PRIORITY

This application claims priority to and the benefit of French Patent Application No. 1852987, filed April 6, 2018, the entire contents of which is incorporated herein by reference.

TECHNICAL FIELD

The invention relates to a self-tapping screw for concrete.

STATE OF THE ART

A self-tapping screw for concrete is a fastening component which is screwed into a pre-drilled hole in concrete. The hole is drilled with a predetermined diameter, and the screw is configured to form a tapping in the peripheral wall of the hole as it is being screwed in.

The prior art proposes numerous possible configurations for a suchlike screw. However, the applicant considers that previously disclosed screws are not entirely satisfactory. Some of them are effective, but very complicated, and are therefore expensive to produce. Others are simpler and more cost-effective, but exhibit inferior performance.

The applicant has thus developed a screw which is of relatively simple design and is as such relatively cost-effective, and which exhibits very good fixing performance in concrete, including reinforced concrete. DISCLOSURE OF THE INVENTION

The invention thus proposes a self-tapping screw for concrete comprising a threaded rod, of which one extremity is free and an opposite extremity is connected to a head for driving the screw causing it to rotate about its axis by means of an appropriate tool, the threaded rod comprising a core having a diameter D1 and at least one thread which extends in a helix around the core and has an external diameter D2, said at least one thread having a generally triangular or trapezoidal form in cross section in a plane passing through said axis, comprising:

- a free point which is radially external with respect to said axis,

- a radially internal base situated in the area of said core, - a first side formed by a lateral flank of the thread situated to one side of said free extremity, which is substantially flat and which forms an angle a with a plane which is perpendicular to said axis, and

- a second side formed by a lateral flank of the thread situated to one side of said head, which is curved inwards in a concave manner with a radius R,

characterized in that:

- the relationship D2/D1 is less than 1.5,

- said angle a lies between 0 and 10°, and

- said radius R is equal to K x (D2-D1 )/2, where K lies between 1 and 1 .5.

The thread thus has an asymmetrical form with parameters D2/D1 , a and R which are optimized in order to impart a good performance to the screw. The advantages of this screw are numerous and include in particular:

• better orientation of the stresses in the concrete; in the quest for improved performance in the resistance to traction of the screw, the inclination of the thread makes it possible to have radial stresses rather than axial stresses, thereby improving the hold in the concrete,

• harmonization of the fitting of the screw depending on the concretes that are encountered and on the wear in the drills that are used; the dissymmetry of the thread, with respect to the equivalent symmetrical straight thread, means that there is less steel to be removed during the rolling process (ease of production) and reduces the interference with the concrete; this difference is all the more important in the case of a hole drilled with a worn drill, of which the external diameter may be less than D2,

• optimization of the durable section of the screw (determined by the diameter D1 ) in order to reduce the risks of rupture when fitting in concrete; the dissymmetry of the thread reduces the quantity of material to be displaced (and facilitates rolling in production) and increases the diameter at the base of the thread D1 ).

The screw according to the invention may comprise one or a plurality of the following characterizing features, taken in isolation one from the other or in combination one with the other:

- said lateral flanks define between them, at the tip of said point, an angle b which is less than 60°, - said angle b is more than 45°,

- said angle a lies between 4 and 8°, and is preferably 6°,

- said angle b is more than Q x a, where Q is a coefficient more than 2,

- said relationship D2/D1 lies between 1.3 and 1 .4,

- the screw comprises a single thread,

- the thread exhibits a pitch P between 0.9 x D1 and 1 .1 x D1 , and preferably between 0.95 x D1 and 1 .05 x D1 ,

- the thread has the same profile for substantially its entire length and is devoid of lead teeth at its extremity situated on the side of said free extremity,

- the screw comprises at least one longitudinal flute which extends through said at least one thread and into said core, from said free extremity,

- said at least one flute has a length of more than 2 x D1 ,

- said at least one flute has the general form of a dihedron in cross section, in a plane perpendicular to said axis,

- said dihedron has an angle g between 60 and 120°, preferably between 80 and 100°, and more preferably in the order of 90°,

- said dihedron comprises a first wall extending substantially in a median longitudinal plane of said screw,

- said dihedron comprises a second wall, of which a plane first portion is substantially parallel to said axis and extends from said free extremity, and of which a second portion curved inwards in a concave manner extends between the first portion and the external periphery of the threaded rod,

- said second portion has a radius F between 3 x D1 and 6 x D1 , and preferably between 4 x D1 and 5 x D1 , and

- the screw comprises a single flute.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be more readily appreciated, and other details, characterizing features and advantages of the present invention will emerge more clearly, from a perusal of the following description, which is given by way of non- exhaustive example, and with reference to the accompanying drawings, in which:

- figures 1 to 3 represent a self-tapping screw for concrete according to the prior art, figures 2 and 3 being detailed views on a larger scale, - figure 4 is a schematic view in perspective of a self-tapping screw according to the invention,

- figures 5 and 6 are schematic side views of the screw in figure 4,

- figure 7 is a schematic view from above of the screw in figure 4,

- figure 8 is a schematic view from below of the screw in figure 4,

- figure 9 is a view on a larger scale of the ringed detail in figure 6, and

- figures 10 and 1 1 are detailed views of the thread of the screw in figure 9.

DETAILED DESCRIPTION

Figures 1 to 3 represent a self-tapping screw 10 for concrete according to the prior art.

The screw 10 is designed to be screwed into a pre-drilled hole in concrete. The screw 10 comprises essentially two parts, these being a threaded rod 12 and a head 14 for driving the screw causing it to rotate about its axis A.

The rod 12 includes a core 16 having a diameter D1 and a thread 18 which extends in a helix around the core 16 and has an external diameter D2. The thread has in cross section, in a plane passing through the axis A, a generally triangular form, of which the point forms an angle b (figure 2). The thread 18 comprises a flank 18a situated to the side of the free extremity of the rod 12, which forms an angle a with the aforementioned plane (figure 2). The opposite flank 18b of the thread 18 exhibits a radius of curvature R (figure 3).

In the case illustrated here, which corresponds to the prior art, the relationship D2/D1 is more than or equal to 1.5, the angle a is more than 10°, and the angle b is more than or equal to 60°. The radius R is equal to K x (D2-D1 )/2, where K is in the order of 2.

Figures 4 to 8 represent a self-tapping screw 1 10 for concrete according to the invention.

The screw 1 10 is designed to be screwed into a pre-drilled hole in concrete. The screw 1 10 comprises a threaded rod 1 12, of which one extremity is free and an opposite extremity is connected to a head 1 14 for driving the screw causing it to rotate about its longitudinal A axis by means of an appropriate tool. The head in this case has a generally polygonal form, for example of type SW10, although this is in no way restrictive. The rod 1 12 includes a core 1 16 having a diameter D1 and at least one thread 118 which extends in a helix around the core 1 16 and has an external diameter D2. The thread has a generally triangular or trapezoidal form in cross section, in a plane passing through the axis A (figure 9).

This triangular or trapezoidal form includes:

- a radially external free point 120 with respect to the axis A and forming an angle b,

- a radially internal base 122 situated in the area of the core 1 16,

- a first side formed by a lateral flank 124 of the thread situated to one side of the free extremity, which is substantially flat and which forms an angle a with a plane perpendicular to the axis A, and

- a second side formed by a lateral flank 126 of the thread situated to one side of the head, which is curved inwards in a concave manner with a radius R.

According to the invention,

- the relationship D2/D1 is less than 1.5; and it preferably lies between 1 .3 and 1.4; - the angle a lies between 0 and 10°; it preferably lies between 4 and 8°, and more preferably it is 6°; and

- the radius R is equal to K x (D2-D1 )/2, where K lies between 1 and 1.5.

Furthermore, the screw 1 10 according to the invention comprises one or a plurality of the following optimized parameters:

- the angle b is measured between the lateral flanks, at the top of the point (that is to say at the radially external free extremity of the point), and is less than 60°; it is preferably more than 45°,

- the thread exhibits a pitch P between 0.9 x D1 and 1 .1 x D1 , and preferably between 0.95 x D1 and 1 .05 x D1 ,

- the point 120 exhibits a thickness e between 0.1 and 0.3 mm, and for example in the order of 0.2 mm.

The screw 1 10 preferably comprises a single thread 1 18, which simplifies its design. The thread may have the same profile for substantially its entire length and is devoid of lead teeth at its extremity situated on the side of the free extremity. The screw in this case is relatively simple to produce, for example by stamping and rolling, these being methods that are well known to a person skilled in the art.

The screw 1 10 thus comprises an asymmetrical thread 1 18. A symmetrical thread with an angle at the tip b necessarily causes stresses that are more axial than those caused by the asymmetrical thread with the same angle b. It is possible to describe the angle (b - a), which orients the stresses in a more or less radial manner.

The radius R of the asymmetrical thread in relation to the equivalent straight thread with the same angle b reduces the interference with the concrete, which is represented schematically by the hatched area in figure 10. The lines T1 , T2 in figure 11 show the diameters of drills that are respectively worn, that is to say having a reduced diameter, and new, that is to say having a maximum diameter. As already mentioned in the foregoing, the dissymmetry of the thread, with respect to the equivalent symmetrical straight thread, reduces the interference with the concrete; this difference is even more important in the case of a hole drilled with a worn drill of which the external diameter would be less than D2.

The screw may comprise inter alia a cylindrical collar 127 for connecting the threaded rod 1 12 to the head 1 14. The diameter D’ of this collar is preferably smaller than the diameter of the hole into which the screw 1 10 must be screwed. The length of the collar depends in particular on the total length of the screw. D’ is more than D1 and may be less than D2. This collar corresponds in general to the diameter of the cylindrical rod of the blank obtained after stamping and before rolling.

The free extremity of the threaded rod 1 12 may be configured as a frustum of a cone and may comprise an end face perpendicular to the axis A, of which the diameter lies between 0.75 x D1 and 0.9 x D1. This frustum of a cone has a length between 0.2 x D1 and 0.3 x D1.

The screw 1 10 may in addition comprise at least one longitudinal flute 128, preferably a single flute, which extends through the thread 1 18 and into the core 1 16 from the free extremity of the threaded rod. This flute 128 is particularly useful in situations where the concrete into which the screw is to be screwed is reinforced and contains reinforcing bars. The flute 128 would then make it easier to address these bars. The flute 128 is produced by machining, for example, after the rolling operation.

This flute may comprise one or a plurality of the following characterizing features and/or optimized parameters:

- the flute 128 has a length of more than 2 x D1 ,

- the flute 128 has the general form of a dihedron 130 in cross section, in a plane perpendicular to the axis A (figure 8), - the dihedron 130 has an angle g between 60 and 120°, preferably between 80 and 100°, and more preferably in the order of 90°,

- the dihedron 130 comprises a first wall 130a extending substantially in a median longitudinal plane of said screw,

- the dihedron 130 comprises a second wall 130b, of which a first portion 130ba plane is substantially parallel to the axis A and extends from the free extremity of the threaded rod, and a second portion 130bb is curved inwards in a concave manner and extends between the first portion and the external periphery of the threaded rod,

- the first portion has a length L1 between 2 x D1 and 2.5 x D1 ,

- the second portion has a radius F between 3 x D1 and 6 x D1 , and preferably between

4 x D1 and 5 x D1 ,

- the flute 128 is situated at a radial distance L2 from the axis 1 , which is between 0.3 x D1 and 0.5 x D1.

In a manner known per se, the drilling diameter of the hole in the concrete depends on the diameter of the screw. In the particular case in which the screw 1 10 is of "diameter 6", the diameters D1 and D2 could be in the order of 5.7 mm and 7.74 mm respectively. The pitch P could be 5.6 mm. The rod 1 12 would then have a length between 35 and 55 mm.

The screw 1 10 is made of tempered steel, for example.