Pneumatic Tire, Having Working Layers Comprising Monofilaments And A Tire Tread With Incisions

Abstract

Technique to increase the endurance of tires comprising two crossed working layers (41, 42), comprising mutually parallel reinforcing elements that form, with the circumferential direction (XX) of the tire, an angle which is at least equal to 20 and at most equal to 50. The reinforcing elements are made up of individual metal threads or monofilaments having a cross section which is at least equal to 0.20 mm and at most equal to 0.5 mm. The tire also comprises axially exterior sipes (24) having a mean width W at most equal to 1 mm, of a depth D at least equal to 5 mm and spaced apart, in the circumferential direction (XX), by a circumferential spacing P at least equal to 4 mm, and axially exterior grooves (25) having a mean width W at least equal to 1 mm, of a depth D at most equal to 5 mm.

Claims

1. A tire for a passenger vehicle, comprising: a tread adapted to come into contact with the ground via a tread surface and having an axial width LT, the tread comprising two axially exterior portions each having an axial width at most equal to 0.3 times the axial width LT, at least one axially exterior portion comprising axially exterior cuts, an axially exterior cut forming a space opening onto the tread surface and being delimited by at least two main faces referred to as lateral faces connected by a bottom face, the axially exterior cuts having a mean width W, defined by the distance between the two main lateral faces, a depth D defined by the maximum radial distance between the tread surface and the bottom face, the tire further comprising a crown reinforcement radially on the inside of the tread, the crown reinforcement comprising a working reinforcement and a hoop reinforcement, the working reinforcement being comprised of two working layers each comprising reinforcing elements which are coated in an elastomeric material, mutually parallel and respectively form, with a circumferential direction of the tire, an oriented angle at least equal to 20 and at most equal to 50, in terms of absolute value, and of opposite sign from one layer to the next, said reinforcing elements in each ply being comprised of individual metal threads or monofilaments having a cross section the smallest dimension of which is at least equal to 0.20 mm and at most equal to 0.5 mm, and a breaking strength Rm, the density of reinforcing elements in each working layer being at least equal to 100 threads per dm and at most equal to 200 threads per dm, the hoop reinforcement comprising at least one hooping layer comprising reinforcing elements which are mutually parallel and form, with the circumferential direction of the tire, an angle B at most equal to 10, in terms of absolute value, wherein when the axially exterior cuts have a mean width W at most equal to 1 mm, these axially exterior cuts, referred to as sipes, have a depth D at least equal to 5 mm and are spaced apart, in the circumferential direction of the tire, by a circumferential spacing P at least equal to 4 mm, and wherein, when the axially exterior cuts have a mean width W at least equal to 1 mm, these axially exterior cuts, referred to as grooves, have a depth D at most equal to 5 mm, and wherein the breaking strength R.sub.c of each working layer is at least equal to 30 000 N/dm, Rc being defined by: Rc=Rm*S*d, where Rm is the tensile breaking strength of the monofilaments in MPa, S is the cross-sectional area of the monofilaments in mm.sup.2 and d is the density of monofilaments in the working layer considered, in number of monofilaments per dm.

2. The tire according to claim 1, wherein at least one said axially exterior cut opens axially to the outside of an axially exterior portion of the tread.

3. The tire according to claim 1, the tread comprising at least one circumferential groove, wherein at least one said axially exterior cut opens axially into a circumferential groove of the tread.

4. The tire according to claim 1, wherein the axially exterior sipes are spaced apart, in the circumferential direction of the tire, by a circumferential spacing P at most equal to 20 mm.

5. The tire according to claim 1, wherein at least one axially exterior sipe is bridged.

6. The tire according to claim 1, wherein the depth D of the axially exterior sipes is at most equal to 8 mm.

7. The tire according to claim 1, wherein the radial distance D1 between the bottom face of the axially exterior sipes and the crown reinforcement is at least equal to 1.5 mm.

8. The tire according to claim 1, wherein the radial distance D1 between the bottom face of the axially exterior sipes and the crown reinforcement is at most equal to 3.5 mm.

9. The tire according to claim 1, wherein all of the axially exterior cuts are sipes.

10. The tire according to claim 1, wherein the two axially exterior portions of the tread each have an axial width at most equal to 0.2 times the axial width LT.

11. The tire according to claim 1, wherein each working layer comprises reinforcing elements made up of individual metal threads or monofilaments having a diameter at least equal to 0.3 mm and at most equal to 0.37 mm.

12. The tire according to claim 1, wherein each working layer comprises reinforcing elements which form, with the circumferential direction of the tire, an angle at least equal to 22 and at most equal to 35.

13. The tire according to claim 1, wherein the density of reinforcing elements in each working layer is at least equal to 120 threads per dm and at most equal to 180 threads per dm.

14. The tire according to claim 1, wherein the reinforcing elements of the working layers are made of steel.

15. The tire according to claim 1, wherein the reinforcing elements of the at least one hooping layer are made of textile, aromatic polyamide or combination of aliphatic polyamide and of aromatic polyamide, polyethylene terephthalate or rayon type.

16. The tire according to claim 1, wherein the hoop reinforcement is radially on the outside of the working reinforcement.

17. The tire according to claim 1, wherein the steel is carbon steel.

18. The tire according to claim 15, wherein the textile is of aliphatic polymide.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0066] The features and other advantages of the invention will be understood better with the aid of FIGS. 1 to 4, the said figures being drawn not to scale but in a simplified manner so as to make it easier to understand the invention:

[0067] FIG. 1 is a perspective view depicting part of the tire according to the invention, particularly its architecture and its tread.

[0068] FIG. 2 depicts the meridian section through the crown and illustrates the axially exterior parts 22 and 23 of the tread, and the width thereof.

[0069] FIGS. 3A and 3B depict two types of radially exterior meridian profile of the tread of a passenger vehicle tire.

[0070] FIG. 4 illustrates the terms interior edge and exterior edge of a tread.

DETAILED DESCRIPTION OF THE DRAWINGS

[0071] FIG. 1 depicts a part of the crown of a tire. The tire comprises a tread 2 which is intended to come into contact with the ground via a tread surface 21. In the axially exterior portions 22 and 23 of the tread there are axially exterior cuts including sipes 24 and grooves 25. The tire further comprises a crown reinforcement 3 comprising a working reinforcement 4 and a hoop reinforcement 5, the working reinforcement comprising two working layers 41 and 42. FIG. 1 further depicts simple and complex blind cuts, double-blind cuts, and cuts opening axially to the outside or to the inside, namely cuts having parallel lateral faces or having lateral faces with zigzag or sinusoidal portions in the main direction of the cut or in the depth thereof, so as to block certain relative movements of the 2 lateral faces.

[0072] FIG. 1 depicts in the axially exterior parts 22 and 23 of the tread, only axial axially exterior grooves, running along the axial axis (YY). In reality, this depiction is pure convenience for the sake of the readability of FIG. 1, it being possible, depending on the performance aims, particularly in terms of wet grip, for the axially exterior grooves in the treads of passenger vehicles to make with the axial direction (YY) an angle of between plus and minus 60.

[0073] FIG. 2 is a schematic meridian section through the crown of the tire according to the invention. It illustrates in particular the widths LS1 and LS2 of the axially exterior portions 23 and 24 of the tread, and the total width of the tread of the tire LT. The depth D of an axially exterior cut 24, and the distance D1 between the bottom face 243 of an axially exterior cut 24 and the crown reinforcement 3, measured along a meridian section of the tire, are also depicted. A meridian section of the tire is obtained by cutting the tire on two meridian planes. By way of example, a meridian section of tire has a thickness in the circumferential direction of around 60 mm at the tread. The measurement is taken with the distance between the two beads being kept identical to that of the tire mounted on its rim and lightly inflated.

[0074] In FIGS. 3A and 3B, the axial edges 7 of the tread, that make it possible to measure the tread width, are determined. In FIG. 3A, in which the tread surface 21 is secant with the exterior axial surface of the tire 8, the axial edge 7 is determined by a person skilled in the art in a trivial way. In FIG. 3B, in which the tread surface 21 is continuous with the exterior axial surface of the tire 8, the tangent to the tread surface at any point on the said tread surface in the region of transition towards the sidewall is plotted on a meridian section of the tire. The first axial edge 7 is the point for which the angle between the said tangent and an axial direction YY is equal to 30. When there are several points for which the angle between the said tangent and an axial direction is equal to 30, it is the radially outermost point that is adopted. The same approach is used to determine the second axial edge of the tread.

[0075] FIG. 4 schematically depicts tires mounted on mounting rims of wheels of a vehicle 200 and having a predetermined direction of mounting on the vehicle. Each tire comprises an exterior axial edge 45 and an interior axial edge 46, the interior axial edge 46 being the edge mounted on the bodyshell side of the vehicle when the tire is mounted on the vehicle in the said predetermined direction of mounting, and the exterior axial edge 45 being the opposite of that. In the document, outboard side of the vehicle denotes the exterior axial edge 45.

[0076] The inventors have performed calculations on the basis of the invention for a tire of size 205/55 R16, inflated to a pressure of 2 bar, comprising two working layers comprising steel monofilaments of diameter 0.3 mm, distributed at a density of 158 threads to the dm and forming, with the circumferential direction XX, angles respectively equal to 27 and 27. The monofilaments have a breaking strength R.sub.m equal to 3500 MPa and the working layers each have a breaking strength R.sub.c equal to 39 000 N/dm. The tire comprises axially exterior cuts on the two axially exterior portions of the tread of the tire having an axial width equal to 0.21 times the axial width of the tread. The radial distance D1 between the bottom face of the axially exterior cuts and the crown reinforcement is at least equal to 2 mm.

[0077] Various tires were calculated, varying the thickness and circumferential spacing of the cuts. Tire A, according to the prior art, comprises grooves of rectangular cross section of constant width in the thickness equal to 3.5 mm and depth equal to 6.5 mm. The circumferential spacing of the grooves is 27 mm for 72 grooves on the circumference of the tire. Tire B, according to the invention, comprises sipes of rectangular cross section of constant width in the thickness equal to 1 mm. The circumferential spacing of the sipes is 7.8 mm for 252 sipes on the circumference of the tire. The spacings of the axially exterior grooves and sipes are such that the volume void ratios of the treads of tires A and B are identical. The conditions used for the calculation reproduce the running conditions of a front tire on the outside of the bend, namely the tire that is most heavily loaded in passenger vehicle. These loadings, for a lateral acceleration of 0.7 g, are as follows: a load (Fz) of 749 daN, a lateral load (Fy) of 509 daN and a camber angle of 3.12. The thickness and the spacing of the cuts, in this instance of the sipes, and their circumferential spacing in the tire B makes it possible, for the same volume void ratio, to reduce the bending stresses in the monofilaments of the working reinforcement by 74% by comparison with tire A, which bending stresses are what causes them to rupture through fatigue loading.