1. Field of the Invention
The present invention refers to rubberized fabrics for tires having zero-degree reinforcing elements and to tires incorporating such fabrics.
2. Description of the Related Art
As is known, a tire includes at least three fundamental components, the carcass, the tread band, and the reinforcing belt between the tread band and the carcass. The carcass, usually at least one ply, is turned out at both ends around a pair of bead cores. Together, the bead cores, the ends of the carcass, and whatever rubberized filler is added between the bead cores and the ends of the carcass cooperate to form the beads on either side of the tire.
When in use, a tire is placed on a wheel rim, which has two seats axially displaced from one another. The two beads on either side of the tire rest on the two rim seats. Each of the rim seats terminates in an end flange, which has an outermost diameter greater than the diameter of the tire beads, which prevents the beads from slipping off of the wheel rim once the tire is installed on the rim.
The belts of a conventional tire generally consist of at least three rubberized fabrics. The first two fabrics comprise fine cords that crisscross each other and are both angled with respect to the equatorial plane of the tire. The third fabric is external radial belt made from fine cords of heat-shrinkable material including synthetic fibers, oriented at 0xc2x0 with respect to the equatorial plane of the tire, such as commonly nylon. In one common embodiment, the third layer consists of nylon cords 0.39 mm in diameter embedded in a 0.7-mm-thick rubberized fabric.
To form the tread pattern on a tire, an uncured or xe2x80x9cgreenxe2x80x9d tire is placed in a mold, which carries on its interior surface the pattern for the tread of the tire. During the tread forming step in the manufacturing process, the mold is pushed into the tire to imprint the tread pattern onto the tire. At the same time, the tire is inflated, causing the layers with crisscrossed cords to expand toward the interior surface of the mold. This expansion helps to push the tread band againsto the surface of the mold so that it can accept the tread pattern from the mold. As the tire expands when inflated, the crisscrossed cords are pushed outwardly, diminishing the angle of inclination of the cords with respect to one another. The third belt layer, with its heat-shrinkable synthetic fiber cords, exerts a force on the layers beneath it to limit their outward movement during the tread-forming stage.
Not only does the third belt layer serve a purpose in the manufacture of a tire, it also is important to the operation of a tire when the tire is mounted on a rim. The third belt layer helps to counteract the outward expansion of the underlying layers, which is caused by the large centrifugal forces that act on the belts at high speeds.
The synthetic fiber cords, however, have at least one disadvantage. They are known to temporarily deform in a tire in a phenomenon known as xe2x80x9cflatspotting.xe2x80x9d When the vehicle is stopped, the entire weight of the car rests on one spot on each of the tires. This causes a flattening of each tire in the footprint or imprint area where the tire contacts the ground. Because the synthetic fibers are prone to creep under stress, they distort in the imprint area. Even after the vehicle begins to move and the tire rotates, the flattened region persists in the imprint area for a prolonged period of time. Such a phenomenon is typical of all synthetic materials. The phenomenon varies from material to material depending on the viscoelastomeric characteristics of the particular synthetic fiber in question. Consequently, at least for a certain period after the tire rotates following flatspotting, the temporary deformation of the synthetic cords generates a noise effect and uncomfortable behavior.
To avoid this phenomenon, it is known to incorporate zero-degree metallic cords (oriented at zero degrees with respect to the equatorial plane of the tire) in the third belt rather than cords of synthetic material. The metallic cords are sufficiently rigid to resist deformation when the vehicle is stopped. Tires made according to this teaching do not exhibit the phenomenon of xe2x80x9cflatspottingxe2x80x9d because they incorporate metallic cords rather than cords made from a viscoelastic material.
When the third belt layer is constructed with metallic cords, it is known to use cords made of some strands twisted together of the so called xe2x80x9clang lay typexe2x80x9d that provides cords with high elongation prior to reaching their breaking point, and, because of this, the cords are also known in the prior art as xe2x80x9cHExe2x80x9d (high-elongation) cords. In such an embodiment, the metallic cords act like a spring wire, which is evident from studying a typical stress-deformation diagram for these materials.
The first segment of the stress-deformation diagram for metallic cords is identified by a small or weak slope with respect to the abscissa. This first segment of the stress-deformation diagram is useful because it can be used to predict the behavioral characteristics of the metal during the tread band forming stage where the material exhibits high elongation at low loads. The next, strongly sloped segment of the stress-deformation diagram is useful for determining the behavioral characteristics of the metal during operation of the tire, where the material exhibits only slight elongation under a high load.
The overall diameter of metallic cords suitable for this purpose may be 0.7 mm in a zero-degree fabric used in the manufacturing of large tires. However, such a cord size is too large to be compatible with the dimensions required for a belt fabric in an automobile tire.
Belt structures with metal cords made from shape-memory materials are also known in the art, e.g., U.S. Pat. No. 5,242,002 and Japanese Patent Application JP 4362401. In U.S. Pat. No. 5,242,002, a tire is described with belts having cords symmetrically inclined with respect to the equatorial plane of the tire. The cords are formed by helically winding several wires together. At least one of the wires in the cord is made from a shape-memory material. The shape-memory wire, before being cabled with the other wires, undergoes a heat treatment at a predetermined temperature while it is in a particular configuration (for example undulated) and is subsequently deformed into a linear configuration below the temperature of the heat treatment; accordingly, said wire recover the undulated configuration above the heat treatment temperature.
Each time the temperature of the tread band increases at high speeds, the temperature of the belt exceeds that of the heat treatment of the shape-memory wire, and the wire tends to take on the undulated shape. However, since the shape-memory wire is corded with the other wires, the shape-memory wire cannot deform but, instead, is subject to tension. As a result, in the shape-memory wire, a stress is established, the effect of which is to increase the rigidity of the belt and, accordingly, avoid an increase in the diameter of the tire caused by centrifugal forces.
Japanese Patent Application JP 4362401 discloses a tire with an outermost belt having an outermost layer comprising a shape-memory expansion element, preferably a spring wire element made from a Nixe2x80x94Ti alloy. The spring wire element is wound at zero degrees over the underlying layers of the belt. The shape-memory element is designed to contract in the peripheral direction of the tire when the wire is heated during high speed use. In this way, at high speeds, the tire becomes more rigid and the phenomenon of tire expansion is controlled. On the other hand, at low speeds, such as those encountered under normal traveling conditions, the shape-memory element returns to and maintains its original shape. The Japanese application describes wires from 0.25 to 0.5 mm in diameter. Finally, the Japanese application discloses that it is not necessary for the shape-memory expansion element in the tire to be spring wire shaped, but that it can be shaped as a belt or cord, for example.
Confronted with the state of the art set forth above, it was believed possible to provide a single solution both to the problem of flatspotting and to the problem of the outward expansion of the belt when the associated tire is subjected to the large centrifugal forces during high speed use. Furthermore, it was believed that the dimensions of the materials used (and, therefore, the thickness of the belt created) could be kept at least at the levels known in the art, if not reduced.
It was believed that the problems of flatspotting and centrifugal expansion could be addressed simultaneously if zero-degree metal reinforcements were used that exhibited both (1) a correct geometric orientation in the rubberized fabric and (2) a high degree of resistance to fatigue.
One problem that had to be addressed, however, was how to maintain the correct geometric orientation of the reinforcing elements in the rubberized fabric. In the prior art, the reinforcing elements are known to have an uneven distribution in the fabric layer. It is believed that this uneven distribution is caused by the application of high pressure to the tire during the manufacturing process. Specifically, it is believed that the inflation pressure exerts a force on the reinforcing elements that displaces them from their intended positions before the tire solidifies.
Maintaining the correct geometry of the reinforcing elements in the fabric during manufacture is only part of the problem, however, because the cords are also prone to move in the fabric during operation of the tire. In use, the cord coils may compress into the elastomeric material. Since the diameter of the cords is so small, they are believed to act like knives, creating small cuts and tears in the elastomer over time. As a result, over time, the cords begin to move about within the elastomer and, in extreme cases, may even exit the material, creating a risk that the reinforcing elements might contact the metallic reinforcement cords in the immediately underlying layer. If the cords in different layers contact each other, they can generate sufficient friction to compromise the integrity of the tire fabric.
Fatigue stress is another factor that must be considered when designing a reinforced tire fabric. Fatigue stress can be attributed to two particular events as the tire rotates. First, in the undeformed portion of the tire, the radially outermost portion of the cord is subject to traction and assumes a bending deformation proportional to the distance from its neutral longitudinal axis and inversely proportional to the radius of curvature of the undeformed tire. Second, in the transition between the undeformed portion of the tire and the area of imprint, the same portion of the cord is subjected to a deformation force considerably greater than the previous one, since the deformation becomes inversely proportional to the new radius of curvature of the tire, which is noticeably smaller than the radius of curvature corresponding to the undeformed tire geometry. Similarly, the innermost portion of the cord deforms according to the same relationship, a deformation that is proportional to the distance of the considered fiber from the neutral axis and inversely proportional to the radii of curvature assumed by the tire, respectively, first in the undeformed condition and then in the deformed condition in proximity to the area of the imprint.
One way to reduce this fatigue is to reduce the diameter of the cords. While the 0.39 mm nylon cord of the typical reinforcing element may seem small, that dimension is actually quite large when examining the cord from the perspective of the fatigue stress on the cord. However, as the diameter of the cords is reduced, the potential for problems related to the possibility that the cords might cut through the elastomeric material, as discussed above, increases.
Once the various factors associated with the interaction of the reinforcing elements in the rubberized fabric were appreciated in accordance with the development of the present invention, a solution was selected that addressed both the need for the correct placement of the zero-degree reinforcing elements into the incorporating fabric and also the need for a fabric with a high resistance to fatigue. One of the improvements selected was the incorporation of zero-degree reinforcements made of shape-memory alloy in the shape of metal straps. For such a configuration, it was thought that a specific width of the metal strap could be determined and used to stabilize the strap in the elastomeric material. Moreover, it was thought that such a configuration would react differently to the air pressure in the tire and would resist the tendency to cut through the elastomeric material. It was also thought that if the thickness of the metal strap was noticeably smaller than that of the corresponding cords used in the prior art, resistance to fatigue as the tire rolls over the ground could be considerably increased.
Therefore, a first aspect of the present invention is characterized by a rubberized fabric for a tire belt incorporating an elastomeric material with at least one continuous reinforcing metal strap. The strap is made from a shape-memory material that is oriented at 0 degrees to the equatorial plane of the tire. The minimum width of the strap is 1 mm. The maximum thickness of the rubberized fabric is 0.4 mm.
In the first aspect of the present invention, the reinforcing elements are held firmly in place or stabilized in a predetermined position in the rubberized fabric because the reinforcing elements are metal straps disposed in a side-by-side arrangement. The minimum width of the metal straps is 1 mm and the thickness of the rubberized fabric incorporating the metal straps has a maximum thickness of 0.4 mm. Each coil of the metal straps is oriented substantially perpendicularly to the direction of the force from the air pressure inside the tire. With such an orientation, the metal straps resist compression into the rubberized fabric.
Preferably, the fabric includes metal straps between 1 and 5 mm wide and between 0.02 and 0.1 mm thick, inclusive. In a preferred embodiment of the present invention, the thickness of each metal strap is no greater than one tenth of its the minimum width. The metal straps of the present invention are preferably made from a shape-memory material such as a NiTi alloy, a NiTiX alloy, where X is selected from a combination of Fe, Cu, or Nb, a CuZnAl alloy, a CuAlNi alloy, a CuAlBe alloy, a FeNiCoTi alloy, a FeMnSi alloy, an alloy with a FeMnSi-base, or an alloy with a FeNiCo-base. Being made of a shape-memory material, the reinforcing straps, when heated, try to return to a predetermined, memorized length.
In a second embodiment of the present invention, the tire comprises a carcass, a tread band on the carcass, and a belt positioned between the tread band and the carcass including at least one layer of rubberized fabric. The belt incorporates at least one continuous reinforcing metal strap made from a shape-memory material that is oriented at 0xc2x0 to the equatorial plane of the tire. The metal strap has a minimum width of 1 mm and is wound with coils arranged side-by-side, perpendicularly to the radius of the tire.
In the second embodiment of the present invention, the reinforcing belt is further characterized by including means to stabilize the reinforcing straps in a predetermined position within the fabric. The position of the reinforcing straps is stabilized by shaping the reinforcing elements as side-by-side metal straps, the minimum width of each metal strap being 1 mm and the maximum thickness of the rubberized fabric being 0.4 mm. Each coil of the metal straps is oriented substantially perpendicularly to the direction of the force from the air pressure inside the tire, thereby resisting compression of the reinforcing strap into the elastomeric material in which the reinforcing strap is incorporated. In one preferred embodiment, the metal straps are distributed evenly in the tire with a 0.1 mm separation between adjacent straps.
In yet another preferred embodiment, the metal straps in the fabric tend to recover a previously memorized shape by exerting contractive forces on the innermost fabrics of the belt between two predetermined temperatures, As and Af, respectively. As corresponds to a temperature at the start of the structural transformation of the metal strap from a martensitic state to an austenitic state. Af corresponds to a temperature at the completion of the structural transformation of the metal strap from the martensitic state to the austenitic state.
It should be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.