial spacing between them. The protrusions may extend for the full depth of the further recess.

It is preferred that the wall of the further recess is tapered downwardly inward towards the bottom of the further recess. It is further preferred that the taper is between 6.0 and 12.0deg, and advantageously the taper is about 10.8333deg It is also preferred that the upper peripheral wall is tapered downwardly inward towards the upper edge of the sloping transition surface. It is further preferred that the taper is between 1 and 5deg _; typically between 1.25 and 1.75deg and

advantageously 1.5deg.

In a preferred form the slot in the upper surface of the screw is hexalobular. In yet another form the protrusion runs round the interior of the screw in a plane at right angles to the axis of the screw.

The invention also provides a tool bit for holding and driving a screw as described above, in which there is at least one protrusion (or rib) on an external surface of the engagement portion of the tool intended to engage the further recess, so rendering the protrusion on the corresponding portion of the screw head unnecessary.

Preferably the tool has three equally spaced protrusions (or ribs).

The invention also provides a system comprising a screw as described above in combination with a tool as described above.

The invention also provides a punch pin for forming protrusions in the wall of the screw as described above.

Brief Description of the Drawings

A diagrammatic example and one specific embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which: Fig 1 is a diagram illustrating a tool bit and screw head recess with protrusions; Fig 2 is a cross section on line AA in Fig 1 ; Fig 3 is a diagram illustrating a punch pin to form protrusions; Fig 4 is a cross section on line BB in Fig 3;

Fig 5 is a detailed side view of a specific embodiment showing a bit engaging in the head of a screw;

Fig 6 is a cross section on line CC in Fig 5;

Fig 7 is a side view of a punch pin; and Fig 8 is a cross section on line DD in Fig 7. Detailed Description of the Embodiment

As shown diagrammatically in Fig 1, a screw head 1 has a slot 2 in its upper surface. (The configuration of the slot is better shown in Fig 5.) The slot 2 forms a straight sided recess 3 which has a cross section in a plane parallel to the upper surface of the screw head 1. The recess has outstanding points with engagement surfaces (not shown) on either side of the points. These points are intended for engagement by a tool 6 of corresponding shape to the straight sided recess 3.

The slot 2 has a further central recess 11 at the bottom of the straight sided six pointed recess 3. The further recess 11 is aligned with the axis of the screw, and so is centrally located therein. This further recess 1 1 has a circular cross section and a smaller diameter than the recess 3. Walls 12 of the further recess 11 are slightly inclined so that the bottom 13 of the further recess 1 1 has a diameter slightly smaller than that of an upper part 14 of the further recess 1 1. A sloping transition surface 15 is formed between the upper part 14 of the further recess 1 1 and the lower part of the walls 9 of the six pointed recess 3.

It will be understood that while the specific embodiment is concerned with a screw head having a six pointed recess 3, screws having recesses of many other regular shapes may incorporate protrusions according to the invention. The tool 6 has an engagement section 16 that is designed to fit within the recess 3. The shape of the engagement section 16 in plan corresponds to the shape of the recess 3. The diameter of the engagement section may be uniform, or, as shown in Fig 1, may be slightly tapered downwardly and inwardly towards its lower end. The taper may be 1.25deg to 1.75deg - typically 1.5deg. The diameter of the

engagement section 16 is sized to allow it to be inserted into the recess 3 without difficulty.

The tool 6 also has a central point 17 with a circular cross section extending downwardly from engagement section 16. The diameter of the central point 17 of the tool 6 is slightly smaller than that of the further recess 11 of the screw head 1. A sloping transition surface 18 on the tool 6 extends outwardly and upwardly from the central point 17 to the engagement section 16. The angle of inclination of the transition surface 18 is smaller than that of the transition surface 15. Thus a space 19 is formed between the surfaces 15 and 18 when the tool is inserted into the slot 2. The depth of the space 19 diverges towards the central point 17. The section 16 of the tool 6 may engage the upper periphery of the walls 9 of the recess 3 when the tool enters the slot 2. The central point (or pilot point) 17 may typically have a length shorter than the height of the recess 3, to enable the engagement section 16 of the tool 6 to enter the recess 3. Tn the case of screws which have undergone surface coating for preservation purposes, the diverging space 19 allows any excess coating to collect in the space 19 without obstructing engagement between the tool 6 and the slot 2. If no space had been provided, excess coating residue might settle on the surface 15 and at the bottom 13 of the recess 1 1, and thereby prevent sufficient engagement between the tool 6 and the slot 2.

Press engagement between the central point 17 and the further recess 11, supported by the engagement section 16 of the tool 6 and the upper periphery of the wall 9, ensures that - due to friction, i.e. stick fit, - the screw can remain on the tool without falling off. This mode of engagement is particularly effective in reducing angular movement between the tool 6 and the screw head 1.

As shown in Fig 2, the present invention differs from the prior art in that the screw head 1 has three inward protrusions 21 on the wall 12 of the further recess 11 of the screw head 1. It is possible to have more than three inward protrusions, but three protrusions 21 are advantageously used to locate the screw head 1 on the tool 6. The protrusions 21 stand out inwardly with respect to the wall 12 as ribs.

In a form not illustrated by way of a specific embodiment, but within the scope of the invention, there may be only one protrusion or rib. This may be formed straight on the wall of the further recess within the screw with its length aligned with the axis of the screw, or may be formed as a regular helix on the wall of the further recess within the screw.

Figures 3 and 4 illustrate a punch pin 22 to form the internal surface of the further recess 1 1 in the screw head 1. The process to create protrusion cavities 23 on the punch pin starts with removal of material from a punch pin to create an oversized 'near net' pilot shape. Protrusion cavities can be extruded, machined or coined in some manner on the 'near net' pilot shape. Then material can be removed from the 'near net' pilot shape to create the final pilot shape. The punch pin therefore comprises at least one protrusion cavity 23 for forming of the at least one protrusion 21 on the inside wall 12 of the further recess 1 1. Consequently the punch pin will form protrusions in the further recess 1 1 in the screw head 1 of a screw during manufacturing of the screw.

More detailed drawings of a practical embodiment of the invention are now shown in Figures 5 to 8. Similar reference numerals are used for similar parts to those shown in Figures 1 and 2. A screw head 101 has a hexalobular slot 102 in its upper surface, forming a straight sided six pointed recess 103 extending axially down into the screw head. The hexalobular shape of the slot 102 has outstanding points with engagement surfaces either side of these points. A further recess 111 of smaller cross section is centrally located on the axis of the screw, and deeper into the head of the screw than the recess 103. The wall 112 of the further recess 11 1 are slightly inclined, so that the further recess 1 11 is slightly tapered downwardly and inwardly toward the bottom 113 of that recess. A sloping transition surface 115 links the base of the recess 103 with the top of the further recess 11 1 of the screw head 101.

Hexalobular slots are internal driving features described in ISO 10664.

A tool 106 has an engagement surface 116 that is designed to fit within the hexalobular recess 103. The shape of the engagement surface 1 16 in plan corresponds to the shape of the recess 103. The engagement surface 1 16 is slightly tapered towards its lower end.

While the taper of the engagement surface may be within the range l .OOdeg to S.OOdeg, the taper is typically been between 1.25deg and 1.75deg, and

advantageously a matter of l .Sdeg.

The central point 1 17 of the tool 106 is slightly tapered for engagement with the further recess 1 11 of the screw head 101. In the further central recess 1 11, there is a downwardly inward taper of just under 1 ldeg. (10.833) However, FEA studies may show that a 7.5deg taper may give a better stick between the screw head and the tool.

Following the invention, the further recess 111 on screw head 101 has three inwardly facing protrusions 121 on its inner surface. These protrusions 121 are parallel to the axis of the screw, and engage with the outwardly facing wall of the central point 117 of the tool 106 creating a stick-fit engagement between the protrusions 121 and the tool due to friction such that the screw can remain on the tool without falling off. This may best be seen in the cross section of Figure 6. Provision of the protrusions or ribs gives the driving tool 106 a better engagement with the screw head 101. In embodiments of the invention not illustrated, there may be just one protrusion. In addition to being aligned with the axis of the screw, the protrusion may run helically on the inner surface of the further recess 1 11.

The heights of the protrusions 121 may be from 5% to 25% of the diameter of the further recess 11 1. The protrusions may occupy from 1% to 10% of the perimeter, but smaller or larger proportions are possible.

The specific embodiment of the invention described above shows how the invention may be applied to one particular (hexalobular) form of screw head. However, many other screw heads may be provided with the protrusions as described with reference to the specific embodiment, and indeed the protrusions may be formed on the tool (screwdriver) used with those other screw heads.

For example, in the case of a Hex or Spline drive screw etc etc. While preferred embodiments of the present invention are shown and described, it is envisioned that those skilled in the art may devise various modifications and equivalents without departing from the spirit and scope of the appended claims.

It may be noted that the engagement section 16 of the tool 6, which engages the six pointed recess 3, will transfer a torque from the tool to the screw. The further recess 1 1 is preferably provided with a generally circular cross section (when the protrusions 21 are not considered).

Advantages of the Invention

Protrusions on the inner surface of the recess pilot cavity are expected to improve press fit adhesion with the bit pilot because of the significantly increased contact pressures, so to improve a craftsman's confidence that a screw will stick to the tool.

Slight deformation of the protrusions will to some extent aid adhesion, with a similar effect.

Furthermore, protrusions will allow excess fastener coating to be displaced into adjacent regions upon bit insertion, thus reducing the negative effect of coating on adhesion.