Groove pattern for friction plates

Abstract

The present invention relates to a groove pattern for a friction plate, characterized in that the groove pattern is formed by friction lining pads, each friction lining pad having a friction surface, the friction lining pads being separated from each other by segmentation grooves, and each friction lining pad additionally having embossed grooves on its friction surface.

Description

[Technical Field]

[0001] The present invention relates to a groove pattern for a friction plate having the features of the preamble of claim 1 . [Background technology]

[0002] Grooves or groove patterns (also referred to in the context of this specification as groove geometry, groove design, or pad geometry or design) are used to cool the plates through oil flow, even when the shifting elements are closed. They break through the oil film, thereby stabilizing the coefficient of friction. This creates the desired friction behavior when shifting. Idling behavior is improved and drag torque is reduced.

[0003] Field of use of the invention Wet-actuated multi-plate clutches and brakes are widely used in conventional power-shiftable transmissions, in new hybrid modules in heavy-duty drivetrains, or in shiftable e-axles, where they represent high-performance, heavy-duty components. The demands for reduced CO2 emissions and improved efficiency of drivetrains in automotive applications are of paramount importance. In addition to reducing load-independent losses in the shifting elements, thermal loads and suitable cooling must be considered. The groove pattern of the friction disc plays a central role in the trade-off between friction properties, thermal balance, and efficiency.

[0004] DE 102021107843 A1 discloses a groove pattern for a friction plate, which groove pattern is formed by friction lining pads, which have a trapezoidal shape and each friction lining pad has waffle-shaped grooves.

[0005] WO 2016 / 180540 discloses an annular wet-operating friction lining having a first set of grooves that connect the inner and outer peripheries and are made up of non-intersecting straight grooves.

[0006] U.S. Patent Application Publication No. 2009 / 0211867 discloses an annular wet-operating friction lining having a first set of grooves and a second set of embossed grooves forming trapezoidal friction pads connecting the inner and outer peripheries and forming the annular friction lining.

[0007] U.S. Pat. No. 3,972,400 discloses an annular wet-operating friction lining having a first set of radial and linear operating grooves and a second set of non-radial and linear operating grooves connecting the inner and outer peripheries. Summary of the Invention [Problem to be solved by the invention]

[0008] The invention is based on the objective of improving the cooling of the friction plates by means of an appropriate groove pattern and optimizing the surface pressure in the frictional contact. [Means for solving the problem]

[0009] This object is achieved by a groove pattern having the features of claim 1.

[0010] Thus, there is provided a groove pattern for a friction plate according to the invention, the groove pattern being formed by friction lining pads, each friction lining pad having a friction surface and the friction lining pads being separated from one another by segmentation grooves, each friction pad additionally having embossed grooves on its friction surface.

[0011] Thus, the groove pattern for the friction plate according to the present invention is formed by friction lining pads, which are attached to a carrier plate and separated from one another by segmentation grooves. The segmentation grooves are therefore limited in depth by the carrier plate, and in particular do not include friction pad material or other friction lining materials. The carrier plate, together with the friction pad, is referred to as the friction plate. In addition to the segmentation grooves, the friction surface of the friction lining, i.e., the friction surface of the friction lining pad itself, also has grooves whose depth does not reach the bottom of the pad and therefore does not reach the metallic carrier element, i.e., the carrier plate of the pad. In this specification, grooves on the friction side of the pad or grooves in the pad are referred to as embossed grooves. The term "embossed grooves" is not understood to mean that embossed grooves are formed only by embossing or over-embossing, but within the scope of this specification, the term "embossed grooves" is also intended to include other manufacturing processes for embossed grooves, such as milling and / or grinding.

[0012] In a preferred embodiment of the present invention, it is provided that the embossed grooves on the friction surfaces of the friction lining pads are formed by over-embossing or by milling and / or grinding, and / or that the segmentation grooves extend radially between the friction lining pads.

[0013] In a further preferred embodiment of the present invention, it is provided that only, i.e. exclusively, the embossed grooves have a kite-rectangular shape, in the sense of being surrounded or separated by a boundary, and that only, i.e. exclusively, the embossed grooves have an area that is surrounded and / or the kite-rectangular edge length increases from the radially inner side to the radially outer side.

[0014] In a further preferred embodiment of the invention, it is provided that the course of embossed grooves across all friction lining pads has a common structure of a kite-shaped rectangle and is interrupted only by segmentation grooves.

[0015] In a preferred embodiment of the present invention, it is provided that each embossed groove is inclined in one of two circumferential directions with respect to its radial course, so that each embossed groove belongs to one of two groove groups.

[0016] The first groove group therefore includes embossed grooves that are inclined in a predetermined first circumferential direction, and the second groove group therefore includes embossed grooves that are inclined in the other of the two circumferential directions.

[0017] In the following, the inclination is not given relative to the radial direction but relative to the tangent to the inner circumference which is perpendicular to the radial direction, and the point of contact between the course of the centerline of the embossed groove and the course of the inner circumference is taken as the starting point of the radial direction and the starting point of the tangent to the inner circumference.

[0018] In a further preferred embodiment of the present invention, it is provided that the inclination of the embossed groove is determined by the angle γ between the centerline of the embossed groove and the inner circumferential tangent at the tangent point between the course of the embossed groove centerline and the course of the inner circumferential.

[0019] The angle γ is always calculated as a mathematical absolute value. Therefore, there is no sign or semantic assignment for the sign of the angle γ. Of the four angles resulting at each intersection of two lines, two are the same, and the smaller of the two values ​​is selected as the angle γ. The two lines here are the center line and inner tangent of the embossed groove. From the angle γ alone, it is not possible to determine which groove group the corresponding embossed groove belongs to.

[0020] In a further preferred embodiment of the present invention, all intersections of the centerlines are located at the inner radius (R i ) and outer radius (R a ) and the radius R x on two embossed grooves (X-intersections) having a 90 degree intersection angle (90 degree X-intersections) on a circle (circle with 90 degree X-intersections) having

number

[0021] In a further preferred embodiment of the present invention, the radius R x It is specified that all intersections of the centerlines of two embossed grooves (X-intersections) that are not on a circle (circle with 90 degree X-intersections) have an intersection angle other than 90 degrees.

[0022] In a further preferred embodiment of the present invention, the two embossed grooves are formed such that the two centerlines of the two embossed grooves have a radius R x It is defined that if the intersection points lie on a circle with γ, then they will have the same value for the respective angle γ.

[0023] In this case, γ(NG1)=γ'(NG2), so both have the same tilt angle γ with respect to their respective circumferential tangents (see Figure 12).

[0024] In a further preferred embodiment of the present invention, all of the centerlines of all of the embossed grooves in groove group 1 have a common radius R k1 It is defined that the two lines form a tangent to a common circle with

[0025] In a further preferred embodiment of the present invention, all centerlines of all embossed grooves of groove group 2 have a common radius R k2 It is defined that the two lines form tangents to a common circle with

[0026] In a further preferred embodiment of the present invention, all centerlines of all embossed grooves have a common radius R k It is defined that the two lines form a tangent to a common circle with R. k1 =R k2 =R k The centerlines of the embossed grooves in groove group 1 and the centerlines of the embossed grooves in groove group 2 have a common radius R k tangent to a common circle with

[0027] This radius R k is the tangent radius R k and the corresponding diameter is designated by "a" (see Figures 9 and 10).

[0028] In a further preferred embodiment of the present invention, it is provided that the angle γ is selected to be in the range of 0 to 45°, preferably 40 to 44°.

[0029] In this way, both cooling and surface pressure are advantageously improved.

[0030] Further advantages and advantageous configurations of the invention are the subject of the following drawings and their description.

[0031] The details of the drawings are as follows: [Brief explanation of the drawings]

[0032] [Figure 1] Demonstrates the lubrication concept of a wet multi-plate clutch / brake with internal lubrication [Figure 2] A schematic diagram of a wet multi-plate clutch with internal lubrication is shown. [Figure 3] 1 shows a schematic diagram of a groove design for a friction lining according to the present invention; [Figure 4] 1 shows a schematic diagram of a groove design for a friction lining according to the present invention; [Figure 5] 1 shows dimensions of groove design according to the present invention [Figure 6]1 shows dimensions of groove design according to the present invention [Figure 7] 1 shows dimensions of groove design according to the present invention [Figure 8] 1 illustrates oil flow in groove designs according to the present invention; [Figure 9] 1 illustrates one embodiment of a groove design according to the present invention on a narrower friction plate with an embossed groove centerline. [Figure 10] 1 illustrates one embodiment of a groove design according to the present invention on a wider friction plate with an embossed groove centerline. [Figure 11] Demonstrates the steps to construct an original groove design for embossed grooves [Figure 12] 1 illustrates a procedure for constructing a groove design for an embossed groove according to the present invention. [Figure 13] 1 shows the dimensions of one embodiment of a groove design according to the present invention on a narrower friction plate. [Figure 14] 1 shows the dimensions of one embodiment of a groove design according to the present invention on a wider friction plate. [Figure 15] 10 illustrates an alternative groove design according to the present invention having group parallel grooving of segmentation grooves. [Figure 16] 1 illustrates the dimensions of the group cross section of the embossed groove design according to the present invention. [Figure 17] 1 shows a comparison of the groove design according to the present invention with conventional waffle grooving. [Figure 18] A simplified thermal balance for convective cooling of friction contacts is presented. DETAILED DESCRIPTION OF THE INVENTION

[0033] Relevant parameters for convective heat transfer: Volume flow (P oil、out ) The thermal mass of oil in the grooves, the oil volume in the grooves of the friction disc (V oil ) Heat transfer coefficient (α) Heat transfer surface (A)

[0034] The groove design according to the present invention is intended to optimize the following points:

[0035] Friction characteristics (closed): Improvement of the increase in friction coefficient by reducing the lubricating wedge effect and the associated fluid pressure at the leading edge of the embossed groove (solid contact surface relief) through an embossed groove angle γ, particularly preferably selected between 40° and 44°. The angled embossed groove reduces the radial component of the groove and therefore the resulting hydrodynamic pressure force component, reducing and improving the structure of the contact pressure in the frictional contact (avoiding hydroplaning).

[0036] Cooling (closed): Using a simplified heat balance for convective cooling of a friction contact (see Figure 18), it can be inferred that in addition to the required cooling oil volume flow, there must also be a groove volume or oil volume sufficient for the friction contact. Heat transfer to the oil thermal mass is determined by the temperature difference, the heat transfer coefficient, and the heat transfer area. The degree to which the groove is filled with oil plays an important role. Additional air in the groove may reduce cooling performance.

[0037] When configuring a groove pattern, in addition to the segmentation of the friction lining, the additional grooving of the lining segments (friction lining pads) plays an important role in the convective heat transfer between the cooling oil and the anti-friction disc. If the additional grooving of the lining segments is configured as cross grooving, the cooling oil is guided rotationally symmetrically with respect to the surface of the anti-friction disc. This improves the distribution of the cooling oil around the friction surface and results in an optimized contact surface for convective heat transfer between the cooling oil and the steel plate.

[0038] Furthermore, the total cross-sectional area of ​​the grooves remains constant radially from the inner friction diameter to the outer friction diameter, resulting in a consistent groove filling of the grooves on the cover segments and reduced air entrapment in the grooves, improving convective heat transfer from the anti-friction disc to the cooling oil and enhancing cooling performance.

[0039] Surface pressure: Optimal cooling of the intersecting embossed grooves (X-grooves) allows the groove area to be reduced or the net friction surface area to be increased, thus reducing the nominal surface pressure of the frictional contact, which also has a beneficial effect on the frictional properties.

[0040] General functional description of the groove pattern: Cooling of the plate by oil flow when the switching element is closed - Breaks through the oil film, thereby stabilizing the coefficient of friction. Creating the desired friction behavior during switching -Improved idling behavior, reduced drag torque

[0041] In Figure 1, the lubrication concept of a wet multi-plate clutch / brake with internal lubrication is shown. The lubrication concept can be implemented in different ways for wet operation of multi-plate clutches and disc brakes depending on the application. Generally, the cooling oil for the friction system is supplied from inside the O1 either actively (e.g. double clutch, pressure lubrication) or passively (shift elements in step automatic transmissions, passive oil distribution within the transmission). Depending on the design of the transmission, the friction system may additionally operate in an oil bath 03. In the special case of disc brakes (step automatic transmissions, hybrid transmissions or e-axles), active external lubrication 02 can be useful.

[0042] Figure 2 shows an application of a wet multi-plate clutch with internal lubrication. Automatic transmission (DCT, AT) DHT (dedicated hybrid transmission) Multi-stage e-axle

[0043] As shown in Figures 3-8: 3 and 4 show schematic diagrams of groove patterns according to the present invention for friction plates, which are formed by friction lining pads 10 fixed to a carrier plate 11 and separated from each other by segmentation grooves. The depth of the segmentation grooves is therefore limited by the carrier plate. The carrier plate, together with the friction pads, is referred to as a friction plate. In addition to the segmentation grooves, the friction lining pads also have embossed grooves 15, 16, which ensure that the groove depth of the embossed grooves does not reach the bottom of the pad and therefore the carrier element, i.e., the carrier plate 11 of the pad. In this specification, the term "embossed grooves" should not be understood to mean that the embossed grooves are formed only by embossing or over-embossing, but within the scope of this specification, other manufacturing processes for embossed grooves, such as milling and / or grinding, are also included.

[0044] A designated number of embossed grooves 15 form a groove set, hereinafter referred to as groove group 1 NG1, and a designated number of embossed grooves 16 form another groove set, hereinafter referred to as groove group 2 NG2. The number of embossed grooves in both groove groups is the same, i.e., the designated number of embossed grooves 15 is the same as the designated number of embossed grooves 16.

[0045] The embossed grooves 15 are inclined relative to the radial direction (for example, between the origin and EN1, between the origin and EN2, as shown in Figure 12), and the radial course is inclined in the circumferential direction (Figure 12: NG1). The embossed grooves 16 are inclined in another circumferential direction relative to the radial direction (Figure 12: NG2). In the schematic illustration, the segmentation grooves separating the friction pads from each other also show such a radial course, connecting the internal contacts to the outer periphery of the friction lining. However, the invention is not limited to the strict radial course of the segmentation grooves. The embossed grooves 15 and 16 also connect the inner and outer periphery, but may be connected to segmentation grooves. The embossed grooves 15 from groove group NG1 do not intersect each other, since they all have the same inclination. The inclination (see Figure 12) is determined by the relationship between the course of the centerline of embossed groove NG1 and the inner periphery (R i ) and the inner tangent IUT1 at the tangent point EN1 between the grooves 15 and 16. For the same reason, all embossed grooves 16 from groove group 2 do not intersect with each other. However, an embossed groove 15 from groove group 1 may intersect with an embossed groove 16 from groove group 2, for example, at intersection point 17, as shown. In this specification, such intersection point 17 is descriptively referred to as an "X intersection point" (FIG. 5).

[0046] The area of ​​the surface of the friction pad, regardless of groove group, that is surrounded on all sides by the embossed grooves, i.e., the area not bounded by segmentation grooves or the outer or inner edges of the pad, forms the geometric shape of a kite rectangle (the two edges of the kite rectangle that adjoin each other at the corner points are of equal length).

[0047] These kite rectangles increase in size from radially inward to radially outward: the edge length of kite rectangle 4 is greater than the corresponding edge length of kite rectangle 3, which is itself greater than the corresponding edge length of kite rectangle 2, which is itself greater than the corresponding edge length of kite rectangle 1 (Figure 6). The same applies to the area of ​​the kite rectangles. Figure 7: The X-intersection of the 90 degree cutting angle 21 (herein referred to as the "90 degree X-intersection") of the embossed groove is located at the inner radius R of the friction lining. i and outer diameter R a and is therefore located on a circle with a radius of 20 on the friction lining in the friction surface. In the context of this specification, the circle with a radius of 20 is called the 90 degree X-intersection circle. · X-intersections that are not on the circle of a 90 degree X-intersection have an intersection angle different from 90 degrees. The width 6 of the embossed grooves is constant along the groove and is the same for all embossed grooves: 0.8 to 1.4 mm, preferably 1.2 mm (Fig. 7). Depending on the pad size and pad shape, the number of X crosses per pad on the friction lining is at least 1 · Oil flow 25 usually occurs from the inside to the outside (Figure 8). The groove area decreases from the inner circumference to the outer circumference. The embossed groove 26 always connects the inner periphery with the outer periphery or leads to a segmentation groove The leading edges of the embossed grooves 26 for the fluid are not aligned perpendicularly (vertically, laterally) to the sliding direction of the friction lining or friction plate, i.e. the embossed grooves 26 do not extend in the radial direction. The teeth and tooth gaps of the internal or external toothing (not shown) of the carrier plate can be adjusted, for example, with regard to the number, width and position of the friction pads or the arrangement of segmentation grooves, but alternatively or additionally with regard to the number, width and position of the embossed grooves, in particular with regard to the inlet openings on the tooth side, as well as their inclination. Adjustment of the position of the oil supply openings (not shown) can also be taken into account for further optimization.

[0048] In Figures 9 and 10, the condition of the embossed groove course is that all the centerlines of the embossed grooves have a common diameter (a) or a common tangent radius R k It has been shown that the tangents to the common circle of are simultaneously expressed as

[0049] Based on Figures 11 and 12, it is perfectly shown how the inventive groove design of the embossed groove is produced: Legend for Figures 11 and 12: [Table 1]

[0050] The groove design of the embossed groove is completely composed of two groove groups: 1. Inner circumference (R i ) evenly distributed over 360°, and the inlet openings EN1 of the embossed grooves of the first group of grooves are i For example, 60 or 64 embossed groove inlet openings EN1 are distributed over the inner circumference at equal distances from one another.

[0051] 2. An angle γ is established (the angle between the embossed grooves NG1 of the first groove group and the inner circumferential tangent IUT1 at the entrance opening EN1).

[0052] 3. Radius R of the circle at the 90 degree X intersection x (R i< <R x< <R a )teeth,

number

[0053] 4. For example, vector

number

number

[0054] When the procedure of this description has been performed for all intended entrance openings EN1 of the embossed grooves of the first groove group, an embossed groove of the second groove group is generated for each embossed groove of the first groove group.

[0055] For an embossed groove NG1 from the first groove group and an embossed groove NG2 determined in this way for this embossed groove NG1 from the second groove group, the following applies (FIG. 12): γ(NG1)=γ´(NG2) Additional explanation for the above γ ranges: When γ=0°, R k =R i and When γ=45°, R x =R i is When choosing the angle γ, the following boundary conditions must be taken into account: ·R k <R i Therefore, γ>0°, ·R x >R i Therefore, γ<45°, Therefore, we obtain 0°<γ<45°.

[0056] In addition, the range of values ​​of γ is further restricted by the following boundary conditions: (R i +2mm) < R x< (R a -2mm)

[0057] This boundary condition arises when production-related reasons for tolerances need to be considered: R x must always be located within the friction surface (friction ring formed by the friction pads). This ensures that the area immediately adjacent to the inner periphery of the friction ring or the outer periphery of the friction ring is within the 90-degree X-intersection (R x ) and the embossed grooves of the first groove group NG1 (see point 4 of the procedure above).

[0058] This further limits γ, so that under these boundary conditions, the angle γ is preferably in the range 40°<γ<44°.

[0059] In Figures 13 and 14 the dimensions of two preferred embodiments of groove patterns according to the invention for friction plates are shown: Dimensions of an embodiment of a groove design according to the invention with a narrower friction ring made of a friction lining pad on a narrower friction plate according to FIG. 13: [Table 2]

[0060] Dimensions of an embodiment of a groove design according to the invention with a wider friction ring made of friction lining pads on a wider friction plate according to FIG. 14 (reference symbols similar to FIG. 13): [Table 3]

[0061] FIG. 15 shows an alternative form of friction pad geometry: As an alternative to the previously shown friction pad geometry resulting from radially extending segmentation grooves, a group parallel arrangement of segmentation grooves is shown, which also represents an advantageous friction pad geometry.

[0062] Instead of the friction pad geometry resulting from radially extending segmentation grooves, the possible friction pad geometries can also be formed from the following friction pad shapes, which are separated from each other by corresponding segmentation grooves, such as trapezoids, squares, rectangles, triangles, circles, etc.

[0063] FIG. 16 shows an alternative form of cross section of an embossed groove as follows: U-shaped, V-shaped, rectangular, W-shaped (e.g. with or without embossed edges).

[0064] Possible variations of the manufacturing process for the embossed grooves: Instead of embossing the grooves, they are milled and / or polished.

[0065] Figure 17 shows the following: Compared to conventional waffle grooving (Fig. 17, left), the implementation of the groove design according to the invention (Fig. 17, right) improves the cooling performance by 7.5% under the same boundary conditions, while reducing the nominal surface pressure in the groove area and therefore in the friction contact by 8%, which has a beneficial effect on the friction characteristics. [Explanation of symbols]

[0066] 01 Internal lubrication (active and passive) 02 External lubrication 03 Immersion bath, oil bath 5. Outer Plate Carrier (Outer Plate Carrier / Housing) 6 Friction Plate (Friction Plate) 7 Steel plate (steel plate) 8. Inner Plate Carrier (Inner Plate Carrier) 10 friction pads 11 Carrier Plate NG1 Groove Group 1 NG2 Groove Group 2 15 Embossed groove from NG1 16 Embossed groove from NG2 17 X intersection, embossed groove intersection 1, 2, 3, 4 Kite-shaped rectangle exclusively bounded by embossed grooves 15, 16 20 R x : Radius of the circle at the 90 degree X intersection 21 Cutting angle of embossed groove 22 Width of embossed groove 25 Oil flow direction 26 Embossed grooves R k Tangent radius (tangent circle) (see Figure 9 and Figure 10(a)) R in Inner Friction Radius R a Outer Friction Radius R x Radius of the circle at the 90 degree X-intersection (see Figure 7 (20)) NG1 Embossed grooves in the first groove group NG2 Embossed grooves in the second groove group EN1 NG1 embossed groove inlet opening EN2 NG2 embossed groove inlet opening IUT1 Inner circumference in contact with EN1 IUT2 Inner circumference in contact with EN2 30 Outer friction diameter (2*R a ) 31 Inner friction diameter (2*R i ) 32 Segmentation (number and width of pads including segmentation grooves) 33 Width of embossed groove 34 Segmentation groove width (groove between pads) 35 Diameter of the circle at the 90 degree X intersection (2*R x ) 36 friction pads 37 Carrier Plate 40 volume of oil 41 Anti-friction disc

Claims

1. A groove pattern for a friction plate (6), characterized in that the groove pattern is formed by friction lining pads (10), each friction lining pad (10) having a friction surface and the friction lining pads are separated from each other by segmentation grooves, and each friction lining pad (10) on the friction surface additionally has embossed grooves (15, 16).

2. 2. A groove pattern according to claim 1, characterized in that the embossed grooves (15, 16) on the friction surfaces of the friction lining pads (10) are formed by over-embossing or by milling and / or grinding, and / or the segmentation grooves extend radially between the friction lining pads (10).

3. 3. A groove pattern according to claim 1 or 2, characterized in that the surface exclusively surrounded by the embossed grooves has a kite-shaped quadrangle shape (1, 2, 3, 4) and the edge length of the surface exclusively surrounded by the embossed grooves and / or of the kite-shaped quadrangles (1, 2, 3, 4) increases from the radially inner to the radially outer side.

4. A groove pattern according to any one of claims 1 to 3, characterized in that each embossed groove (15, 16) is inclined in one of two circumferential directions with respect to its radial course, so that each embossed groove belongs to one of two groove groups.

5. The inclination of the embossed grooves (15, 16, NG1, NG2) is such that the center line of the embossed grooves (NG1, NG2) and the course of the center line of the embossed grooves (NG1, NG2) and the inner circumference (R ｉ 5. A groove pattern according to claim 1, characterized in that the angle γ is determined by the angle between the inner tangent (IUT1, IUT2) at the tangent point (EN1, EN2) between the courses of the grooves.

6. All intersections (17) of the centerlines of two embossed grooves (15, 16, NG1, NG2) having an intersection angle of 90 degrees are located at the inner radius (R ｉ ) and outer radius (R ａ ) and the radius R ｘ lies on a circle with [Equation 1] and R ｋ =R ｉ 6. The groove pattern according to claim 5, wherein sin α, α=90°-γ.

7. Radius R ｘ 7. The groove pattern according to claim 6, characterized in that all intersections (17) of the center lines of two embossed grooves (15, 16, NG1, NG2) that are not on a circle have an intersection angle other than 90 degrees.

8. The two embossed grooves (NG1, NG2) are such that the center lines of the two embossed grooves (NG1, NG2) have a radius R ｘ 8. A groove pattern according to claim 6 or 7, characterized in that they have the same value for each of said angles γ when they have their intersection points (17) on the circle with

9. All centerlines of all embossed grooves (NG1, NG2) have a common radius R ｋ A groove pattern according to any one of claims 1 to 8, characterized in that it forms a tangent to a common circle having

10. A groove pattern according to any one of claims 5 to 9, characterized in that the angle γ is selected to be in the range of 0 to 45°, preferably 40 to 44°.

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