Method for Producing a Screw Anchor Comprising a Metric Connection Thread FIELD OF THE INVENTION
The present invention lies in the field of anchoring technology.
It in particular relates to a method for producing a screw anchor comprising a metric connection thread, as well as a corresponding screw anchor.
BACKGROUND AND PRIOR ART Screw anchors are known in the prior art, which comprise an anchoring thread, which is suitable for direct screwing into concrete, stone or masonry, a metric connection thread, and a drive.
The “anchoring thread” can also be referred to in — short as “concrete thread” herein.
The term “direct screwing into” refers to the fact that the thread cuts directly into the concrete or the like, without using a dowel.
Such a screw anchor 10 is shown in Fig. 1e.
As can be gathered from Fig. 1e, the screw anchor 10 has an anchoring thread 12, a metric connection thread 14 and a drive 16, in this case a hex drive.
Fig. 1a to 1e further show manufactur- — ing steps as part of the production of a screw anchor 10 of this type.
A cylindrical blank 18 is created by pressing and placing in Fig. 1a.
The diameter of the blank 18 is then reduced, whereby a stepped blank 18 is formed, as it is shown in Fig. 1b.
The geometry of the stepped blank 18 is thereby selected so that afirst section 20, on which the anchoring thread 12 is to be formed, and a second section 14, on which the metric connection thread 14 is to be formed, each have a suitable rolling diameter.
When using the drilling diameters for concrete, which are standardized on the market, and nominal diameters of the metric threads, the rolling diameter of the first section 20 is smaller than the rolling diameter of — the second section 22 in the prior art.
This diameter difference is carried out by reduction in one or several steps on a press or multi-stage press, respectively, i.e. by cold forming.
To be able to carry out the cold forming process efficiently and without excessive stress on the tools, a low carbon steel with a carbon content of less than 0.45%, preferably even of less than 0.25%, is typically used for the — blank 18 of the prior art.
As can further be gathered from Fig. 1b, the stepped blank 18 furthermore contains a section 24 with reduced diameter, which is pro- vided for forming the drive 16. In a following method step, the drive 16 is likewise formed by cold forming, typi- cally by flow processing (Fig. 1c). The metric thread 14 (Fig. 1d) and the anchor- ing thread 12 (Fig. 1e) are subsequently formed by rolling.
Lastly, the screw an- chor 10 of Fig. 1e is hardened.
For this purpose, the screw anchor 10 is typically carbonized in the course of a heat treatment in order to obtain a carbon content of more than 0.65%, is hardened, and tempered.
Typically, the leading end of the anchoring thread 12 is furthermore additionally hardened, for example by induc- tion hardening.
The described manufacturing method from the prior art is comparatively com- plex and cost-intensive.
WO 2007/012417 A1 discloses a thread-cutting concrete anchor for self-tapping anchoring in hard substrates, such as concrete, masonry or the like.
The concrete anchor has an essentially cylindrical shaft, the core diameter of which is formed in a stepped manner.
The shaft has a cutting section, which comprises a helically revolving cutting thread and which, with respect to a setting direction, forms a front shaft section with a first core diameter.
A fastening section extends in the opposite direction to the setting direction and forms a rear shaft section, which has a load engagement means and a second core diameter.
The shaft is further equipped with an engagement means for transferring a torque to the shaft.
The — cutting thread, which revolves helically on the front shaft section, has thread flanks, which enclose an acute angle with one another.
US 5,531,553 discloses a masonry fastening device, which comprises a steel shaft, which has a circular cylindrical form.
A ridge-groove-ridge construction extends helically along the lower section of the shaft and comprises a pair of parallel, opposed ridges, which rise up from an adjacent region.
Each ridge forms a groove with the adjacent ridge.
At least the front end of the lower section of the shaft is formed in a self-tapping manner.
During use, the fastening device is in- troduced into a pre-drilled bore in masonry, such as, e.g., brickwork, in that it is — turned so that it forms a thread on the inner wall of the bore.
The axial dimen-
sion of the ridge 9 is at least 50% of the blank diameter, so that relatively large amounts of substrate material are located between the ridge-groove-ridge con- figuration when the fastening device is inserted.
A high pull-out strength of the fastening device in masonry structures is achieved thereby.
BRIEF DESCRIPTION OF THE INVENTION The invention is based on the object of providing a method for producing a screw anchor, which is more efficient and less cost-intensive.
This object is solved according to a method according to claim 1. Advantageous embodiments are specified in the dependent claims.
According to the invention, the method for producing a blank comprising an anchoring thread, a metric connection thread and a drive comprises the follow- ing steps: - providing a blank with a first section, on which the anchoring thread is to be formed, and a second section, on which the metric connection thread is to be formed, - forming the drive, - forming the metric thread by rolling, and
- forming the anchoring thread by rolling, wherein the first section is not reduced before rolling the anchoring thread, and wherein the anchoring thread has one or more of the following properties:
- the anchoring thread has a first flank angle in a radially outer region and a second flank angle in a radially inner region, wherein the second flank an- gle is larger than the first flank angle,
- the anchoring thread has a tip radius of at least 0.15 mm, preferably of at least 0.18 mm and particularly preferably of at least 0.2 mm, - the anchoring thread has
. a height of at least 1.8 mm, preferably of at least 1.9 mm with a nominal diameter of 14 mm or UNF 9/16”, . a height of at least 1.9 mm, preferably of at least 2.05 mm with a nominal diameter of 16 mm or UNF 5/8”, . a height of at least 2.0 mm, preferably of at least 2.15 mm with a nominal diameter of 20 mm or UNF 3/4",
15 . a height of at least 2.0 mm, preferably of at least 2.2 mm with a nominal diameter of 24 mm or UNF 7/8”, . a height of at least 2.2 mm, preferably of at least 2.5 mm with a nominal diameter of > 30 mm or > UNF 11/8”. In a preferred embodiment, the blank has a carbon content of at least 0.5%, preferably of at least 0.6%. In this preferred embodiment, the drive is further also formed by a machining process, by a joining process or by hot forming at a temperature of at least 300°C, preferably at least 350°C and particular prefera- bly at least 400°C. In contrast to the production method of the prior art, a blank is provided accord- ing to this embodiment, which already has a comparatively high carbon content. This means that the above-described heat treatment with carbonization, harden- ing and tempering, which is carried out in the prior art after rolling the metric thread and the anchoring thread, can be omitted, whereby the production pro- cess becomes significantly faster and more cost efficient. In the case of such a high carbon content, however, the formation of the drive by cold forming is sig- nificantly more difficult and typically leads to an excessive wear of the corre- sponding tools. According to this embodiment, the drive is thus not formed by cold forming, but by a machining process, by a joining process, for example welding, or by hot forming at a temperature of at least 300°C, preferably at least 350°C and particular preferably at least 400°C, for example by forging.
As a whole, the production process can be simplified and accelerated significantly by 5 means of the method according to the invention.
In an advantageous embodiment, the drive is formed by polygon turning, in par- ticular by hexagonal hammering.
The polygon turning is an example of the above-mentioned machining process.
It allows for a quick formation of the drive
— even in the case of comparatively high carbon steel.
It has turned out, in fact, that specifically in the case of larger lengths of the screw anchor, the formation of the drive by a machining process, specifically polygon turning, is faster and more efficient than the common cold forming, in the case of which the screw anchor has to be received with its entire length in a corresponding press.
In the case of longer screw anchors, for example those with a length of 200 mm or more, very large, long-stroke presses are thus used, which typically have a low output rate.
These problems can be avoided when using a machining process, in particular polygon turning.
— In preferred embodiments, the first section is not hardened and tempered after rolling the anchoring thread, as it is the case in the above-described prior art according to Fig. 1. This is made possible in that, according to this aspect of the invention, the utilized blank already has a comparatively high carbon content of at least 0.5%, preferably of at least 0.6%. After rolling the anchoring thread, the
— first section is preferably not hardened over its entire length.
It is possible, how- ever, that only the leading end of the anchoring thread is hardened, in particular by induction hardening.
A hardness of at least 60 HRC is thereby preferably achieved in the region of the leading end of the anchoring thread, which leads to a comparatively low wear resistance.
The metric thread is preferably formed by rolling.
The formation of the drive and the formation of the metric thread is thereby respectively carried out on a CNC machine, preferably on the same CNC machine.
The drive and the metric thread can be formed quickly and cost-efficiently in this way.
The method according to the invention turns out to be particularly advantageous for screw anchors with a length of at least 200 mm, preferably of at least 250 mm and particularly preferably of at least 300 mm.
In the case of lengths of this size, the advantages are particularly effective compared to conventional meth- ods, in the case of which the drive is formed by cold forming using correspond- ingly long-stroke presses.
In the case of an advantageous embodiment, the metric thread has a nominal diameter according to DIN 13 > 14 mm.
Similarly as in the case of the length, the advantages of the invention with regard to the diameter are also effective in par- ticular in the case of larger diameters.
The reason for this is, inter alia, that screw anchors with higher diameters more easily withstand the required insertion tor- ques even without additional heat treatment.
It is important to note that in the case of the preferred embodiments, the nominal diameter of the metric thread and of the anchoring thread are identical.
However, while the “nominal diame- ter” in the case of a metric thread is defined highly precisely, the same does not apply equally for the anchoring thread, thus, for example, a concrete thread.
While, for example, a “nominal diameter” of 20 mm in the case of a metric thread means that the outer diameter corresponds to 20 mm with high precision, a concrete thread with a “nominal diameter” of 20 mm can indeed deviate from this value by around 1 mm.
In the case of a concrete thread, the “nominal diame- ter” is nonetheless a common and sufficiently clear specification for the person of skill in the art, which he follows.
Commercially available concrete screws are thus sold with reference to such a nominal diameter, which, in turn, is adapted to a certain borehole size (for example, a concrete screw with a nominal diameter of 20 mm is adapted to a borehole size of 18 mm). In alternative embodiments, the metric thread is a UNF thread according to AN- SI B1.1, the nominal diameter of which is > 9/16”. It is important to note that in terms of the present disclosure, a UNF thread according to ANSI B1.1 is thus also considered to be a “metric thread”. According to the invention, the first section is not reduced before rolling the an- choring thread.
A graded blank, as it is shown in Fig. 1b, is thus not used thereby, — but a cylindrical blank, in the case of which the diameter in the first section 20 and in the second section 22 is identical.
While the person of skill in the art cur- rently assumes that he has to adapt the first section 20 to the rolling diameter of the anchoring thread, thus has to select it to be smaller than the rolling diameter for the metric thread 14 in the second section 22, the inventor has realized that — this can be avoided when the cross section of the anchoring thread 12 is simply chosen skillfully, namely so that a sufficient amount of material is transported into the thread during the rolling in order to achieve a sufficiently slim core di- ameter — even without prior reduction.
The additional operating step of reduc- ing, that is, the step between the manufacturing stages of Fig. 1a and Fig. 1b, can thus be saved in this way.
It is surprising for the person of skill in the art that a screw anchor with metric connection thread can in fact be produced without prior reduction of the first section 20 of the blank 18.
The invention has a particular synergetic effect in the above-described further development, which relates to the use of a comparatively high carbon blank be- cause the harder the starting material of the blank 18, the more difficult it is to accomplish the reduction described in connection with Fig. 1. In order to reduce — the diameter of a blank with a carbon content of more than 0.5%, or even more than 0.6% by cold forming, enormously high forces are required, which lead to a significant wear of the tools.
If the reduction is carried out by a machining pro- cess, a significant wear is to likewise be expected due to the comparatively high hardness.
It turns out to be extraordinarily simple in this respect if a reduction of — the blank can be omitted.
The inventor was able to confirm that the anchoring thread 12 can still be rolled even in the case of an increased carbon content of more than 0.5%, and also of more than 0.6%. According to the invention, the anchoring thread has a first flank angle in a radi- ally outer region and a second flank angle in a radially inner region, wherein the second flank angle is larger than the first flank angle.
The cross sectional surface of the thread can be enlarged in this way compared to conventional threads, or — in other words — more material can be moved into the thread during the rolling, without a torque increasing significantly.
In preferred embodiments, the height ofthe radially inner thread section is thereby between 0.35 and 0.7 mm.
According to the invention, the anchoring thread 12 has a tip radius of at least
0.15 mm, preferably of at least 0.18 mm and particularly preferably of at least 0.2 mm. A comparatively large tip radius of this type has the effect that the thread — cross section can be widened with consistent height and consistent flank angle, whereby more material can likewise be accommodated in the thread. Additionally or alternatively, the anchoring thread can have a comparatively large height, whereby more material can likewise be accommodated in the thread. In embodiments according to the invention, the anchoring thread con- cretely has: o a height of at least 1.8 mm, preferably of at least 1.9 mm with a nominal diameter of 14 mm or UNF 9/16”, o a height of at least 1.9 mm, preferably of at least 2.05 mm with a nominal diameter of 16 mm or UNF 5/8”, o a height of at least 2.0 mm, preferably of at least 2.15 mm with a nominal diameter of 20 mm or UNF 3/4", o a height of at least 2.0 mm, preferably of at least 2.2 mm with a nominal diameter of 24 mm or UNF 7/8”, n a height of at least 2.2 mm, preferably of at least 2.5 mm with a nominal diameter of > 30 mm or > UNF 11/8”. In a preferred embodiment, the second flank angle is larger by a factor of at least
1.5, preferably of at least 1.8 and particularly preferably of at least 2 than the first flank angle. It goes without saying for the person of skill in the art that the average value of the insertion torque tends to enlarge when the flank cross section is enlarged in one or more of the above-described ways. However, the inventor was surprised — to discover that even though an enlargement of this type of the flank cross sec-
tion does in fact lead to an increase of the average value of the insertion torque, the 95% fractile value of the insertion torque does not necessarily increase, for example, but, in contrast, was even smaller than in the case of anchoring threads from the prior art with smaller flank cross section.
As an explanation, the inven- — tor assumes that slimmer anchoring threads are more likely to collapse when being inserted, whereby the insertion torque is then increased to a particularly large extent.
In the case of the more voluminous anchoring threads proposed here, this risk is obviously significantly lower, which leads to a reduction of the 95% fractile value.
BRIEF DESCRIPTION OF THE FIGURES Figure 1a-1e shows a sequence of manufacturing stages during the production of a screw anchor with metric connection thread according to the prior art.
Figure 2a-2c shows a sequence of manufacturing stages during the production of a screw anchor with metric connection thread according to an embodiment of the invention.
Figure 3a shows a cross sectional view of an anchoring thread from the prior art.
Figures 3b/c show cross sectional views of an anchoring thread according to embodiments of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS — Fig. 3 shows a sequence of manufacturing stages during the production of a screw anchor 10 with metric connection thread according to a preferred embod- iment of the invention.
In the shown exemplary embodiment, a cylindrical blank 18 made of steel with a carbon content of more than 0.6%, of the so-called C- quality, is used.
The cylindrical blank 18 has a first section 20, in which an an- — choring thread 12 (see Fig. 2c) is to be formed, and a second section 22, in which a metric thread 14 (see Fig. 2b) is to be formed. Due to the fact that the cylindri- cal blank 18 has a uniform diameter, however, the first and the second section 20, 22 are not differentiated from one another by any structural features, but are initially only virtual sections of the same cylindrical blank 18. First of all, the drive 16 is formed by polygon turning on a CNC machine (not shown), in the shown exemplary embodiment by so-called hexagonal hammer-
ing. The cylindrical blank 18 with drive 16 formed thereon is shown in Fig. 2a. The metric thread 14 is then formed by rolling on the same CNC machine (Fig.
2b). It is important to note that the order is not mandatory, but that, in contrast to the order illustrated in Fig. 2, the metric thread 14 can be rolled first, and the drive 16 can be formed afterwards. It is further important to note that the order, in which certain method steps are listed in the description or the claims, is not to suggest that they are in fact carried out in that order, if a different order is tech- — nically possible, without reference being made expressly thereto. It is particularly advantageous that in the case of the embodiment of the inven- tion, the drive 16 and the metric thread 14 can be formed guasi in one step on the same CNC machine. An extraordinarily high throughput with comparatively — moderate machine effort can be achieved thereby. As has been mentioned above, it is increasingly difficult with the increasing length of the screw anchor 10 to efficiently form the drive 16, i.e. with high throughput, by cold forming because very large, long-stroke presses, in which the workpieces are received with their entire length, have to be used for longer piece sizes. Then, the anchoring thread 12 is formed in the first section 20 by rolling (Fig.
2c). As can be seen by comparing Fig. 2c with Fig. 1e, the anchoring thread 12 in the case of the screw anchor according to the invention (Fig. 2c) is more volumi- nous than the anchoring thread 12 of the conventional screw anchor (Fig. 1e). — This means that more material is moved into the thread during the rolling, and that the core diameter in the first section 20 thus decreases due to the rolling itself. A sufficiently small core diameter can be achieved in this way, without the blank 18 having to first be reduced in the first section 20 in a separate step, as it is the case in Fig. 1a/1b.
Due to the fact that in the case of the method of Fig. 2, the blank 18 already has a comparatively high carbon content of more than 0.6% in preferred embodi- ments, a further heat treatment and hardening of the first section 20 as a whole, for example case hardening, can be omitted. Instead, only the leading end of the anchoring thread 12 is hardened by induction hardening, in the shown exempla- ry embodiment to a hardness of more than 60 HRC. This induction hardening at the tip can be carried out comparatively quickly and cost-efficiently. With reference to Fig. 3, preferred forms of the anchoring thread 12 are de- scribed with reference to dimensions, which are only exemplary, but which are specified concretely for illustration purposes. Fig. 3a shows a section of a longi- tudinal section of a conventional concrete screw with a nominal diameter of 20 mm, in which the thread cross section, i.e. the cross section of a thread turn, can be seen on the right. As can be gathered from Fig. 3a, the actual outer diameter of the thread is 20.50 mm, and the core diameter of the concrete screw is 17.10 mm. The thread has a flank angle of 40° and a tip radius of 0.1 mm. The height of the thread, i.e. the radial distance between the thread tip and the core of the screw, is (20.50 — 17.10):2 = 1.7 mm. The thread turn thus has a cross sectional surface of 1.29 mm2.
Fig. 3b shows the cross section of a modified thread, which differs from the thread of Fig. 3a in two aspects. On the one hand, the tip radius, with 0.2 mm, is higher than in the case of the conventional thread of Fig. 3a. With the same height and the same flank angle, the increase of the tip radius by 0.1 mm leads to — awidening of the thread cross section by 0.2 mm. On the other hand, in the cross section the thread has a radially outer region with the same flank angle of 40° as the thread of Fig. 3a, but additionally a radially inner region with a larger flank angle, in the shown exemplary embodiment of 90”. In the shown exemplary em- bodiment, this radially inner region has a height of 0.5 mm. In other exemplary embodiments, the height of the radially inner region can be between 0.35 and
0.7 mm. Due to this increased flank angle in the radially inner region, the cross sectional surface of the thread turn enlarges further, in the concretely shown exemplary embodiment to a surface area of 1.68 mm2.
Fig. 3c shows the cross section of a further modified thread, which, similarly to the thread of Fig. 3b, has an enlarged tip radius of 0.2 mm and a radially inner region with an enlarged flank angle of 90°. However, the height of the thread is also enlarged additionally, namely to (21.30 — 17.10):2=2.1 mm.
The cross sec- — tional surface of the thread turn can be increased further in this way, in the con- cretely shown exemplary embodiment to 2.35 mm?. List of Reference Numerals
10 screw anchor 12 anchoring thread 14 metric connection thread 16 drive 18 blank first section 22 second section