THREAD ROLL DIE

FIELD OF THE INVENTION

This invention relates to a thread forming screw thread fastener with high torque retention for forming an internal thread in a workpiece material such as synthetic plastics, resins and the like and corresponding thread roll die to manufacture the thread of the screw.

BACKGROUND

Conventional screws often damage the material of the workpiece, in particular when the material of the workpiece is a synthetic plastic, resin and the like. Relatively high driving torques are typically required to fasten the screw in the workpiece, which contribute to the likelihood of damaging the material of the workpiece when the screw is fastened to the workpiece. The screw may also strip-off, burst, become stuck in the material, and/or stress cracks may form in the workpiece material. This damage to the workpiece material and screw weakens the integrity of the joints formed between the workpiece material and the screw. Additionally, screw fasteners that are fastened to the workpiece may loosen when subject to vibrations and other external factors.

Attempts have been made to improve screw thread design. However as mentioned, conventional screw threads do not adequately meet critical functional requirements, and the strength and integrity of the joints has been compromised. For example, some previous threads that have been disclosed include, United States Patent Number 4,527,932, United States Patent Number 5,544,993, United States Patent Number 6,672,813, and United States Patent Number 5,061,135. These previous screw thread designs have been employed in a few applications on workpiece materials as attempts to provide sure insertion of the screw into the workpiece. However, optimum results have not been achieved with conventional screw thread designs in areas such as the positive engagement, failure torque, torsional strength and the like. In particular, the assembly efficiency and flexibility of the screw into the workpiece of conventional screw thread designs are not easily applied in product miniaturization applications where the design solution is hampered by weak integrity of joints of the screw thread formed with the workpiece material.

Therefore, a thread forming screw thread and corresponding thread roll that improves torque retention of the fastener in the workpiece and alleviates the problems and drawbacks associated with the prior art is needed.

SUMMARY

An aspect of the invention is a screw thread fastener for fastening into a material of a workpiece, the fastener comprising a head having a drive to be engaged with a driver bit; a shank with threads having a symmetrical thread profile with a minor diameter and a major diameter, the major diameter defining a thread crest of the screw thread, the minor diameter defining the thread root of the screw thread, the profile of the screw thread defined by the surface of the screw thread between adjacent thread crests forming a thread flank; and a retention element formed on the surface of the thread flank of the screw thread for retaining the screw in the material of the workpiece.

In an embodiment, the retention element may be integrally formed from the surface of the screw thread. The retention element may be a protrusion from the surface of the screw thread, and may be arranged on the thread root of the thread flank of screw thread. The thread flank may have a first central portion of constant radius along the thread root, the radius common at the thread root, at each adjacent thread crests, and at two transition intersection points, the profile of the thread flank having a second and third portion defined by surfaces having different radii of curvature between each transition intersection point and each adjacent thread crest, the retention element extending from each intersection point.

In an embodiment, the retention element has a surface forming a leading surface and a trailing surface of the insertion direction of the screw thread fastener into the material of the workpiece, the leading surface and the trailing surface being uniform. The leading surface may form an angle and the trailing surface may form an angle wherein the angle of the leading surface is substantially equal, less than or greater than the angle of the trailing surface. The leading surface may form an angle of 30 degrees and the trailing surface may form an angle of 60 degrees. The leading surface may form an angle of 45 degrees and the trailing surface may form an angle of 45 degrees. The different radii of curvature of the second and the third portion may vary relative to the constant radius of the central portion of the screw thread profile along the thread root.

In an embodiment, the torque retention elements are arranged in line parallel with a central axis of the fastener along the length of the fastener in adjacent thread flanks. The torque retention elements may be arranged in an evenly spacing in the screw flank. The torque retention elements may be arranged every 120 degrees along the screw flank.

In an embodiment, the surface of the threads, thread crest, thread root and thread flanks are arranged to deform the ductile material without cracking or damaging the surrounding ductile material of the workpiece. The threads may be arranged to deform the material of the workpiece against the surface of the thread flank and the surface of the torque retention elements. The second and third portions of the thread flank may be arranged to deform the ductile material of the workpiece against the surface of the torque retention elements of the thread flank. The radius of the central portion of the thread flank having constant radius may define a construction circle which determines the pitch of the thread. The pitch of the thread may be determined by the tangents between adjacent construction circles sharing a thread crest. The thread pitch of the fastener may be arranged to urge ductile material displacement towards the torque retention element. The profile of the torque retention element may be arranged to urge the ductile material displacement towards the trailing edge of the torque retention element.

In an embodiment the material of the workpiece is synthetic plastic, resin, a ductile material or the like. The material of the fastener may be steel, stainless steel, or the like. The fastener may be arranged to be self-tapping into the material of the workpiece.

An aspect of the invention is a thread roll die for manufacturing the thread of a screw thread fastener for fastening into a material of a workpiece, the thread roll die comprising a profile corresponding to the profile of the fastener, the fastener profile comprising a shank with threads having a symmetrical thread profile with a minor diameter and a major diameter, the major diameter defining a thread crest of the screw thread, the minor diameter defining the thread root of the screw thread, the profile of the screw thread defined by the surface of the screw thread between adjacent thread crests forming a thread flank; and a retention element formed on the surface of the thread flank of the screw thread for retaining the screw in the material of the workpiece. In an embodiment, the thread roll die comprises a die face having the same profile as the thread forming screw.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that embodiments of the invention may be fully and more clearly understood by way of non-limitative example from the following description taken in conjunction with the accompanying drawings in which like reference numerals designate similar or corresponding elements, regions and portions, and in which:

FIG. 1A-E shows the side perspective view of the torque retention thread forming screw (FIG. 1A), a cross-section of the screw taken along line A -A in FIG. 1A (FIG. 1B), enlarge view of the thread profile (FIG. 1 C), basic construction and geometry of the torque retention thread forming screw (FIG. 1 D), and the cross-section of the screw taken along line A - A in FIG. 1 A, like FIG. 1 B, of another embodiment (FIG. 1E), in accordance with an embodiment of the invention;

FIG. 2A-B shows the side view and the cross sectional view of the torque retention screw where the thread forming screw is rolled from a cylindrical header blank (FIG. 2A), and the cross section of the screw taken along line A - A in FIG. 2A (FIG. 2B) in accordance with an embodiment of the invention;

FIG. 3A-B shows the side view of the roll thread die to form the thread (FIG. 3A) and the face of the roll thread die (FIG. 3B) in accordance with an embodiment of the invention;

FIG. 4A-B shows the cross sectional view of an assembled thread forming screw fastener with engagement and material flow of the material of the workpiece that the fastener is attached (FIG. 4A) and a cross-section of the screw taken along line A - A in FIG. 4A (FIG. 4B) in accordance with an embodiment of the invention; and

FIG. 5A-B shows a cross sectional view (FIG. 5A) and a graph (FIG. 5B) of a thread forming screw fastener showing the torque and insertion depth at three points during insertion into a workpiece in accordance with an embodiment of the invention.

DETAILED DESCRIPTION

A torque retention screw thread fastener and a corresponding thread roll die for manufacture of the torque retention screw thread of the fastener are disclosed. An embodiment of invention is a screw thread fastener namely for fixing to workpiece such as plastic is disclosed and shown in FIG. 1A-E. In addition, a corresponding roll thread die as shown in FIG 3A-B to manufacture the thread is disclosed.

In FIG 1A, a torque retention thread forming screw is shown having a symmetrical thread profile around the central axis 11 with a constant radius transition from intersection 46 and to the root of the thread 18. The flanks 20 of the thread are developed from the intersections with two circles points 46 and meet the common point 50 that make's an isosceles triangle 50 of 30 degree 48 as per shown in FIG 1D. The flanks 20, threads root radius 18 and pitch 32 are constant throughout the threads. However the lead thread 30 and the thread end 28 can be manufactured with various shapes based on application needs. With the basic thread geometry described above, the torque retention thread-forming screw comprises a special feature in the minor diameter 24 and or the thread valley 24. The torque retention feature or retention element 17 and 19 locks the material that flows during installation towards the thread root 18.The torque retention features or retention elements 17 and 19, are placed for example 120 degrees apart equally at the circumference 25, of the thread valley. The torque retention features

17 are designed in this embodiment with and angle of 60 degrees to form the trailing edge of the retention elements which linearly increase in radius 15 from the thread root

18 to the intersection 23, and linearly decreases in radius 13 from the mid point 21 towards the opposite side of the thread minor diameter 24 at an angle of 30 degrees to makes a steep radius forming the leading edge of the retention elements which effectively prevent retro-grade rotations after the screw is assembled fully in a plastic joint.

Embodiments of the invention are improved torque retention thread forming screws, which have the capability of cutting and deforming plastic material such as ABS, thermoplastic and the like. The sharp thread flank 20 with 30-degree angle 48 promotes cutting action, which will split through the plastic material and facilitates the material of the workpiece to flow easily in the circular valley of the threads and flow out over to the root 18. This minimizes radial stress in the pilot hole and or boss and reduces stress cracks. The cross sectional configuration of the thread major diameter 26 and minor diameter 24 is cylindrical in shape and the thread locking features may be placed 120 . degrees apart 25. It will be appreciated that the torque retention elements or features 17,19 may have different configurations. For example, the retention elements may by spaced apart at different spacings, and the leading/trailing edges may vary. The retention elements may also be symmetrical with a 45 degree/45 degree configuration as shown in FIG. 1E. Of course the torque retention features can be created through out the thread length as well by specific number of thread as per application requirements and needs. For example, the torque retention elements may be arranged in line along the length of the fastener parallel with the central axis 11. The torque retention elements may be arranged in each adjacent thread valley, alternate thread valleys or the like. The torque retention elements may be in any number of thread valleys along the length of the fastener, for example, as shown in FIG. 1 A the torque retention elements are arranged in the three thread valleys adjacent the top region 16 of the fastener with no other torque retention elements in any of the other thread valleys from the central towards the bottom region of the fastener. It will be appreciated that the torque retention elements may be arranged in different configurations of spacing and angles.

In an embodiment shown in FIG. 1A, the screw shank of the screw fastener 10 is surrounded by a helically circumferential cutting-edge which projects radially from the root 18 of the thread. The thread flank 20 is constructed with, for example, 30 degree angle, sharp crest which is intended to deform and cut the base material. In this configuration the workpiece does not need to be tapped, however, in other embodiments the workpiece may be tapped. The thread root is in circular shape which is connected radially from one flank to the other to promote efficient flow of deformed material towards the root of the thread. The flanks themselves are provided with a constant pitch from the root of the thread so that a constant threads angles between the two flank lines, an additional locking feature or retention element 19 is added in between the thread valley. The retention element 19 increases the locking torque retention of the screw thread fastener. In conventional screws without the retention elements in the thread valley, the material of the workpiece which deforms and flows to the cylindrical root is not being held and tends to loosen when subjected to vibrations. With the additional locking features or retention elements 19 in the thread valley the material that flows to the root will be trapped and promotes higher retention torque upon full assembly and after material relaxations.

In the embodiment the torque retention thread forming screw 10 in FIG 1A - D and FIG 2A - B, having a head 12 and a recess drive 14. The fastener may have a flat head shape and can be extended to various shapes such as circular, countersunk, oval and the like. The torque retention thread forming screw 10 can be formed from any suitable material, such as steel, stainless steel and the like, having appropriate requisite properties and is rolled from a cylindrical cold formed blank 40 with variable diameter and length based on requirements. The thread is extended from the bearing surface 44 of the head 12 and towards the thread end 28. The thread may extend the total length of the screw. The top region, from bearing surface 44 having fillet radius 42 to the first complete thread 36, is the thread run-out 16. In the bottom region of the thread end 28 is the lead thread 30. The thread end 28 is not limited to a flat surface as shown but also may be manufactured with variable shapes such as tapered, round, sharp, dog point and others as per intended application. The central region of the thread is the full formed threads, with thread flank 20, circular thread root 18, major thread diameter 26, minor thread diameter 24, thread pitch 32 and flank or thread height 22. The torque retention thread forming screw has torque retention features 17,19 in the thread root 18 to reduce retro-rotations after assembly. The cross section configuration of the torque retention thread forming screw is cylindrical and allows the mating part with an engagement by a full circumferences engagement enable to achieve strong contact in the mating members, which prevents vibration loosening, high pull out force, high failure torque, high stripping torque and ability to use in multiple insertions.

The torque retention thread forming screw thread according to an embodiment of the invention has a flank 20 which forms an isosceles triangle of 30 degree angle 48, connected to intersection point 46 of basic construction circle and the concavity from intersection point 46 is continuous since it is formed by the basic root radius 18 having constant radii and whose centers are disposed in the same planes, as shown in FIG. 1D. The thread flank 20 is able to cut the plastic material 64 and the like efficiently without any cracks, with low torque transmission during installation and maximize material engagements in the mating parts and this extended with a circular thread root radius 18 which allows smooth displacement flow of material 66 towards the root without any interruption and with an addition of torque retention features in the thread root 18, the material that flows towards the roots will be interrupted and folds at counter-clockwise lock feature 13, which has an angle of 30 degrees and prevent from retro-grade loosening when the screw is fully tightened. The plastic material 64, material engagement 65 and material flow 66 are shown in FIG. 4A-B. The retention element has a clock-wise lock angle 15 that forms a leading edge and a counter clockwise lock angle 13 that forms a trailing edge. The retention element has a midpoint of locking features 21, and an intersection of locking features 23 along the flank of the screw thread. The locking or retention elements are spaced apart by a locking feature angle 25. In the embodiment shown in FIG. 1 B the locking feature angle 25 is 120 degrees.

In FIG. 2A-B, the torque retention thread forming screw is shown with a blank end 38 and cold formed blank 40. The helix angle 34 is shown defining the angle from the thread intersection point 46 of the retention element to the apex of the crest.

The torque retention thread forming screw in accordance with an embodiment of the invention has a high torsional strength, a high stripping torque, an increased assembly efficiency, and a retention of high break-out torque which reduces retro-grade rotation due to for example vibration loosening. The thread forming screw in accordance with an embodiment of the invention also reduces hoop stress and is reusable with multiple assembly. The physical characteristics of embodiments of the present invention also contribute to a reliable joint formed between the screw thread and the material of the workpiece, which provides comfortable margin between the assembly and failure torques. This wide torque range assures that all embodiments of the torque retention screw that are fully tightened provide superior clamp load in the joint and reduces the possibility of stripped assemblies. The torque retention thread forming screw in accordance with an embodiment is engineered primarily for plastic applications, and compatible with miniaturization applications. With the design of the torque retention thread forming screw in accordance with an embodiment of the invention, torque retention may be increased such that the screws may be designed with a smaller diameter and shorter length which contributes to lower manufacturing cost. In an embodiment, the torque retention screw thread fastener is for fastening into a hole made by the fastener into a workpiece composed of a low ductility material such as synthetic plastic, resin, glass filled nylon, polypropylene, polyphenelyne oxide, polystyrene, polyethylene, acrylonitrite butadiene styrene (ABS), polycarbonate and the like.

In an embodiment the torque retention screw fastener may be a self-tapping thread which is able to tap an internal thread in an unthreaded workpiece which may be made by, for example, die casting, molding, drilled hole and the like. The torque retention screw may have a symmetrical thread profile. The crest of the screw thread may take different forms for different applications. Embodiments of the invention relate to a torque retention thread forming screw having a symmetrical thread profile with a sharp transition flanks, circular thread root and torque retention locking features in between the thread valley, i.e. between the flanks and circular thread root of the screw fastener. An aspect of this invention is to form an internal thread in the workpiece with low torque, promotes efficient deformation of material during installation, having high failure torque, high clamping load, improves resistance to vibration loosening, and provides high torque retentions.

The crest shown and discussed is one embodiment has a sharp transition crest and circular thread root which forms an internal thread in the workpiece with low installation torque. It will be appreciated that other crest configurations may be implemented with aspects of the invention. In screw thread fasteners, a low installation torque is defined as the amount of torque applied to the thread forming screw configured in accordance with an embodiment of the invention during installation or engagement between the workpiece and the screw, whereby the resulting in less stress cracks in the workpiece, avoiding thread strip, while a higher clamp load is retained in the joint or the like. A lower installation torque may result in a lower friction that is transmitted on the workpiece, whereby an effective cutting action is promoted by the specific engineered geometric design of the thread flank, while promoting efficient deformation of the workpiece material during installation and having a high failure torque to enhance high clamping loads and improve joint integrity with mating parts of the plastic material of the workpiece.

Torque retention is defined as the amount of torque that must be applied to a screw fastener that is fixed in a workpiece material to dislodge the screw fastener. Given an example of an embodiment of the invention, if the torque retention the thread forming screw is 1.0Nm, the targeted installation torque is expected to be below 0.25Nm, which is may lead to an improvement of 10-15% over some attempts made with previous threads in conventional fasteners. Of course, the value of the torque retention of the thread forming screw and the targeted installation torque may depend on several factors including for example the type of material, length of engagement, boss diameter, size of screw, type of finishing of the thread forming screw and the like.

The thread may be constructed with single helix and double helix configuration with the same pitch and geometry. When used in plastics and other like materials, most known conventional screws have the drawback on plastic damage, such as, strip-off, burst, screw stuck, vibration loosening, retro-grade rotation, torque retention and or stress cracks are formed, with relatively high driving torques being required at the same time. In an embodiment of the invention, however, a universal screw is provided having a relatively high retention torque that assures a highly stressable and secure connection between screw and plastic. Secure connection is achieved even with different consistencies of the plastic. This is accomplished by having a thread crest, which is able to deform the material to form a mating thread's and efficiently promote the flow of material towards the root with a circular core and retention elements. This reduces the radial stress between the screw and plastic, and limits premature failure such as, boss burst, cracks and stripping. The torque retention thread forming screw comprises for example a sharp crest with a 30-degree isosceles triangle as flank, which is half of total flank height which intersects with the basic fundamental circle that form the thread root.

In an embodiment, due to smooth transition from sharp thread crest of the flank and continuing towards circular root of thread, the stress and friction during installation is reduced to enhance smooth material flow. The torque retention features are added to lock the material that flows to the root. The torque retention feature is placed 120 degree apart equally at the circumference of the thread valley. The torque retention features or retention elements are designed with an angle of 60 degrees which linearly increase from the root towards the intersection of the flank, when in the motion of tightening and or towards clockwise direction and 30 degrees angle in the opposite side which increase linearly from the root towards the intersection of the thread flank in the opposite side, anticlockwise. This configuration reduces friction and enhances smooth material flow during installation with an angle of 60 degrees in clockwise direction. The 30 degrees angle in opposite side will entrap the material that flows after full tightening and material relaxation, to restrict retro-grade rotation, vibration loosening and improves torque retention in the joint.

In another aspect of the invention, a thread roll die 52 is disclosed for manufacturing the torque retention thread forming screw fastener 10. FIG 3A - B show the thread forming screw roll thread die 52 for manufacturing the thread. The thread roll die 52 comprises of the profile to form thread of the fastener with thread die depth 56, thread root radius 58 for forming the retention element in the thread flank area, surface of the die face for forming the thread flank areas without a retention element 59, die face 54, thread pitch 60, thread crest 62, locking features 51 and 53 and the undercut to form the locking features 55. The thread roll die 52 comprises the mirror image profile of the thread forming screw with thread depth 56, thread root radius 58, thread roll die face 54, thread pitch 60, thread crest 62 and special undercut or serration radius 55 to create the torque retention features. The angle to manufacture the torque retention features of 30 degrees and 60 degrees is shown in FIG. 3A and FIG. 3B and indicated 51,53 in the roll thread die which would create the profile of 17 and 19 in the screw. Of course the angle to manufacture the torque retention features may be altered to the desired angle for the desired application.

FIG 4A-B shows the cross sectional view of an assembled torque retention thread forming screw in the material 64 of the workpiece such as a plastic material 64 in FIG. 4A. FIG. 4A-B shows the cross sectional view of an assembled thread forming screw fastener with material engagement 65 and material flow 66 of the material of the workpiece that the fastener is attached to in FIG. 4A, and a cross-section of the screw taken along line A - A in FIG. 4A is shown in FIG. 4B in accordance with an embodiment of the invention. The design of the torque retention thread forming screw reduces the need for a tapping operation and forms a strong thread by perfect material engagement of thread flank 20 and deformation of material towards the thread root 18 and shows material engagement 65 with the torque retention screw thread fastener. The torque retention elements function as a locking feature with the material of the workpiece after complete assembly of the screw.

Referring to FIG. 5A-B, the torque between point 2 to point 3 provides a clamping force in the joint, if the gap between point 2 and 3 is narrow, it means either the installation torque is high and or the failure torque in the joint is low and thus provides a lower clamping force or load. This may occur because there is an insufficient load carrying capability which may result in loosening when subjected to shock or vibration in real time applications, which contributes to failure in assembly joints.

Embodiments of the invention prevent or at least alleviate the drawbacks associated with prior conventional screw thread fasteners, as well provide alternative options on assembly efficiency, for example, by double helix threads for faster insertions. The torque retention thread forming screw comprises retention elements in the thread valley of the screw flank of the screw thread which reduces retro-grade rotation due to vibration loosening, and contributes to a reliable joint, to provide superior clamp load in the joint when the screw thread fastener is fully tightened and reduces the possibility of stripped assemblies. While embodiments of the invention have been described and illustrated, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention.