GB2375153A

GB2375153A - Screw fastening

Info

Publication number: GB2375153A
Application number: GB0110801A
Authority: GB
Inventors: Garry Roy Enticott; Nick Gordon; Callum Ernest Mcminn
Original assignee: Trifast PLC
Current assignee: Trifast PLC
Priority date: 2001-05-02
Filing date: 2001-05-02
Publication date: 2002-11-06
Also published as: GB0110801D0

Abstract

A screw fastening (10) comprises a shaft (12) having a helical thread (14). The thread (14) has a leading surface (20) which is radiussed in cross section, the cross section being taken through and parallel to a central axis (22) of the shaft (12). The fastening (10) can be push inserted into a hole but must be unscrewed to remove the fastening (10) from the hole.

Description

A Screw Fastening The present invention relates to a screw fastening, in particular a screw fastening for fastening a first article to a second article. The screw fastening of the present invention is particularly adapted for use in fastening one article to another article, the second article being made of an amorphous thermoplastic.

Many types of screw fastening exist. They generally comprise a shaft having a first end portion for penetrating into an article to which a second article is to be secured. A head is usually provided at the other end of the shaft for applying a torque thereto. A thread is provided on the surface of the shaft. The thread usually extends at least partially from the first end of the shaft towards the head.

A screw fastening in the form of a screw is usually provided with a point at the first end. A screw fastening in the form of a bolt is usually provided with a flat or rounded first end.

The thread may, for example, be a single helix or a double helix.

Existing threads generally have a triangular section, more usually in the form of an isosceles triangle. Typically the peak angle, i. e. the angle formed between the leading surface of the thread and the rear surface of the thread, is between 45 and 90 , dependent on the application to which the screw fastening will be applied. Self-tapping screws, for example, usually define an angle of 60 .

In conventional use, screw fastenings are screwed into a material to secure a first article to that material. To remove the article from the material, the screw fastening is then unscrewed from the material. For this purpose, the head is provided with a torque applying means for engagement with a screwdriver. Such means, include a cross head, a Phillips (RTM) head, a slotted head or a TORX (RTM) head. A square or hexagonal head may be provided for applying a torque with a spanner.

Another type of screw fastening exists that can be driven into a material with a hammer. In order to remove the screw fastening, however, it is necessary to unscrew it. Such screw

fastenings are sold, for example, by Continental/Midland Inc., of 24000 S. Western Park Forest, IL 60466, USA under the trademark PUSHTITE (RTM). Figure 1 shows such a screw fastening 100. The screw fastening 100 has a shaft 102 having a trilobular section,

i. e. a rounded triangle (not shown). The shaft 102 has formed thereon a non-isosceles triangle sectioned thread 104. The peak angle 106 of the thread 104 is approximately 90 , the leading angle 108 of the thread, i. e. the angle between the leading surface 110 of the thread 104 and the axis 112 of the shaft 102 is approximately 20 , and the rear angle 114 of the thread, i. e. the angle between rear surface 116 of the thread and the shaft axis 112 is approximately 70 .

The problem with screw fastenings of the prior art is that, in order to grip the material, the thread tends to key into the material. This defines a sharp and deep indentation in the material. Amorphous thermoplastics, although being high strength engineering plastics materials, unfortunately do suffer from notch sensitivity. Therefore, using existing screw fastenings to mount a first article into or onto an amorphous thermoplastic article can be problematic. For example, with the screw fastenings of the prior art, there is a tendency for cracks to form and spread through the amorphous thermoplastic material upon insertion of the fastening into the material. It is believed that the crack propagates from the point of insertion of the screw fastening, or, more precisely, at the peak of the thread. This is believed to be due to the stress concentrations occurring due to the insertion of the screw or due to stress concentrations forming at the peak of the thread.

To overcome this, it has been proposed to pre-tap a thread into a hole before driving a screw fastening into the hole, or to provide intermittent threads on the screw fastenings for assisting the self-tapping of a hole during insertion of the screw fastening. However, neither of these solutions is ideal for amorphous thermoplastics.

It would therefore be desirable to provide a new form of screw fastening designed specifically to be used with amorphous thermoplastics that can be used without propagating cracks, or the like, in the receiving material.

When thread forming screw fastenings are installed into a material, compression between the thread pitches will affect the performance of the screw fastening. Certain factors must therefore be considered when designing a thread form for screw fastenings.

Firstly, the boss geometry needs to be considered. Boss geometry relates to the geometric design of the plastic which the fastener will be installed into. Thread friction will be

encountered during assembly. Similarly, the thread engagement value will determine the assembly torque and torque safety margins required to give optimum performance.

It is important for the product to be reusable. The boss geometry and the assembly torque will be determining factors relating to this. Reusability is necessary so that the article, such as a mobile telephone or some other product, can be serviced.

The material itself needs to be considered since plastic under load due to forces applied by application of torque will relax with time. When a torque is applied to screw fastenings to insert the screw fastening fully into a material, a clamp load is generated under the head of the screw fastening. Plastic relaxation occurs over time and this reduces this clamp load.

Where a higher torque is needed to be applied to a screw to tighten it up, there is an increased compression. Therefore there is more scope for plastic relaxation. By push fitting the screws, the pushing value determines the amount of clamp load, and there is no torque applied load to relax. It would therefore be preferable to provide a screw fastening that can be push fitted.

According to the first aspect of the present invention there is provided a screw fastening comprising a threaded shaft, the thread on the shaft having a generally helical leading surface, wherein the leading surface is generally radiused in cross-section, the crosssection being taken through and parallel to the axis of the shaft. The surface is convex.

The radiused section may be approximated by a set of straight sections. Preferably, however, the leading surface has a continuous arcuate radius extending from the shaft to a radially outermost tip of the thread relative to the shaft.

Preferably the radially outermost tip of the thread is an edge defined by the intersection line of the leading surface of the thread and the rear surface of the thread. The edge may be slightly radiused.

Preferably a tangent of the leading surface of the thread, taken at the outermost tip of the leading surface of the thread, is not parallel to the axis of the shaft. Preferably the tangent defines a leading angle of less than 50 from the axis of the shaft.

Preferably the screw fastening is straight, having a central linear axis extending all the way therethrough.

Preferably, the screw fastening has at opposite ends thereof a first end portion and a head.

Preferably the screw fastening has a blunt first end.

Preferably the thread terminates short of the first end of the screw fastening.

Preferably the thread terminates short of the head.

Preferably the first end portion of the screw fastening is tapered radially inwards.

Preferably the first end portion of the screw fastening is chamfered.

Preferably the screw fastening is suitable for use in joining a first component e. g. of a mobile telephone to a second, amorphous thermoplastic, component, e. g. of the mobile telephone.

Preferably the shaft of the screw fastening has an overall length of between 4mm and 7.8mm and a thread outermost diameter of about 1. 8mm.

Preferably the generally radiused leading surface of the thread approximates an arc having a radius of 1 mm.

Preferably the thread comprises a continuous helical thread.

Preferably there is only a single helical thread on the shaft.

Preferably the thread has a thread pitch and a thread width, the thread pitch being greater than the thread width, thereby leaving a radially flat helical portion of the shaft between adjacent coils of the thread.

Preferably the thread width to thread pitch ratio is about %.

Preferably the thread has a thread height which is less than the thread width.

Preferably the thread height to thread width ratio is about 1/3.

Preferably the peak angle of the thread is between 85 and 1000.

Preferably the rear angle of the thread is between 80 and 90 .

Preferably the leading angle of the thread at the shaft surface is between 25 and 50 .

Preferably the leading angle of the thread at the shaft surface is about 35 . The screw fastening may be made of any stiff, strong material such as steel, stainless steel, aluminium, brass, titanium or special purpose plastics.

A second aspect of the present invention concerns the application of screw fastenings. In , applications in which a plurality of screws need to be used to connect a first article to a second article, it is normal practice either to screw in each screw individually or to have an adapter that can apply a variety of screws in a single application, the adapter having an assembly of rotating parts thereon for rotating each of the screws for driving the screws into the receiving material. Such methods are either time consuming or involve a costly or complicated adapter. It would therefore be desirable to provide a new method of joining a first article to a second article using screw fastenings. One such application would be in the mobile telephone industry, where, for example, a front cover of a phone is fixed to a back cover of a phone. In these applications the first article may be made of any material.

Preferably, however, the second article is made of an amorphous thermoplastic.

According to the second aspect of the present invention there is provided a method of inserting a plurality of screw fastenings into a plurality of holes formed in an article, the screw fastenings being of the push-in type, comprising the steps of assembling a plurality of screw fastenings in a formation, in a position translated from their desired inserted position, and, using an adapter, pushing or pulling the screw fastenings into their desired inserted position without screwing the screw fastenings into the article.

Preferably, the screw fastenings are in accordance with the first aspect of the present invention.

Preferably the holes are formed in an amorphous thermoplastic component of the article.

Preferably the article is made of an amorphous thermoplastic.

Preferably the method attaches a second article to a first article.

Preferably the first article is a mobile telephone component, and the screw fastenings attach another mobile telephone component to that mobile telephone component.

The present invention also provides an apparatus for carrying out the method described above, the apparatus comprising an adapter for pushing or pulling a plurality of push/pull in, unscrew out, screw fastenings, such as those in accordance with the first aspect of the present invention or PUSHTITE (RTM) screws, into inserted positions in an array of holes in a first component, a second adapter for holding the first component in a position relative to the first adapter, the first component having holes for receiving the plurality of screw fastenings, and means for moving the first adapter relatiye to the second adapter to push or pull the screw fastenings into inserted positions in the holes of the first component.

Preferably the first component is a mobile telephone component.

The present invention provides a unique solution of particular benefit within the mobile telephone industry. It offers benefits over the existing methods of construction that uses inserts for screws or individually driven screws. Benefits such as improved weight, e. g. due to the improved useability of different materials such as amorphous thermoplastics, cost reductions due to decreased inventory levels and the improvement of the assembly and installation cycle times are all features of the present invention.

Upon installation of the screws of the present invention into the receiving material, the elasticity of the material, which is typically a plastics material, will allow the material to form around the threads securing the screw fastenings in a tight, mating manner, thus forming a strong product. When the screw needs to be removed, it can be unscrewed out.

Due to the shape of the thread of the present invention, the material into which the screw fastening was pushed will display minimal deformations due to the screw fastening. Due to this lack of deformation of the material, using the screw fastenings of the present invention, achieves improved reusability. In a preferred case, the screw might be insertable up to ten times without any significant deformation or degradation of the receiving material, and therefore without any significant deformation or degradation of the holding force achievable by the screw fastening.

The convex, radiused leading surface of the first aspect of the present invention is an entirely novel concept in screw fastenings. It more gently permits screw fastenings to be inserted into materials which thereby reduces the risk of failure of the article into which the screw fastening is to be inserted.

Since no positive sharp edge engages into the material, there is a reduction in the risk of end product failure. Such failures need not occur at the time of manufacture. Such a failure might occur after a period of time due to ageing of the material or environmental reasons (resulting, for example, in increased brittleness of the material). For example heat, age hardening, ultra violet degradation, application of solvents or other causes can change the properties of the material.

The screw fastening of the present invention attempts to retain the benefits of a conventional screw, e. g. secure construction and serviceability. However, it is important to decrease the assembly times, to optimise clamp loads by reducing the plastic compression between threads, and to minimise the risk of joint failure. By push (or pull) inserting screw fastenings into the material, plastic compression between threads is reduced or removed and no torque applied loads are created. This is achieved without sacrificing the other advantages of screw fastenings.

These and other aspects of the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a prior art PUSHTITE (RTM) screw fastening, as described above ; Figure 2 shows a first embodiment of a screw fastening in accordance with the present invention; Figure 3 shows an end view of the fastening of Figure 2; Figure 4 shows an alternative embodiment of the present invention.

Referring first to Figure 2 the screw fastening 10 comprises a shaft 12 having a central axis 22. On the shaft 12 there is provided a helical thread 14. The thread 14 has a leading surface 20 having a convex, radiused profile in cross section (the cross section is taken through a plane extending through and parallel to the central axis 22 of the shaft). The thread 14 has a rear surface 26 having a linear profile through the same plane.

In the embodiment shown, the radiused shape of the leading surface 20 of the thread 14 can be defined by two angles 18, 19, taken from the cross section, relative to the central axis 22. The first angle, that being the one at which the leading surface 20 extends away from the shaft 12, is the shaft leading edge angle 18. This angle 18 is preferably in the

order of 250 to 50 , and, in the preferred embodiment, as shown, is approximately 350. The second angle, that being the leading angle 19, is that angle formed between the tangent of the leading surface at the outermost tip of the thread, i. e. usually at the intersection point of the leading surface 20 and the rear surface 26, and the central axis 22 (which is parallel to a line 21 drawn along the peaks of the thread 14). In practice, the intersection point will be radiused, in which case the tangent is taken just forward, i. e. just distally from the screw-head side, of the intersection point.

Preferably, this leading angle 19 is between 0 and 5 .

The screw fastening can have a head portion 30. As shown in Figure 3, the head portion 30 has provided therein a torque applying means 32. Figure 3 shows a torque applying means in the form of a TORX (RTM) type aperture. Other torque applying means would also be suitable.

Although the screw fastening 10, as shown, has a flat head 30, a counter-sunk head or a round head, as shown in Figure 1, could instead be provided.

Referring now to Figure 4 there is shown a second embodiment of the present invention, which corresponds largely with that shown in Figure 2, but in which the shaft has a shorter length (4mm instead of 7. 8mm). The thread section is identical.

In accordance with the preferred embodiment of the present invention, which will be used in the manufacture of mobile phones, the pitch of the thread, i. e. the axial distance between adjacent coils of the thread, is 0. 72mm. The width of each thread section in the shaft's axial direction is 0.54mm. The shaft's outermost diameter, i. e. excluding the thread, is 1.44mm. Including the thread, but excluding the head, the screw fastening's outermost diameter is 1. 8mm. This defines a thread height of 0. 18mm.

The radius of the leading surface 20 is about Imm.

For larger and smaller screw fastenings in accordance with the present invention, it is preferable to use size ratios for the shaft and the thread approximating the relative ratios used in the two examples described above.

As shown in the Figures, it is preferable that a single helix is used. With the thread proportions as described above, a portion of the shaft is thereby not covered by thread, i. e. it is radially flat. The radius of curvature of the thread's surface, however, can be varied to provide different proportions, perhaps thereby covering the entire shaft with thread.

In both Figure 2 and Figure 4, the screw is provided with a blunt tip at a first end thereof.

In Figure 2, the first end portion of the screw fastening has a slightly tapered profile. No thread is provided at the taper. In Figure 4, a chamfered profile is provided at the first end portion of the screw fastening. These shaft diameter reducing features facilitate insertion of the screw fastenings into holes provided in an article.

A person skilled in the art will appreciate that a point can be provided on the first end of screw fastenings in accordance with the present invention without departing from the unique thread shape provided by the first aspect of the present invention. It is by virtue of this thread shape that the screw fastenings of the present invention can be, for example, push-fitted into a hole formed in a material to secure the screw fastening in the hole. By virtue of the steep rear surface, and a shallow leading angle 19 formed at the outermost part of the leading surface of the thread due to the near axial orientation thereof, the screw can easily be push-fitted, but it has to be unscrewed out.

Due to the radiused profile of the leading surface and the near axial orientation of the leading angle of the thread, the screw fastening of the present invention is particularly suitable for use in amorphous thermoplastic materials. There is a reduced stress concentration imposed on the material upon insertion of the screw fastening of the present invention into the material as compared to screw fastenings provided in the prior art.

Function Theory Using the mechanical property of elasticity within a polymer, it is possible to minimise plastic compression by entrapping plastic into retention features of a screw fastening.

To enable the screw fastener to be taken out, i. e. for servicing, there is a need to rotate the screw fastening. Therefore a helix angle on the thread is needed. The acuteness of the helix angle will also effect the assembly time and the performance of the screw fastening when resisting a pull out force.

As amorphous materials are notch sensitive, it is important that sharp edges do not penetrate the polymer. Therefore, the rake angle of entry of the thread is the radius. This enables the polymer to be manipulated by stretching to its elastic capabilities. This radius may be changed, pending the screw fastening's diameter, and possibly pending the composition of the polymer material into which the screw fastening is to be inserted.

To entrap the polymer between the threads and to optimise the screw fastening's resistance to pull out forces (and also to enhance the clamp load forces, a load bearing face (i. e. the rear surface) should be optimised. It has been found that this is when the rear surface is horizontal, i. e. perpendicular to the shaft axis. However, due to thread rolling processes, horizontal, although achievable, is not generally practical. It should be noted that the rear angle may be altered, pending the screw fastening's diameter and the properties of the polymer material.

In theory, as the screw fastening is utilising the elasticity properties of polymers, the boss geometry will be under less stress, giving the opportunity to reduce the amount of plastic used to form the boss.

The present invention has been described above purely by way of example. It should be noted, however, that modifications in detail may be made within the scope of the invention.

Claims

CLAIMS 1. A screw fastening comprising a threaded shaft, the thread on the shaft having a generally helical leading surface, wherein the leading surface is generally radiused in cross-section, the cross-section being taken through and parallel to the axis of the shaft.
2. A screw fastening according to claim 1, wherein the radiused section is , approximated by a set of straight sections.
3. A screw fastening according to claim 1, wherein the leading surface has a continuous arcuate radius extending from the shaft to a radially outermost tip of the thread relative to the shaft.
4. A screw fastening according to claim 1, 2 or 3, wherein the radially outermost tip of the thread is an edge defined by the intersection line of the leading surface of the thread and a rear surface of the thread.
5. A screw fastening according to claim 4, wherein the edge is slightly radiused.
6. A screw fastening according to any one of the preceding claims, wherein a tangent of the leading surface of the thread, taken at the outermost tip of the leading surface of the thread, is not parallel to the axis of the shaft.
7. A screw fastening according to claim 6, wherein the tangent defines a leading angle of less than 50 from the axis of the shaft.
8. A screw fastening according to any one of the preceding claims, wherein the screw fastening is straight, having a central linear axis extending all the way therethrough.
9. A screw fastening according to any one of the preceding claims, wherein the screw fastening has at opposite ends thereof a first end portion and a head.
10. A screw fastening according to any one of the preceding claims, wherein the screw fastening has a blunt first end.
11. A screw fastening according to any one of the preceding claims, wherein the thread terminates short of a first end portion of the screw fastening.

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12. A screw fastening according to any one of the preceding claims, wherein the thread terminates short of a head of the screw fastening.
13. A screw fastening according to any one of the preceding claims, wherein a first end portion of the screw fastening is tapered radially inwards.
14. A screw fastening according to any one of the preceding claims, wherein a first end portion of the screw fastening is chamfered.
15. A screw fastening according to any one of the preceding claims, wherein the screw fastening is suitable for use in joining a first component of a mobile telephone to a second component of a mobile telephone, the shaft of the screw fastening having an overall length of between 4mm and 7. 8mm and a thread outermost diameter of about 1. 8mm.
16. A screw fastening according to any one of the preceding claims, wherein the generally radiused leading surface of the thread approximates an arc having a radius of 1mm.
17. A screw fastening according to any one of the preceding claims, wherein the thread comprises a continuous helical thread.
18. A screw fastening according to any one of the preceding claims, wherein there is only a single helical thread on the shaft.
19. A screw fastening according to any one of the preceding claims, wherein the thread has a thread pitch and a thread width, the thread pitch being greater than the thread width, thereby leaving a radially flat helical portion of the shaft between adjacent coils of the thread.
20. A screw fastening according to claim 19, wherein the thread width to thread pitch ratio is about 3/4.
21. A screw fastening according to any one of the preceding claims, wherein the thread has a thread height which is less than the width of the thread.
22. A screw fastening according to claim 21, wherein the thread height to thread width ratio is about 1/3.

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23. A screw fastening according to any one of the preceding claims, wherein the peak angle of the thread is between 850 and 1000.
24. A screw fastening according to any one of the preceding claims, wherein the rear angle of the thread is between 80 and 90 .
25. A screw fastening according to any one of the preceding claims, wherein the leading angle of the thread at the shaft surface is between 25 and 50 .
26. A screw fastening according to claim 25, wherein the leading angle of the thread at the shaft surface is about 35 .
27. A screw fastening according to any one of the preceding claims, wherein the screw fastening is made of steel, stainless steel, aluminium, brass, titanium or a special purpose plastics material.
28. A method of inserting a plurality of screw fastenings into a plurality of holes formed in an article, the screw fastenings being of the push-in type, comprising the steps of assembling a plurality of screw fastenings in a formation, in a position translated from their desired inserted position, and, using an adapter, pushing or pulling the screw fastenings into their desired inserted position without screwing the screw fastenings into the article.
29. The method of claim 28, wherein the screw fastenings are in accordance with any one of claims 1 to 27.
30. The method of claim 28 or 29, wherein the holes are formed in an amorphous thermoplastic component of the article.
31. The method of claim 28,29 or 30, wherein the article is made of an amorphous thermoplastic.
32. The method of any one of claims 28 to 31, wherein the method attaches a second article to a first article.
33. The method of claim 32, wherein the first article is a mobile telephone component, and the screw fastenings attach another mobile telephone component to that mobile telephone component.

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34. An apparatus for carrying out the method of any one of claims 28 to 33, the apparatus comprising an adapter for pushing or pulling a plurality of push/pull in, unscrew out, screw fastenings into inserted positions in an array of holes in a first component, a second adapter for holding the first component in a position relative to the first adapter, the first component having holes for receiving the plurality of screw fastenings, and means for moving the first adapter relative to the second adapter to push or pull the screw fastenings

into inserted positions in the holes of the first component. i
35. The apparatus of claim 34, wherein the first component is a mobile telephone component.
36. The apparatus of claim 34 or 35, wherein the screw fastenings are in accordance with any one of claims 1 to 27.
37. A screw fastening substantially as hereinbefore described with reference to Figure 2.
38. A screw fastening substantially as hereinbefore described with reference to Figure 4.
39. A method of assembling a mobile telephone substantially as hereinbefore described.
40. A method of fastening a first article to an amorphous thermoplastic component substantially as hereinbefore described.