GB2142566A - Screw fasteners - Google Patents

Abstract

A screw has a head with a recess 1 of equilateral triangular cross-section having rounded apexes and flat sides parallel to the axes of the screw to eliminate cam-out. The driving tool 6 has a substantially identical prismatic shape, with minimal clearance in the recess throughout its circumference. <IMAGE>

Description

SPECIFICATION Screw fasteners This invention relates to screw fasteners, more commonly called screws.

Screws fasteners are provided with recesses for receiving the tip of a screw driving tool.

The commonest forms of recess are the straight slot, the hexagon socket, the multi-lobed socket and the cross-shaped recess which exists in various forms such as the "Posidriv" (Trade Mark) screw.

Conventional screws are all susceptible to failure when they are being fastened or unfastened. In the case of straight slot and cross-shaped recess screws, failure by camming out of the tool is common. In the case of hexagon and multi-lobed socket screws, failure by the tool reaming out the recess, or conversely by the tool becoming rounded and thereby losing its grip, are common. The hexagon recess is particularly susceptible to failure by reaming of the recess or crushing of the tool faces, owing to the large drive angle (60 ) between the tool and recess. In the hexagon recess, the driving force acts outwards in a bursting manner destroying the screw recess.

Other recessed heads use tapered recesses, which increases the risk of cam-out and makes it necessary for the user to apply an axial load while turning the screw. Slotted screws also suffer from cam-out and furthermore do not provide self-alignment of the tool, which may consequently damage the material surrounding the screw head and may damage the screw head itself because the driving force becomes undesirably localised if the tool is not correctly centred in the slot.

The object of the present invention is to provide a novel screw fastener which overcomes the disadvantages of known screw fasteners.

According to the present invention I provide a screw fastener and a driving tool therefor, in which the screw has a recess and the tool has a tip region corresponding in shape to the recess, whereby the driving tool can be engaged in the screw for rotating the latter, and in which the said recess and region have substantially identical prismatic shapes each comprising an equilateral triangular cross section with blunt apexes and side faces parallel to a common longitudinal axis.

The invention is applicable to screw fasteners of all types or materials, including (but not exclusively) wood screws, machine screws, grub screws, selftapping screws; the screw head may be of any desired type, including counter-sunk, cap head, cheesehead, roundhead, domed head.

The invention will be further described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is an end view of a screw, Figure 2 is a cross-section of the screw head on the line ll-ll, Figure 3 is a partial perspective view of a corresponding screw driving tool, and Figure 4 shows the tool engaged in the screw head, on a larger scale.

Figures 1 and 2 show the head of a countersunk screw. The nature of the screw shank and threads is immaterial and therefore these are not shown in detail.

Coaxial with the screw head and shank, there is a recess 1 in the shape of an equilateral triangular prism. That is to say, the recess has a cross-section, perpendicular to the axis of the screw, which is an equilateral triangle and the side walls 2 of the recess are flat planes parallel to the longitudinal axis 3 of the screw. The bottom of the recess is flat or slightly dished, depending on the manufacturing process used to form the recess in the screw head.

In cross section, the apexes 4 of the triangular recess are radiused, these radii at the apexes are positioned so as to provide sufficient supporting material behind the contact point.

A simple tool or screw driver for turning such a screw is shown in Figure 3, comprising a handle 5 and a metal bar 6 of prismatic form (or with a circular shank tapering into a prism) with a cross-section corresponding closely to that of the recess 1 in the screw head. The tool is designed to fit closely into the recess as illustrated in Figure 4. It is designed to have no more clearance in the recess, than is necessary for insertion of the driver into the recess.

Clearance between the tool and screw is substantially uniform throughout the cross-section of the tool and screw, and is kept as small as possible, subject to inevitable manufacturing tolerances. In particular, the tool and recess are designed to fit closely at the apexes of the triangle, without any avoidable gaps there.

My triangle recess and tool provide an efficient drive angle of approximately 30 which is superior particularly to that of hexagon socket screws. This reduces any levering action between the driver and the recess faces, and thus reduces radial forces on the screw head. Such forces would tend to reduce the depth of radial engagement and would round the recess, as happens commonly with hexagon recesses. The triangular recess provides a depth of radial engagement (difference between maximum and minimum tool radii) substantially greater than that of a hexagon recess. This deep radial engagement provided by the invention increases the resistance of the screw to deformation of the recess, and also the resistance to deformation of the tool.

A further advantage of the equilateral triangular recess is that all three faces of the recess and tool must come into contact when a torque load is applied, so that the driving load is necessarily distributed substantially equally among the three faces even if the faces of the recess or of the tool are imperfect. In the case of a recess with a greater number of faces, it is all too easy for the load to be concentrated on only some of the faces.

The use of a prism-shaped recess, with its side faces parallel to the axis of the screw, eliminates cam-out under torque, since the driving load is transmitted positively to the recess in the plane of rotation, producing no axial resultant forces. In contrast, any recess with tapered driving faces will inevitably lead to the production of a resultant force in the axial direction under torque, tending to force the tool out of the recess and obliging the user to apply an axial load, which is tiring. If cam-out is not overcome, the resulting relative movement of the tool and screw will damage either or both.

The radii at the apexes of the recess and tool enhance the strength of both components. Under torque load, tne tool and recess make contact at the driving side of each apex. The amount of driving tool material supporting the contact point would be very small if the apex were sharp, leading to deformation and failure of the apex and damage to the corresponding region of the recess. The radii at the apexes of the tool are positioned so as to provide sufficient supporting material behind the contact point. The radiused apexes of the recess eliminate local stress concentrations under tensile load which might lead to shearing of the screw head. On both the screw head and tool, the radii at the apexes provide continuous blending between the adjacent driving faces, reducing local stress concentrations and risk of damage.Furthermore, in special uses such as orthopaedic screws, sharp apexes of the recess would tend to attract moisture and foreign bodies by capillary attraction, leading to corrosion and contamination of the recess, which would be unsuitable for surgical use. The provision of radii at the apexes also makes manufacture of the tool and recess easier.

Alternatively, instead of radii the apexes may be provided with narrow flats or lands. Such a shape may be easierto manufacture than radiused apexes, but will be somewhat less efficient and in particular may lead to load concentrations at the edges of the flats or lands.

The base of the recess or socket may be flat, conical or dished, depending on the manufacturing process used to form the recess. Any convenient manufacturing process can be used, for example casting, forging, or broaching.

To facilitate insertion of the tool into the recess, the entrance edges 7 of the recess may have a radius or chamfer. Similarly, the leading edges 8 of the tool may be provided with a radius or chamfer.

Several advantages of the recess and tool form have already been described. Afurther advantage arises from the provision of only minimal clearance between the tool and recess throughout the circumferences of these and in particular the avoidance of clearances at the apexes. Any gaps at these points, which are where the load is highest under torque, would allow the material at the apexes of the tool cross-section to become distorted by flow into the gaps, and conversely it would be possible for the material of the screw head to become distorted at the gaps. Such distortion would weaken the compo nents and ultimately would lead to failure of the driving surfaces. I avoid this by providing substantially identical radii at the apexes of the recess and of the tool.

Screw fasteners and tools according to the present invention can be used for any purpose. One field for which they are particularly advantageous is aeros pace equipment where fasterners are removed for maintenance purposes by hand. Recess failure can be expensive and dangerous if adjacent components are damaged during the process of extraction.

Another field for which they are particularly advantageous is that of orthopaedic fixings, in particular bone screws.

Current designs of bone screws, although made of high-strength materials such as stainless steel or titanium, are nevertheless prone to failure when being inserted or removed. Insertion into dense sclerotic bone is liable to cause failure of the screw recess or socket, and such problems are exacerbated if the screw is to be extracted, because ingrowth of bone takes place and the bony ingrowths have to be broken down, involving heavy torque loadings.

Screws with triangular recesses as described herein have been found to be easier to use and less liable to fail, than conventional screws which have hexagonal recesses.

Tests have been carried out, comparing screws which embody the present invention, relative to conventional screws, in particular screws with hexagonal recesses.

These tests have shown that screws according to the present invention are completely free from cam-out, and therefore need no axial load during screwing or unscrewing.

In one test, Sherman bone screws of BS 3531:1980 Stainless Steel were compared, respectively having hexagon recesses, Tri-drive recesses, and triangular recesses in accordance with the present invention.

When driving torque was applied with no axial load, using a Sturtevanttesting machine, the Tri-drive recess failed by cam-out at a load of ten pounds whereas the hexagon and triangular recesses exceeded the registering capacity of the machine at one hundred pounds, and failure eventually occured in the screw-threaded portions of the screws. Under two-handed axial pressure, the Tri-drive screw again failed by cam-out, at a load of forty five pounds. The "Tri-drive" recess is effectively a combination of a Philips recess with a cross slot.

In another test, mild steel quarter inch UNF countersunk screws were compared, one having a conventional hexagon recess and the other having a triangular recess according to the present invention.

The hexagon recess screw failed at an applied torque of 175 inch-pounds force, by plastic deformation of the recess in the screw head. The triangular recess did not fail, however the driving failed by plastic deformation at an applied torque of 317 inch pounds force.

In another series of tests on quarter inch UNF countersunk steel screws, a hexagon recess screw failed at a torque of 104 inch-pounds force, by shearing the corners of the hexagon recess. A corresponding screw with a triangular recess failed at a torque of 398 inch-pounds force, owing to a bearing failure of the corners of the triangular recess.

These test results clearly establish the superiority of my triangular recess over conventional hexagon recesses, which are the standard form of recess for orthopaedic screws and are widely used in other fields, and other conventional recesses.

Claims (8)

1. A screw fastener having a head in which is provided a recess for receiving a driving tool, which recess has an equilaeral triangular cross section with blunt apexes, and has side faces parallel to a common longitudinal axis.

2. A screw fastener and a driving tool therefor, in which the screw head has a recess and the tool has a tip region corresponding in shape to the recess, whereby the driving tool can be engaged in the screw for rotating the latter, and in which the said recess and region have substantially identical prismatic shapes each comprising an equilateral triangular cross section with blunt apexes and side faces parallel to a common longitudinal axis.

3. The invention claimed in claim 2 in which the said tip region has rounded or chamfered edges to facilitate engagement of the tool in the screw.

4. The invention claimed in claim 2 or 3 in which the said tip region is a close fit in the recess having minimal clearance throughout the circumference thereof.

5. The invention claimed in any of claims 1 to 4 in which the said apexes are radiused.

6. The invention claimed in any of claims 1 to 4 in which the said apexes are flattened.

7. The invention claimed in any preceding claim in which the recess has a rounded or chamfered entrance.

8. A screw fastener and driving tool, substantially as herein described with reference to the accompanying drawings.