Patent Application
20230051293
The present invention relates to rubber caterpillar tracks of vehicles, and more particularly rubber caterpillar tracks for agricultural, construction plant, or military vehicles with improved traction over loose ground, and particularly relates to treads for caterpillar tracks made of rubber.
The invention will be more particularly described with reference to a multipurpose vehicle, that is to say a vehicle that can be driven both in the fields on loose ground and on roads, such as an agricultural tractor.
What is meant by rubber or rubbery material or rubber compound is any elastomeric material containing at least an elastomer that may be reinforced by at least a reinforcing filler such as carbon black for example.
A tread for a caterpillar track made of rubber for a vehicle notably comprises:
a. a tread, containing rubber and a reinforcement, forming a closed loop in a longitudinal direction having a given width in a transverse direction and having an interior surface intended to collaborate with drive means for driving the caterpillar track, and an exterior surface intended to come into contact with the ground,
b. a plurality of relief elements, hereinafter referred to as tread pattern blocks or, more simply, as blocks, which are intended to come into contact with the ground in order to give the vehicle equipped with this caterpillar track good traction, and which extend outwards from a bearing surface as far as the exterior surface,
c. a plurality of teeth, extending from the interior surface towards the inside of the caterpillar track and intended at least for guiding this caterpillar track, or any other device which performs the role of transmitting caterpillar track drive force, the drive being performed by means such as at least one wheel positioned on the inside of the caterpillar track.
Given the forces to which a caterpillar track mounted on a vehicle is subjected, it is known practice to provide, within the tread, at least one reinforcement providing the caterpillar track with dimensional stability in the longitudinal direction that corresponds to the direction of the loop formed by said caterpillar track. This reinforcement is generally made up of a plurality of cords laid in the longitudinal direction or else a cord wound at a relatively shallow angle to said direction so as to confer upon the tread a suitable tensile stiffness in the longitudinal direction to allow it to withstand the forces to which it is subjected during use. It is also commonplace to envisage adding an additional reinforcement comprising a superposition of plies each one formed of a plurality of reinforcing filaments or cords laid parallel to one another and possibly criss-crossing from one layer to another.
The ability of the caterpillar track to transmit the rotational energy of the transmission to the ground is very closely dependent on the tread pattern of the tread situated on the exterior face in contact with the ground. A caterpillar track must not slip, which is to say must not slide without advancing through loss of traction. Hence, the tread pattern is generally provided with raised blocks situated on the exterior face of the tread and separated from one another by cuts.
The tread of the caterpillar track is intended to run on various types of ground such as the more or less compact soil of the fields, unmade tracks providing access to the fields, and the tarmacked surfaces of roads. Bearing in mind the diversity of use, in the fields and on the road, the tread of the caterpillar track needs to offer a performance compromise between traction in the field, resistance to chunking, resistance to wear on the road, rolling resistance, and vibrational comfort on the road.
Associated with the caterpillar track is an overall frame of reference (O, X, Y, Z) in which O is the geometric centre of the caterpillar track, the axis (OY) is the axis of rotation of the wheel that drives the caterpillar track, the axis (OX) is perpendicular to (OY) and orientated in the direction of the greatest dimension or length of the caterpillar track, and finally the axis (OZ) is orthogonal to the plane formed by the axes (OX) and (OY).
A transverse direction means a direction parallel to the axis (OY) of rotation of the caterpillar track.
A longitudinal direction (OX) means a direction oriented along the greatest length of the caterpillar track in the direction of running.
A vertical direction means, in the present document, any direction that is perpendicular to the axis of rotation of the wheel that drives the caterpillar track and to the longitudinal axis. This direction corresponds to the direction of the thickness of the tread and is generally referenced (OZ).
A direction that is normal means the direction perpendicular to the exterior surface and oriented towards the outside of the caterpillar track.
A “sipe” means a cut that generates opposing walls of material. The distance between the walls of material of the sipe is suitable for allowing these walls to come at least partially into contact with one another as they enter the contact patch in contact with the ground.
A “groove” means a cut that generates opposing walls of material. The distance between the walls of material of the groove is such that these walls cannot come into contact with one another under usual running conditions.
The tread pattern elements of the tread are volume elements that are in raised relief with respect to the bearing surface, and also referred to as “blocks” and comprise a leading face, a contact face and a trailing face.
By way of example, in the case of tread pattern elements comprising two rows of blocks of the V-configured or chevron-configured lug type, the caterpillar track has a preferred direction of rotation according to the point of the chevrons. The leading face is, by definition, the face of which the edge corner directed outward in the normal direction, or leading edge corner, is first to come into contact with the ground when the lug enters the contact patch in which the caterpillar track is in contact with the ground, as the caterpillar track rotates. The trailing face is, by definition, the face of which the edge corner directed outward in the normal direction, or trailing edge corner, is last to come into contact with the ground when the lug enters the contact patch in which the caterpillar track is in contact with the ground, as the caterpillar track rotates. In the direction of rotation, the leading face is said to be forward of the trailing face.
The voids volume ratio of the tread pattern of the tread is defined as being the ratio between the total volume of the grooves that separate the raised elements and the total volume of the tread assumed to be free of voids, comprised between the bottom surface and the tread surface. The voids volume ratio thus implicitly defines the volume of elastomer material of which the tread is made that is intended to become worn. It also has a direct impact on the contact patch over which the tread is in contact with the ground and, therefore, on the contact pressures for contact with the ground, both of which govern caterpillar-track tread wear.
The “level of wear” of the tread means the ratio between a thickness that the tread has lost through wear and the total thickness that the tread is able to lose before it has to be replaced. Thus, a 25% level of wear means that the tread has lost one quarter of the thickness of rubbery material that can be worn away.
The “tread surface” of a tread here means the set of points of the tread that come into contact with the ground when the caterpillar track is running on this ground.
The “voids area ratio” of a tread means the ratio of voids present on the tread surface of this tread. This voids area ratio is notably generated by the grooves of the tread.
The voids area ratio is closely connected with the grip of the tread with the ground. Said ratio determines the surface area of rubber in contact with the ground in order to provide the grip required to cause the vehicle to advance by transmitting motive torque.
A lack of grip results in the tread of the caterpillar track slipping, which means the caterpillar track turning without supplying sufficient traction.
The level of slip, as a percentage (%), corresponds to the ratio between the actual speed minus the theoretical speed, divided by the actual speed. In other words, the level of slip is equal to (Vactual−Vtheoretical)/Vactual. The amount of traction with respect to the level of slip is an indication of the performance of the tread pattern.
The hardness of an elastomeric compound after curing, such as the one involved in the composition of the tread, is one of the mechanical data points used for characterizing this. The Shore A hardness is measured in accordance with the standard ASTM 2240 using a durometer instrument and is indicative of the ability of the elastomeric compound to resist the penetration of an indentor. The stiffer the compound, the higher the Shore A hardness. In order to design a rubber caterpillar track, the elastomeric compound is selected with consideration to, amongst others, the Shore A hardness which needs to be suitable for the tread pattern of the exterior surface in order to withstand scratching and tearing, given the diversity of the use of the vehicle.
In order to compare the traction performance of tread patterns of caterpillar tracks it is possible to conduct compression-shear tests in a laboratory on test specimens equipped with said tread pattern, on previously compacted loose ground. The result of the test demonstrates how the traction changes with respect to the level of slip of the test specimen.
The shear-compression test conducted on the test specimens on previously compacted loose ground involves three steps:
i. a first step of compacting the previously-expanded soil in a tray. The compaction is performed at a pressure of 0.7 bar;
ii. a static load is applied to the test specimen by applying weights;
iii. the test specimen is made to move, beginning with an acceleration phase at 0.5 m/s2, followed by a steady-state phase at a speed adjustable from 1 to 100 mm/s.
The postprocessing of the measurement result allows the tread patterns to be classified just as a traction test under actual conditions would, providing access to the traction potential of the tread pattern as a function of slip.
Treads for caterpillar tracks for agricultural or construction plant vehicles conventionally comprise elongate blocks referred to as lugs which extend from the centre of the tread towards the shoulders. These lugs are disposed on both sides of the transverse median plane, such as to form a pattern in the form of a “V”, with the point of the V-pattern (or chevron pattern) being designed to first to enter the contact patch which is the area of contact with the ground. The lugs are spaced apart from one another so as to form furrows or grooves; the width of these furrows is determined so as to allow good operation both on the road and on loose ground. The lugs exhibit symmetry with respect to the transverse median plane of the tread, usually with an offset between the two rows of lugs, similar to that obtained by one half of the tread being rotated about the axis of the caterpillar track with respect to the other half of the tread. Moreover, the lugs may be continuous or discontinuous, and may be distributed longitudinally with a pitch spacing that is constant or variable. The lugs have a contact face intended to run on firm ground, such as, for example, the roads taken to reach the vicinity of the fields, and a leading face intended to transmit the driving force when the agricultural vehicle is travelling over loose ground, typically the soil in the fields.
A recurring dissatisfaction relates to the traction performance of the known treads on loose ground. The productivity of vehicles, such as that of agricultural vehicles for example, is directly linked to the traction capability thereof on loose ground. Improving this performance would allow the user not only to improve the efficiency of their equipment but also to reduce their operating costs, for example by reducing the fuel consumption per unit of area covered. Of course, the improvement in traction should not be at the expense of other performance aspects such as for example wear resistance, comfort and the possibility of running at a sufficiently high speed on the road.
According to the invention, this objective is achieved by a rubber caterpillar track forming a closed loop in a longitudinal direction (OX) having a given width in a transverse direction (OY) and having an interior surface intended to collaborate with drive means, and a tread of which the exterior surface is intended to come into contact with the ground, said tread comprising a tread pattern formed of N relief elements made of an elastomeric material which are intended to come into contact with the ground in order to give the caterpillar track traction, each element of said tread pattern extending outwards from a bearing surface as far as the exterior surface and comprising a contact face intended to be in contact with the ground and a leading face intended to transmit force:
a. said leading face of n tread pattern elements, the number n being at least equal to 0.2×N, is inclined by an angle α comprised between 45 degrees and 75 degrees with respect to a direction (OZ) normal to the bearing surface;
b. the tread is made of an elastomeric material of which the shore A hardness, measured in accordance with the standard ASTM D2240, is greater than or equal to 65.
The principle of the invention is to have a caterpillar-track tread pattern that is multifunctional in recognition of the fact that the vehicle is used both on loose ground and on made-up roads. On loose ground, in order to obtain good traction and therefore optimal power transfer the tread pattern blocks need to be able to bear on the ground over their entire height without slipping. According to the invention, if at least 20% of the tread pattern blocks have a leading face that is inclined with respect to the normal direction (OZ) by an angle α comprised between 45 degrees and 75 degrees, then the traction of the vehicle is markedly improved.
Again according to the invention, the tread is made of an elastomeric material of which the shore A hardness, measured in accordance with the standard ASTM D2240, is greater than or equal to 65.
The rubber tread pattern blocks need to be stiff enough to bite into the ground without bending in order to transmit force. Furthermore, the tread pattern blocks need to be resistant to scratching and, in general, to all forms of attack from the environment when the vehicle is being used. For that reason, the Shore A hardness needs to be at a suitable level of at least 65 according to the invention.
Preferably, the angle α is between 55 degrees and 65 degrees.
Also preferably, the number n is at least equal to 0.4×N.
Also preferably, the leading face is made up mainly of two surfaces, a first surface being adjacent to the contact face and a second surface being adjacent to the bearing surface, the angle of inclination α being that of the linear regression line of the profile of the leading face, the first surface forming an angle α1 with the vertical direction Z, α1 being greater than α and between 45° and 75°, a height h of the first surface being at least equal to one third of the height H of the tread pattern block.
Preferably, the tread pattern blocks have a quadrilateral base and form, between one another, rows that are inclined with respect to the transverse direction.
According to a first variant, the tread pattern blocks within each row are disposed such that their leading faces are aligned with one another.
According to a second variant, the tread pattern blocks within each row are disposed such that the leading faces of adjacent blocks are offset with respect to one another in the longitudinal direction.
In this second variant, the tread pattern blocks within each row are preferably disposed such that, starting from the most central block of the tread, the front edge corners of the successively adjacent blocks are offset in the longitudinal direction and in the opposite direction to the direction of running by an offset “a” of between 45% and 65% of the pitch spacing “p” of the tread pattern.
In a third variant, the tread pattern blocks also have transverse sipes.
In a fourth variant of the invention, the tread pattern elements of the exterior surface that is intended to come into contact with the ground are lugs with a leading face which extends across the entire transverse width of the caterpillar track.
Still according to the fourth variant of the invention, the tread pattern elements of the tread are curved lugs distributed in the longitudinal direction (OX) in two rows that are symmetrical with respect to the transverse plane (XOZ), said lugs being inclined with respect to the direction (OX) so as to form chevron patterns oriented towards the centre of the tread of the caterpillar track.
Further features and advantages of the invention will become apparent from the following description given with reference to the appended drawing which shows, by way of non-limiting example, an embodiment of the subject matter of the invention.
Each element of said tread pattern 21 extends outwards from a bearing surface 22 and comprises a contact face 211, intended to be in contact with the ground, a leading face 212, intended to transmit the motive torque from the vehicle during running, said leading face 212 being inclined, with respect to the direction normal to the bearing surface 22, by an angle α comprised between 45 degrees and 75 degrees.
In
In this first embodiment, all of the blocks of the tread 2 have leading faces inclined at an angle α of between 45° and 75°, meaning that the number n of blocks that comply with this inclination characteristic of their leading face is equal to N. However, the invention can also be implemented when only some of the tread pattern blocks of the tread comply with this inclination characteristic. For example, when the number n of these blocks is at least equal to 0.2×N, that is to say when at least 20% of the blocks comply with this inclination characteristic of their leading face, the benefit in terms of traction on loose ground is already substantial.
Still in this first embodiment, the blocks are disposed across the width of the tread in a four-block pattern. Within each row, the blocks are disposed such that their leading faces are aligned with one another, meaning that together they are almost continuous, only being interrupted by the cuts 23. The tread shown here is perfectly symmetric with respect to the median transverse plane of the caterpillar track. In a variant that is not shown, the patterns of the two halves of the tread that are situated on either side of the median transverse plane can, by contrast, be offset with respect to one another in the longitudinal direction, as is often the case for lug tread patterns of prior art agricultural tyres.
In this second embodiment, the disposition of the blocks differs from that of the first embodiment mainly in that the blocks are no longer aligned within each row but rather are disposed such that the leading faces of adjacent blocks are offset with respect to one another in the longitudinal direction. One way of characterizing this offset is best visible in the view of
The traction versus slip performance as defined above was evaluated on test specimens indicative of the embodiment with lugs, as illustrated in
According to the inventors, a tread pattern voids volume ratio of the tread greater than or equal to 30% guarantees the invention good functionality. Likewise, a voids area ratio lower than 70% makes it possible to obtain optimal grip on made-up (asphalted) roads.
The invention is not intended to be limited to just these described exemplary embodiments, and various modifications can be made thereto while remaining within the scope as defined by the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR1913146 | Nov 2019 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2020/052153 | 11/24/2020 | WO |
