The invention concerns the field of tyres intended for fitting to transport vehicles. More particularly, it deals with the problem of the flow of electric charges in tyres made with rubber mixes that are very poor electrical conductors.
To improve their rolling resistance and reduce fuel consumption, modern tyres comprise mixes containing as their principal filler electrically non-conducting fillers such as silica, or even mixes with small amounts of carbon black.
The use of such mixes is widespread for the production of tyre treads, granted the advantages of such mixes in also improving performances related to grip on dry, wet or icy ground, resistance to wear, and even running noise. As an illustrative example, a tyre of this type is described in European Patent Application EP 0 501 227 B1.
However, the use of these mixes goes together with a difficulty related to the accumulation of static electricity as the vehicle is rolling, and the absence of any escape of these charges into the ground because of the very high resistivity of the mixes constituting the tread. When certain particular conditions exist, the static electricity accumulated in a tyre can give rise to a nasty electric shock to the occupant of a vehicle when he touches the body of the vehicle. This static electricity can also speed up the ageing of the tyre because of the ozone generated by electrical discharge. Depending on the nature of the ground and of the vehicle, it can also cause the on-board radio to malfunction because of the interferences it generates.
Accordingly, numerous solutions have been proposed for allowing the charges to flow away from the crown block into the ground.
U.S. Pat. No. 5 518 055 describes a tyre whose tread, produced from a mix with very low conductivity, is coated with a fine layer of conductive mix. This layer is in contact with the sidewall mixes, themselves electrically conductive, to allow the electric charges to flow away.
Another solution is disclosed in the application EP 0 658 452 A1, which describes the addition of a strip of mix or insert, made from a conductive rubber mix, which extends radially preferably over the full circumference of the tyre and which connects the outer surface of the tread either to one of the crown plies, or to the carcass reinforcement, or to any other part touching the tyre tread that is a sufficiently good conductor of electricity, the necessary conductivity being conferred by the presence of a suitable carbon black.
Numerous improvements have been made to this principle depending on whether the tread comprises one or more layers of materials that are conductive or very poorly conductive, as disclosed for example in the patents EP 0 925 903 B1 or EP 0 963 302 B1.
However, the aim of all these methods is to connect the outer surface of the tread with part of the internal zone of the tyre's crown just adjacent to it, such as the sidewall, a crown reinforcement ply or a carcass reinforcement ply, which has electrically conductive properties. The electric charges are then drained away into the ground from the wheel rim, which is connected to the vehicle, passing through the bead area in contact with the rim and then flowing along the sidewalls through the mixes constituting the carcass reinforcement ply or the sidewall protection rubbers towards an internal part of the crown zone, and finally flowing from the said internal part of the crown zone into the ground through the crown reinforcement and the tread.
However, recent tyre developments aiming to improve the rolling resistance have led to the use of mixes based on silica, or ones with very low carbon contents, in most compositions forming the tyre and able to perform mechanical work during the tyre's use. This can be the case with mixes used to make treads, sidewalls, carcass reinforcement plies or crown reinforcement plies, crown-reinforcing profiled elements and bead-reinforcing profiled elements, and the interior sealing rubber. Thus, all these materials are characterised by very low electrical conductivity.
Only elements of the tyre such as the bead reinforcement ring or bead protection rubbers whose fuiction is to ensure contact between the wheel rim and the lower part of the tyre, which are made of mixes containing a carbon-based filler, still have the property of conducting electricity.
The purpose of the invention is to provide a solution for the conduction of electric charges in tyres essentially made from mixes with very low electrical conductivity.
However, as will be seen later, the invention can concern both outer tyre casings that enclose a volume of air under pressure to support the load, and so-termed non-pneumatic tyres formed of one or more self-supporting elements such as those described as examples in the patent EP 1 056 604.
A material with very low electrical conductivity is understood to mean one whose resistivity is greater than or equal to 108 Ohms/cm. Similarly, a material that conducts electricity is one whose resistivity is lower than 106 Ohms/cm. These materials have the appropriate elastic properties and may or may not be of rubber-like nature.
Thus, in a general way the invention can be applied to a tyre intended for fitting on a wheel which itself conducts electricity, and designed to support a load, the said tyre consisting of:
This tyre is characterised in that:
In what follows, the description will refer to the application of the invention to a tyre of the pneumatic type in which the crown zone (S) comprises a circumferential crown reinforcement consisting of at least one crown reinforcement ply, and a tread arranged circumferentially around the crown reinforcement, whose outer surface is in contact with the ground, the said crown zone being attached to a carcass reinforcement ply anchored at its two ends to the bead area (B), the said bead area comprising a bead protection rubber designed to ensure contact with the wheel and made from an electrically conductive material. Besides the carcass reinforcement ply, the sidewall zone can comprise a profiled element that protects the tyre against transverse impacts.
The mixes used to make the sidewall protection and the carcass reinforcement ply rubbers have very low electrical conductivity. Under these conditions there is no electrically conducting path between the crown zone S and the bead area B since, besides, the carcass reinforcement ply constitutes an insulator with the constituents located radially below it, such as the sealing rubber.
It does not matter whether the mixes used for making the tread or the crown reinforcement conduct electricity or have very low conductivity. In the latter case the tyre comprises means, known from the prior art, which enable electric charges to flow from the outer surface of the tread to its radially inner surface. The flow of charges from the inside portion of the crown zone to the ground can take place through the tread alone, or through the crown reinforcement and the tread.
The mixes used for making the bead protection rubbers are electrically conductive.
A conductive strip of small thickness and width, made from a material preferably of the nature of rubber and which conducts electricity, is arranged radially along a path extending from the said interior part of the crown zone S with which it is in contact, to the bead area B where it is in contact with the constituent in contact with the wheel.
Depending on whether or not the mixes used to make the crown reinforcement are electrically conductive, the end of the conductive strip in contact with the interior part of the crown zone is positioned above or below the crown reinforcement so as to allow charges to flow between the end of the strip and the ground passing either through the assembly formed by the crown reinforcement and the tread, or directly through the tread, the latter being able to be provided or not with an insert positioned radially between the surface of the tread and the said interior part, depending on the nature of the mix used to make the tread.
The advantages and nature of the invention will emerge more clearly on reading the description of an example of its implementation and referring to the figures, which show:
The tyre shown in
Dotted lines surround respectively the crown zone S, the bead area B and the sidewall zone F.
In this example, only the mixes need to make the bead protection rubber 5 and the insert 43 are electrically conductive. Thus, electric charges cannot flow freely between the crown zone S and the bead area B.
To ensure the conduction of electric charges between the crown zone and the bead area, the conductive strip 8 is arranged between the sidewall protection rubber and the carcass reinforcement in such manner that one of its ends is in contact with the bead protection rubber 5 and its other end is in contact with the part of the insert 43 which meets the radially inside surface of the tread and corresponds to the part of the tread located radially nearest to the rotation axis of the tyre.
The conductive strip 8, of small width and thickness, is made from an electrically conductive mix. The width of the strip can range from a few millimetres up to two or three centimetres, and is adapted to the manner to which the said strip is produced and used. This does not exclude the production of strips having smaller or larger widths. Likewise, and in a non-limiting way, the thickness can range from one or two tenths of a millimetre and one, or even two millimetres. Here too, the limits are imposed by the production process and the intended use.
The embodiment illustrated in
The embodiment illustrated in
When the end of the sidewalls 6 is positioned under the lateral wing of the tread 4, the conductive strip 81, one of whose ends is in contact with the bead reinforcement rubber 5, can be turned up around the radially outer end of the sidewall protection rubber so that the end of the strip 81 is in contact with the end of the conductive strip 82 positioned under the tread, whose other end is in contact with the part of the insert 43 that meets the radially inner surface of the tread, as illustrated in
All these embodiments demand that the crown and the shaped carcass should be assembled with sufficient circumferential precision to enable the ends of the conductive strips 81 and 82 to be placed in contact.
When the mixes used to make the tread 4 and the crown reinforcement 3 are electrically conductive, it suffices for the end of the conductive strip to be in contact with the radially inner part of the crown reinforcement 3 as illustrated in
An analogous configuration, illustrated in
A last configuration illustrated in
Under these conditions, to enable the flow of electric charges it suffices for the conductive strip 81 to be in contact at one end with the underlying layer 42 and at its other end with the inside surface of the bead protection rubber 5.
Implementation of this last solution is particularly advantageous in that there is no need to adopt special precautions when circumferentially orientating the crown block relative to the shaped carcass. Besides, the underlying layer 42 can consist of a simple connecting rubber only a few tenths of a millimetre thick, with all the conductivity properties desired.
It does not matter whether the conductive strip 8, 81, 82 is positioned on one side of the tyre, on the other side, or on both sides.
Likewise, it is entirely possible to create an electrically conductive path similar to those described earlier and equivalent to the positioning of a conductive strip, by spraying onto the desired path a rubber solution made from an electrically conductive mix. This thin solution layer has the same conductive properties as a conductive strip of small thickness, and can be put in place using for example a spray or brush.
The purpose of the embodiments described above is to illustrate particular ways of implementing the invention and the adaptations that are possible in the various most common ways of manufacturing tyres. Thus, without deviating from the concept of the invention, those engaged in the field will have no difficulty in finding adaptations appropriate for more particular implementations.
Thus, while remaining within the scope of the invention, a non-pneumatic tyre consisting of self-supporting elements with very low electrical conductivity as depicted in
Number | Date | Country | Kind |
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04/12281 | Nov 2004 | FR | national |