RELATED APPLICATIONS

This is a U.S. National Phase Application under 35 USC 371 of International Application PCT/FR2017/053641 filed on Dec. 18, 2017.

This application claims the priority of French application no. 1662816 filed Dec. 20, 2016, the entire content of which is hereby incorporated by reference.

FIELD OF THE INVENTION

The subject of the present invention is a tire for an aeroplane, more particularly the tread thereof and, even more particularly, a tread wear indicator.

BACKGROUND OF THE INVENTION

It is known that the tread of a tire, intended to come into contact with the ground via a rolling surface, is the wearing part of the tire, that is to say that it undergoes a reduction of thickness resulting from the rolling of the tire. The tread is generally composed of elements in relief separated from one another by hollows, this combination constituting what is commonly called a sculpture. In the case of a tire for aircraft, the elements in relief are more often than not circumferential ridges, continuous over all the circumference of the tire and separated from one another by hollows called circumferential grooves. The wear causes the height of the circumferential ridges and, correlatively, the depth of the circumferential grooves to be reduced, thus degrading the performance of the tire. The wear of a circumferential ridge, at a given instant in the life of the tire, is generally quantified by a wear ratio. The wear ratio is equal to the ratio (Ho−H)/Ho, where Ho and H are, respectively, the initial height Ho of the circumferential ridge, measured on the new tire, and the remaining height H of the circumferential ridge, measured on the worn tire.

Regarding the specific field of tires for an aeroplane, the person skilled in the art has highlighted two types of wear depending on the phase of use of the tire. On landing, the median part of the tread, whose axial width is at least equal to 50% and at most equal to 80% of the total axial width of the tread, is subjected, when the rolling surface enters into contact with the ground, to a wear called “touchdown wear”, resulting from a significant abrasion and from a high thermal heating, because of the speed differential between the speed of rotation of the tire and the speed of the aeroplane. In taxying phase, before the aeroplane has taken off or after it has landed, the lateral parts of the tread, positioned axially on either side of the median part and whose respective axial widths are at least equal to 10% and at most equal to 25% of the total axial width of the tread, are subject to a wear called “taxi wear”, resulting from the braking forces exerted on these lateral parts because of their speed of rotation, which is greater than that of the median part. Thus, the tread is mainly worn, in its median part, on landing, and, in its lateral parts, in taxi phase.

The wear described previously often leads to the presence of a non-uniform wear of the tread, called uneven wear, resulting from the stresses during the different phases of use of the tire: take-off, taxying and landing. More particularly, a differential wear of the tread between the median part and the two lateral parts of the tread has been revealed, the wear of this median part being generally greater. The differential wear of the median part of the tread leads to a limitation on the life of the tire, therefore on its use and to its early removal, even though the tread generally exhibits only a relatively low wear of the lateral parts of the tread: which is disadvantageous economically.

In practice, a tire for an aeroplane is removed when the wear ratio of one or more circumferential ridges of the tread reaches a maximum acceptable value, generally benchmarked as base 100, and the worn tire is then replaced.

One difficulty known to any operator of tires for aeroplanes is how to predict, reliably, the replacement of the worn tires and schedule the corresponding maintenance operations. A poor scheduling of the maintenance operations can in fact cause untimely downtimes of the aeroplane inducing additional operating costs for the user. To avoid these downtimes, a storage of replacement tires is often implemented, on each of the airports frequented by the aeroplane fleet operator, to be able to perform tire replacements, at any moment, which is disadvantageous economically.

The scheduling of the tire replacement operations is complex, because, in particular, of the difficulty in predicting of the wear a tire for an aeroplane. In effect, the remaining thickness of the tread as a function of the number of landings performed, which characterizes the speed of wear of the tread and more specifically the speed of touchdown wear, does not vary linearly. The remaining wear potential, measuring the remaining number of landings that can be performed before complete wear, cannot be determined by a simple measurement of the remaining height of the circumferential ridges of the tread. By way of example, the inventors have been able, in a particular case, to show that a tread, theoretically allowing 400 landings to be performed before complete wear, corresponding to a wear ratio of 100%, in reality allows 160 landings to be performed, corresponding to 40% of the total forecast number of landings, with a wear ratio of 75%. In other words, the trend of the wear ratio as a function of the number of landings is not linear.

In practice, the respectively initial Ho and remaining H heights of a circumferential ridge are often determined by a direct measurement of the corresponding depth of the adjacent circumferential groove, using a suitable measurement tool, which presents the drawback of giving a certain tediousness to the tracking of the wear.

The wear ratio of the tread can also be determined using wear indicators, also called wear tell-tales. Conventionally, these wear indicators can be of two types: wear indicators in relief relative to a circumferential groove bottom and wear indicators positioning in the thickness of the circumferential ridges.

A wear indicator, composed of an element in relief relative to a circumferential groove bottom, generally has a height substantially lower than the initial height of the circumferential ridge. When the remaining height of the circumferential ridge, adjacent to the groove comprising the wear indicator, reaches an allowable minimum value, corresponding to the top level of the wear indicator, the allowable maximum wear ratio is reached and triggers the replacement of the tire. Such a wear indicator does not however make it possible to quantify the progressive trend of the wear of the tread during use: it simply indicates that the allowable maximum wear ratio is reached.

It has also been proposed to position wear indicators in the thickness of the circumferential ridges, such that the trace of these wear indicators on rolling surface can visually warn that the allowable minimum height of the circumferential ridges, defining the allowable maximum wear ratio of the circumferential ridge, was reached.

A first type of wear indicator positioned in the circumferential ridges is composed of rubbery elements, the rubbery composition of which is of non-black colour. As an example, the document FR 1480472 provides a tread incorporating coloured materials, contrasting with the black colour of the tire, which appear when the allowable wear limit has been reached. The use of coloured rubbery compositions does however have a certain number of drawbacks, particularly in terms of the complexity and of the cost of the manufacturing method. Accurately positioning them in the tread is also difficult to ensure. Furthermore inserts of coloured rubbery compositions can constitute irregular areas of wear. Finally, this type of wear indicator does not make it possible to quantify the progressive trend of the wear of the tread during use.

A second type of wear indicator positioned in the circumferential ridges is composed of grooves or cavities hollowed out in the circumferential ridges, or, more generally, in the relief elements of the tread. The trace of these wear indicators on the rolling surface is characterized by an optical contrast between the rubbery composition forming the tread and the grooves or cavities formed in this same tread.

To illustrate a first example of this second type of wear indicator, the document EP 0250113A2 describes wear indicators, composed of groups of grooves of different depths, such that the traces of the grooves on the rolling surface disappear at different wear ratios. The visual detection of these grooves can however be difficult because of the small thickness of these grooves and the absence of contrast between the grooves and the rest of the tread.

As an illustration of a second example of the second type of wear indicator, the documents EP 1705034B1 and WO 2010012735A1 describe a tread comprising a rolling surface and at least one wear indicator formed by at least one cavity emerging on the rolling surface, the trace of the cavity on the rolling surface comprising a bearing surface and a plurality of parts connected to a bearing surface and protruding on the bearing surface, the number of the protruding parts being a function of the wear ratio of the tread, the geometrical form and the area of the trace of the cavity on the rolling surface being modified as a function of the wear ratio of the tread, the geometrical form of said trace being an analogous expression of the wear ratio of the tread. “Analogous expression” of the wear ratio of the tread should be understood to mean a form of the trace of the wear indicator that makes it possible to express the wear ratio by itself and directly, without it being necessary to introduce another reference. The forms of these wear indicators are relatively complex and, assuming that these wear indicators are manufactured by moulding, in the baking of the tire, the corresponding moulding elements are consequently relatively complex.

Finally, according to a third example of the second type of wear indicator, the document U.S. Pat. No. 9,355,346B2 describes a tread wear indicator whose appearance changes as a function of the wear. The wear indicator thus contains a multidimensional pattern, arranged in the tread, such that different levels of the pattern are exposed as a function of the wear. As in the preceding example, the complexity of form of such a multidimensional pattern implies relative manufacturing complexity.

The inventors set out to determine the wear ratio of a tread of a tire for an aeroplane, using wear indicators positioned in the circumferential ridges of the tread, said wear indicators being simple to manufacture and allowing for a direct and reliable reading of the wear ratio, over the life of the tire.

SUMMARY OF THE INVENTION

One aspect of the present invention is directed to a tire for an aeroplane comprising a tread, intended to come into contact with the ground via a rolling surface:the tread comprising at least three circumferential ridges axially separated from one another by at least one circumferential groove,at least one circumferential ridge having an initial height H0, measured on the tire in the new state, and being intended to be worn to an allowable remaining height HR,the at least one circumferential ridge comprising at least one wear indicator,the wear indicator comprising N cylindrical cavities emerging on the rolling surface and independent of one another,the N cavities of the wear indicator being divided up into a first family of N1wear cavities C1ihaving respective initial depths P1iin pairs that are distinct and equal to i*(H0−HR)/N1, i being an integer number varying from 1 to N1, and into a second family of N2positioning cavities C2j, j being an integer number varying from 1 to N2, each positioning cavity C2jhaving an initial depth P2jstrictly greater than (H0−HR), such that only the N2positioning cavities C2jemerge on the rolling surface when the allowable remaining height HR of the circumferential ridge is reached.

The wear indicator of a circumferential ridge of the tread of a tire for an aeroplane, according to the invention, is composed of N cylindrical cavities emerging on the rolling surface, that are divided up, essentially, into two distinct families respectively of N1wear cavities and of N2positioning cavities.

The first family, composed of N1wear cavities C1i, i being an integer number varying from 1 to N1, is intended to quantify the wear ratio of the circumferential ridge at any instant in the life of the tire. Each wear cavity C1iis a cylinder of radial axis, at right angles to the axis of revolution of the tire, emerging on the rolling surface and having a depth P1iequal to i*(H0−H)/N1. Consequently, a wear cavity C1iis visible on the rolling surface, as long as the height of the circumferential ridge lies between its initial value H0and a remaining value Hiequal to H0−(i*(H0−HR)/N1), and disappears below this remaining value Hi. The corresponding wear ratio Ui=(H0−Hi)/Ho is then equal to i*(1−HR/H0)/N1. When i is equal to N1, the remaining height HN1of the circumferential ridge is then equal to HR, a minimum remaining height that is allowable with respect to the wear, corresponding to an allowable maximum wear ratio equal to 1−HR/H0. There is thus a gradual disappearance of the wear cavities C1i, when i varies from 1 to N1.

The second family, composed of N2positioning cavities C2j, j being an integer number varying from 1 to N2, is intended to mark the axial positioning of the wear indicator, that is to say its positioning in the axial width of the tread, the axial direction being, by convention, parallel to the axis of revolution of the tire. Each positioning cavity C2jis a cylinder of radial axis, at right angles to the axis of revolution of the tire, emerging on the rolling surface and having a depth P2jstrictly greater than (H0−HR), so as to remain emerging, therefore visible, when the allowable minimum remaining height HR of the circumferential ridge is reached, that is to say when the N1wear cavities C1iof the first family have disappeared. In the general case, there is no particular relationship between the respective depths P2jof the positioning cavities C2j, but the latter are advantageously equal to one another to simplify the manufacturing of said positioning cavities by moulding.

Advantageously, the number N1of wear cavities C1iis at least equal to 1 and at most equal to 10. The higher the number N1, the greater the number of intermediate wear ratios between the new tire, corresponding to a zero wear ratio, and the totally worn tire, corresponding to a maximum wear ratio equal to 1−H0/HR, that can be determined. The number N1thus conditions the resolution of the tracking of the trend of the wear ratio of the circumferential ridge, over the life of the tire.

Advantageously too, the number N2of positioning cavities C2jis at least equal to 1 and at most equal to 5. The number N2must be sufficiently high to make it possible to identify the wear indicator in a differentiated way depending on whether it is positioned in a circumferential ridge of the median portion of the tread, also called median circumferential ridge, or in a circumferential ridge of a shoulder portion of the tread, also called shoulder circumferential ridge.

Preferentially, any cavity has, on the rolling surface, an emerging surface inscribed in a circle of diameter D at least equal to 2 mm and at most equal to 10 mm. Below 2 mm, the emerging surface of the cavity has an insufficient diameter that does not allow for easy visual detection and, as appropriate, that can be sensitive to blocking by foreign materials. Beyond 10 mm, the emerging surface can have a local impact on the wear of the circumferential ridge.

According to a preferential embodiment of the wear cavities, all the wear cavities C1ihave, on the rolling surface, an emerging surface inscribed in a circle of identical diameter D1. An identical emerging surface diameter makes it possible to easily identify that the wear cavities belong to the same first family and to have, for the manufacturing, moulding parts of the same section, hence a gain in standardization.

According to a preferential embodiment of the positioning cavities, all the positioning cavities C2jhave, on the rolling surface, an emerging surface inscribed in a circle of identical diameter D2. An identical emerging surface diameter makes it possible to easily identify that the positioning cavities belong to the same second family and to have, for the manufacturing, moulding parts of the same section, hence a gain in standardization.

Advantageously, the respective diameters of the wear and positioning cavities, identical within one and the same family, are distinct from one family to another. This difference makes it possible to easily distinguish whether a given cavity belongs to one or other of the two families.

Preferentially, the tread comprises two shoulder circumferential ridges and one median circumferential ridge, each comprising at least one wear indicator having the same number N1of wear cavities C1iand a different number N2of positioning cavities C2j, such that the total number N=N1+N2of cavities is different for each of the two shoulder circumferential ridges and the median circumferential ridge. A number N2of positioning cavities C2jthat is different according to the circumferential ridge concerned makes it possible to clearly identify that the wear indicator belongs to a given circumferential ridge and therefore the area of the tread in which the wear is measured.

Even more preferentially, the tread comprises two shoulder circumferential ridges and one median circumferential ridge, each comprising at least three wear indicators, distributed over the circumference of the circumferential ridge, identical to one another for each circumferential ridge, and having, from one circumferential ridge to the other, the same number N1of wear cavities C1iand a different number N2of positioning cavities C2j. Advantageously, each circumferential ridge for which the wear ratio is to be determined over time comprises at least three wear indicators, distributed over the circumference of the tire, preferably with an equidistant distribution. This distribution of at least three wear indicators for a given circumferential ridge makes it possible to determine the local wear ratios of said circumferential ridge at various points of the circumference of the tire, and therefore identify any uneven wears on the circumference, and also to determine a mean wear ratio, corresponding to the average of the local wear ratios recorded. Moreover, as previously, a number N2of positioning cavities, different according to the circumferential ridge concerned, makes it possible to clearly identify that the wear indicator belongs to a given circumferential ridge and therefore the area of the tread in which the wear is measured.

It is also advantageous for the respective at least one wear indicator of the two shoulder circumferential ridges and of the median circumferential ridge to be positioned circumferentially relative to one another in a circumferential angular sector at most equal to 20°. In effect, the fact that there are wear indicators for the various circumferential ridges grouped together in a limited angular sector facilitates the capture of the visual information concerning these wear indicators.

