Title of Invention

"FASTENING SCREW WITH SELF-TAPPING THREAD"

Abstract

The invention relates to a retaining screw produced by cold rolling, having a self tapping thread and designed to be screwed for example into thermoplastic material. Said screw has a virtually end-to-end cylindrical thread as well as thread teeth, the thread crest of which has the same height on the whole length, while the free space between two teeth remains the same all along the thread. The screw presents the following characteristics: 1) the thread outer diameter Do and the thread inner diameter gives a ratio Ql=Do/Dk, in a magnitude of 1,25 to 1,65; 2) the axial distance P between two thread teeth gives with the height H of the latter a ratio Q2=P/H, in a magnitude of 2,35 to 2,7; 3) the outer thread flank angle of the thread teeth is about 30°; 4) the fillet flank surface features an inward curve in its central third and forms an outer and an inner thread flank angle, whereby the inner thread flank angle in the central portion is substantially equal to or larger than a third of the outer thread flank angle.

Full Text

The invention relates to a screw, formed by cold rolling, with a self-tapping single-flight thread, for screwing, in particular, into thermoplastics, with an essentially continuously cylindrical thread bottom and with thread teeth which have a vertex of continuously equal height, the free spaces between adjacent thread teeth being made the same along the thread. A screw of this type is known from German patent specification 39 26 000. This screw, regarding which the patent specification expressly points out that it is particularly suitable for setting and adjusting devices, that is to say for so-called movement threads, is to have a high-quality thread, by means of which a continuous and exactly meterable screw-in torque can be achieved. The outlay in terms of forming during the production of the screw by cold rolling is, at the same time, to be lower than in the case of conventional types of screw. In order to achieve this object, the screw possesses a vertex angle of about 40°, thus making it easier for the screw material to flow during the cold-rolling process. The present invention is to provide a screw which, in contrast to the action of a movement thread, is particularly suitable for screwing, in particular, into thermoplastics for fastening purposes. When screws of this type are screwed into plastic, the latter is displaced by the thread teeth penetrating into the plastic, for which purpose the thread cross section must make sufficient free space available. During this time, the plastic displaced by the thread teeth is squeezed into this free space. In this case, German patent specification 27 54 870, column 2, paragraph 1, started from the idea of penetrating as deep as possible into the respective plastic workpiece (high bearing depth), for which purpose, in the case of a flank angle of about 30°, a correspondingly large free space must then be made available. To this effect, in the screw according to German patent specification 27 54 870, the thread bottom has a contraction, so that a correspondingly large free space is available for the plastic displaced by the thread teeth. In this case, the plastic displaced by the thread teeth has to cover, from the region of the thread tooth which is penetrated into the plastic, into the thread bottom, a distance over which the displaced plastic loses the direct intimate bond with the non-displaced plastic due to the length of this distance, thus making the said displaced plastic less capable of contributing to the pull-out force. The term "pull-out force" refers to the force necessary for tearing out the screwed-in screw. However, a large free space for receiving displaced plastic results in a correspondingly small thread-bottom diameter (core diameter), and a consequence of this may be that, when such a screw is screwed in, in particular when it is screwed in over a relatively large number of thread flights, it is overloaded by the torque acting on it and breaks off. The idea of displacing the plastic is also of primary importance in the design of the screw according to European patent specification 0, 589, 399. This screw, which is designed expressly for a high bearing depth of the thread flanks, is to achieve a material displacement such that the material is compacted on the thread flanks, specifically on those flanks which, as so-called load flanks, have to absorb tear-out forces acting on the screw. In order to influence the flow of material in this way, the screw possesses a conical thread bottom, the smaller core diameter of which is adjacent to the said load flank. The flow of the plastic is thereby to be directed towards the load flank, where the plastic is then to be compacted. This intended flow of plastic presupposes that the material is sufficiently heated and converted into flow and solidification, while the screw is being screwed in. The design of the known screw is therefore based on considerably softening, shifting and compacting the plastic while the screw is being screwed in, as a result of which the plastic is completely released from its original bond and its structure is changed. As a consequence of this, the load flanks of the screws have to be supported on this material, which has been changed in such a way, but precisely this leads to a reduction in the tear-out forces of a screw thus screwed in. The present invention departs from the idea, prevalent hitherto in the prior art, of the thread teeth penetrating as deep as possible into the plastic, which at first sight, seems plausible in order to achieve high tear-out forces. For this purpose, the following dimensioning may be used: 1. the outside diameter Do and the core diameter Dk of the thread form a quotient Ql=Do/Dk of a magnitude of 1.25 to 1.65; 2. the axial spacing P of adjacent thread teeth forms, with the height H of the thread teeth, a quotient Q2=P/H which is between 2.35 and 2.7; 3. the vertex angle of the thread teeth is about 30°; 4. the screw consists of hardable or refined steel. This dimensioning leads to the screw having relatively low thread teeth, so that the screw sits in the plastic with a relatively low depth of penetration. The advantage of this is that the structure of the displaced plastic is not impaired to any considerable extent. In order to achieve high tear-out forces in spite of the relatively low depth of penetration, the thread is produced with a relatively low pitch, so that a large number of thread flights engaged with the plastic are provided. This dimensioning has, in addition, the following effect: the core diameter possesses a dimension which makes it possible to take into account the forming behaviour of a respective screw blank during its production by rolling, specifically in that the height of the thread teeth, which is building up on the core diameter, is still sufficiently large to displace the material of the screw in an advantageous way, in the region between the thread teeth, during rolling by the pressure exerted on the blank and, consequently, to produce accurately shaped thread teeth which, on the one hand, also have the advantage that they do not impair the plastic when they penetrate into it, but still have a height such that they can be shaped with considerable accuracy during rolling. So that these effects, made possible by the dimensionings explained above, can be fully utilized, the screw according to the invention possesses a vertex angle of the thread which is about 30°. A vertex angle of this kind for screws to be screwed into plastic is known per se, as shown, for example, by German patent specification 27 54 870. The aim, with this screw, is to make a relatively large free space between the thread flights available for the plastic into which the screw is screwed. The tendency followed by this screw amounts, therefore, to displacing as much plastic as possible by means of the thread teeth, that is to say, to working with a high depth of penetration, in order thereby to achieve high tear-out forces. The teachings of this patent specification do not therefore suit the prior art defined by the German patent specification 39 26 000, since this prior art, apart from its particular suitability for movement threads, when it is applied to plastic screws, amounts to displacing as little plastic as possible with these screws. The combination of the dimensioning features relating to the height and spacing of the thread teeth with the use of a particularly small vertex angle known per se results in a reinforcement of the principle of displacing as little plastic as possible when the screw is screwed into plastic, since the plastic mass to be displaced on account of the small vertex angle of 30° is substantially smaller than in the case of a vertex angle of 40°, as emerges from German patent specification 39 26 000, the result of this being that the interspace existing between the thread flights, this being defined by the pitch of the thread can be kept shorter, the consequence of this being that the number of thread flights to be anchored in a plastic part is increased correspondingly, if the screw-in length remains the same. This then leads to correspondingly increased tear-out forces. High tear-out forces necessitate a corresponding lead-bearing capacity of the screws both in the axial and in the tangential direction. The thinner the cross section of the screw relative to its outside diameter, the greater is the load on the screw, in relation to its cross section, both when it is being screwed in an when it is being tightened. So that, in this case, high tear-out forces can be withstood, it has hitherto been necessary for the screw material used for the known screws to have very high strength, the result of this being that highly tempered materials have had to be used for the screws in question here, these materials, on the one hand, being costly and, on the other hand, reacting sensitively to embrittlement occurring due to hydrogen diffusion. The consequence of this known effect may be that, in the known firmly tightened screws made of highly tempered material, time-delayed brittle fractures occur after they have been screwed in and tightened, for example by their heads splitting off after a few days. The screw dimensioning according to the invention provides screws having a relatively large cross section in relation to their outside diameter (as, for example, a look at Fig. 1 clearly shows). The screws according to the invention, by virtue of their design over their cross section, can therefore absorb considerably higher torques and axial forces than has been possible hitherto. This affords the possibility of producing the screw according to the invention from a material which has lower strength, as compared with conventional screws, that is to say materials can be used, which because of lower tempering do not have the tendency to absorb hydrogen by diffusion and are therefore inclined to brittle fractures. A further object of the invention is for the self-tapping thread of a cold-rolled screw to be configured, in terms of the axial section through the thread turns, such that the plastic material which is displaced when the screw is screwed into plastic can flow away in particularly favourable manner. This is achieved according to the invention in that a flank surface of the thread turns has an inwardly directed inflection approximately in the central third and forms an outer flank angle of approximately 30° (between the inflection and the thread crest) and an inner flank angle (between the inflection and the thread root), it being the case that the inner flank angle, on average, is essentially equal to or greater than 1/3 of the outer flank angle. EP Patent 133 773 describes a screw which is intended for screwing into plastic, has a self-tapping thread and in the case of which, for the purposed of increasing the release torque, the thread flanks, in axial section are rectilinear on one side and are provided with an outwardly directed inflection on the other side. With this thread configuration, it is not possible, during displacement of the plastic, to counteract the loss of contact between displaced and undisplaced plastic. Furthermore, EP Patent 476 831 likewise presents a non-symmetrical configuration of the thread of a screw which is intended for screwing into plastic and has a self-tapping thread, it being the case that this configuration results in the thread turns, in axial section, being rectilinear on one side and being allowed to run in arculate form into the thread root on the other side, the flank angle of the thread thus increasing constantly in the region of the arc. This is intended to improve the displacement of the plastic material with the effect of increasing the drawing-out force. You are additionally referred to German utility Model 79 25 469, which discloses a welding tip which is provided with a thread and in the case of which the thread serves for pressing a workpiece, which is provided with an opening, onto the welding tip and for securing the workpiece in that sharp-edged thread crests press into the material of the workpiece and secure the latter against a drawing-out force. In the case of the known welding tip, the thread is provided with sharp edges in that a peripheral phase [sic] is pressed laterally onto the thread crest, said phase [sic] merging with a sharp-edged formation into one thread flank. This necessarily gives a particularly large flank angle of approximately 75° on the thread creast. This configuration means that the thread provided on the known welding tip is not suitable for a screw which is intended for screwing into plastic and has a self-tapping thread since screws for screwing into plastic only ensure a sufficient penetration depth of the thread turns when the cross section of the thread turns is designed approximately in the manner of knife blades (see above mentioned German Patent 27 54 870). On account of the inwardly oriented inflection of the flank surface of the thread turns, the plastic material which is displaced by the thread turns is displaced into the region of the inner flank angle, it being the case that this region, on account of the inwardly oriented inflection and the clearance which thus extends relatively far outwards, directly adjoins the displaced plastic material protruding convexly from the bore wall without any material accumulation, with the result that the displaced plastic material, on account of the short displacement distance, is only heated to a minimal extent and remains in direct intimate contact with the undisplaced, and thus unimpaired plastic material. This means that these properties, in the region of the inner flank angle can counteract the drawing-out force and thus makes a significant contribution to the loading capacity of the relevant screw connection. It is possible for the inflection to be provided in each case just on one flank surface of a thread turn, in which case, over the entire thread, the inflection is arranged on one side throughout. However, it is also possible for the inflection to be provided. This configuration means that the thread provided on the known welding tip is not suitable for a screw which is intended for screwing into plastic and has a self-tapping thread since crews for screwing into plastic only ensure a sufficient penetration depth of the thread turns when the cross section of the thread turns is designed approximately in the manner of knife blades (see above mentioned German Patent 27 54 870). On account of the inwardly oriented inflection of the flank surface of the thread turns, the plastic material which is displaced by the thread turns is displaced into the region of the inner flank angle, it being the case that this region, on account of the inwardly oriented inflection and the clearance which thus extends relatively far outwards, directly adjoins the displaced plastic material protruding convexly from the bore wall without any material accumulation, with the result that the displaced plastic material, on account of the short displacement distance, is only heated to a minimal extent and remains in direct intimate contact with the undisplaced, and thus unimpaired plastic material. This means that this displaced plastic material, which largely maintains its properties, in the region of the inner flank angle can counteract the drawing-out force and thus makes a significant contribution to the loading capacity of the relevant screw connection. It is possible for the inflection to be provided in each case just on one flank surface of a thread turn, in which case, over the entire thread, the inflection is arranged on one side throughout. However, it is also possible for the inflection to be provided in each case on both flank halves of a thread turn. The arrangement of the inflection on just one flank surface depends on the plastic material into which the relevant screw is to be screwed. If the plastic material is heat-sensitive, then the inflection is advantageously arranged on that flank surface which is directed away from the screw head (rear flank). In this case, there is a deformation of plastic material essentially on the side of the rear flank where the deformed plastic material flows into the clearance provided by the inflection, with the result that on the flank surface which is directed towards the screw head (load flank) there is largely unheated plastic material for absorbing the forces acting on the screw. If, on the other hand, the plastic material is temperature-resistant (as is the case, in particular, as a result of glass-fibre reinforcement), then the inflection is advantageously arranged on the load flank since, in this case, plastic material which is displaced when the screw is screwed in is accumulated to a considerable extent in front of the load flank, said plastic material, on account of its quantity and, if appropriate, as a result of the reinforcement provided by its filler, then withstands particularly well the loading to which it is subjected by the screw. The outer flank angle may bound the thread turns symmetrically or non-symmetrically in axial section. A symmetrical boundary is favourable for the production of the rolling jaws necessary for the cold-rolling operation; a non-symmetrical boundary give increased drawing-out forces in the case of certain plastic materials. The configuration of the flank surfaces in the region of the inner flank angle may be selected such that the flank surfaces run rectilinearly or in a concavely curved manner from the inflection in axial section. The selection of this configuration depends on the plastic into which the screw is to be screwed. As far as the configuration of the inner flank angle of the thread turns its concerned, it is also possible for said angle to bound the thread turns symmetrically or non-symmetrically in axial section. The selection of this boundary likewise depends essentially on the plastic into which the screw is to be screwed. In order to increase the stability of the thread turns and to improve the material flow during the production of the screw by cold rolling, the screw is expediently configured such that the flank surfaces in the region of the thread base merge into the thread root in the form of a widening of the thread base, the inner flank angle being increased in the process. In this case, the widening forms a flank angle which, on average, is greater than the outer flank angle. It is possible here for the widening to run rectilinearly in axial section, but it is also possible for the widening to run in a concavely curved manner in axial section. Accordingly the instant invention provides for a fastening screw, formed by cold rolling, with a self-tapping thread for screwing, in particular, into thermoplastics, with an essentially continuously cylindrical thread bottom and with thread teeth which have a vertex of continuously equal height, the free spaces between adjacent thread teeth being made the same along the thread, characterized in that the outside diameter Do and the core diameter Dk of the thread forms a quotient Ql=Do/Dk of a magnitude of 1.25 to 1.65; the axial spacing P of adjacent thread teeth forms, with the height H of the thread teeth, a quotient Q2=P/H which is between 2.35 and 2.7; the vertex angle of the thread teeth is 30° Brief description of the accompanying drawings Figure 1 shows a fastening screw screwed into a tube, for fastening a plate Figure 2 shows a detail of the single-flight thread of the screw according to Figure 1 in an enlarged illustration, Figure 3 shows a detail of a two-flight thread, Figure 4 shows a screw in section, Fig. 5 shows thread turns in which both flank surfaces of a thread turn contain an inflection, the receiving plastic material also being shown, Fig. 6 shows thread turns with an inflection on just one flank surface, Fig. 7 shows thread turns with an inflection on just one flank surface and a non-symmetrical outer flank angle, Fig. 8 shows thread turns with a widening of the inner flank angle in the region of the thread base, Fig. 9 shows thread turns with a widening of the flank surfaces in the region of the thread base in conjunction with a non-symmetrical outer flank angle, Figure 1 shows a screw 1, formed by cold rolling, with the self-tapping thread 2 which extends uniformly over the shank 3. On that side of the screw 1 which is located opposite the end of the shank 3, the said screw possesses the screw head 4 which is provided, here with, with a profiled recess 5 for the insertion of a suitable spanner, by means of which the screw 1 is screwed into a workpiece, here the tube 6. The screw 1 serves for fastening the plate 7 to the tube 6. For this purpose, the screw is tightened relative to the tube 6 in such a way that its head 4 at the same time presses the plate 7 against the tube 6. The particular feature of the fastening screw 1 according to the invention can be seen clearly from Figure 1 specifically its relatively low depth of penetration into the material of the tube 6 and, as compared with this, the relatively large core diameter Dk of the thread 2, the continuously cylindrical thread bottom 8 of which leaves only a relatively small free space relative to the inner face 9 of the tube 6. Figure 2 illustrates a detail of the shank 3 according to Figure 1 in an enlarged illustration. This details illustrates the core diameter Dk, the outside diameter Do and the axial spacing and the height H of the thread teeth 10. Moreover, the flank angle of the thread teeth is specified as 30° in Figure 2. It may be gathered from Figure 2 that, in the screw, illustrated enlarged, the quotient Ql = Do/Dk is 1.43 Here, the quotient Q2 = axial spacing p/height H has the value 2.57. Figure 3 illustrates a detail of a two-flight thread 11 which, apart from the number of flights, is otherwise largely identiacal to the thread 2 according to Figure 2. The axial spacing P in the thread 31 according to figure 3 is measured, here, between the two adjacent thread teeth of the one thread flight and of he other. This results, as in the exemplary embodiment according to Figure 2 in the quotient Q2 = axial spacing P/height H with a value of 2.57. The fastening screws illustrated in Figures 1 and 2 are those which are usually produced with an outside diameter of 1 mm to 10 mm. The screw according to the invention is suitable for screwing not only into the currently customary thermoplastics, but also into all other materials which have properties similar to those of thermoplastics. Fig. 4 illustrates a cold-rolled screw with the screw head 21 and the screw shank 22, which has the thread turns 23. As far as the cross section of the thread turns 23 is concerned, you are referred to the explanation relating to Fig. 5. The inner wall of a bore in a plastic part is also indicated in Fig. 4, by the chain-dotted lines 24. It can be seen from the position of the chain-dotted lines 24 that the thread turns 23 have more than 1/3 of their height penetrating into the material of a plastic part (not illustrated). In this case, the line 24 more or less intersects the location of the thread turns 23. It should be pointed out, however, that it is also possible to select a different position for the line 24, which illustrates the wall of a bore, this position depending on the relevant plastic material into which the screw is to be screwed. Figure 5 illustrates thread turns 23 which are configured symmetrically in axial section, this resulting from the fact that both the flank surface 34/35 and the flank surface 31/33 are provided with an inflection, namely the inflection 36 and the inflection 25. The inflection 25 and the inflection 36 form the boundary between the outer flank angle a (between the inflection 25 or 36 and the thread crest 47) and the inner flank angle  (between the inflection 25 or 36 and the thread root 27). Both the outer flank angle a and the inner flank angle  bound the thread turns 23 symmetrically. It can also be seen from Figure 5 how the thread turns 23 penetrate into the plastic material 46 belonging to some component. This penetration of the thread turns 23 results in the formation of the beads 29 alongside the thread flanks 34/35 and 31/33. In this case, the plastic material 46 is displaced in the direction of the thread root 27, the narrowness of the thread turns 23 resulting in just minimal displacement of the plastic material 46. The inflection of the thread flanks 34/35 and 31/33 approximately in the region of the line 24 results in an additional clearance on both sides, which is indicated by the dashed line 32. The dashed line 32 forms the imaginary continuation of the flank surfaces 33 and 35 in the region of the inner flank angle . The clearance 30, which is correspondingly increased in size by the inflection of the flank surface 33 and 35, i.e. the transition into the smaller flank angle  Figure 6 illustrates a shaping of the thread turns 23 which is modified with respect to the configuration according to Figure 5. According to Figure 6, the thread turns 23 are configured non-symmetrically in axial section, which results from the fact that the flank surface 26 runs rectilinearly throughout as far as the thread root 27, while the other flank surface 28 has the inflection 25. Figure 7 shows a further modification of the illustration according to Figure 6. Figure 7 has thread turns 23 in which one flank surface 38 runs rectilinearly and the other flank surface 37 runs with an inflection, the difference from Figure 6 being that the non-symmetry is in the region of the outer flank angle a. Figure 8 shows a modification of the configuration according to Figure 5. The screw according to Figure 8 has the widenings 42 and 43, which run rectilinearly in each case here, in the region of the thread base. The widenings 42 and 43 may also be of concavely curved design. Figure 9 shows a configuration, similar to that according to Figure 7, with an asymmetrical outer flank angle a, the transitions into the flank base being designed in each case as a rectilinearly running widening 42 and 43. It should also be pointed out that the non-symmetry in the region of the outer flank angle may also be provided in the region of the inner flank angle, as already specified above, and in this case may relate, if appropriate, to widenings provided in the region of the thread base. Particularly suitable for soft plastics is a screw according to the invention in the case of which, within the range of variation of the flank angle of approximately 30°, the value selected results in particularly acute-angled thread turns, namely with a flank angle of approximately 25°. We claim: 1. Fastening screw, formed by cold rolling, with a self-tapping thread for screwing, in particular, into thermoplastics, with an essentially continuously cylindrical thread bottom and with thread teeth which have a vertex of continuously equal height, the free spaces between adjacent thread teeth being made the same along the thread, characterized in that the outside diameter Do and the core diameter Dk of the thread forms a quotient Ql=Do/Dk of a magnitude of 1.25 to 1.65; the axial spacing P of adjacent thread teeth forms, with the height H of the thread teeth, a quotient Q2=P/H which is between 2.35 and 2.7; the vertex angle of the thread teeth is 30° 2. Fastening screw as claimed in claim 1 wherein the said screw has a self- tapping thread where the thread turns are configured in the manner of knife blades in cross section, such that the flank surface of the thread turns has an inwardly directed inflection in the central third and forms an outer flank angle of 30° between the inflection and the thread crest and an inner flank angle, between the inflection and the thread root, such that the inner flank angle, is equal to or greater than 1/3 of the outer flank angle. 3. Fastening Screw as claimed in Claim 2, wherein the said the inflection on each thread is provided only on one flank surface of a thread turn. 4. Fastening Screw as claimed in Claim 2, wherein the said inwardly directed inflection on each thread is provided on both flank surfaces of a thread turn. 5. Fastening Screw as claimed in any of the Claims 2 to 4, wherein the outer flank angle bounds the thread turns symmetrically or non-symmetrically in axial section. 6. Fastening Screw as claimed in any of Claims 2 to 5, wherein the inner flank angle bounds the thread turns symmetrically or non-symmetrically in axial section. 7. Fastening Screw as claimed in any of Claims 2 to 6 wherein the flank surfaces in the region of the thread base merge into the thread root in the form of a widening of the thread base, the inner flank angle being increased in the process. 8. Fastening screw formed by cold rolling with a self-tapping thread substantially as herein described with reference to and as illustrated by the accompanying drawings.

Documents:

1143-del-1999-abstract.pdf

1143-del-1999-claims.pdf

1143-del-1999-complete specification (granted).pdf

1143-del-1999-correspondence-others.pdf

1143-del-1999-correspondence-po.pdf

1143-del-1999-description (complete).pdf

1143-del-1999-drawings.pdf

1143-del-1999-form-1.pdf

1143-del-1999-form-19.pdf

1143-del-1999-form-2.pdf

1143-del-1999-form-3.pdf

1143-del-1999-pa.pdf

1143-del-1999-petition-137.pdf