Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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IMPROVEMENTS IN OR RELATING TO TYRES
This invention concerns improvements in or relating
to tyres.
More specifically, the invention relates to tyre
casings, in par-ticular tyre casings having a radial carcass
and especially those reinforced with at least one metal ply.
The lower part of each side of the tyre casing, called the
"bead", includes -the end part of the carcass which is
folded towards the outside around a bead core. In general,
a shaped element made of rubber in one or two parts, called
the "apex" is disposed on the bead core, along the side,
between the casing fabric and its fold. The apex extends
along the side to a certain height, its thickness gradually
tapering off Such tyre casings are used in particular for
heavy goods vehicles.
In the known casings, one or~two steel cord
reinforcing plies inclined in relation to the bead core,
provide reinforcement and increase the rigidity in this
area. In addition, the bead has a shaped element made of
relatively soft rubber joined to the or each reinforcing
ply ac-ting as a shock absorber and then a hard rubber cover
layer which withstands -the friction between the bead and a
wheel rim on which the tyre is mounted in use.
Figure 1 of the accompanying drawings shows in
partial cross-section an example of such
a known tyre casing, specifically showing
its bead;
Figure 2 is a diagrammatic cross-section of part of
a tyre casing in accordance with one
embodiment of the invention; and
Figure 3 is a cross-section similar to that of
Figure 2, in accordance with a second
embodiment of the invention.
This casing has a radial carcass 109 the top of which
has folded breaker layers 12 on the side nearest the tread
14. At the wheel rim, marked with reference 15 on the
drawing, the casing ends in a bead 16. More specifically,
this bead is formed by the end portion of the carcass which,
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as indicated by re~erences 18 and 20, is folded around a
bead core 22 and extends upwards towards the outside
forming fold 100 An apex 24 of a relatively hard rubber
is disposed between the carcass 10, the bead core 22 and
-the fold 20, and tapers off at
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its external radial part.
In accordance with -the known state of the art,
reinforcing plies 26 (only one shown in Figure 1) are
placed on the outside of fold 20 in order to strengthen
and stiffen this a.rea~ More specifically, the. plies
are usually reinforced by metal o.r -textile cords which
are parallel with one ano-ther and inclined at a slight
angle to the longitudina] direction, thus forming a
rigid ne-twork with fold 20 of the carcass which they
cross. ..
Figure 1 shows that the bead 16 also has a shaped
element 28 made of a relatively soft rubber.at the edge
of -the s-trip, and a hard rubber cover layer 30 which
extends around the entire bead and withstands the ~ear
caused by friction between the bead and the wheel rim
in which the tyre is mounted in use.
Tyre casings of the type indicated above have the
disadvantage of need:i.ng many precautions and a high
level of precision during manu~acture. In fact, the
reinforcing plies have to be precisely cut, tran~ported
and ~it-ted, using expensive tooling. The int~rnal
mo~ement which inevitably occurs during ~ulcanisation
: can lead to displacement of these plies so that the
tyre cas.ings produced a.re somet:imes completely unsui~able
~or use. For this reason, systematic in-depth monito~ing
techniques have -to be used so tha-t these de~ecti~e -tyre
casings~can be eliminated.
The elimination of reinforcing.plies from this
type of tyre casing has already been proposed~ ~Thus,
30 French Patent No 1 343 402 described -the production of
a tyre casing ha~ing a bead with an apex o~ very hard
rubber, tapering off at~i.ts external radial:par-t into
: the side, and able to extend towards the internal radial
:~ part of the bead, thus forming the rim rubber. These
tyres have a certain number o~ disadvantages. First of
all, the assembly of the section on the fold of the
carcass, before vulcanisation, causes problems and the
union between the edge of -the fabric and the rubber is
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often far from adequate. In addition, during utilisation
it has been noticed that tyres with this type of bead do
not have the qualities required.
Tyre casings are also known where the outside
of the bead is protected with a hard rubber section. This
section is placed on the outside of the fold but the bead
has no apex. Such a bead is not sufficiently rigid.
According to one aspect of the present invention,
a tyre casing comprises a carcass extending between a pair
of beads wherein at least one bead includes a bead core and
a rubber apex strip around which a respective one end portion
of said carcass is folded and a rubber element positioned in
direct contact with the folded end portion of said carcass
and at least part of said apex strip, said element rubber
having a modulus which is greater than that of said apex
rubber, said rubber element having a radial dimension between
20% and 60% of the radial dimension of the tyre and extending
radially outwards from its respective bead heel.
Preferably, the modulus of the element rubber is
at least double that of the apex rubber. More specifically,
it is advantageous for the stabilised modulus at 20% of
the element rubber to be at least equal to 1.5 MPa, the
Shore A hardness of the rubber being at least equal to 75,
whilst the stabilised modulus at 20% of the apex rubber is
only of the order of 0.65 MPa, its Shore A hardness being
of the order of 65.
In one particular embodiment, the element is
extended around part of the bead core and forms the cover
layer which contacts the rim on which a tyre incorporating
the tyre casing is mounted in use.
It is advantageous for the radial dimension of
the element, measured in a plane perpendicular to the axis
of rotation of the casing, to be ~etween 20 and 60% of the
radial dimension of the casing, also measured in a plane
perpendicular to the axis of-rotation. It is advantageous
for the thickness of the element to be between 5 and 30%
of its radial dimension.
The design indicated above has several advantages.
First of all, the fatigue life of the shaped casings
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is very high. Then, the manufacture of the tyre
casings does not pose any particular problems because
the in-ternal movements of the elements during
vulcanisation are smaller -than those of reinforcing
plies and in any event have fewer disadvantages. Thus
the production accuracy can be increased. Furthermore
when -the shaped element also forms the cover layer,
-the number of elements incorporated in the tyre casing
is reduced so -that the cost of the latter can be
decreased.
Preferably the carcass is of radial construction
comprising one or more plies each comprising a plurality ~
of parallel reinforcement members e.g. cords of flexible
material such as nylon,Kevlar,lor no~-textile material
such as steel wire. Preferably the rein~orcement
members have a high modulus, for example a modulus
greater than 2500 Kg/cm2.
The bead core may be of any known cons-t~uction
e.g. cable, creel or close-packed wires and have any
desired cross-section e.g. circular, hexagonal~
rectangular. Preferably the bead core is subsia~ltially
inextensible.
The invention will now be described in more
detail , b~ way of example only, with reference to Figures
2 and 3 of the accompanying drawings,(Figure 1 already
having been described)"~ hiehl-
Figure 2 is a diagrammatic cross-section of part
of a tyre casing in accordance with one
embodiment of the~invention; and
Figure 3 is a cross-section similar to that of
Figure 2, in accordance with a second
embodiment of the invention.
Referring to Figure 2 -the casing shown comprises
a radial carcass 30 comprising one or more reinforcement
plies the end portion of which, indicated by reference
numerals 39 and 40, is ~olded towards the outside around
a bead comprising a bead core 42 and an apex strip 44 of
rubber. The apex strip 44 is tapered tawards its
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radially outer extremity and pro~ec~s beyond the end of
the carcass fold 40. An element 46 of rubber having a
higher modulus than the apex rubber is positioned nex~
axially outwards of and in direc-t contact with the
carcass fold 40 and part of the apex strip 44. Element
46 extends from the bead heel to a point below the
radially outer extremity of -the apex s-trip 44, A
cover layer 50 of hard rubber is positioned next axially
ou-twards of -the element 46 and extends below the bead
base and around the bead toe. Layer 50 contacts the
bead seat of a rim 35 on which the -tyre is mounted in
use.
Assuming'the radial dimension of the casing,
measured in a plane perpendicular to the axis of
rotation, is H as shown in Figure 1, the element 46 has
a radial dimension h which, advantageously, is between
Z0 and 600/o of the total radial dimension H, depending
on the particular dimensions and the aspect ratio of
the casing. It is advantageous for -the thickness of
the element 46 to be between 5 and 30% of its radial
dimension h.
In the particular example shown, which is that of
a tyre casing of standard dimensions 8.5-17.5, the
element 46 is made of a rubber, or rather of a mixture
of rubbers, where the stabilised modulus at 2Q% is
greater than or equal to 1~5 MPa 3 i-ts Shore A hardness
being at leas-t equal to 75, As a comparison, the
apex 44 is made of a rubber or rather a mixture o~
rubbers where the s-tabilised modulus at 20% is of the
ordér o~ 0.65 ~Pa, its Shore A hardness being of the
order of 65.
I~e modulus indicated above is measured a-t just
Z0% extension. In fac-t, this extension is ver~
representative of the conditions of use o~ the rubber
in the apex and in the shaped element. This modulus is
called "stabilised" because each value is measured
several times in succession and it is noted ~that the
measurements s-tabillse, generally after the fifth one.
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It is this s-tabilised value which is indica-ted in the
present specification.
I-t will be apparent from the foregoing
descrip-tion th~-t -the element 46 made o~ hard rubber
replaces -the reinforcing plies 26 and the element 28
of` soft rubber in -the casing shown in Figure 1.
Figure 3 shows ano-ther embodiment, intended for
a tyre casing of similar dimension -to that shown in
Figure 2. Those elements ln Figure 3 which are similar
to those in the embodiment in Figure 2 have been given
iden-tical re~erence numbers. Thus it will be seen tha~
the only difference between the embodiments in Figures
2 and 3 is that the element 46 of hard rubber and the
cover layer 50 in the embodiment in Figure 2 are
replaced by a single component 52 made of a rubber
having properties corre~s.ponding to those of the rubber
indicated for element 4~.
The advan-tage of the embodimen-t in Figure 3
over the one in Figure 2 is that it provides a reduction
in the number of elements used to make up -the tyre
casing. It therefore allows a certain reduction in the
cost of the casing to be made~
In each of the e~bodiments describe~ with
reference to Figures 2 and 3 the apex strip 44 and
element 46 or 52 o~ high modulus, harder rubber provide
a rigidity in the bead wh.ich varies progressively in a
-transverse direction enabling the bead to fully play
its part as a connection between the rim and a tyre
incorpora-ting the cas.ing.
Tests performed on tyre casings in accordance
with the invention, compared with tyre casi.ngs of the
known type, show the significant advantages of the
inventionf
First of all, a stud~ of the distrlbution o~
deformation s-tresses in loaded tyre casings shows that
the de~ormation stresses are better distributed in
casings made in accordance with the in~entionO This
better distribu-tion of the deformation stresses allows
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an improvement in -the fa-tigue li~e.
More specifically~ the fatigue life of the tyre
casings in accordance with the invention is determined
during rolling trials on a rolling machine, with -the
application o~ very high loads, the maximum loads
reaching 200% of the nominal load standardised by the
ETRT0 organisa-tion.
More specifically, tyre cas.ings in accordance
wlth -the invention, and traditional casings~ of the
standard 11 R 22.5 SC type, have been subjected to
rolling tests on overload~ The casings of the
traditional type all split in the bead region before
25~ hours o~ rolling. Tyres in accordance with the
invention withs-tood -this test for a longer period,
one Of them reading 518 hours of rolllng under the same
conditions.
Moreover, the tests have been carried out on the
-tyre casings in accordan oe with the invention under real
conditions of use. These tyres o~ standard dimensions,
8.5 R 17,5 SC, were fitted to a vehicle with a total
load equal -to 6250 kg. I-t was confirmed that the beads
of the casings were intact af-ter an initi.al utilisation
until the tread was worn, and also after a second
period of utilisation un-til complete wear permitted by
retreading the crown.
Therefore, the previous tests show that the
inv.e~tion provides tyre casings having a bett~r fatigue
life than -traditiona]. tyre casings, and yet allows these
casings to be produced at reduced cost, thanks to the
new structure used.
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