Patent Application
20100065183
The present invention relates to pneumatic tires and, more particularly, to means of impeding cracking at ply turnup ends of a pneumatic tire.
A pneumatic vehicle tire typically includes a pair of axially separated inextensible beads. A circumferentially disposed bead filler apex extends radially outward from each respective bead. At least one carcass ply extends between the two beads. The carcass ply has axially opposite end portions, each of which is turned up around a respective bead and secured thereto. Tread rubber and sidewall rubber are located axially and radially outward of the carcass ply.
The bead area is one part of the tire that contributes a substantial amount to the rolling resistance of the tire, due to cyclical flexure which also leads to heat buildup. Under conditions of severe operation, as with truck tires, the flexure and heating in the bead region can be especially problematic, leading to cracking of surrounding rubber. In particular, the ply turnup ends are prone to separation from adjacent structural elements of the tire. The ply is reinforced with materials such as nylon, polyester, rayon, and metal which have much greater stiffness (i.e., modulus of elasticity) than does the adjacent rubber compound of which much of the tire is made. The difference in elastic modulus of mutually adjacent tire elements leads to cracking and separation when the tire is stressed and deformed during use.
A variety of conventional structural design approaches have been used to manage the cracking and separation of tire elements in the bead regions of tires. For example, one method has been to provide a “flipper” surrounding the bead and a bead filler. The flipper works as a spacer that keeps the ply from making direct contact with the inextensible beads, allowing some degree of relative motion between the ply, where it turns upward under the bead, and the respective beads. In this role as a spacer, the flipper reduces the inevitable disparities of strain on the ply and on the adjacent rubber components of the tire (e.g., filler apex and sidewall rubber in the bead region and the elastomeric portions of the ply itself).
Prior to the use of steel-reinforced radial ply construction, conventional plies were reinforced with materials having substantially lower moduli of elasticity than that of steel. Accordingly, the stresses associated with heavy-duty tire use were more easily accommodated by the respectively adjacent components, such as the ply reinforcing materials and the adjacent rubber polymeric materials. Such tires were less durable than are those having metal reinforced plies. Still, disparities of respective moduli of elasticity led to cracking and ply separation under severe conditions, beginning at the ply turnup ends.
In addition to the use of flippers as a means by which to reduce the tendency of a ply to separate, another conventional method involves the placement of “chippers.” A chipper is a circumferentially deployed metal or fabric layer disposed within the bead region in the portion of the tire where the bead fits onto the wheel rim. More specifically, the chipper lies inward of the wheel rim (i.e., toward the bead) and outward (i.e., radially outward relative to the bead, viewed in cross section) of the portion of the ply that turns upward around the bead. Chippers stiffen and increase the resistance to flexure of the adjacent rubber material, which is typically adjacent to the turnup ends.
Also, given that the ply is, on each side of the tire, clamped around, or anchored to, or “turned up” about, the respective bead, there exists a “turn-up end” (as viewed in the cross section of a tire) that extends radially outward within, and circumferentially about, each sidewall. Limits on the length of the ply turnup ends are made in order to locate the ends of the ply in positions where radial deformations of the tire are relatively small.
Stresses that result in the deposition of energy (i.e., the generation of heat) in the bead region and in the region where the turnup ends terminate are frequently accompanied by strains that contribute to cracking and separation failures at the turnup ends. A balanced design for a reinforced bead assembly of a tire has stress characteristics that lead to reduced flexural energy generation (heat buildup) and to strain characteristics that can be uniformly borne by mutually adjacent tire components in the bead region, including the turnup ends.
Conventional radial-ply truck tires, in which the one or more plies are reinforced with steel cables or cords, are prone to turnup cracking and separation when exposed to severe service. Part of the cause of cracking and separation is related to the stresses described above and to the disparate moduli of elasticity of the respective metal and adjacent polymeric rubber compounds. As the tire undergoes flexure during heavy-duty use, flexure of the sidewalls in the region near to and immediately radially outward of the beads experience repeated flexural deformations in one or more directions, such as the radial and axial directions. Also, cracking and ply separation is especially problematic if the tire is overinflated or underinflated.
As stated above, a high stress/strain concentration region exists at ply turn-up ends. In addition, bonding between the sharp and narrow ending of ply wires and adjacent compounds may be inadequate, due to lack of adhesive brass at tips of cut wires. Therefore, rubber is prone to crack initiation and propagation adjacent to the ply end.
It would be desirable to provide a bead region design that can reduce ply end cracking and separation initiation and propagation within radial tires exposed to severe service conditions. Particularly, it would be desirable to reduce the flexural heat buildup associated with the cyclical shearing stresses and concomitant cyclical shearing strains in the ply end regions of truck tires exposed to severe operating conditions.
Conventional approaches for reducing cracking have added a gum strip-type compound adjacent the ply turnup ends. These conventional approaches have not significantly altered the geometry or amount of crack initiation and propagation. Cracks still initiate and propagate through the gum strip-type compound.
A pneumatic tire in accordance with the present invention comprises a tread, two inextensible annular beads, a carcass ply having two turnup end portions, and an annular cap structure encompassing one of the turnup end portions. Each turnup end portion is wrapped around one of the annular beads. The cap structure has a U-shaped cross-section for surrounding the turnup end portion. The cap structure is constructed of reinforced fabric with fibers oriented in the range from −45° to +45° with respect to a radial direction of the pneumatic tire.
According to another aspect of the present invention, the fibers of the cap structure are aramid fibers.
According to still another aspect of the present invention, the fibers of the cap structure are nylon fibers.
According to yet another aspect of the present invention, the pneumatic tire further includes nylon fabric flippers for absorbing strain between the annular beads and the carcass ply.
According to still another aspect of the present invention, the pneumatic tire further includes steel cord chippers for absorbing strain between the turnup ends and a wheel rim on which the pneumatic tire is mounted.
According to yet another aspect of the present invention, axially outwardmost portions of the turnup end portions of the carcass ply extend radially outward beyond the top of a wheel rim flange of the wheel rim.
The structure, operation, and advantages of the invention will become more apparent upon contemplation of the following description taken in conjunction with the accompanying drawings, wherein:
“Apex” or “bead filler apex” means an elastomeric filler located radially above the bead core and between the plies and the turnup plies.
“Axial” and “Axially” means the lines or directions that are parallel to the axis of rotation of the tire.
“Bead” or “Bead Core” generally means that part of the tire comprising an annular tensile member of radially inner beads that are associated with holding the tire to the rim; the beads being wrapped by ply cords and shaped, with or without other reinforcement elements such as flippers, chippers, apexes or fillers, toe guards and chafers.
“Carcass” means the tire structure apart from the belt structure, tread, undertread over the plies, but including the beads.
“Casing” means the carcass, belt structure, beads, sidewalls and all other components of the tire excepting the tread and undertread, i.e., the whole tire.
“Chipper” refers to a narrow band of fabric or steel cords located in the bead area whose function is to reinforce the bead area and stabilize the radially inwardmost part of the sidewall.
“Circumferential” most often means circular lines or directions extending along the perimeter of the surface of the annular tread perpendicular to the axial direction; it can also refer to the direction of the sets of adjacent circular curves whose radii define the axial curvature of the tread, as viewed in cross section.
“Cord” means one of the reinforcement strands, including fibers, with which the plies and belts are reinforced.
“Equatorial Plane” means the plane perpendicular to the tire's axis of rotation and passing through the center of its tread; or the plane containing the circumferential centerline of the tread.
“Flipper” refers to a reinforcing fabric around the bead wire for strength and to tie the bead wire in the tire body.
“Gauge” refers generally to a measurement and specifically to thickness.
“Inner Liner” means the layer or layers of elastomer or other material that form the inside surface of a tubeless tire and that contain the inflating fluid within the tire.
“Lateral” means a direction parallel to the axial direction.
“Normal Load” means the specific design inflation pressure and load assigned by the appropriate standards organization for the service condition for the tire.
“Ply” means a cord-reinforced layer of rubber-coated radially deployed or otherwise parallel cords.
“Radial” and “radially” mean directions radially toward or away from the axis of rotation of the tire.
“Radial Ply Structure” means one or more carcass plies of which at least one ply has reinforcing cords oriented at an angle of between 65° and 90° with respect to the equatorial plane of the tire.
“Radial Ply Tire” means a belted or circumferentially-restricted pneumatic tire in which at least one ply has cords which extend from bead to bead are laid at cord angles between 65° and 90° with respect to the equatorial plane of the tire.
“Section Height” means the radial distance from the nominal rim diameter to the outer diameter of the tire at its equatorial plane.
“Section Width” means the maximum linear distance parallel to the axis of the tire and between the exterior of its sidewalls when and after it has been inflated at normal pressure for 24 hours, but unloaded, excluding elevations of the sidewalls due to labeling, decoration or protective bands.
“Sidewall” means that portion of a tire between the tread and the bead.
“Toe guard” refers to the circumferentially deployed elastomeric rim-contacting portion of the tire axially inward of each bead.
“Tread width” means the arc length of the tread surface in the plane includes the axis of rotation of the tire.
“Turnup end” means the portion of a carcass ply that turns upward (i.e., radially outward) from the beads about which the ply is wrapped.
The carcass ply 14 may be a rubberized ply having a plurality of substantially parallel extending carcass reinforcing members made of such material as polyester, rayon, or similar organic polymeric compounds. Axially outwardmost portions of the turnup end portions 30a, 30b of the carcass ply 14 may extend radially outward by a distance of between about 15 millimeters and about 30 millimeters beyond a top of a wheel rim flange of a wheel rim.
The turnup end portions 30a, 30b of the carcass ply 14 may engage axial outer surfaces of flippers 32a, 32b and axial inner surfaces of chippers 34a, 34b. The chippers 34a, 34b may consist of narrow bands of steel cloth located in the bead area for the purpose of reinforcing the bead area and stabilizing the axially inwardmost part of the sidewalls 15, 17.
The flippers 32a, 32b wrap around the beads 28a, 28b and extend radially outward into the sidewall regions of the tire 10. The axially inward portion of flippers 32a, 32b terminate within the bead-filler apexes 26a, 26b. The axially outward portions of the flippers 32a, 32b lie radially inward of the turnup end portions 30a, 30b, which are also located radially beyond the radially outermost reach of the chippers 34a, 34b. An axially outermost portion of each flipper 32a, 32b may extend radially to within between about 7 mm and about 15 mm of the radially outermost reach of the turnup end portions 30a, 30b of the carcass ply 14.
The flippers 32a, 32b may be made of nylon fabric or other suitable thermoplastic polymers capable of extension when woven into fabrics, sheets, etc. of extreme toughness, strength and elasticity. The nylon fabric may be woven, or it can be of a monofilament or multifilament type of material in which the cords run in the same direction. The nylon fabric of the flippers 32a, 32b may have a thread pitch of between about 5 and about 30 ends per inch (about 2-12 ends/cm) and an overall thickness in the range of about 0.3 to about 1.2 mm, preferably about 10 to about 20 ends per inch (about 4-8 ends/cm) and 0.5 to about 1.0 mm gauge. The nylon cords of the flippers 32a, 32b may be oriented at an angle of between about 20 degrees and about 50 degrees with respect to the radial direction, preferably at an angle of between 25 degrees and 35 degrees. The flippers 32a, 32b may be termed “active” because they actively absorb (i.e. during tire deflection) differential strain between the very rigid beads 28a, 28b and less rigid metal reinforced carcass ply 14.
The chippers 34a, 34b may be made of steel cords. Each chipper 34a, 34b may be disposed adjacent to the portion of the carcass ply 14 that is wrapped around the beads 28a, 28b. Further, the chippers 34a, 34b may be disposed on opposite sides of the portion of the carcass ply 14 from the flippers 32a, 32b. The axially inwardmost portion of the chippers 34a, 34b may be disposed in a portion of the bead regions 24a, 24b that, when the tire 10 is mounted on a wheel, would be closest to a circularly cylindrical part of the wheel. The axially and radially outwardmost portion of the chippers 34a, 34b may be disposed in a portion of the bead regions 24a, 24b that, when the tire 10 is mounted on a wheel, would be inward of a circular portion of a wheel-rim flange, being separated from the circular portion of the wheel-rim flange by tire rubber. In other words, the chippers 34a, 34b are disposed circumferentially about the radially inwardmost portion of carcass ply 14 where it turns up around the beads 28a, 28b. The chippers 34a, 34b may extend radially outward, being more or less parallel with the turned up ends 30a, 30b of the carcass ply 14. The disposition of the chippers 34a, 34b may be mirror-symmetric with respect to the bead-regions 24a, 24b.
The chippers 34a, 34b protect the portion of the carcass ply 14 that wraps around the beads 28a, 28b from strains in the rubber that separates the chippers from a wheel rim. The chippers 24a, 24b reinforce the bead regions 24a, 24b and stabilize the radially inwardmost part of the sidewalls 15, 17. In other words, the chippers 34a, 34b, being constructed of relatively flexible steel cords encompassed with an elastomeric material, may absorb deformation in a way that minimizes transmission of stress-induced shearing strains that arise inward from a wheel rim, through the rubber portion to the turnup ends 30a, 30b of the carcass ply 14 where the chippers are most immediately adjacent to the rigid beads 28a, 28b.
In accordance with the present invention, the tire 10 may further include a cap structure 100 encompassing, or wrapping around, each turnup end 30a, 30b. The cap structure 100 has demonstrated superior advantage to improve fatigue life of the turnup ends 30a, 30b and surrounding area. The cap structure 100 may be U-shaped in cross-section (
Thus, the cap structure 100 has been shown to accomplish two goals: 1) the cap structure contains existing cracks adjacent the turnup ends 30a, 30b within the U-shaped cap structure; and 2) the cap structure relocates the cracks. The cap structure 100 replaces the sharp and narrow interface area at the turnup ends 30a, 30b with the rounded and larger U-cap structure thereby smoothing the transition or interface both in terms of material and geometry.
The reinforced U-cap structure 100 thus greatly increases crack resistance capacity and, in effect, stops crack propagation through the U-cap structure. Further, crack driving forces at the exterior of the U-cap structure 100 are greatly contained and substantially reduced by the U-cap structure, thereby significantly delaying, if not completely eliminating, crack initiation at the exterior of the U-cap structure for the life of the tire. In other words, the reinforced U-cap structure 100 shifts and greatly delays crack initiation from the ply turnup ends 30a, 30b to the exterior of the U-cap structure.
One example bead durability test has shown that mileage may be increased from an average of 11,150 km to an average of 13,069 km, or an almost 2,000 km (17%) improvement, by incorporating a cap structure 100 in accordance with the present invention at the turnup ends 30a, 30b of a radial truck tire, such as the example pneumatic tire 10. Thus, the cap structure 100 may be applied to turnup ends 30a, 30b to improve bead area durability. Nylon, PET, PEN, rayon, or any suitable material, or any suitable combination of materials (i.e., hybrid) may be used for reinforcing the cap structure 100. Less costly material may obviously reduce cost of the tire.
As stated above, conventional turnup ends have sharp, narrow, and bare transitions between the metal ply ends and adjacent polymer compounds. Thus, macro cracks appear and propagate from the adjacent polymer compounds, typically surrounding rubber. The unique cap structure 100 may shield or contain the cracks within the U-shaped cap, and further introduce a relatively large and smooth interface transition between the fabric of the cap structure and the adjacent polymer compound, thus greatly improving bead and overall tire durability
While the invention has been described in combination with embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing teachings. Accordingly, the invention is intended to embrace all such alternatives, modifications and variations as fall within the spirit and scope of the appended claims.
