GB1567614A

GB1567614A - Radial carcass tyres

Info

Publication number: GB1567614A
Application number: GB52910/76A
Authority: GB
Original assignee: Industrie Pirelli SpA; Pirelli SpA
Current assignee: Industrie Pirelli SpA; Pirelli and C SpA
Priority date: 1976-01-05
Filing date: 1976-12-17
Publication date: 1980-05-21
Also published as: FR2337053B1; IE44434L; FI58457B; NO144733C; ES225966U; LU76515A1; DK3877A; NO770006L; DE2659629A1; GR71445B; IT1054931B; BR7700018A; SE421765B; ES225966Y; AU506874B2; FR2337053A1; ZA767643B; AU2093676A; JPS5284610A; IE44434B1

Description

(54) IMPROVEMENT TO RADIAL CARCASS TYRES (71) We INDUSTRIE PIRELLI SPA, an Italian Company of Centro Pirelli, Piazza d'Aosta No. 3, Milan 20100, Italy do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to pneumatic tyres for vehicle wheels and in particular to tyres provided with a radial carcass (i.e. a carcass whose cords lie on radial planes or substantially radial planes), particularly suitable for heavy vehicles for normal and heavy duty and intended to be used for normal and heavy duty including long distances at normal and high speed.

It is known that radial tyres are usually provided with a structure constituted by one or more reinforcing layers arranged at the crown zone of the carcass. below the tread. This structure is provided mainly to withstand the tension stresses to which the tyre is subjected, either because of the inflation pressure or in heavy service conditions due to the high speed.

Clearly, in the case of tyres of large sizes intended to be used on motor vehicles for normal and heavy duty, such as buses or coaches, or motor vehicles for industrial use, the tension stresses which are exerted on the reinforcing structure are very high.

Hence, it is necessary to provide such tyres with a reinforcing structure which is especially strong. However, the strong reinforcing structure should not be constituted by too high a number of layers, since this would produce an increase of weight and! or of the tyre size and would impose an increase in the required inflation pressure with the consequent increase of the tension stresses on said reinforcing structure.

In view of the aforesaid problems the trend in the art is to form the reinforcing structure with the least number of layers each layer utilising a material having a high modulus of elasticity. Theoretically, the reinforcing structure which would assure the maximum resistance to tension stresses is that constituted by one or more layers of metallic cords, having an ultimate elongation equal to 3% and oriented in a direction substantialy parallel to the circumferential direction of the tyre. However, such a reinforcing structure is not satisfactory as regards the driving behaviour of the tyre, particularly as regards the transversal stability when the tyre runs both along a curved trajectory and along a straight one.

Consequently, the reinforcing structure usually used to achieve the above described aims is constituted by two layers of metal lic cords, arranged so that the cords, parallel to one another in each layer, are sym metrically inclined with respect to the circumferential direction at a relatively small angle, i.e. between 5 and 30".

In such a way, the intrinsic rigidity of the material constituting the two layers is attenuated by a certain flexibility of the resulting reinforcing structure. In fact, the tension stresses dut to tyre inflation result in a reduction of the value of the respective inclination angle of the cords of the two layers and a consequent increase of the tyre diameter. Consequently, the reinforcing structure is also quite satis factory with regard to the torsional or transversal stiffness which leads to a tyre which offers a good driving behaviour, par ticularly as regards both the driving sta bility when the tyre runs along a straight trajectory and the lack of drift when the tyre runs along a curved trajectory.

However, especially in the case of large size tyres, which utilise a radial carcass constituted by a mono-ply of metallic cords, it has been found that, although the aforesaid reinforcing structure effli ently withstands the tension stresses to which the tyre is subjected, it is not sufficient to counter-balance the high carcass flexibility; consequently the tyre does not provide sufficient steering response. Therefore it is necessary to add other layers to the reinforcing structure acting as a stabilizer; in general a further strip of metallic cord is used. The strip is arranged so that the cords are inclined with respect to the circumferential direction at a high angle, for example 60".

The Applicant has now surprisingly found that, contrary to the aforesaid, the application to a reinforcing structure constituted by two layers of metallic cords as described of a layer of metallic cords having a relatively high elongation and arranged in a direction substantially parallel to the circumferential direction of the tyre, not only gives the driving stability effects which previously were considered due exclusively to the strips of metallic cords having high inclination with respect to said circumferential direction, but also a very high resistance to tension stresses.

More specifically, the reinforcing structure thus constituted has a high enough resistance so that it withstands not only the tension stresses due to the inflation pressure of the tyre, but also those which occur when the tyre runs at high speed.

According to one aspect of the present invention pneumatic tyre for motor vehicles, specifically intended to be used for normal and heavy duty, having a radial carcass is provided with a (tread) reinforcing structure comprising two layers which are formed by metalic cords having an ultimate elongation of 3%, said cords being parallel to one another in each layer and crossed with respect to those of the other layer, said two layers being arranged so that said cords are inclined with respect to the circumferential direction of the tyre at an angle between 5 and30" characterised by the fact that radially outwards of the said two layers there is at least one layer comprising metallic cords parallel to one another and substantially parallel to the circumferential direction, said metallic cords having an ultimate elongation between 4 and 8%.

The tyres having the assembly of reinforcing layers as described above can be of the type intended to be mounted on the vehicle with or without an inner tube; moreover, although said assembly of reinforcing layers is extremely advantageous for tyres having large sizes provided with a carcass constituted by a radial metallic mono-ply, it is understood that it may be applied to tyres provided with a radial carcass having a plurality of plies and constituted by cords of textile material.

Preferably, the layer of metallic cords belonging to the reinforcing structure, which is immediately below the layer of metallic cords arranged in the circumferential direction, has a width at least equal to that of said layer. The other layer constituting said reinforcing structure may have a slightly smaller width.

As aforesaid, it has been found that the large size tyres, particularly intended to be used for normal and heavy duty, which are provided with said assembly of reinforcing layers, are able to offer high performances not only in service conditions at normal speed but also at higher speeds, and they have a very good driving behaviour.

The effect of said assembly of reinforcing layers appears to be still more surprising, if it is taken into account that tests carried out on similar tyres, having the same reinforcing structure constituted by two layers of metallic cords having an ultimate elongation equal to 3% to which a layer of metallic cords of the same elongation to said two layers and arranged in the longitudinal direction had been superimposed, have shown a clear worsening of the driving and comfort characteristics.

In other words, having to constitute an assembly of reinforcing layers which comprises a layer of metallic cords arranged in the circumferential direction, which per se produces, as aforesaid, an improvement in respect of the resistance to the tension stresses, it has been found that utilising metallic cords having a low ultimate elongation it is undoubtedly advantageous for the abovermentioned aid. However, it has been found that in the assembly of the three layers, this layer of circumferential metallic cords supports almost all the stresses. Therefore, the other two layers, not being under tension, do not intervene to affect positively the driving stability characteristics of the tyre.

The effect of the assembly of reinforcing layers according to the present invention is probably due to the fact that the relative extensibility of the metallic cords chosen for the radially outer layer or layers in the reinforcing structure according to the invention results in that when the tyre is in service conditions, said layer or said layers have a strengthening effect which does not exclude the effect of the other two layers of the reinforcing structure which, although being per se constituted by metallic cords which are relatively inextensible, having a certain extensibilitv because of the inclined arrangement of the cords with respect to the circumferential direction.

Examples of metallic cords having an ultimate elongation between 4 and 8%, which hereinafter we will call "extensible" are per se widely known to a person skilled in the art. They may be constituted by strands formed by a plurality of filaments or wires twisted together, and the individual strands of the cord each being twisted in the same sense.

Vice versa, the metallic cords having an ultimate elongation equal to 3%, which are also per se known, may be constituted by strands formed by a plurality of filaments or wires twisted in opposite sense to the twist of the strands of the cord.

According to a preferred embodiment of the present invention, the layer or the layers of the extensible metallic cords may be formed by a single extensible cord helically wound, with the convolutions of the helix arranged in the circumferential direction.

According to another preferred embodiment, said layer or each of said extensible metallic cords layers may be formed by strips placed side by side, each strip having a width equal to a portion of the tread width and each strip extending circumferentially on the whole development of the tyre.

The invention will be further understood by the following description, by way of example, of the embodiments of the invention in conjunction with the attached drawings in which: Figure 1 shows partially sectioned in a radial plane, a tyre according to the present invention; Figure 2 shows partially sectioned a perspective view of the tyre shown in figure 1, with the section to better illustrate the arrangement of the various cords; Figure 3 shows partially sectioned in a radial plane a tyre according to an alternative embodiment of the present invention.

More particularly, figure 1 shows a tubeless tyre of the size 11R22.5 which comprises a tread 1 and a carcass 2 constituted by a mono-ply of metallic cords lying in radial or substantially radial, planes.

Between the tread and the carcass an assembly of reinforcing layers is inserted, comprising two layers 3 and 4 of metallic cords having an ultimate elongation equal to 3% and a radially outer layer 5 with respect to the other two layers constituted by metallic cords having an ultimate elongation of 7%. The layers 3 and 4, as shown more clearly in figure 2, are arranged so that their metallic cords are parallel to one another in each layer and inclined respectively according to an angle and and P of 18 with respect to the circumferential direction. The layer 5 has its metallic cords arranged in the circumferential direction of the tyre.

The aforesaid assembly of layers has a width only slightly less than that of the tread 1 as is customary; the reinforcing layers are in addition arranged to provide a certain graduation and it is preferred according to the present invention that the layer 4 has, in the reinforcing structure, the greater width so as to project slightly at both sides with respect to the layer 5, whilst the layer 3 has a slightly smaller width with respect to that of the layer 4.

The layer 5 may either be formed on the layer 4 by helically winding an extensible metallic cord or it may be formed by placing side by side strips of cord fabric consisting of extensible metallic cords on said layer (as will be described below). The layer 5 has said metallic cords completely supported by said layer 4 across its whole width which ensures less elongation of the reinforcing assembly at its lateral ends, with consequent high performance of the tyre in service at high speeds.

Specifically as regards the metallic cords, those constituting the layers 3 and 4 which, as aforesaid, have an ultimate elongation equal to 3%, are each formed by seven strands, twisted together. Each strand is formed by the winding of four steel wires having a diameter equal to 0.22 mm and the strands are twisted together according to a winding sense opposite to that of the twisting of the individual strands. The metallic cords forming the layer 5 and having an ultimate elongation of 7% are each constituted by three strands, twisted together, each strand being formed by winding seven steel wires having diameter equal to 0.20 mm. The strands are twisted together according to a winding sense equal to that of the twisting of the individual strands, with the twisting pitch of each strand equal to about 4 mm and with the twisting pitch of the cord equal to about 6.5mm.

Figure 3 illustrates an alternative embodiment of the present invention according to which the tyre 6 has the assembly of reinforcing layers constituted by two layers of metallic cords 7 and 8 the same as the two layers 3 and 4 of figures 1 and 2 and by a layer 9 of metallic cords having an ultimate elongation equal to 7%. The layer 9 consists of three strips, respectively 10, 11 and 12, placed side by side with respect to one another to form the layer 9.

The central strip 11 is at least twice the width of each of the two other strips 10 and 12 (for convenience of representation, the cords of the said strips, although they are the same to one another, are indicated in figure 3, respectively, with filled circular section, the cords of the strips 10 and 12) and mere circular section, (the cords of the strip 11, for clarity).

Because of the curvature of the assem bly of the reinforcing layers, the strips 10 and 12 have a circumferential development lower than the corresponding development of the strip 11. Preferably the join of the ends of each strip is carried out so that each junction in the tyre is staggered with respect to the others, for example of 1200.

The pneumatic tyres according to the present invention, as described as embodiments 1 and 2 above, were compared both with conventional tyres, and with tyres having a reinforcing structure comprising, in a way per se known of circumferentially arranged reinforcing elements different from those foreseen in the present invention.

More particularly, the tyres subjected to the tests were as follows: Tyres A Tubeless tyres, size llR22.5 (in accordance with the present invention) provided with a mono-ply radial metallic carcass, with a reinforcing structure, arranged between the tread and the carcass, consisting of two layers of metallic cords having an ultimate elongation of 3%, crossed with one another and inclined with respect to the circumferential direction according to an angle of 18 and with a layer of metallic cords having an ultimate elongation of 7%, said layer being arranged radially outside the aforesaid layers and having its cords arrranged in circumferential direction.

Tyres B Tubeless tyres, size 11R22.5, provided with a mono-ply radial metallic carcass, with a reinforcing structure consisting of two layers of metallic cords having an ultimate elognation of 3 %, crossed with one another and inclined with respect to the circumferential direction according to an angle of 18 , and with a stabiliser strip, arranged in a radially inner position as regards to the aforesaid layers, constituted by metallic cords having an ultimate elongation of 3% and inclined according to an angle of 60 with respect to the circumferential direction (conventional tyres).

Tyres C Tubeless tyres, size 11R22.5, provided with a mono-ply radial metallic carcass and with a reinforcing structure consisting of three layers of metallic cords, all the cords having an ultimate elongation of 3%, two of said layers showing the cords inclined according to an angle of 18 and crossed with one another, whilst the third layer, radially outer to the first ones, has the cords arranged in circumferential direction.

Tyres D Tubeless tyres, size 11R22.5, provided with a mono-ply radial metallic carcass, with a reinforcing structure consisting of two layers of metallic cords having an ultimate elongation of 3% crossed with one another and inclined according to an angle of 18 as regards to the circumference direction, and with a layer of polyamide cords arranged in a circumferential direction, in a radially outer position in respect of the first ones.

Tests The above indicated tyres were first of all subjected to laboratory tests in order to value the detachment resistance in the reinforcing structure, particularly at the high speeds. In practice, the test consisted of subjecting to increasing speeds the different tyres under examination, each tyre being mounted on a rim, inflated to the pressure of 8 kg/cmz and put under a load of 2,550 kg (pressure and load comparable with those maximum admissible in use).

The imposed speeds were at first of 70 km/h for 5 hours, with an increasing of the speed of 10 km/h every 2 hours. The obtained results were: Tyres A: they reached 160 km/h Tyres B: they reached 140 km/h Tyres C: they reached 160 km/h Tyres D: they reached 140 km/h From the above, it is noted that tyres whose reinforcing structure comprises metallic cords arranged in the circumferential direction (both having a low and high elongation) give the better results. Then the power consumption resulting from using the different types of test tyres was measured showing that tyres A, C and D gave a power consumption analogous and lower than that resulting from using tyres B.

Then the steering response (an extremely important characteristic, as the test tyres were intended to be used for duty at high speeds) was evaluated showing that tyres A and B showed said characteristic high, whilst tyres C and D had it very low.

Therefore, the appliance of these latter types of tyres was excluded from the subsequent road tests, since they were shown unsiutable for the high performances foreseen for the tyres according to the aim of the present invention.

The road tests with the tyres A and B were carried out on a running-track; a truck mounting each type of test tyres, each tyre having an overload of 20% in respect of its maximum capacity, had at first covered 30,000 km, at a speed of 122 km/h, along a straight stretch. At the end of this first part of the test, it was noted that the tread of tyres A and B showed an equivalent wear, and neither one type, nor the other had significant crackings or damages. In the second part of the test, the truck was run on a curve and reverse curve run; after a run between 1100 and 1900 km, the tyres B showed detachments among the layers of the reinforcing structure so that the test had to be interrupted. The tyres A instead ran in said conditions until the whole wear of the tread was used, without detachment occurring in the reinforcing structure. On the ground of the results obtained from all these tests, it is clear that the tyres according to the present invention, are those which, in comparison with other known tyres, provide the better performances in every sense.

WHAT WE CLAIM IS: 1. A pneumatic tyre for motor vehicles specifically intended to be used for normal and heavy duty, having a radial carcass and provided with a tread reinforcing structure comprising two layers which are formed by metallic cords having an ultimate elongation equal to 3%, said cords being parallel to one another in each layer and crossed with respect to those of the other layer, said two layers being arranged so that said cords are inclined with respect to the circumferential direction of the tyre at an angle of between 5 and 30 , characterised by the fact that radially outwards of the said two layers there is at least one layer comprising metallic cords parallel to one another and substantially parallel to the circumferential direction, said metallic cords having an ultimate elongation between 4 and 8%.

2. A pneumatic tyre tyre as in claim 1, characterised by the fact that said layer or said layers of metallic cords having an ultimate elongation comprised between 4% and 8% is supported by the layer underneath belonging to the reinforcing structure, this latter layer having a width at least equal to that of said layer or said layers of metallic cords having the said ultimate elongation, the second layer of said reinforcing structure being inwardly graduated with respect to the first one.

3. A pneumatic tyre as in claims 1 or 2 wherein said metallic cords having an ultimate elongation between 4 and 8% consist of cords comprising strands wound in the same sense as the cord is wound.

4. A pneumatic tyre as in any one of claims 1 to 3, characterised by the fact that the layer or the layers of metallic cords having an ultimate elongation comprised between 4 and 8% are constituted by a single cord helically wound, with the convolutions of the helix arranged in the circumferential direction of the tyre.

5. A pneumatic tyre as in any one of claims 1 to 3, characterised by the fact that the layer or the layers of metallic cords having an ultimate elongation comprised between 4 and 8% are each constituted by strips placed side by side, each of said strip has a width equal to a portion of the tread width and extends circumferentially on the whole development of the tyre.

6. A pneumatic tyre constructed substantially as described herein and illustrated in Figures 1 and 2 of the accompanying drawings.

7. A pneumatic tyre constructed substantially as described herein and illustrated in Figure 3 of the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. tyres A instead ran in said conditions until the whole wear of the tread was used, without detachment occurring in the reinforcing structure. On the ground of the results obtained from all these tests, it is clear that the tyres according to the present invention, are those which, in comparison with other known tyres, provide the better performances in every sense. WHAT WE CLAIM IS:

1. A pneumatic tyre for motor vehicles specifically intended to be used for normal and heavy duty, having a radial carcass and provided with a tread reinforcing structure comprising two layers which are formed by metallic cords having an ultimate elongation equal to 3%, said cords being parallel to one another in each layer and crossed with respect to those of the other layer, said two layers being arranged so that said cords are inclined with respect to the circumferential direction of the tyre at an angle of between 5 and 30 , characterised by the fact that radially outwards of the said two layers there is at least one layer comprising metallic cords parallel to one another and substantially parallel to the circumferential direction, said metallic cords having an ultimate elongation between 4 and 8%.