This disclosure relates to a method for manufacturing a drive belt for a continuously variable transmission, as well as to thus manufactured drive belt. The drive belt is, as such, well-known, for example from the British patent number GB1286777 (A) and from the more recent international patent publication WO2015/177372 (A1 ). This known drive belt consists of a number of mutually nested endless flexible metal bands or rings, i.e. that are mutually concentrically stacked into a set of rings or ring-set, and a number of metal transverse segments that are arranged along the circumference of such ring-set in an essentially contiguous row. The transverse segments each define a central opening defined by and between a base part of the transverse segment and two pillar parts, each extending from a respective axial side of the base part in radial outward direction, in which central opening a respective circumference section of the ring-set is accommodated, while allowing the transverse segments to move, i.e. slide along the circumference thereof. For containing the ring-set in the central opening, the central opening is partly closed in radial outward direction by a respective axially extending portion of at least one and possibly both of the pillar parts. In particular, such axially extending portion of a respective pillar part extends partly over the ring-set towards the other, i.e. axially opposite, pillar part of the transverse segment and is denoted a hook portion of the pillar part hereinafter. It is noted that alternative measures and/or means for containing the ring-set in the central opening of the transverse segments, i.e. alternative to such hook portion or hook portions are known in the art, such as a containment ring (see e.g. US5123880) and a closing pin (see e.g. EP0122064).

In the above and below description, the axial, radial and circumference directions are defined relative to the drive belt when placed in a circular posture. A thickness direction and thickness dimension of the transverse segments are defined in the circumference direction of the drive belt, a height direction and height dimension thereof are defined in the radial direction of the drive belt and a width direction and width dimension thereof are defined in the axial direction of the drive belt. A thickness direction and thickness dimension of the ring-set and the individual rings thereof are defined in the radial direction of the drive belt, a width direction and width dimension of the ring-set and the individual rings thereof are defined in the axial direction of the drive belt and a length direction and length dimension of the ring-set and the individual rings thereof are defined in the circumference direction of the drive belt. Up and down directions and above and below positions are defined relative to the radial or height directions.

In the continuously variable transmission the drive belt is wrapped around and in friction contact with two pulleys that each define a V-groove of variable width, in which pulley V-grooves respective parts of the drive belt are held at a variable radius. By varying such belt radius at the transmission pulleys, a speed ratio of the transmission can be varied. This type of transmission is well-known and is commonly applied in the drive train of passenger cars and other motor vehicles.

The above-described drive belt is set apart from another known design thereof, whereof the transverse segments each define two lateral openings, one on either lateral side of a central or neck part of the segment, which neck part is located between and connects a bottom or body part and a top of head part of the segment. This type of drive belts includes two sets of nested rings, each accommodate in a respective one of the lateral openings of the transverse segment. In this latter known design that is for example known from WO2015/097293, the two ring-sets are considerably less wide individually than the single ring-set of the said above-described drive belt. For a given application of the continuously variable transmission, this latter single ring-set of the so-called single ring-set belt and its constituent rings will typically be about twice as wide as the rings of either the two ring-sets of the so-called double ring-set belt, at least when these ring-sets contain the same number of rings individually. Generally speaking, the rings of the double ring-set belt have a width of up to 12 mm with a typical value of around 10 mm, whereas the width of the rings of the single ring-set belt exceeds 14 mm with a typical value in the range between 16 and 20 mm.

As part of the known overall manufacturing process of the drive belt, the rings are subjected to a process step of rolling, wherein their thickness is decreased and their diameter, i.e. circumference length is increased by rotating the rings in their circumference direction between while being compressed between a pair of rolls. For example, a semi finished ring product before rolling has a thickness of 0.4 mm, which thickness is then reduced to between 200 and 150 micrometre in rolling. Such a process step of ring rolling is described in detail in W02004/050270. In addition to providing the rings with a desired thickness and diameter, ring rolling also provides the ring with a desired cross-sectional shape and/or a desired surface relief that are both mentioned in W02004/050270 as well.

Although the basic setup of the known overall manufacturing process of the drive belt is well-known, longstanding and satisfactory, it can still be improved upon according to the present disclosure, in particular in terms of the robustness thereof in the mass- production of the drive belt. Also according to the present disclosure, by such improvement of the manufacturing process thereof, the performance of the drive belt in the transmission can be improved.

According to the present disclosure, an additional process step is included in the said overall manufacturing process of either turning around or turning inside out a selection, i.e. a part of the successively rolled rings relative to another, i.e. a remaining part thereof, before these rings are mutually nested to build the ring-set.

It is noted that within the context of the present disclosure, turning around means turning the ring about its radial direction for a half rotation (i.e. for 180 degrees), whereas turning inside out means pushing one axial side face of the ring via the radial inside of the ring to the opposite axial side of the ring, while simultaneously pulling the other axial side face of the ring via the radial outside of the ring to the respective opposite side of the ring. In either case, the axial sides of the ring are swapped, i.e. are switched around.

Underlying the present disclosure is the discovery that, at least in mass-production, the rings are provided with a minimally, but consistently tapered tangentially oriented cross- section, meaning that a thickness of the rolled ring at an axial side thereof can be either less or more than such thickness at the other axial side thereof. Such a systematic deviation of the ring thickness in the axial direction is believed to result from a misalignment between the respective axes of rotation of the said pair of rolls applied in ring rolling. In this case, the amount of such ring taper is proportional to the width of the ring. Moreover, when nesting the rings after rolling to build the ring-set, the said systematic rings thickness deviation disadvantageously adds up. Therefore, as a result of even a small ring taper, a stress level experienced by the rings during operation of the drive belt may be considerably and disadvantageously higher than without such taper. In these circumstances, by turning around or turning inside out some of the rings of the ring-set relative to the other rings thereof, the said systematic rings thickness deviation is favourably compensated, at least partly, between the rings of the ring-set instead of added-up. As a result, the said ring stress level during operation is favourably lowered. Additionally or alternatively, a required process accuracy of ring rolling can be favourably relaxed. Preferably, according to the present disclosure, every other ring of the ring-set is turned around. In this case, the said systematic rings thickness deviation is optimally compensated within the ring-set.

The drive belt manufacturing method according to the present disclosure will now be explained further with reference to the drawing figures, whereof:

Figure 1 is a schematic illustration of a known transmission incorporating two variable pulleys and a drive belt;

Figure 2 illustrates two known drive belt types in a schematic cross-section, each provided with a set of nested, flexible metal rings and with a plurality of metal transverse segments that are slidably mounted on such ring-set along the circumference thereof; Figure 3 provides a diagrammatic representation of the presently relevant part of the known overall manufacturing process of the drive belt;

Figure 4 is a schematic representation of a rolling device for rolling the metal ring as part of the overall manufacturing process of the drive belt;

Figure 5 is a cross-section of the metal ring schematically illustrating the desired geometry thereof after rolling;

Figure 6 is a cross-section of the metal ring schematically illustrating an actual geometry thereof after rolling;

Figure 7 is a cross-section of the ring-set schematically illustrating a problem associated with the actual ring geometry illustrated in figure 6;

Figure 8 is a cross-section of a novel ring-set; and

Figure 9 illustrated a novel process step in the overall manufacturing process of the drive belt according to the present disclosure.

Figure 1 shows the central parts of a known continuously variable transmission or CVT that is commonly applied in the drive-line of motor vehicles between the engine and the driven wheels thereof. The transmission comprises two pulleys 1 , 2 that are each provided with a pair of conical pulley discs 4, 5 mounted on a pulley shaft 6 or 7, between which pulley discs 4, 5 a predominantly V-shaped circumferential pulley groove is defined. At least one pulley disc 4 of each pair of pulley discs 4, 5, i.e. of each pulley 1 , 2, is axially moveable along the pulley shaft 6, 7 of the respective pulley 1 , 2. A drive belt 3 is wrapped around the pulleys 1 , 2, located in the pulley grooves thereof, for transmitting a rotational movement and an accompanying torque between the pulley shafts 6, 7.

The transmission typically also comprises activation means (not shown) that -at least during operation- impose on the said axially moveable pulley disc 4 of each pulley 1 , 2 an axially oriented clamping force that is directed towards the respective other pulley disc 5 of that pulley 1 , 2, such that the drive belt 3 is clamped between each such disc pair 4, 5. These clamping forces not only determine a friction force that can maximally be exerted between the drive belt 3 and a respective pulley 1 , 2 to transmit the said torque, but also radial positions R of the drive belt 3 in the pulley grooves. These radial position(s) R determine a speed ratio of the transmission. This type of transmission and its operation are well-known per se.

In figure 2, two known examples of the drive belt 3 are schematically illustrated in a cross-section thereof facing in the circumference direction thereof. In both examples, the drive belt 3 comprises transverse segments 32 that are arranged in a row along the circumference of an annular carrier in the form of one or two sets 31 of metal rings 41 . In either example of the drive belt 3, the ring-set 31 is laminated, i.e. is composed of a number of mutually nested, flat, thin and flexible individual rings 41 . A thickness of the transverse segments 32 is small relative to a circumference length of the ring-set 31 , in particular such that several hundred transverse segments 32 are comprised in the said row thereof.

Although in the accompanying figures the ring-set 31 is illustrated to be composed of 5 nested rings 41 , in practice, mostly 6, 9, 10 or 12 rings 41 are applied in such ring-set 31 , each with a nominal thickness of 185 micrometre.

On the left-side of figure 2 an embodiment of the drive belt 3 is illustrated including two such ring-sets 31 , each accommodated in a respective laterally oriented recess of the transverse segment 32 that opens towards a respective, i.e. left and right, axial sides thereof. Such lateral openings are defined between a body part 33 and a head part 35 of the transverse segment 32 on either side of a relatively narrow neck part 34 that is provided between and interconnects the body part 33 and the head part 35.

On the right-side of figure 2 an embodiment of the drive belt 3 is illustrated incorporating only a single ring-set 31 . In this case, the ring-set 31 is accommodated in a centrally located recess of the transverse segment 32 that opens towards the radial outside of the drive belt 3. Such central opening is defined between a base part 39 and two pillar parts 36 of the transverse segment 32 that respectively extend from either axial side of the base part 39 in radial outward direction. In such radial outward direction, the central opening is partly closed-off by respective, axially extending hook parts 37 of the pillar parts 36.

On either side thereof, the transverse segments 32 of both of the drive belts 3 are provided with contact faces 38 for arriving in friction contact with the pulley discs 4, 5. The contact faces 38 of each transverse segment 32 are mutually oriented at an angle f that essentially matches an angle of the V-shaped pulley grooves. The transverse segments 32 are typically made from metal as well.

It is well-known that, during operation in the transmission, the individual rings 41 of the drive belt 3 are tensioned by a/o a radially oriented reaction force to the said clamping forces. A resulting ring tension force is, however, not constant and varies not only in dependence on a torque to be transmitted by the transmission, but also in dependence on the rotation of the drive belt 3 in the transmission. Therefore, in addition to the yield strength and wear resistance of the rings 41 , also the fatigue strength is an important property and design parameter thereof. Accordingly, maraging steel is used as the base material for the rings 41 , which steel can be hardened by precipitation formation (ageing) to improve the overall strength thereof and additionally be surface hardened by nitriding (gas-soft nitriding) to improve wear resistance and fatigue strength in particular.

Figure 3 illustrates a relevant part of the known manufacturing method for the ring- set 31 , as it is typically applied in the art for the production of metal drive belts 3 for automotive application. The separate process steps of the known manufacturing method are indicated by way of Roman numerals.

In a first process step I a thin sheet or plate 20 of a maraging steel base material having a thickness of around 0.4 mm is bend into a cylindrical shape and the meeting plate ends 21 are welded together in a second process step II to form a hollow cylinder or tube 22. In a third step III of the process, the tube 22 is annealed in an oven chamber 50. Thereafter, in a fourth process step IV, the tube 22 is cut into a number of rings 41 , which are subsequently -process step five V- rolled to a larger diameter while the thickness thereof is reduced to, typically, around 0.2 mm. The thus rolled rings 41 are subjected to a further, i.e. ring annealing process step VI for removing the work hardening effect of the previous rolling process step V by recovery and re-crystallization of the ring material at a temperature considerably above 600 degrees Celsius, e.g. about 800 °C, in an oven chamber 50. At such high temperature, the microstructure of the ring material is completely composed of austenite-type crystals. However, when the temperature of rings 41 drops again to room temperature, such microstructure transforms back to martensite, as desired.

After annealing VI, the rings 41 are calibrated in a seventh process step VII by being mounted around two rotating calibration rolls and stretched to a predefined circumference length by forcing the said rolls apart. In this seventh process step VII of ring calibration, the ring 41 is typically also provided with a slight transverse curvature, i.e. crowning, and an internal stress is imposed upon the rings 41 . Thereafter, the rings 41 are heat-treated in an eighth process step VIII of combined ageing, i.e. bulk precipitation hardening, and nitriding, i.e. case hardening. More in particular, such combined heat treatment involves keeping the rings 41 in an oven chamber 50 containing a process atmosphere composed of ammonia, nitrogen and hydrogen. In the oven chamber the ammonia molecules decompose at the surface of the rings 41 into hydrogen gas and nitrogen atoms that can enter into the microstructure of the rings 41 . These nitrogen atoms partly remain as interstitial atoms in the microstructure and partly bond with some of the alloying elements of the maraging steel, such as molybdenum in particular, to form intermetallic precipitates (e.g. Mo2N). These interstitials and precipitates are known to remarkably increase the resistance of the rings 41 against wear as well as against fatigue fracture. Inter alia it is noted that such combined heat treatment can alternatively be followed or preceded by an aging treatment (without simultaneous nitriding), i.e. in a processing gas that is free from ammonia. Such separate aging treatment is applied when the duration of the nitriding treatment is too short to simultaneously complete the precipitation hardening process.

A number of the thus processed rings 41 are assembled in a ninth process step IX to form the ring-set 31 by the radially nesting, i.e. the concentrically stacking of selected rings 41 to realize a minimal radial play or clearance between each pair of adjoining rings 41 . It is noted that it is also known in the art to instead assemble the ring-set 31 immediately following the seventh process step VII of ring calibration, i.e. in advance of the eighth process step VIII of ring ageing and ring nitriding.

The process step V of rolling the ring 41 is illustrated in more detail in figure 4 that depicts a known ring rolling device comprising two rotatable bearing rolls 8, 9, a rotatable rolling roll 10, a pair of rotatable supporting rolls 1 1 and a rotatable pressure roll 12. The pressure roll 12 acts upon the supporting rolls 1 1 that in turn act upon a first 8 of the two bearing rolls 8, 9. The first bearing roll 8 is placed centrally in the rolling device, whereas the other, second bearing roll 9 is movably accommodated in the rolling device, in such a way that it can be moved away from (and back towards) the first bearing roll 8 to exert a pulling force FI on the ring 41 that is mounted on and around the two bearing rolls 8, 9. Also the pressure roll 12 is movably accommodated in the rolling device, in such a way that it can be moved towards (and away from) the supporting rolls 1 1 to exert a pushing force Fs on the inside of the ring 41 via the supporting rolls 1 1 and the first bearing roll 8. Said pushing force Fs is balanced by a reaction force Fr exerted by the rolling roll 10 on the outside surface of the ring 41 opposite the first bearing roll 8. Other embodiments of the ring rolling device are known as well. During the actual rolling of the ring 41 , it is rotated by and around the two bearing rolls 8, 9 in direction of the arrow marked RD in figure 4, while being compressed by the pushing force Fs between the first bearing roll 8 and the rolling roll 10 and being stretched by the pulling force FI.

The ring rolling process (step V) is primarily aimed at achieving a desired cross- sectional shape and circumference length of the ring 41 . An example of such a desired cross-sectional shape of the ring 41 is illustrated schematically and not to scale in figure 5. As shown in figure 5 the ring 41 is provided with a largely symmetric, so-called barrel shape by the thickness Tm in the middle of the ring 41 being more than its thickness Ts at or near its axial sides (in particular as measured within 1 mm of a respective axially oriented side face of the ring 41 , for example at a distance of 0.5 mm from such respective side face). It is noted that, in figure 5, the said thickness dimensions Tm, Ts of the ring, as well as the said barrel shape thereof have been exaggerated for illustrative purposes.

According to the present disclosure, the actual cross-sectional shape of the ring 41 after ring rolling may deviate from the desired shape in terms of the axial symmetry thereof, as schematically illustrated in figure 6. In particular, either the thickness Tsr at one axial side of the ring 41 (in figure 6: right side, as seen in the rolling direction RD and relative to the radial outward of the ring 41 ) may be somewhat less than the thickness T si at the other axial side thereof (in figure 6: left side) and/or the thickest part Tmax of the ring 41 may not occur in the middle thereof, but somewhere between its middle and one of its axial sides (i.e. Tm Tmax).

Such an imperfection in the cross-sectional shape of the ring 41 after rolling and isolated from the said barrel shape, is illustrated in figure 7, however, to an exaggerated extend and not to scale. In figure 7, the ring 41 is illustrated in cross-section showing (only, i.e. without the said barrel shape) a wedge shape or taper in its width direction that results from the said thickness difference between its axial sides. When a number of these rings 41 are mutually nested to form the ring-set 31 , the said thickness difference of the individual rings 31 disadvantageously accumulates in the thickness direction of the ring-set 31 . For example, in case of a thickness difference of 8 micrometre in relation to a nominal ring thickness of 185 micrometre, a ring-set 31 with 12 rings can show a taper of almost 100 micrometre as whole. Therefore, as a result of even a relatively minimal taper of the individual rings 41 of the ring-set 31 , a stress level experienced during operation of the drive belt 3 will be considerably higher than without such taper.

According to the present disclosure, the taper of the ring-set 31 as a whole can be favourably and cost effectively minimised by turning around every other ring 41 b of the rings 41 a, 41 b of the ring-set 31 , relative to the rest of the rings 41 a thereof, before or as a part of the assembly of the ring-set 31 in the said ninth process step IX, as is schematically illustrated in figure 8. As a result, after being turned around, the left axial side of such every other ring 41 b 8 is thinnest, whereas the rest of the rings 41 a are thinner on their right axial side. In particular in this way, a thickness difference of at least 5 micron, potentially in the range between 10 and 25 micron can be allowed for the individual rings 41 according to the present disclosure.

Due to the resulting alternating taper and/or the resulting alternating asymmetry of the barrel-shape of the adjacent rings 41 in the ring-set 31 , these rings 41 will have a tendency to displace somewhat in mutually opposite axial directions, i.e. alternating^ to the left and to the right in figure 2, during operation of the drive belt 3. As a result the width of the ring-set 31 as a whole will be somewhat more than the width of an individual ring 41 . In case of the embodiment of the drive belt 3 with a single ring-set 31 (depicted on the right- side of figure 2), such axial spreading of the individual rings 41 of the ring-set 31 (relative to the perfect axial alignment thereof illustrated in figure 2) favourably increases an overlap in axial direction with the hook parts 37 and thus supports the robustness of the drive belt 3, because the transverse segments 32 are thereby increasingly prevented from inadvertently separating from the ring-set 31 during operation.

In theory (i.e. in case of a ring-set 31 with an even number of rings 41 that each show an identical taper), the resulting taper of the ring-set 31 as a whole can be reduced to zero in this way, however, in practice the ring-set 31 will show a taper in the same order of magnitude as the individual rings 41 thereof on average. In any case, the taper of the ring-set 31 manufactured according to the present disclosure as a whole, is reduced considerably relative to the prior art ring-set 31 .

Alternative to such turning around, but to the same effect, every other ring 41 b can be turned inside out.

The turning around or turning inside out in accordance with the present disclosure of some of the rings 41 b, e.g. of every other ring 41 b, relative to other rings 41 a is implemented as an additional process step APS in the said overall manufacturing process after the said fifth process step V of ring rolling and before or ultimately as part the said nine process step IX of ring-set assembly. Preferably and as schematically illustrated in figure 9, such additional process step APS is implemented before the rolled rings 41 proceed to the ring calibration (process step VII). In doing so, the crowning radius and the internal stress that are imposed upon the rings 41 in the said seventh process step VII of ring calibration are applied in a corresponding manner (e.g. in terms of calibration rotation direction, crowning transverse symmetry, internal stress distribution, etc.) to all rings 41 , i.e. regardless of whether it is turned around (ring 41 b) or not (ring 41 a) in accordance with the present disclosure. Furthermore, the said additional process step APS is implemented after the rings 41 have been subjected to ring annealing (process step VI). In doing so, the rings 41 are more compliant and less susceptible to being damaged while being handled in the said additional process step APS, at least compared to immediately after ring rolling (process step V), because in ring annealing (process step VI) the work hardening effect of ring rolling is removed by recrystallization and normalizing.

Within the context of the present disclosure, the said turning around of a ring 41 entails turning the ring 41 over 180 degrees (i.e. over half a full rotation) about its radial axis RA (as illustrated by the arrow marked © in figure 9), such that it axial side faces are switched around, i.e. are swapped. Turning the ring 41 inside out entails pushing the left axial side face of the ring 41 via the radial inside of the ring 41 to the right side of the ring 41 , while simultaneously pulling the right axial side face of the ring 41 via the radial outside of the ring 41 to the left side of the ring (as illustrated by the arrows marked © in figure 9), or vice versa. It is noted that in the latter case, i.e. by turning the ring 41 inside out, not only its axial sides are swapped, but also its radially inner and outer sides are swapped, i.e. the radially inward facing surface of the ring 41 becomes its radially outward facing surface and vice versa. It is, however, noted that such turning inside out of the ring 41 may be less preferred than the said turning around thereof, because in practice one radial side of the ring 41 , typically the radially inner side, is provided with a surface relief or increased roughness in ring rolling, whereas the other radial side is not and is relatively smooth. Preferably, this latter radial side bears against the opposite radial side of an adjacent ring 41 in the ring-set 41 that is relatively smooth and/or without surface relief, which is no longer possible if one of these adjacent rings 41 is turned inside out.

The present disclosure, in addition to the entirety of the preceding description and all details of the accompanying figures, also concerns and includes all the features of the appended set of claims. Bracketed references in the claims do not limit the scope thereof, but are merely provided as non-binding examples of the respective features. The claimed features can be applied separately in a given product or a given process or method, as the case may be, but it is also possible to apply any combination of two or more of such features therein.

The invention(s) represented by the present disclosure is (are) not limited to the embodiments and/or the examples that are explicitly mentioned herein, but also encompasses amendments, modifications and practical applications thereof, in particular those that lie within reach of the person skilled in the relevant art.