GB2075927A

GB2075927A - A pneumatic tire

Info

Publication number: GB2075927A
Application number: GB8106619A
Authority: GB
Original assignee: Goodyear Tire and Rubber Co
Current assignee: Goodyear Tire and Rubber Co
Priority date: 1980-03-24
Filing date: 1981-03-03
Publication date: 1981-11-25
Also published as: BR8101624A; CA1144464A; IT8120670A0; JPS56146401A; FR2478527B1; FR2478527A1; AU540803B2; MX151927A; GB2075927B; IT1135705B; AU6800681A; DE3108825A1

Abstract

A pneumatic tire (10) is cured in a configuration such that in at least part of the tire the lateral neutral bending axis of the tire coincides with a natural equilibrium curve of the tire, which is defined by the curve formed in a meridian plane by a toroidal membrane which represents the composite tire structure under design boundary conditions and loads when the circumferential and meredial elongation properties of the toroidal membrane are substantially equivalent to that of the composite tire structure. The local stiffness angle may be substantially equal to the local stress angle at any given point in the tire part were the neutral axis and natural equilibrium curve are coincident. The tire may have a carcass with cords at 75 DEG -90 DEG to the circumferential, and a belt with cords at 15 DEG -30 DEG to the circumferential. The entire cross- section of the tire between its bead portions may be curved in the configuration such that the neutral axis and equilibrium curve coincide, or the part of the tire between points in the shoulder regions may be cured in this configuration whilst from these points to the radially outer region of the bead portions the carcass follows a different profile as prescribed by an equation given in the specification. <IMAGE>

Description

SPECIFICATION A pneumatic tire The foregoing abstract is not to be taken as limiting the invention ofthis application, and in order to understand the full nature and extent of the technical disclosure of this application, reference must be made to the accompanying drawings in the following detailed description.

Background of the Invention This invention relates to a pneumatic tire, and more particularly, to a novel and improved pneumatic tire.

It is well known in the prior art that it is desirable to cure a tire in a configuration such that the carcass ply structure follows the natural equilibrium curve for at least a portion of its length. The natural equilibrium curve is the shape the carcass ply structure tends to assume as prescribed by its cord path and dimensions when mounted on the rim for which it is designed and inflated to design inflation pressure. This concept is thoroughly discussed and explained in "Mathematics Underlying The Design of Pneumatic Tire" by John F.

Purdy. U.S. Patent Nos. 3,757,844; 3,910,336; and 3,938,575 are illustrative of the prior art wherein at least a portion of the carcass ply structure follows the natural equilibrium curve.

The prior art has been limited to describing the natural equilibrium curve for those areas of the tire in which no other fabric reinforcement is present other than the carcass ply structure, that is, the prior art has not been able to define the natural equilibrium curve for those areas of the tire which are reinforced with additional reinforcing structures such as a belt reinforcing ply structure in the tread area and/or a reinforcing ply structure in the bead area. Additionally, the prior art does not take into consideration the stiffening effect of the rubber in the tire in determining the natural equilibrium curve.

In accordance with the present invention, the natural equilibrium curve can be determined for the composite tire structure from bead area to bead area including those areas of the tire reinforced with other reinforcing ply structures and takes into account the stiffening effect of the rubber. The natural equilibrium curve for the purposes of this invention is the curve defined by a toroidal membrane under design boundary conditions and loads as viewed in a plane which passes through the axis of rotation wherein the circumferential and meridional elongation properties of the toroidal membrane are equivalent to that of the composite tire structure.The tire has a natural shape when the locus of points on the lateral neutral bending axis of the tire cross-section and the natural equilibrium curve of the composite tire structure coincide when mounted on the rim for which the tire is designed and inflated to design inflation pressure. Applicants are able to achieve this result by having the local stiffness angle of the composite laminate tire structure substantially equal to the local stress angle ofthe composite laminate tire structure in the area wherein it is desired to curve the tire in its natural shape.

Description of the Drawings Figure 1 is a radial cross-sectional view of a pneumatic tire made in accordance with the present invention; and Figure 2 is a diagrammatical representation of a doubly curved surface of a tire carrying a pressure load; and Figure 3 is a graphical representation of the acceptable and preferred difference between the local stiffness and stiffness angles.

Figure 4 is a radial cross-sectional view of a modified pneumatic tire also made in accordance with the present invention.

Detailed Description of the Invention Referring to Figure 1,there is illustrated in the cross-section a pneumatic tire 10 made in accordance with the present invention. The tire 10 has a circumferentially extending ground engaging tread portion 12 which terminates in a pair of shoulder portions 14, 15 at the lateral ends thereof. A pair of sidewall portions 16, 17 extend radially inward from the shoulder portions 14, 15 respectively terminating in a pair of bead portions 18, 19. The bead portions 18, 19 each have embedded therein a substantially inextensible bead core 20,21 respectively. In the embodiment illustrated there is shown only a single bead core in each of the bead portions, it being understood that the application is not limited to such. The bead cores 20,21 in the embodiment illustrated are preferably made of steel wires.A carcass reinforcing ply structure 22 extends from bead core 20 through sidewall 16, tread portion 12 and sidewall 17 to bead core 21. The carcass reinforcing ply structure 22 comprises at least one layer of rubber coated cord fabric. The cords of the ply structure 22 may be comprised of any material used for rubber reinforcement, for example, and not by way of limitation, rayon, nylon, polyester, and steel and may form any desired angle with respect to the mid-circumferential centerplane CP of the tire. In the embodiment illustrated the cords of the carcass ply structure 22 may form an angle of from about 75" to 90" with respect to the mid-circumferential centerplane CP. While the particular embodiment illustrated shows one layer of cord fabric, it is understood that the carcass reinforcing structure 22 may be comprised of as many layers as desired.

Placed circumferentially about the radially outermost surface of the carcass ply structure 22 is a tread reinforcing belt structure 23 having two cut reinforcing layers 24, 26 of parallel cords coated with a thin layer of rubber. The cords of layers 24,26 may be comprised of any material used for rubber reinforcement, for example, and not by way of limitation nylon, rayon, steel and aramid. While the belt structure 23 is shown as having two cut layers 24, 26, belt structure 23 may be of any desired configuration and be omitted entirely if desired. The cords of layers 24 and 26 preferably form an angle of from about 15" to 30" with respect to the mid-circumferential centerplane CP of the tire and in the particular embodiment illustrated the angle is approximately 23".Preferably the layers 24 and 26 are oriented such that the cords of ply 24 form an angle equal and opposite to the angle formed by the cords of belt ply 26.

In the embodiment illustrated bead portions 18, 19 are each provided with a cord reinforcing fabric strip 28 which extends circumferentially about the tire 10 axially outside the axially outermost portion of the carcass ply. The cords of the fabric strip 28 may be comprised of any materials used for rubber reinforcement and in the particular embodiment illustrated may be of steel. Referring to Figure 1, the radially inner end 29 of reinforcing strips 28 start at a point radially inward of the radially outermost point of the associated bead cords 20,21 respectively and extend radially outward with respect to the axis of rotation of the tire such that the radially outer end 30 of strip 28 terminates at a point located radially outward from the tire rim contact point 31.For the purpose of this invention, the tire rim contact point 31 is the radially outermost point at which the rim and tire first meet in the bead area as determined when the tire is mounted on the wheel for which it is designed and inflated to design pressure.

The tire 10 is cured in its natural shape, that is, the configuration in which the natural equilibrium curve of the tire composite structure coincides with the lateral neutral bending axis of the tire. For the purposes of this invention the natural equilibrium curve is the curve defined by a toroidal membrane which represents the composite tire structure under design boundary conditions and loads as viewed in a plane passing thru the axis of rotation wherein the circumferential and meridional elongation properties of the toroidal membrane are equivalent to that of the composite tire. Curing the tire in this configuration minimizes the stresses and tension variations in the tire that would be present if the tire were not cured in this configuration.Also, for the purposes of this invention the lateral neutral bending axis of the tire is defined by a line as viewed in a plane which passes through the axis of rotation of the wire wherein no change in length occurs when a lateral curvature change is imposed on the tire. It is believed that in order four a tire to be molded in its natural shape, the geometry of the tire as cured should match the physical construction of the tire. The geometry of the tire can be represented by the local stress angles of the composite structure and the physical construction of the tire can be represented by the local stiffness angles.The local stiffness angle of the composite structure is the angle of a hypothetical pair of ply layers that have elongation characteristics equal to the tire composite structure at any given point even though the component layers may be of several different modulii and cord angles. The local stress angle can be represented by the angle of a hypothetical pair of plies which carry the lateral and circumferential membrane stresses with only elongational deformation.

The local stiffness angle and the stress angle are determined for plurality points about the cross-section of the tire. Generally, the local stress angle and stiffness angle need to be calculated for at least 25 points about the half cross-section of the tire. That is the local stiffness angle and stress angle are determined for at least 25 different points from the area of the tire adjacent the bead area radially outward to the midcircumferential centerplane. Most pneumatic tires that are made today are symmetrical about the mid-circumferential centerplane, therefore, it is only necessary to calculate the local stress and stiffness angles for one-half of the tire, that is, the part of the tire on one side of the mid-circumferential centerplane.

Because of the symmetry, the values for the local stress and stiffness angles are symmetrical with respect to an equatorial plane. In the event the tire is of the asymmetric variety then the local stiffness and stress angles should be calculated for approximately 50 different points about the entire cross-section ofthetire.

Applicants have found for a symmetrical passenger tire having a size designation of FR78-14, this is a passenger tire having the aspect ratio of about 78 in a nominal rim diameter of approximately 14 inches, the determination of local stress and stiffness angles for approximately 60 different points for a half tire section provides a satisfactual representation of the configuration in which the tire tends to assume for natural shape.

It is believed that when the tire is cured in the configuration in which the local stiffness angle of the composite tire structure is substantially equal to the local stress angle of the composite tire structure for a plurality of points about the entire cross-section of the tire, the tire will then have a natural shape.

Accordingly in those areas of the tire in which it is desired to have natural shape, the local stiffness angle should be substantially equal to the local stress angle. For the purposes of this invention, what is meant by substantially equal stiffness angle and stress angle is dependent upon which area of the tire the values are being determined. The greater the value of the local stiffness angle for any particular point, the greater the difference between the stiffness angle and the stress angle may be at that point Referring to Figure 3 there is illustrated in graphical form the acceptable difference between the stiffness angle and stress angle for calculated values of stiffness angles. The acceptable values being those areas below the curve. The solid line indicates the acceptable values for the difference between the local stiffness angle and local stress angle for any calculated stiffness angle at any point, and the dash line indicates the preferred maximum difference.

For example when the stiffness angle is approximately 30" the maximum acceptable difference between the stiffness angle and the stress angle is about approximately 2" and preferably is less than about 1". As the calculated stiffness angle increases the maximum allowable difference also increases. When the stiffness angle for the composite structure approaches 90", the maximum acceptable difference is approximately 20".

For the purpose of this invention the stiffness angle 05 is defined by the following relationship:

wherein the meridional membrane stiffness and the circumferential membrane stiffness are equal to the Al, and A22 terms, respectively, as determined by the Halpin-Tsai relations and laminate theory.

For the purposes of this invention, the stress angle Ok iS represented by the following relationship:

wherein the lateral membrane stress and circumferential membrane stress resultants satisfy the following relationship: N1+ N2 p =-+ P1 P2 wherein P is equal to the pressure and N1 and N2 are stress resultants in the circumferential and lateral directions and p1 and p2 are the radius of curvature of the elements in the circumferential and lateral directions respectively. The subscripts can, of course, be reversed depending on the coordinate system being used.The meaning of stress resultant and radius curvature of the element can be better understood by reference to Figure 2 wherein there is illustrated a doubly curved surface carrying a pressure load, such as would a pneumatic tire.

In the designing of a pneumatic tire there are certain physical parameters of the tire that are generally selected. They are: 1. The radially outermost point of the carcass ply structure; 2. The maximum axial distance from the mid-circumferential centerplane of the tire to the neutral carcass ply; 3. The coordinates of the bead core and geometry of the rim; 4. Construction of material to be used for the carcass ply structure; 5. The type and amount of rubber to be used in the sidewalls, tread portion and bead areas; and 6. The configuration of the belt structure, if any, and the material from which the cords of the belt structure are to be made.

Knowing the foregoing physical parameters the local stress angle can be readily obtained through finite element analysis techniques such as that illustrated in "The Finite Element Method" by O.C. Zienkiewicz published by McGraw Hill and "Primer on Composite Material" by Halpin, Tsai, Ashton and Pettit published by Technomic Publications.

Applicants, by using finite element analysis and laminate theory, take into consideration the carcass ply structure in determining natural shape for that particular tire wall also taking into account the other physical features of the tire such as the rubber compound, other reinforcing belt structures. Through applicant's invention it is now possible to design and construct a pneumatic tire so that the entire cross-section of the tire can be cured in the natural shape. For general application of the present invention the shape in which a tire is molded can be considered the shape in which it is cured.However, under normal tire manufacturing practices the tire undergoes some degree of shrinkage and deformation after it is removed from the mold, therefore, it is preferred to mold the tire such that when the tire is completely cured it is in its natural shape configuration as taught by applicant.

In a modified form of the present invention, the natural shape configuration of the tire is determined by a combination of applicant's method and the prior art. Applicant's method is used to determine the configuration in areas of the tire reinforced with additional reinforcement, such as the tread portion. The configuration of the carcass structure from the bead area to a point ps in the shoulder area is determined in accordance with the prior art. Referring to Figure 4, there is illustrated tire 110 wherein the carcass ply structure 120 has a natural shape from Yb,pb in the bead area of the tire to the point ps in the shoulder area.

These points, where the carcass deviates from the natural shape, are selected by the tire designer as desired.

The carcass ply structure 120 in this area follows a path in accordance with the following relationship:

wherein y is the perpendicular distance from the mid-circumferential plane of radius po to any point on the tire carcass for any specified p value; pm is the radius from the axis of rotation to the point on the tire carcass where y is maximum; p is the distance from the axis of rotation to a point on the tire; a is the cured cord angle, the outer angle which the cord makes with the circumference of any circle in the circumferential direction of the tire; po is the radius from the axis of rotation of the tire to a point determined by an iterative method such that the curve defined by equation (1) will pass thru a point ps (specified in the shoulder area) and Ym, pb, Yb determined for the carcass structure in the bead area;; ps is the distance from the axis of rotation to a point in the shouideharea where the carcass contour deviates from equation (1); Yb,pb is the point at which the carcass contour deviates from equation (1); pb is the distance from the axis of rotation to the point on the tire carcass; Yb is the distance from the mid-circumferential plane of radius po to the point on the tire carcass.

The central portion of the tread in between the two meeting points ps has a configuration in the cured state such that the local stress angle and local stiffness angles are substantially equal for a plurality of points across the central portion L.

While certain representative embodiments and details have been shown for purposes of illustrating the invention, it will be apparent to those skilled in the art that various changes and modifications may be made.

For example, the entire composite may not necessarily be cured in its natural shape, but only the portion of the tire composite wherein it is determined to have natural shape.

Claims

1. A pneumatic tire comprising: a ground engaging tread portion, said tread portion terminating in a pair of shoulder portions at each lateral edge thereof, a pair of sidewall portions extending radially inward from said shoulder portions terminating in a pair of bead portions respectively, each of said bead portions having at least one inextensible bead core, a carcass reinforcing ply structure extending from bead core to bead core, characterized by at least a portion of said tire is cured in a configuration such that the natural equilibrium curve of the tire coincides with the lateral neutral bending axis of said tire, said natural equilibrium curve of said tire is the curve defined by a toroidal membrane which represents the composite tire structure under design boundary conditions and loads as viewed in the plane passing through the axis of rotation of said tire wherein circumferential and meridional elongation properties of the toroidal membrane are substantially equivalent to that of the composite tire structure.

2. A pneumatic tire comprising: a ground engaging tread portion, said tread portion terminating in a pair of shoulder portions at each lateral edge thereof, a pair of sidewall portions extending radially inward from said shoulder portions terminating in a pair of bead portions respectively, each of said bead portions having at least one inextensible bead core, a carcass reinforcing ply structure extending from bead core to bead core, characterized by at least a portion of said tire is cured in a configuration such that the local stiffness angle and local stress angle for any given point in said portion are substantially equal.

3. A pneumatic tire comprising: a ground engaging tread portion, said tread portion terminating in a pair of shoulder portions at each lateral end thereof, a pair of sidewall portions extending radially inward from said shoulder portions terminating in a pair of bead portions respectively, each of said bead portions having at least one inextensible bead core, a carcass reinforcing ply structure extending from bead core to bead core, characterized by said tire being cured in a configuration such that said carcass reinforcing ply structure from a point adjacent each associated bead area radially outward to a preselected point in said shoulder portion follows the relationship:

the remaining portion of said tire being cured in a configuration such that the stiffness angle is substantially equal to the stress angle for any given point.

4. A pneumatic tire according to claim 1, 2 or 3 wherein said carcass structure comprises a plurality of parallel cords, said cords forming an angle of from about 75" to 90 with respect to the mid-circumferential centerplane of said tire.

5. A tire according to claims 1, 2 or 3 characterized by each of said bead portions having a circumferentially extending reinforcing structure located axially outward of said carcass ply structure, said reinforcing structure extending radially outward from a point radially inward of the radially outermost point of said bead core in said respective bead portion to a point, radially outward of the tire rim contact point.

6. A tire according to claims 1 or 2 wherein said tire is further characterized by a belt reinforcing structure placed radially outward of said carcass ply structure in said ground engaging tread portion of said tire.

7. A tire according to claim 1 further characterized by said entire cross-section of said tire between said bead portions being cured in the natural shape of said tire.

8. A tire according to claim 2 wherein said plurality of points comprises at least twenty-five parts.

9. A tire according to claims 1 or 2 wherein said tire is further characterized by a belt reinforcing structure being placed radially outward of said carcass ply structure in said ground engaging tread portion of said tire, said entire cross-section of said tire between said bead portions being cured in the natural shape of said tire.

10. A pneumatic tire comprising: a ground engaging tread portion, said tread portion terminating in a pair of shoulder portions at each lateral end thereof, a pair of sidewall portions extending radially inward from said shoulder portions terminating in a pair of bead portions respectively, each of said bead portions having at least one inextensible bead core, a carcass reinforcing ply structure extending from bead core to bead core, characterized by said tire being cured in a configuration such that said carcass reinforcing ply structure from a point adjacent each associated bead area radially outward to a preselected point in said shoulder portion follows the relationship: :

the remaining portion of said tire being cured in a configuration such that the natural equilibrium curve of said tire coincides with the lateral bending axis of said tire, said natural equilibrium curve of said tire is the curve defined by a toroidal membrane which represents the composite tire structure under design boundary conditions and loads as viewed in the plane passing through the axis of rotation of said tire wherein circumferential and meridional elongation properties of the toroidal membrane are substantially equivalent to that of the composite tire structure.