Abstract:
The invention relates to a tire with radial carcass reinforcement, intended for equipping a heavy vehicle such as a transport vehicle or construction machinery. It refers in particular to a tire having an axial width greater than 37 inches. The tire according to the invention has a working crown reinforcement, composed of at least two continuous working plies and completed on each side of the circumferential center plane by at least two half-plies whereof the metal reinforcing elements form angles greater than the smallest of the angles of the reinforcing elements of the continuous plies with the circumferential direction, with that half-ply extending axially furthest outwards being in contact with the axially widest continuous working crown ply, and the two half-plies radially covering the axially outer end of the said axially widest working ply.

Description:
This application is a continuation of PCT/EP02/07621 filed on Jul. 9, 2002, which claims priority from patent application Ser. No. 01/09573 filed in France on Jul. 16, 2001, the entire content of which is hereby incorporated by reference. 

   BACKGROUND OF THE INVENTION 
   The invention relates to a tire with radial carcass reinforcement, intended for equipping a heavy vehicle such as a transport vehicle or construction machinery. It refers in particular to a tire having an axial width greater than 37 inches. 
   A tire of this kind, usually intended to bear heavy loads, comprises a radial carcass reinforcement and a crown reinforcement composed of at least two working crown plies which are formed by non-extensible reinforcement elements that cross from one ply to the next, forming equal or unequal angles of between 10° and 45° with the circumferential direction. 
   Crown reinforcements of radial tires, in particular as regards very large tires, undergo pronounced deformations which give rise to longitudinal and transverse shear stresses between the edges of two crossed plies (the longitudinal shear is greater than the transverse shear when the cords of crossed plies are at small angles with the circumferential direction) at the same time as a delaminating stress, a radial stress tending to separate the edges of two plies radially from one another. The said stresses are primarily due to the inflation pressure of the tire, which has the effect that the so-called belt pressure between the carcass reinforcement and the crown reinforcement tends to cause the said crown reinforcement to expand circumferentially. The said stresses are moreover due to the load borne by the tire during travel, once a contact surface is produced between the ground and the tire. The said stresses are, furthermore, due to the slip angle of the tire during travel. These stresses produce cracks in the rubber mix adjoining the end of the shortest ply, and these cracks are propagated through the said mix, having an adverse effect on the service life of a crown reinforcement and hence of the tire. 
   A clear improvement to the service life has been achieved by using in the crown reinforcement at least one protective crown ply having an axial width greater than the width of the axially widest working ply. 
   The term “axial” means a direction parallel to the axis of rotation of the tire, and the term “radial” means a direction intersecting the axis of rotation of the tire and perpendicular thereto. The axis of rotation of the tire is the axis about which it turns in normal use. The circumferential center plane is a plane perpendicular to the axis of rotation of the tire, dividing the tire into two halves. A radial plane is a plane containing the axis of rotation of the tire. 
   Another solution, as described in the patent FR 2 421 742, consists in more favorable distribution of the stresses which give rise to separation between working crown plies and which result from the slip angle of the tire, by multiplying the number of working plies. 
   Multiplying the working plies is not without disadvantages, in particular at the center of the crown reinforcement, where the number of plies has a direct influence on the stiffness in flexure of the crown of the tire. When this stiffness increases, the result is that impacts on the crown of the tire, such as in particular on passing over large stones, may lead to irreparable damage to the tire as a result of this increased stiffness. 
   Patent application WO 00/54992 has moreover proposed, as a way of avoiding this disadvantage, making a working crown reinforcement composed of at least three continuous working plies and at least one half-ply on either side of the circumferential center plane, disposed between the edges of at least two radially adjacent continuous working plies, and whereof the special feature is in particular the fact of forming an angle with the circumferential direction greater than 25° and greater by an amount of between 5° and 15° than the largest of the angles of the continuous working plies. The results obtained with this type of structure have been perfectly satisfactory for the sizes of tire tested. 
   BRIEF SUMMARY OF THE INVENTION 
   In their research, and in particular in researching the manufacture of larger tires, in particular tires whereof the axial width is greater than 50 inches, the inventors aimed to define a crown structure for tires for heavy vehicles which allows a satisfactory service life to be obtained, in particular by improving the circumferential and transverse stiffness values in order to restrict the shear stresses while maintaining flexibility of the crown. 
   This object has been achieved in accordance with the invention by a tire for a heavy vehicle, comprising a radial carcass reinforcement radially surmounted by a working crown reinforcement, composed of at least two continuous working crown plies formed by metal reinforcing elements which cross from one ply to the next, forming angles α, α′ of between 10° and 35° with the circumferential direction, the working crown reinforcement being completed on each side of the circumferential center plane by at least two half-plies whereof the metal reinforcing elements form angles α, α′ greater than the smallest of the angles α, α′ with the circumferential direction, with that half-ply extending axially furthest outwards being in contact with the axially widest continuous working crown ply, and the two half-plies radially covering the axially outer end of the said axially widest working ply. 
   A tire as just defined, according to the invention, that is to say having a crown reinforcement as described, enables the service life of tires for heavy vehicles to be improved. It has in fact been found that the structures proposed reduce the shear stresses while maintaining the flexibility of the tire, particularly at its crown, enabling good impact resistance to be obtained, which in turn has a favorable effect on the service life of the tire. 
   The continuous plies and the working half-plies are preferably composed of non-extensible metal reinforcing elements, such that the function of hooping, or binding, the carcass ply is ensured as effectively as possible. 
   According to an advantageous embodiment of the invention, at least one of the half-plies has a zone covering the end of the narrowest continuous crown ply. According to an embodiment of this kind, it is possible to further improve distribution of the forces in the crown reinforcement as a whole by optimizing the coupling between plies. 
   Also advantageously, the reinforcing elements of one of the half-plies are at an angle at least 10° greater than the smallest of the angles α, α′. A ply of this kind in particular enables a contribution to be made to the transverse stiffness of the tire without having an adverse effect on flexibility. 
   According to a preferred embodiment of the invention, in order to improve the circumferential stiffness of the tire while increasing the transverse stiffness, the reinforcing elements of the half-plies cross one another. 
   In a manner known per se, in particular to improve the resistance of the tire to cuts and perforations, the working crown reinforcement is completed by a protective reinforcement. The latter is advantageously composed of at least two plies of resilient metal reinforcing elements. Variant embodiments of the invention provide protective plies formed by strips partly covering one another. Whatever the type of protective plies used, the resilient reinforcements used may be elements arranged rectilinearly or in a sinusoidal shape. 
   At least one of the said protective plies, preferably the radially inner ply, has an axial width greater than the axially largest width of the continuous working plies. Also advantageously, the said protective ply covers all the continuous working plies and working half-plies. 
   According to an advantageous variant on the invention, the radially outer protective ply has an axially outer end between the axially outer end of the half-ply extending axially least far outwards and the axially outer end of the widest continuous working ply. 
   Also preferably, the reinforcing elements of the protective plies cross one another. 

   
     BRIEF DESCRIPTION OF THE DRAWING 
     Other details and advantageous features of the invention will become apparent below from the description of example embodiments of the invention, with reference to  FIGS. 1 to 4 , in which: 
       FIG. 1  shows a diagram in radial section of a tire for construction machinery, 
       FIG. 2  shows a diagrammatic illustration in radial section of a first crown structure according to the invention, 
       FIG. 3  shows a diagrammatic illustration in radial section of a second crown structure according to the invention, and 
       FIG. 4  shows a diagrammatic illustration in radial section of a third crown structure according to the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION  
   The figures are not shown to scale, to simplify understanding thereof.  FIGS. 2 to 4  show only half of the structures, which extend symmetrically with respect to the axis XX′, which shows the circumferential center plane of a tire. 
   The dimensional values given are theoretical values, that is to say they are reference values given on manufacture of the tires; the actual values may be slightly different, in particular because of the uncertainty associated with the manufacturing process for this type of tire. 
   Moreover, as regards the angles of the ply reinforcements, the values given are minimum values, that is to say the values corresponding to the zone of a ply closest to the center axis of the tire. In reality the angle of the said reinforcements varies in dependence on the axial direction of the tire, in particular as a result of the camber thereof. 
   In  FIG. 1  a radial section through a tire  1  commonly used in construction machinery is shown diagrammatically. 
   This tire  1  is a large tire whereof the H/S ratio is 0.80, where H is the height of the tire on the rim and S is the maximum axial width of the tire when the latter is mounted on its operating rim and inflated to its recommended pressure. 
   This tire  1  comprises a carcass reinforcement  2  composed of a ply of non-extensible metal cords made of steel, anchored in each bead to a bead wire  3  in order to form an upturn  4  whereof the end is located substantially at the point of greatest axial width of the carcass reinforcement  2 . The carcass reinforcement  2  is radially surmounted by layers  5  and profiles of rubber mix  6 , and then by a crown reinforcement  7 . The said crown reinforcement  7  is conventionally composed, in the case of  FIG. 1 , on the one hand of two plies  8 ,  9 , called working plies, and on the other of two protective plies  10 ,  11 . The working plies  8 ,  9  are themselves composed of non-extensible steel cords parallel to one another in each ply  8 ,  9 , and crossing from one ply to the next to form angles which may be between 15° and 45° with the circumferential direction. The protective plies  10 ,  11  are generally composed of resilient metal cords made of steel, parallel to one another in each ply  10 ,  11  and crossing one another from one ply  10  to the next  11 , also making angles which may be between 15° and 45°. The cords of the radially outer working ply  9  conventionally cross the cords of the radially inner protective ply  10 . Finally, the crown reinforcement is surmounted by a tread  12  which is connected to the two beads  13  by two sidewalls  14 . 
   In  FIG. 2 , a stack according to the invention of plies  15  to  22  forming a working crown reinforcement  24  of a tire, this reinforcement being surmounted by a protective reinforcement  23 , is shown diagrammatically. The carcass reinforcement and the different zones of rubber mix are not shown in this  FIG. 2  or the following figures, in order to simplify understanding of the invention. 
   The working reinforcement  24  thus comprises in the first instance the four axially continuous plies  15 ,  16 ,  17  and  18 . These plies have the respective theoretical widths L 15 , L 16 , L 17 , L 18  equal to 0.40 S 0 , 0.35 S 0 , 0.65 S 0 , 0.25 S 0 , where S 0  is the maximum axial width of the carcass reinforcement when the latter is mounted on its operating rim and inflated to its recommended pressure. 
   These four continuous working plies are formed from non-extensible metal cords which are parallel to one another in each ply  15  to  18  and cross from one ply  15 ,  17  to the next  16 ,  18 , forming theoretical minimum angles α 15 , α 16 , α 17  and α 18  respectively equal to +15°, −22°, +22° and −22° with the circumferential direction of the tire. 
   The working crown reinforcement is then radially completed by superposing two half-plies  19 ,  20 . As explained above, these two half-plies are located symmetrically with respect to the circumferential center plane on that part of the stack of the tire which is not shown. These two half-plies also have non-extensible metal cords which are parallel to one another and cross from the ply  19  to the ply  20  at theoretical minimum angles β 19 , β 20  which are respectively equal to −22° and +34°. 
   The theoretical axial distance separating the radial plane from the inner end of the half-ply  19  closest to the said circumferential center plane is equal to 0.07 S 0 . 
   The two half-plies  19 ,  20  extend axially further outwards than the axially widest continuous working ply  17  of the continuous working plies. 
   Moreover, according to this embodiment of  FIG. 2 , the half-ply  19  which is the only one to be in direct contact with the continuous working plies  17 ,  18  has cords which cross those of the ply  17 , with respect to which the said half-ply  19  has the largest area of cover. 
   The cover zones between the half-plies and the continuous working plies, including that between the half-ply  19  and the axially least extensive continuous working ply  18 , are sufficiently large to ensure continuity of the forces. 
   According to other features of the invention, the half-ply  19  extending axially furthest outwards is in contact with the axially most extensive continuous working ply  17 . Moreover, the two half-plies  19 ,  20  cover the axially outer end of the said axially most extensive continuous working ply  17 . 
   The protective reinforcement  23 , which radially covers the working crown reinforcement  24 , is formed from two plies  21 ,  22  comprising resilient steel cords. Those cords which have a relative elongation at least equal to 4% under a tractive force equal to the breaking load are called resilient, whereas cords are called non-extensible when their relative elongation, measured at 10% of the breaking force, is less than 0.2%. The cords of the said two plies cross from one ply  21  to the other  22 , forming theoretical minimum angles respectively equal to −24° and +24° with the circumferential direction. The cords of the protective ply  21  which is radially closest to the working reinforcement cross the cords of the working half-ply  20  radially furthest from the carcass reinforcement; in accordance with other variant embodiments, the invention provides for inverting the radial order of the two protective plies, while the cords of the said plies remain crossed with one another. 
   The axial width of the widest protective ply  21  is such that it covers the axial width of the working reinforcement  24 ; that is to say that it radially covers all the working plies, as a function of its axial extent. In other words, the end of the widest protective ply  21  is axially outside the half-ply  19 . 
   The axially outer end of the narrowest protective ply  22  is between the axially outer end of the half-ply  20  extending axially least far outwards and the end of the widest continuous working ply  17 . 
     FIG. 3  shows a diagrammatic illustration in radial section of a different crown reinforcement structure according to the invention. According to this second illustration of the invention, the stack has plies  25  to  32  forming a working crown reinforcement  33  of a tire, this reinforcement being surmounted by a protective reinforcement  34 . 
   The working reinforcement  33  comprises the four axially continuous plies  25 ,  26 ,  27  and  28 . These plies have respective theoretical widths L 25 , L 26 , L 27 , L 28  equal to 0.50 S 0 , 0.45 S 0 , 0.55 S 0 , 0.40 S 0 , where as mentioned above S 0  is the maximum axial width of the carcass reinforcement when the latter is mounted on its operating rim and inflated to its recommended pressure. 
   These four continuous working plies are formed from non-extensible metal cords which are parallel to one another in each ply  25  to  28  and cross from one ply  25 ,  27  to the next  26 ,  28 , forming theoretical minimum angles α 25 , α 26 , α 27  and α 28  respectively equal to +18°, −24°, +20° and −20° with the circumferential direction of the tire. 
   The working crown reinforcement  33  is then radially completed by superposing two half-plies  29 ,  30 . As above, these two half-plies are located symmetrically with respect to the circumferential center plane on that part of the stack of the tire which is not shown. These two half-plies also have non-extensible metal cords which are parallel to one another and cross from the ply  29  to the ply  30  at theoretical minimum angles β 29 , β 30  which are respectively equal to −23° and +34°. 
   The theoretical axial distance separating the radial plane from the inner end of the half-ply  29  closest to the said circumferential center plane is equal to 0.10 S 0 . 
   The protective reinforcement  34  which radially covers the working crown reinforcement  33  is identical to that in  FIG. 2  and is formed by plies  31 ,  32 . 
   By comparison with  FIG. 2 , the structure in  FIG. 3  proposes continuous working plies which are wider, at least as far as the plies  25 ,  26 ,  28  are concerned. This difference in structure has the effect in particular of increasing the coupling surfaces between the continuous working plies and the working half-plies. This increase in the coupling surfaces between the plies allows the tire to be given greater resistance to slip angle forces. 
   This structure also proposes greater binding in the central zone than in the case of  FIG. 2 . 
   Moreover, the structure shown in  FIG. 3  allows the flexibility of the crown to be increased in the center, since the half-ply  29  is further from the circumferential center plane than the half-ply  19  in  FIG. 2 . 
     FIG. 4  shows a diagrammatic illustration in radial section of a third crown structure according to the invention. According to this third illustration of the invention, the stack has plies  35  to  40  forming a working crown reinforcement  41  of a tire, this reinforcement being surmounted by a protective reinforcement  42 . 
   The working reinforcement  41  comprises two axially continuous plies  35  and  36 . These plies have respective theoretical widths L 35  and L 36  equal to 0.45 S 0  and 0.65 S 0 , where as mentioned above S 0  is the maximum axial width of the carcass reinforcement when the latter is mounted on its operating rim and inflated to its recommended pressure. 
   These two continuous working plies are formed from non-extensible metal cords which are parallel to one another in each ply  35  and  36  and cross from one ply  35  to the next  36 , forming theoretical minimum angles α 35  and α 36  respectively equal to −15° and +24° with the circumferential direction of the tire. 
   The working crown reinforcement  41  is then radially completed by superposing two half-plies  37 ,  38 . As above, these two half-plies are located symmetrically with respect to the circumferential center plane on that part of the stack of the tire which is not shown. These two half-plies also have non-extensible metal cords which are parallel to one another and cross from the ply  37  to the ply  38  at theoretical minimum angles β 37 , β 39  which are respectively equal to −18° and +34°. 
   The theoretical axial distance separating the radial plane from the inner end of the half-ply  37  closest to the said circumferential center plane is equal to 0.08 S 0 . 
   The protective reinforcement  42  which radially covers the working crown reinforcement  41  is identical to that in the previous two examples, and is formed by plies  39 ,  40 . 
   The essential difference from the two examples above is that the crown structure has only two continuous working plies. This embodiment allows greater flexibility at the center of the crown of the tire to be retained than in the previous cases. 
   These examples should not be interpreted restrictively, and there are numerous variant embodiments; it is in particular possible to provide for the working half-plies to be interposed between the continuous working plies. It is also possible to have three continuous working plies. It is also conceivable to invert the orientation of the cords of the protective plies.