GB2143297A - Shear resistant screw - Google Patents

Abstract

Part of the shank of a woodscrew bears a thread 12 while part 20 is unthreaded and between the unthreaded part and the head 22 of the screw is a frustoconical region 28 which merges at one end directly into the head and at the other end directly into the shank, thereby serving to increase the strength of this region. <IMAGE>

Description

SPECIFICATION Improved fastening Field ofinvention This invention concerns fastenings, particularly a metal screw for securing in wood, wood composition and plastics materials.

Background to the invention Screws for wood and like materials include a shank having a single or multi-start thread profile formed therein extending from a pointed tip at one end to an unthreaded region at the other end thereof which includes a head of enlarged diameter. The latter is conventionally countersunk.

It is a common problem associated with such screws that the torque exterted on the head during the act of screwing-in can result in the head shearing from off the shank. When this occurs with the shank almost completely buried in the material, removal of the broken shank can prove almost impossible.

It has been proposed that a short section of the shank just below the countersunk head be of greater diameter than the remainder of the shank section leading to the thread profile so that the resistance of the screw to shearing at the junction with the head is increased.

Screws incorporating this cylindrical section of greater diameter just below the head have nevertheless still been found to shear under high torque loads and it is an object of the invention to provide an alternative screw design which possesses an even greater torque resistance than previous designs.

Summary ofthe invention According to the present invention a screw for wood and like materials comprising a generally cylindrical shank having a single or multi-start thread profile formed therein extending from a pointed tip at one end to an unthreaded region at the other end thereof and which includes a head of enlarged diameter containing a screwdriver slot at the other end, is formed with a shank section just below the head having a frusto-conical form so that the diameter of the material joining the head to the cylindrical shank progressively increases in diameter between the shank and the underside of the head.

Preferably, the screwdriver slot is of a cross-slot form.

By providing a progressive increase in the diameter of the material leading between the shank and the head, so the radial shoulder which would otherwise exist between the head and the shank is eliminated and the point of weakness presented by such a shoulder is thereby eliminated.

Surprisingly, the increase in torque resistance of the head/shank junction relative to an unmodified screw is much greater than the increase in torque strength obtained by the prior art cylindrical diameter increase.

According to a preferred feature of the invention a double-start thread profile is formed in the shank and the included angle of the triangular thread profile is in the range 50 to 70 , preferably 60 ".

According to a further preferred feature of the invention the diameter of the unthreaded shank section between the frusto-conical region leading to the head and the threaded section thereof is of greater diameter than the mean shank diameter of the threaded section.

In this way, the diameter of the screw is kept as large as possible and thus the torque-resisting capability of the screw is kept as high as possible.

According to a yet further preferred feature of the invention, where countersunk, the included angle of the countersunk head is 90 whilst that of the frusto-conical section between it and the cylindrical shank is in the range 25 to 35 , preferably 30 .

Where the head is so-called round-head, the included angle of the frusto-conical section is in the range 55 to 65 , preferably 60 .

The invention will now be described by way of example with reference to the accompanying drawings in which: Figure 1 is a side view of a screw constructed in accordance with the invention; Figure 2 is an axial end view of the head; Figure 3 is an enlarged view of the head and of the screw of Figure land Figure 4 is an enlarged view of a round-headed screw embodying the invention.

The screw shown in Figures 1 to 3 includes a generally cylindrical shank 10 which is formed with a double-start thread profile 12 which extends overthe shanktothe pointed tip 14 having an included angle of 34 . The thread angle is 60 .

The section 20 of the shank beyond the thread, towards the head 22, is of a greater diameter than the mean diameter of the threaded section and a small frusto-conical section 24 bridges the junction between the threaded section and the section 20.

The head 22 is countersunk at 26 with an included angle of 90 and a second frusto-conical section 28 is provided between the head countersink 26 and the shank section 20, having an included angle of 30 .

As shown in Figure 2 the head 22 includes a crossed screwdriver slot 30.

Where a round-head is provided as in Figure 4 a similarfrusto-conical region 32 is provided between the shank 34 and the head 36 again to increase the torque strength of the screw, particularly where the head joins the shank.

Tests carried out on screws constructed in accordance with the invention and those to conventional design have indicated a significant increase in torsional strength and a decrease in the time required for insertion using a constanttorque driving device. The results are tabulated below.

TORSIONAL STRENGTH Screw size Screws incorporating Fourother the invention makes of the screw Best 2.4 2.0 4x 1 Worst 2.2 1.6 Mean 2.3 1.76 Best 4.9 3.2 7 x 1 Worst 4.2 2.2 Mean 4.55 2.84 Best 5.9 3.8 8 x 1 Worst 5.6 2.8 Mean 5.75 3.28 Best 5.7 3.8 8 x 2 Worst 5.4 2.8 Mean 5.55 3.4 Insertion times varied as foliows: (seconds) Screw Screws incorporating Four makes ofotherscrew Size the invention 4 x 1 19.9 40.7 44.3 23.1 24.00 7 x 1 14.1 31.5 32.00 18.1 18.1 8x1 12.00 30.2 29.1 18.1 17.4 8 x 2 25.6 56.6 59.7 36.1 34.8 Although quantitative information relating to pull-out values is not available, tests to-date indicate a significant increase in the force required to pull out a screw constructed in accordance with the invention as compared with prior art designs.

Claims (8)

1. A screw for wood and like materials, the screw comprising a head and a shank, the shank having a single or multi-start thread profile formed therealong and extending from a pointed tip at one end of the screw to an unthreaded shank section adjacent the head of the screw, the head being of enlarged diameter relative to the shank and containing a screwdriver recess, wherein the unthreaded shank section includes a frusto-conical region which merges at its narrow end into the shank and at its wide end directly into the head so that the diameter of the material at the junction between the head and the shank progressively increases in diameter between the shank and the underside of the head.

2. A screw as claimed in Claim 1, wherein the screwdriver recess is of a crosshead form.

3. A screw as claimed in Claim 1 or Claim 2, wherein a double-start thread profile is formed along the shank.

4. A screw as claimed in Claim 3, wherein the thread profile is triangular and the included angle of the thread profile is in the range 50 to 70 .

5. A screw as claimed in any preceding claim, wherein the diameter of the unthreaded shank section between the frusto-conical region and the threaded shank section is of greater diameter than the mean shank diameter of the threaded section.

6. A countersunk head screw wherein the included angle of the countersunk head is about 90 , whilst that of the frusto-conical region is in the range 25 to 35 .

7. A round head screw wherein the included angle of the frust-conical section is in the range 55 to 65O.

8. A screw substantially as herein described with reference to any one embodiment shown in the accompanying drawings.