GB2138367A - Self-supporting pneumatic vehicle tyre - Google Patents

Abstract

The tyre comprises a radial carcass having at least two piles 1,2 or groups of plies between which is incorporated, in each sidewall of the tyre, a supporting component (10) made of a soft elastomeric material, of lenticular cross-sectional shape, which extends from a filler (8) (which overlies the bead core (3)) to a location under the belt (5,6,7) which underlies the tread band (4). The elastomeric material of component (10) has an ISO hardness not exceeding 47 and a thermal stability not less than 20 minutes for a temperature increase not exceeding 25 DEG C. Instead of the load, in the deflated tyre, being borne by the material of the sidewalls, the component (10) tends to maintain the tyre shape. <IMAGE>

Description

SPECIFICATION Self-supporting tyre This invention relates to a self-supporting tyre.

A self-supporting tyre is one which is capable of continuing its travel for an appreciable distance and at a non-negligible speed, even under conditions of complete deflation, without this bringing about any contact between the inner sidewall surface and the inner tread surface; this is a characteristic condition in a tyre which is able to endure the so-called "run-flat condition".

In particular, the present invention relates to radial tyres, i.e. to those tyres whose carcass cords, extending from one bead to theother across the crown zone of the tyre, lie in radial planes which contain the rotational axis of the tyre or in planes which make small angles with said radial planes.

Tyres of the self-supporting type are already known to technicians. Such a tyre has a carcass structure which comprises a component made of an elastomeric material, said component having a considerable thickness and having a high degree of hardness and being inserted into the tyre sidewall. This component can be disposed, as a matter of choice, radially outwardly of the carcass plies, or between said plies, or even radially inwardly of said plies.The purpose of said component, which normally has a lenticular cross-section tapered towards its extremities, namely, tapering respectively in the directions both of the associated bead and of the tread, is that of developing an efficient supporting action for the load that bears-down upon the wheel when the supporting action which is normally provided by the air at correct inflation pressure inside the tyre is lacking due to deflation which may be only partial.

These tyres have a drawback in that, during use under deflated conditions, the lenticular rubber component (which becomes greatly over-stressed by cyclical compressions due to the effect of the weight of the vehicle bearing down upon the tyre sidewalls) absorbs a considerable amount of horsepower; this leads to the generation of a lot of heat (difficult to dissipate) which causes progressive lowering of the resistance characteristics or strength of the entire tyre structure and, in particular, the resistance of the rubberized fabric, or of the cords in the carcass plies, and of the profile itself.

This progressive degeneration of the characteristics of the constituent elements of the tyre (and, hence, of the general tyre structure) puts a limit on the distance which can be covered by the tyre and on the speed at which the tyre can travel under deflated conditions. Moreover, in general, at the end of a period of use under the above-mentioned conditions, the tyre is seen to be greatly damaged and is no longer usable.

It has now been found possible to raise the aforesaid limits of tyre use to an appreciable extent. So much so, that the aim of the present invention is to provide a tyre of the self-supporting type, provided with a new structure which functions in a new way and which is such as to allow the tyre to run under load, in a deflated condition, for long distances and at high speeds, not once but several times during the service-life of the tyre.

Accordingly, the present invention consists in a radial tyre for vehicle-wheels, provided with a textile-carcass comprising two plies, or groups of plies, with a tread-band placed in the crown-tone of said carcass, and with an annular circumferentially inextensible reinforcing structure inserted between said carcass and said tread-band, with sidewalls and beads for anchoring said tyre to a corresponding mounting-rim, with each of said beads comprising an annular, circumferentially-inextensible reinforcing core and an elastomeric-material filler of substantially triangular section radially overlying said annular reinforcing core, said carcass plies being turned-up, from the inside towards the outside, around said annular reinforcing core; said sidewalls comprising an annular, elastomeric-material component having a substantially lenticular cross-section and extending between said beads and said crown zone, the maximum thickness of said component being in the radially outer half-portion of said sidewall, said component being inserted in-between said plies or groups of plies, and having an ISO-hardness not exceeding 47, and a thermal stability (defined as 'destruction-time' of a test sample of said elastomeric-material on the "Goodrich flexometer") of not less than 20 minutes.

Preferably, the elastomeric-material of said component has a tgb mechanical value which does not exceed 0.0750, and a dynamic modulus of elasticity which does not exceed 4 Megapascals (MPa).

In a preferred embodiment of said tyre, the radially outer extremity of said component extends axially towards the inside of the tyre, thus overlapping the annular reinforcing structure, over an axial distance which does not exceed 15% of the total width axially of the tyre of said annular structure; whereas the radially inner extremity of said component overlaps the filler, with the interposition of the ply or group of plies which is axially outwardly of said radially inner extremity, over a distance which does not exceed 20% of the tyre section height. As for the filler, quite opportunely, it may extend radially outwardly from said core to a height of not less than 15% of the section height of the tyre.

The radially inner extremity of said component may surmount, through the filler, one edge of a textile-fabric reinforcement which is disposed in a position axially outwardly of the turn-up of the carcass plies and which extends radially outwardly from the annular core to a height which does not exceed 40% of the section height of the tyre. Moreover, the carcass plies may be reinforced with textile cords (natural and/or artificial) which are resistant to heat-action, and which are for preference of rayon or aromatic polyamide.

One embodiment of a self-supporting tyre according to the present invention will now be described with reference to the accompanying drawing which shows said tyre in cross-section. In the following description, there will generally be a description of one element of structure at a time although there may be two of them in fact (e.g. in each sidewall); this is, of course, due to the fact that the Figure only shows one portion as a true section with the various elements hatched or otherwise marked.

The tyre comprises a bi-ply textile radial cord carcass which is consitituted by two rayon plies 1 and 2, turned-up at their extremities, from the inside towards the outside, around an annular bead core 3, which reinforced the beads of the tyre.

As an alternative, the number of carcass plies could also be more than two, but these will always be divided into two groups, which are separate from each other.

In the crown zone, there is disposed a material tread band 4, made of an elastomeric material, in which the tread pattern is impressed by means of moulding. Between this tread band and the carcass, there is inserted an annular, circumferentially inextensible, reinforcing structure which is of commonly known type known as a belt or cincture; for example, in the tyre illustrated in the drawing, this conveniently comprises two metallic-cord layers 5 and 6, the cords being parallel to one another in each layer and the cords in one layer extending in a direction crossing that in which the cords of the adjacent layer are laid. The cords in the two layers cross symmetrically with respect to the mid-circumferential plane of the tyre.The belt also has another layer 7 of textile-material cords, (for example, of nylon which is disposed radially outwardly of the layers 5, 6 and which is wide enough to cover the axially outermost extremities of these layers.

The total width L, of this belt (in the Figure, only the part U2 is shown) corresponds to the distance between two planes which are parallel to said mid-circumferential plane and which are respectively placed halfway between the offset between the widest metallic layer and the narrowest.

In the position radially outwardly of said bead core 3, there is disposed an annular filler 8 which is made of a high-hardness compound and which extends radially outwardly. The bead is further reinforced by a strip 9 of textile cords (preferably, nylon) inclined with respect to the mid-circumferential plane of the tyre at an angle which falls within the range from 20 to 45 , preferably at 22". The strip 9 is disposed axially outwardly of said filler 8 and of the turn-ups of the carcass plies 1,2, and it extends radially outwardly from the bead core 3 up to a height which does not exceed 40% of the section-height H of the tyre.

Between the carcass plies 1, 2 in the tyre sidewall, there is a component 10 which is made of an elastomeric material and which has a lenticular cross-section. The maximum thickness of said component occurs in the radially outer half-portion of the sidewall and said component tapers towards its extremities, namely, in directions towards the treadband and towards the bead core, respectively.

The elastomeric material which constitutes said component is a very soft material which, in itself, is substantially unsuited for developing any support for the load which bears on the wheel when the tyre is in a deflated condition. Quite conveniently, the hardness of this material, measured according to the Standard ASTM 1415-81, must not exceed 47. Likewise, even the dynamic modulus of elasticity must not be greater than 6 Megapascals, and preferably not greater than 4 Megapascals.

The main characteristic of this material must be that of standing up to a very high number of working-cycles (namely, whenever the sidewall of the deflated tyre bends under load during travel of the vehicle) without absorbing excessive amounts of horsepower and without generating any great amount of heat, but whilst maintaining its desirable performance characteristics constant or unchanged, even at high temperatures, throughout the passage of time and notwithstanding repeated use of the tyre.

These desirable performance characteristics are guaranteed whenever the said elastomeric material has a thermal stability (defined as the time necessary for destroying a test-sample of said elastomeric material under predetermined and carefully controlled conditions upon a Goodrich flexometer and according to the Standard ASTM 623-78) of not less than 20o, and a mechanical tg3 not exceeding 0.075.The thermal stability of a material, is precisely its capacity to resist severe dynamic stresses over a long period of time, i.e. to maintain its physical and chemical characteristics constant or unchanged notwithstanding the prolonged dynamic fatigue it has to endure, and the consequent temperature increases - (which temperature increases tend to destroy the chemical cross-linking nodes of the cure or vulcanization, resulting in a deterioration of the structure itself) and, hence, its behaviour characteristics.

It results from what has been said above that the compound should have such characteristics as to minimize any temperature increases due to said dynamic stresses, namely, it must present low values for the dynamic modulus, and for the mechanical tg5 which represents the "hysterical loss"; this known parameter is proportional to the amount of energy dispersed - with respect to the maximum elastic energy imposed.

For determining which compounds would satisfy the requirements of a tyre according to the invention as regards thermal stability, it was found to be opportune to utilize a Goodrich flexometer according to the Standard ASTM, as already mentioned. Briefly, a "Goodrich flexometer" is comprised by a bar which is placed in such a way as to balance on a knifeblade, in the manner of a pair of scales. Two weights of 24 Kg each, are applied to the two ends of this beam in order to raise the level of the moment of inertia of the system, and, in this manner, to damp any oscillations existing therein. The test-sample to be measured is disposed between the bar (on the side opposite to the weights and at a point very near to the fulcrum) and the head of a piston which is to be used to stress the test-sample, axially under compression, in a sinusoidal way, with the form and frequency value as imposed. A test-load is disposed on the bar extremity on that side thereof which is opposite the test-sample with respect to the fulcrum. A micrometrical screw allows for shifting the supporting base of the test-sample, in both directions perpendicularly to the bar, in the direction of the piston-head so that, through the choice of this distance and of the weight of the test-load, it becomes possible to subject the test-sample to an initial pre-loading whilst maintaining the direction of the stresses absolutely axial.

The test is carried out by subjecting the test-sample to a continuous series of cyclical compressions until the rupture-point (blow-out) is reached. Moreover, the temperatures of the sample (both before and after the test) are taken. The time required for taking the test-sample to the blow-out point, as well as the noted increase in temperature, define adequately the characteristic of thermal stability for the elastomeric material under examination.It has been found that the most significant testing conditions are comprised by a frequency of sinusoidal oscillations on the piston equal to 30Hz with an imposed deformation, between one peak and the next of the sinusoidal wave, equal to 6.35 mm, with a static load on the bar equal to 48.46 pounds (weight) corresponding to a pre-load on the test-sample of 489 Newtons, and an initial temperature of the test-sample equal to 100,C. Subjected to these testing conditions, the compounds undergo a testing-time of not less than 20 minutes and show, at the termination of the test, an increase in temperature that does not exceed 25"C.

As far as concerns the dynamic modulus of elasticity and the mechanical tg8, the above-mentioned characteristics have been determined by the Applicants through subjecting the test-sample of elastomeric material which is to be examined (cylindrical; height 26 mm; diameter 29 mm; pre-compression 20%), to cyclical sinusoidal deformations having an amplitude equal to 6% of the height of the undeformed test-sample, said deformations being generated by a servo-hydraulic apparatus, controlled by a computer that measures the area W of the hysteresis cycle (expressed in Joules/m3), the maximum stress values (in Pascals), and the maximum deformation value with hence, calculating the mechanical W tgb = tg arc sln,,,sd and the dynamic modulus of elasticity S E' =S cos 6.

Solely by way of example, there are given below the constituents and proportions for a compound that has proved satisfactory for making components for use in the tyre according to the invention, for showing through it the most important physical characteristics, as measured on the vulcanized compound:: Constituents Parts by weight per 100 of rubber Standard Malaysian Rubber 10 100.00 Zinc Oxide 5.00 Stearic Acid 2.50 Flectol-M (by Monsanto) 1.00 Isopropyl-phenyl-paraphenylenediamine 1.50 Microcrystalline Wax 1.50 Polyplastol-6 (by Bozzetto) 3.00 Carbon-black N-326 20.00 N-oxydiethylenebenzothiazole-2-sulfenamide 1.50 Diphenyl-dimethylthiuramdisulfide 0.375 Sulfasan-R (by Monsantoo 2.00 Characteristics Thermal stability 26' Dynamic modulus of elasticity 3.175 MPa Mechanical Tg8 0.072 Temperature increase at termination of Test 18"C In the preferred embodiment of the self-supporting tyre, shown in the accompanying drawing, the component which has a lenticular cross-section and which is made of an elastomeric material having the above-indicated characteristics, has its radially outer extremity extending in a generally axial direction towards the inside or tyre cavity and also extending radially inwardly of or below the belt and up to a point whose distance b from the nearby axially outermost extremity of the belt is equal to 12.5% of the total axial width of the belt structure; said distance b must in any case be less than 15% of said total axial width.

Therefore, the belt and the lenticular component are mutually overlapping within the above-indicated limit.

Even in the lower or radially inner zone of the sidewall, towards the bead, said lenticular component and the filler and the strip 9 of reinforcing fabric mutually overlap in such a way as to create a progressive stiffening of this zone, said stiffening being necessary for establishing good behavioural characteristics in the tyre when it travels on the road. Thus, the component overlaps the filler (through the outermost ply or plies 2) over a distance c which does not exceed 20% of the section-height of the tyre. The extent of the zone in which the aforementioned overlapping occurs may vary,-within certain limits, inside the tyre sidewalls; in any case, the radial height fof said filler 8 must not be less than 15% of the section-height of the tyre.

With regard to the strip 9 which is provided to protect and to reinforce the bead, it is preferable for the radial height of said strip not to exceed 40% of the section-height of the tyre.

Moreover, the component 10 has the portion thereof which is of maximum thickness in the radially outermost portion of the sidewall and said thickness preferablyfalls within the range from 3% to 6% of the maximum width (chord) of the tyre.

The self supporting tyre according to the invention has made it possible for a vehicle to cover a substantially increased distance compared with what has been possible until now, and also at a higher speed, under conditions of complete tyre-deflation; moreover, a substantial increase in the service-life of the tyre has also been obtained. These advantages stem from the fact that we have abandoned the old concept of causing the load which is being carried by the deflated tyre to be borne by the component which is introduced into the sidewalls in favour of the concept of causing the carcass plies (and not the interposed component) to do the greater part of the necessary work.

It must be pointed out that an elastomeric material is, to all intents and purposes, a volumetrically incompressible material. So much so that, when this material is deformed by being compressed in a certain direction, it expands in other directions as a consequence of this substantial 'incompressibility' effect. As a consequence of this, when a compound (as is the case in the tyre according to the invention) is enclosed between the carcass plies and is therefore in a substantially closed container, said compound is subjected to tri-axial compression during travel of the deflated tyre; this imparts a rigidity, particularly flexional rigidity, to the assembly which is considerably higherthan the rigidity inherent or actually found in the compound in question.

It has been noted, moreover, that whenever the tyre is subjected to a load under deflation conditions, the compound which is inserted in-between the carcass plies is strongly compressed in the radial direction due to the effect of the bending in the tyre-sidewall, so much so that, owing to the effect of its being substantially incompressible, it has to be able to expand in the other directions. However, the compound cannot expand circumferentially of the tyre because, in this direction, the tyre does not admit any dimensional variations.

Hence, for expansion, there remains only the transverse (or, more accurately, the axial) direction and, in this latter direction, any tendency of the compound to expand towards the outside is impeded by the axially outer or outermost carcass ply or plies, which is/are subjected to great tension; thus, the compound is indeformable in said direction. Therefore, the compound expands axially towards the inside ortyre cavity where said carcass ply or plies, due to the effect of the sidewall bending, are subjected to a state of compression which is permissive of such an expansion. Due to the effect of this axial (inwards) expansion of the component 9, the axially inner plies or group of plies are therefore, put into a state of tension or traction, thus contributing efficaciously towards supporting the load upon the tyre in the absence of inflation pressure.

In other words, in the tyre according to the invention, the purpose of the lenticular shape of the component 10 is not so much that of directly supporting the load as that of maintaining the geometry of the tyre structure. Apart from anything else, what will be evident from what has been said above is how essential it is for this component 10 to be inserted between the carcass plies and not in any alternative dispositions (whether outside or inside) with respect to said plies.

It will be readily understood why the compound of which said component 10 is made must absorb the minimum of horsepower and must have the maximum thermal stability; from this point of view, the values mentioned above have proved to be critical values for the purpose of providing good functioning of the tyre.

As a matter of fact, it is clear that should these values vary, i.e. when the hysteresis of the compound of the component 10 increases, the heat generated by this component also increases during the running of a deflated tyre. Moreover, this heat proves to be quite difficult to get rid of, precisely owing to the thickness of the component 10, which thickness lessens the conduction of heat towards the outside, and consequently, the cooling of the tyre sidewalls during running of the tyre. Thus, with the temperature increasing in the component 10, the desirable characteristics of the compound of said component could be compromised within a short time together with the characteristics of resistance of the entire tyre structure, and in particular, those of the carcass plies. Precisely for this purpose, it is preferable for the reinforcing cords of the carcass plies to be constituted by a material which is resistant to heat-action; thus, these cords may conveniently be made out of natural and/or artificial textile materials such as, for example, rayon or aromatic polyamide, one example ofthe latter being well-known under the Registered Trade Mark KEVLAR.

In the above description and in the appended Claims, the word "textile" used to describe material such as cords used in the tyre carcass has the meaning conventionally attributed to it in the tyre industry and comprises, by way of example, organic materials such for example as cotton and rayon, and fibres from synthetic polymers, such for example as polyamide fibres and polyester fibres.

Claims (12)

1. A radial tyre for vehicle wheels which has a textile (as defined above) carcass, said carcass comprising two plies, or group of plies; a tread band placed in the crown zone of said carcass; an annular, circumferentially inextensible reinforcing structure inserted between said carcass and said tread band; sidewalls; beads for anchoring said tyre to a corresponding mounting-rim and each comprising a circumferentially inextensible annular reinforcing core; and a filler of elastomeric material having a substantially triangular cross-sectional shape, said filler being located radially outwardly of said bead core; said carcass plies being turned-up, from the inside towards the outside, around the respective bead core; each of said sidewalls comprising an annular component made of an elastomeric material and having a substantially lenticular cross-sectional shape and extending between said beads and said crown zone; said component having its maximum thickness at or in the radially outer half-portion of said sidewall and being inserted in-between said plies or groups of carcass plies; and said component also having an ISO-hardness not exceeding 47 and a thermal stability (defined as 'destruction time' of a test-sample of said elastomeric material tested on a Goodrich flexometer) of not less than 20 minutes.

2. A tyre according to Claim 1, wherein said elastomeric material presents a mechanical tg5 value not exceeding 0.0750.

3. A tyre according to Claim 1 or Claim 2, wherein said elastomeric material presents a dynamic modulus of elasticity not exceeding 6 Megapascals.

4. A tyre according to any one of the preceding Claims, where the radially outer extremity of said component extends axially inwardly so as to lie radially inwardly of said annular reinforcing structure over a distance, axially of the tyre, which does not exceed 15% of the total axial width of said annular structure.

5. A tyre according to any one of the preceding Claims, wherein the radially inner extremity of said component overlaps said filler, with the interposition of the axially outer ply or group of plies, over a distance which does not exceed 20% of the section-height of the tyre.

6. A tyre according to Claim 5, wherein said filler extends radially towards the outside up to a height that is not less than 15% of the section-height of the tyre.

7. A tyre according to any one of the preceding Claims, wherein the radially inner extremity of said component overlaps, with the interposition of said filler, a reinforcing strip of textile fabric, which strip is disposed axially outwardly of said turn-up of the carcass plies and extends radially outwardly from said annular bead core up to a height not exceeding 40% of the said section-height of the tyre.

8. A tyre according to any one of the preceding Claims, wherein said carcass plies are rubberised and contain rayon cords.

9. A tyre according to any one of Claims 1 to 7, wherein said carcass plies are reinforced with aromatic polyamide cords.

10. A tyre according to any one of the preceding Claims, wherein the maximum thickness of said component falls within the range from 3% to 6% of the maximum width (chord) of said tyre.

11. A radial tyre for vehicle wheels constructed, arranged and adapted to operate substantially as hereinbefore described with reference to and as illustrated in the accompanying drawing.

12. Any features of novelty, taken singly or in combination, of the embodiments of the invention hereinbefore described with reference to the accompanying drawing.