This disclosure relates to a method for manufacturing a metal ring for a ring set of a drive belt for a continuously variable transmission, as well as to a drive belt including such ring. 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.

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.

The basic setup of the overall manufacturing process of such drive belts is well- known as well. In particular in relation to the ring-set, such basic setup is for example described in WO2018/122397. The known manufacturing process entails a considerable number of intermediate steps that are, moreover, carried out within very narrow tolerances, to realise a high quality end-product with an exceptional fatigue strength. One such intermediate process step is the rolling of the rings, wherein their thickness is decreased and their diameter, i.e. circumference length is increased by rotating the rings in their circumference direction, 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 200 to 150 micrometre in rolling. Such a process step of ring rolling is described in detail in W02004/050270.

In relation to such known drive belt and known manufacturing process a surprising discovery was made. Namely, according to the present disclosure, the fatigue strength of the rings -and thus the service life of the drive belt as a whole- could be unexpectedly improved by introducing a new process step that is favorably straightforward and easy to implement. In particular, such improvement was realized by turning the rings inside out after rolling, whereby its radially inwardly oriented, i.e. inner ring surface becomes its radially outwardly oriented, i.e. outer ring surface and vice versa. In particular, 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.

Although it was not intended nor foreseen to improve the fatigue strength of the ring by turning it inside out, with hindsight a possible explanation for such improvement can be found as follows. Both during ring rolling and during use of the drive belt in the transmission, the highest tensile stress occurs at the outer surface of the ring. This implies that surface imperfections or defects, such as (microscopic) cracks, dents, foreign particles or the like, mostly originate and/or become more severe on the outer surface of the ring during ring rolling than on its inner surface. Likewise, any such surface defect on the outer surface of the ring is more detrimental to the ultimate fatigue strength of the ring during use than on its inner surface. In this case, by turning the rings inside out after the said rolling thereof, the most and/or the most severe surface defects are favorably relocated to the inner surface of rings before use.

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 represents the metal ring after rolling;

Figure 6 illustrates a novel process step of turning the ring inside out; and

Figure 7 illustrates how the novel process step of ring turning can be implemented in the otherwise known manufacturing process of the drive belt.

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 residual 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 rolling 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 thickness and circumference length of the ring 41 . Additionally, at least one of the inner and the outer surface of the ring 41 may be provided with a surface relief or increased roughness in ring rolling, where to either one or both of the first bearing roll 8 and the rolling roll 10 a provided with a corresponding (but inverse) relief or roughness. Typically, the surface relief is provided only to the inner surface 42 of the ring 41 by the first bearing roll 8, such that its outer surface 43 is flat and smooth in comparison, as is schematically illustrated by the hatching in figure 5 (not drawn to scale).

According to the present disclosure, a new process step NPS is added to the overall manufacturing method for the ring 41 , in which new process step NPS the ring 41 is turned inside out, as is schematically illustrated in figure 6 in relation to a ring 41 that is provided initially, i.e. in ring rolling (process step V), with a surface relief on its inner surface 42. As illustrated in figure 6 by the dashed arrows, such ring turning (process step NPS) can be accomplished by pushing the left side face of the ring 41 via the radial inside of the ring 41 to the right, while simultaneously pulling the right side face of the ring 41 around the ring 41 to the left. After ring turning (process step NPS) the surface relief is located on the outer surface 43 of the turned ring 41 a. According to the present disclosure, such a turned ring 41 a was found to exhibit a favorably higher fatigue strength after it has been turned inside out (process step NPS) in its application in the drive belt 3, as compared with a ring 41 that has not been turned inside out after ring rolling.

Figure 7 illustrates the preferred implementation of the above novel process step NPS of turning the ring 41 inside out in the manufacturing process of the ring-set 31 . Preferably, ring turning (process step NPS) is carried out after ring annealing (process step VI) rather than immediately after ring rolling (process step V). In ring annealing (process step VI) the work hardening of the ring 41 by the plastic deformation thereof in ring rolling (process step V) is removed, such that less effort and lower stress levels are involved in turning the ring 41 inside out (process step (NPS). Preferably also, ring turning (process step NPS) is carried out before ring calibration (process step VII). After all, in ring calibration (process step VII) the ring 41 , 41 a is provided with the crowning and the internal residual stress that are specifically defined for the inner and outer surfaces 42, 43 thereof in the (ring-set 31 of the) drive belt 3.

Preferably all rings 41 of the ring-set/ring-sets 31 of the drive belt 3 are turned inside out, to realize the maximum improvement of the fatigue strength of the drive belt 3 as a whole. However, to minimize the impact of ring turning process step NPS on the overall manufacturing process, it may also be opted to only turn the radially innermost ring 41 of the ring-set/ring sets 31 inside out. After all, such radially innermost ring 41 is typically subjected to the highest stress levels and/or stress amplitudes during operation of the drive belt 3, such that the fatigue strength thereof is most critical to that of the drive belt 3 as a whole. In this case, the said radially innermost ring 41 is preferably either not provided with the said surface relief (as illustrated in figure 5) or the said surface relief is provided in ring rolling (process step V) to the outer surface 43 of the ring 41. Hereby, it is avoided that in the ring-set 31 , the surface relief of the said radially innermost ring 41 would arrive in contact with the surface relief of/on the inner surface of an adjacent ring 41.

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.