This invention relates to a flexible ring that is destined to be part of a drive belt for a continuously variable transmission with two pulleys and the drive belt. Such a transmission is commonly known and is, for example, applied in the drive train of passenger cars and other motor vehicles. In the transmission, the drive belt runs around and between the pulleys that are each provided with two conical sheaves that define a V- groove wherein a respective circumference part of the drive belt is held. The width of the V-groove of the pulleys can be changed in mutually opposite directions, by moving the pulley sheaves towards, respectively away from one another, to control a radius at which the drive belt is (effectively) in friction contact with the respective pulleys, i.e. to control a speed ratio provided by the transmission within a continuous range between a smallest and a largest speed ratio.

A known type of drive belt comprises an essentially contiguous row of transverse segments made of steel that are mounted on and around the circumference of a ring stack composed of a number of flexible endless bands or rings that are mutually stacked, one around the other, and that are likewise made of steel.

In the above and below description, the axial, the radial and the circumference directions are defined relative to the drive belt when placed in a circular posture. A thickness direction and a thickness dimension of the transverse segments are defined in the said circumference direction, a height direction and a height dimension of the transverse segments are defined in the said radial direction and a width direction and a width dimension of the transverse segments are defined in the said axial direction. A thickness direction and a thickness dimension of the rings and of the ring stack are defined in the said radial direction, a width direction and a width dimension of the rings and of the ring stack are defined in the said axial direction and a length direction and a length dimension of the ring stack is defined in the said circumference direction. Up and down directions and above and below positions are respectively defined in radial outward and radial inward direction.

The known flexible ring is provided with an essentially rectangular cross-section, albeit with rounded side surfaces, such that its thickness is much smaller than its width, typically by a factor of at least forty to one hundred or more. Also in absolute terms, the thickness of the ring is small and typically has a value of 185 to 200 micrometer, such that it can bend relatively easily in its circumference direction. In the ring stack, a number of such rings are arranged mutually concentric, i.e. are nested with minimal play, such that these share the load when the drive belt is operated in the transmission.

It is common practice in the art to roughen the inner surface of the rings, as part of the manufacturing process thereof, in the sense that a regular or irregular pattern of indentations and/or ridges is impressed on it, with the aim of drawing-in and/or capturing lubricant between the adjacent rings in the ring stack. The indentations of such a surface profile can for example be in the form of closed valleys or of intersecting grooves, but in any case the depth thereof remains small, typically within the range from 2.5 to 10 micrometer on average. Moreover, the surface profile is typically provided on a relatively fine scale such that a resulting ISO-standard Ra roughness of the respective ring surface lies between the ISO grades N5 and N7 (i.e. 0.4 pm < Ra < 1.6 pm).

In particular, the rings are provided with the surface profile as part of a process step of ring rolling, in which process step the thickness of a semi-finished ring is reduced to its final thickness, at least substantially, by being compressed in thickness direction between two rolling rolls while being stretched and rotated in circumference direction by a tensioning roll. Thereto, the circumference surface of the rolling roll that is located on the inside of the ring is not completely smooth, but is instead provided with a similar surface profile. During ring rolling such roll profile is impressed on the inner surface of the ring, as is for example described in the patent application publication W02005/053867.

The known transverse segments each define a central opening that is open towards the radial outside of the drive belt and that accommodates and confines a respective circumference section of such ring stack, while allowing the transverse segment to move along the circumference thereof. This central opening is defined by and between a base part of the transverse segment that is located radially inward of the ring stack and two pillar parts thereof that respectively extend from a respective side of the base part in radial outward direction. The two pillar parts thus define respective axial boundaries of the central opening, whereas in radial inward direction the central opening it is bounded by the base part. In radially outward direction the central opening is at least partly closed by some means, in order to confine the ring stack to the central opening. This type of drive belt is, for example, known from the British patent GB1286777-A and, more recently, from the international patent publication WO2018/210456-A1. It is noted that according to these documents, the said means for confining the ring stack in radial outward direction are embodied by respective hook portions of the pillar parts that each extend axially towards the respectively other, i.e. axially opposite, pillar part at some distance away from the base part. These hook portions of the transverse segment can have equal axial extent or such axial extent can be different between the two hook portions, in which case these respective hook portions are preferably provided on opposite sides of the transverse segment for successive transverse segments in the drive belt, as a/o taught by WO2018/210456-A1.

As seen in radial direction, an outer portion of the known transverse segment is provided with an essentially constant thickness, whereas a thickness of an inner portion thereof decreases in radially inward direction. In between the said inner and outer portions, a front surface of the transverse segment, facing in a circumference direction of the drive belt, includes a width-wise extending surface part that is curved in radial direction and that is often referred to in the art as a rocking edge or a tilting zone. The rocking edge allows successive transverse segments in the drive belt to mutually rotate about the axial direction, while these remain in contact at the rocking edge, whereby the drive belt as a whole follows a curved trajectory. Although the rocking edge can be located in the base part of the transverse segment, it is preferably located at least partly in the pillar parts thereof.

During operation in the transmission, the ring stack is tensioned by the transverse segments being urged in radial outward direction at the two pulleys by being clamped between the conical discs thereof. At these pulleys, the drive belt thus follows a curved trajectory, in which curved trajectory parts the transverse segments bear against the radial inside of the ring stack through, at least, a part of the surface of their base part that is located between the pillar parts, which surface part is denoted a bearing surface hereinafter. Due to the said tensioning thereof at the pulleys, the ring stack extends essentially straight between the two pulleys, while guiding the transverse segments as these traverse from the one pulley to the other in such straight trajectory parts.

This type of drive belt is commonly designed such that its service life in the transmission is determined by the fatigue strength of the ring stack that is cyclically bend and stretched under tension during operation. Therefore, it is a general development aim to improve upon the existing drive belt design and existing design considerations in terms of, at least, the resistance against metal fatigue of the ring stack.

The present invention relies on the observation that under life-cycle test conditions, fatigue fracture initiation sometimes occurs in or near the middle of the outer surface of one of the rings of the ring stack other than the radially outermost ring thereof. This observation seems to contradict the prevailing technical insights that, during operation of the drive belt, the rings experience the highest combined tensile and bending stress at their inner surface. This apparent contradiction inspired applicant to carry out extensive investigations that led to the discovery that the said initiation is indirectly related to the said surface profile of the inner surfaces of the rings. In particular, applicant found that in ring rolling, the surface profile that is impressed on the inner ring surface, at least to a certain extent, results in a deformation of the outer ring surface as well. Such unintended surface profile, i.e. the unevenness of the outer ring surface is less pronounced than the intended surface profile of the inner ring surface. For example, the pattern of indentations and/or ridges is less fine and the indentations are more shallow on the outer ring surface. In fact, where the surface profile of the inner ring surface is normally visible with the unaided eye, the surface profile of the outer ring surface is hardly distinguishable without an instrument. Nevertheless, even such minimally unevenness of the outer ring surface can disadvantageously raise the contact stress between two adjacent rings in the ring stack. In particular, when two asperities of the contacting ring surfaces happen to interact near the axial centre of the rings, where a contact force there between the rings is highest, a high, local contract stress occurs. Ultimately, such contact stress can result in surface fatigue, such as micro- pitting, of the outer surface of the inner of the two adjacent rings. The innermost ring of the ring stack is typically most susceptible to such phenomenon, because the outer surface of this ring typically experiences the highest normal force during operation. Also, wider rings, such as applied in the presently discussed drive belt type with a single, centrally located ring stack, are particularly susceptible to this phenomenon, since the force applied in ring rolling is higher such wider rings.

Based on the above observations and compared to the known ring, the present invention proposes to apply the surface profile to a lesser extent to, at least, a central part of, at least, the inner surface of the innermost ring of the ring stack, i.e. opposite the location of the observed fatigue fracture initiation. In other words, according to the present invention, at least the said central part of the inner surface of the radially innermost ring of the ring stack is made relatively smooth and, in particular, is provided with an Ra surface roughness of less than 0.4 pm (ISO grade N4 or smaller). This relatively smooth, central part of the innermost ring of the ring stack preferably spans at least 33%, more preferably around 50% of the width thereof. More in particular, according to the present invention, the surface profile is either not applied to the inner surface of the radially innermost ring of the ring stack at all, or it is applied only to the side edges thereof on either side of the said central part.

Alternatively or additionally, the manufacturing process of the ring can be modified to avoid or at least reduce the said unevenness of the outer ring surface. The present invention proposes two such modifications:

- firstly according to the present invention, the thickness reduction ratio in ring rolling is limited to less than 50%, in particular in relation to rings having a width after rolling of more than 13 mm, preferably more than 15 mm that are typically applied in the presently discussed drive belt type, more in particular in relation to a base material for the rings having a thickness (i.e. before ring rolling) of 0.4 mm or more;

- secondly according to the present invention, the process step of ring rolling includes a smoothing phase, i.e. following a main thickness reduction phase, in which smoothing rolling phase the ring is rotated between the rolling rolls with a compressive, rolling force being applied between these rolls that is reduced by at least 50% relative to the highest applied rolling force in the main rolling phase and preferably is reduced by between 60% to 90%. In particular in such smoothing rolling phase, a supply flow of a cooling and lubrication agent to the contact between the ring and the rolling rolls is reduced relative to such supply flow in the main rolling phase and preferably is reduced by at least 50%.

The above-described invention and the technical working principles underlying the invention will now be explained further with reference to the drawing figures, whereof:

- figure 1 is a simplified and schematic side elevation of a known transmission with two pulleys and a drive belt consisting of a ring stack and a row of transverse segments mounted on the ring stack along the circumference thereof;

- figure 2 schematically illustrates the known drive belt in a cross-section thereof facing in its circumference direction and also includes a separate, transverse cross- section of only the transverse segment thereof;

- figure 3 is a schematic representation of an individual ring of the ring stack;

- figure 4 provides a diagrammatic representation of the presently relevant part of the known overall manufacturing process of the ring including a process step of rolling;

- figure 5 provides a contour plot of an enlarged section of a radially inner surface and a radially outer surface of the ring depicted in figure 3, such as can be obtained in practice with the overall manufacturing process illustrated in figure 4; and

- figure 6 schematically illustrates the radially innermost ring of the ring stack, in particular a surface profile of the radially inner surface thereof, according to the present invention; and

- figure 7 provides a graph illustrating the rolling force applied in the process step of rolling in accordance with the present invention.

Figure 1 schematically shows, in a cross-section thereof, the central parts of a continuously variable transmission 51 for use in a driveline of, for example, passenger motor vehicles. This transmission 51 is well-known and comprises at least a first variable pulley 52, a second variable pulley 53 and a drive belt 50 fitted around these pulleys 52, 53. In the driveline, the first pulley 52 is coupled to and driven by a prime mover of the vehicle, such as an electric motor or a combustion engine, and the second pulley 53 is coupled to and drives a driven wheel of the vehicle, typically via a number of gears. The pulleys 52, 53 each typically comprise a first conical sheave that is fixed to a respective pulley shaft 54, 55 and a second conical sheave that is axially displaceable relative to such respective pulley shaft 54, 55 and that is fixed thereto in rotational direction. As appears from figure 1, the trajectory of the drive belt 50 in the transmission 51 includes two straight parts ST, where the drive belt 50 crosses over between the pulleys 52, 53 and two curved parts CT where the drive belt 50 is wrapped around the two pulleys 52, 53 while being accommodated between the conical sheaves thereof.

The drive belt 50 is composed of a ring stack 8 and a plurality of transverse segments 1 that are mounted on the ring stack 8 along the circumference thereof in an, at least essentially, contiguous row. For the sake of simplicity, only a few of the transverse segments 1 of the drive belt 50 are shown in figure 1, which transverse segments 1 are, moreover, not drawn to scale in relation to, for example, the diameter of the pulleys 52, 53. In the drive belt 50, the transverse segments 1 are movable along the circumference of the ring stack 8, which ring stack 8 is composed of a number of relatively thin and flexible endless steel bands or rings that are mutually nested, as can be seen more clearly in figure 2 that shows the ring stack 8 with eight individual rings.

During operation of the transmission 51, the transverse segments 1 of the drive belt 50 can be driven by the first pulley 52 in the direction of rotation thereof by friction. These driven transverse segments 1 push preceding transverse segments 1 in the circumference direction of the ring stack 8 and, ultimately, rotationally drive the second pulley 53, again by friction. In order to generate such friction (force) between the transverse segments 1 and the pulleys 52, 53, the said pulley sheaves of each pulley 52, 53 are urged towards each other, whereby these clamp the transverse segments 1 between them in the respective curved trajectory part CT of the drive belt 50. To this end, electronically controllable and hydraulically acting movement means (not shown) that act on the moveable pulley sheave of each pulley 52, 53 are provided in the transmission 51. These movement means also control respective radial positions R1 and R2 of the drive belt 50 at the pulleys 52, 53 and, hence, the speed ratio that is provided by the transmission 51 in the driveline between the pulley shafts 54, 55 thereof.

Also during operation of the transmission 51 drive belt 50, the transverse members are urged radial outward by being clamped between the conical pulley sheaves and are being forced into contact with the radial inside of the ring stack 8 that is tensioned thereby. Since, as mentioned hereinabove, in the drive belt 50 the transverse segments 1 can move relative to the ring stack 8 along the circumference thereof, the ring stack 8 is tensioned to a relatively low level in relation to a torque transmitted by the drive belt 50 between the pulleys 52, 53, at least compared to other types of drive belt.

In figure 2 the known drive belt 50 is schematically illustrated in more detail. On the left side of figure 2 the drive belt 50 is shown in a cross-section thereof facing in circumference direction and on the right side of figure 2 a cross-section A-A of only the transverse segment 1 is included. From figure 2 it appears that the transverse segments 1 of the drive belt 50 are generally shaped similar to the letter "V", i.e. are generally V- shaped. In other words, side faces 12 of the transverse segments 1 through which it arrives in (friction) contact with the pulleys 52, 53, are mutually diverging in radial outward direction by being oriented at an angle that closely matches an angle that is present between the conical sheaves of these pulleys 52, 53. The pulley contact faces 12 of the transverse segment 1 are typically either corrugated by a macroscopic profile or are provided with a rough surface structure, such that only the higher lying peaks of the corrugation or of the surface roughness arrive in contact with the pulleys 52, 53. This particular feature of the transverse segment design provides that the friction between the drive belt 50 and the pulleys 52, 53 is optimised by allowing cooling oil that is applied in the known transmission 51 to be accommodated in the lower lying parts of the corrugation or of the surface roughness.

Each transverse segment 1 includes a base part 10 and two pillar parts 11, whereof the base part 10 extends mainly in the axial direction of the drive belt 50 and whereof the pillar parts 11 extend mainly in the radial direction of the drive belt 50, each from a respective axial side of the base part 10. In its thickness direction, the transverse segment 1 extends between a front main body surface, i.e. front surface 2 and a rear main body surface, i.e. rear surface 3 thereof that are both oriented, at least generally, in the circumference direction of the drive belt 50. An opening 5 is defined centrally between the pillar parts 11 and the base part 10 of each transverse segment 1, wherein a circumference section of the ring stack 8 is accommodated. In radial outward direction the central opening 5 is partly closed-off by respective hook portions 9 of the pillar parts 11. Each such hook portion 9 extends from a respective pillar part 11 generally in the direction of the respectively opposite pillar part 11. Thus, the hook portions 9 confine the ring stack 8 to the central opening 5 of the transverse segment 1 in radial outward direction. In between the pillar parts 11, the base part 10 defines a bearing surface 13 for confining and supporting the ring stack 8 in radially inward direction.

As illustrated in figure 2, the bearing surface 13 is a central part of a boundary surface of the central opening 5 that is defined by the base part 10 in radially inward direction and that thus predominantly extends in the axial and circumference directions of the drive belt 50. The bearing surface is convexly curved in, at least, the axial direction in a well-known manner, for realising, or at least promoting, a desired contact and interaction between the transverse segment 1 and the ring stack 8. On either side of bearing surface 13 the said boundary surface of the base part 10 further includes a transition surface 15 forming a transition between the bearing surface 13 and a side surface of a respective pillar part 11 facing the central opening 5. Typically, such transition surfaces 15 include a convexly curved part adjoining the bearing surface 13 and a concavely curved part adjoining the said side surface of the respective pillar part 11. It is noted that convexly curved part of the transition surfaces 15 is curved according to a much smaller (e.g. by factor of 0.1 or less) radius of curvature than the bearing surface 13 is curved. The width of the ring stack 8 can be somewhat larger than a width of the bearing surface 13, as this allows the design of the drive belt 50 to be as compact as possible.

Both pillar parts 11 of the transverse segment 1 are provided with a protrusion 6 that protrudes in thickness direction from the front surface 2 of the transverse segment 1 and with a corresponding, however somewhat larger cavity 7 in the opposite side of the respective pillar part 11, i.e. in the rear surface 3 of the transverse segment 1. In the row of transverse segments 1 in the drive belt 50, the protrusions 6 of a first transverse segment 1 are received in the cavities 7 of a second, successive transverse segment 1. By this engagement of the protrusions 6 and the cavities 7 of successive transverse segments 1, the transverse segments 1 mutually link to and align one another in radial direction and in axial direction in the said row thereof in the drive belt 50. In figure 2, the diameter of the cavity 7 is exaggerated relative to the diameter of the protrusion 6 to illustrate a play that exists there between.

Also in the said row of transverse segments 1 in the drive belt 50, at least a part of the front surface 2 of a first transverse segment 1 abuts against at least a part of the rear surface 3 of a second, successive transverse segment 1. Abutting transverse segments 1 are able to tilt relative to one another, while remaining in mutual contact at and through an axially extending, convexly curved surface part 4 of the front surface 2 thereof that is denoted rocking edge 4 hereinafter. Above, i.e. radially outward of such rocking edge 4, the transverse segment 1 has an essentially constant thickness, whereas below, i.e. radially inward of such rocking edge 4, the transverse segment 1 is tapered, i.e. has a thickness that decreases in radially inward direction (whether gradually, stepwise or by a combination thereof), to allow for the afore-mentioned relative tilting without interference between the respective base parts 10 of the abutting transverse segments 1.

It is noted that, although in figure 2 the rocking edge 4 is located partly in the pillar parts 11 and partly in the base part 10 of the transverse segment 1 such that it overlaps with the bearing surface 13 in radial direction, it is also known to locate the rocking edge 4 fully in the base part 10, i.e. radially inward of the bearing surface 13. In either case, the rocking edge 4 is preferably provided in two parts 4a, 4b separated by the central opening 5 and/or by a recessed area 14 in the front surface 2 of the transverse segment 1 that is recessed in thickness direction relative to the rocking edge 4. The recessed area 14 provides a channel between the abutting transverse segments 1, allowing lubricant to flow from radially inside the drive belt 50 to the radial inside of the ring stack 8. Such lubricant is supplied to the transmission during operation, not only for cooling it, but also for lubricating the dynamic contact between the transverse segments 1 and the ring stack 8, as well as between the individual rings of the ring stack 8. It is further noted that in the embodiment of the transverse segment 1 illustrated in figure 2, wherein the rocking edge 4 is located partly in the pillar parts 11 and the base part 10 of the transverse segment 1, the recessed area 14 is, in part, formed as a curved transition surface between the front surface 2 of the transverse segment 1 and the bearing surface 13 as an inevitable side- effect of the preferred manufacturing method of fine-blanking the transverse segment. In fine-blanking, the transverse segment 1 is cut from basic material by pressing a punch, having a contour corresponding to that of the transverse segment 1, through the basic material into a transverse segment-shaped hole of a die plate, while being supported by a counter punch on the opposite side thereof. An end face of the counter punch that contacts the basic material is a/o shaped to form the rocking edge 4 and is provided with a recess that serves as a mould for forming the protrusion 6, while the end face of the punch that contacts the basic material is protruding part to form the cavity 7.

In figure 3 an individual ring 41 of the ring stack 8 is depicted. A fine-scale surface profile of mutually intersecting, continuous ridges with valleys there between is present on the inner surface 41i of the ring 41 and is schematically indicated in figure 3 by the cross hatching. The outer surface 41o of the ring 41 is without such surface profile and is thus relatively smooth. The surface profile of the inner surface 41i reduces friction between the individual rings 41 of the ring stack 8 by reducing the surface area of the sliding contact there between and by drawing-in and/or capturing lubricant in such sliding contact.

Figure 4 illustrates a relevant part of the known manufacturing method for the drive belt ring component 41, as it is typically applied in the art for the production of metal drive belts 50 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 60. Thereafter, in a fourth process step IV, the tube 22 is cut into a number of annular rings 41, which are subsequently -in a fifth process step V- rolled between two rolling rolls to reduce the thickness thereof by a factor of 2 or more, while being elongated. During ring rolling, a cooling and lubrication agent is supplied to the contact between the ring 41 and the rolling rolls. In the process step of ring rolling also the said surface profile is impressed on the inner surface 41i of the ring 41 by the respective rolling roll being provided with an appropriately profiled rolling surface, as is for example described in the patent application publication W02005/053867-A1.

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 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 60. 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 rollers and stretched to a predefined circumference length by forcing the said rollers apart. In this seventh process step VII of ring calibration, also internal stresses are imposed on 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 60 containing a controlled process atmosphere of a mixture of ammonia, hydrogen and nitrogen gas at a controlled temperature. It is known in the art to control the ammonia concentration in the process atmosphere to a value between 5 and 25% by volume, to control the hydrogen concentration in the process atmosphere to a value between 5 and 15% by volume and to control the temperature of the process atmosphere to a value between 450 and 525 °C. Practically applied values in this respect are around 10% by volume ammonia gas, around 5% by volume hydrogen gas and 470 °C. 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 crystal structure of the rings 41. By these interstitial nitrogen atoms the resistance against wear as well as against fatigue fracture is known to be increased remarkably. Typically, the eighth process step VIII of combined ring ageing and nitriding is carried out until a nitrided layer or nitrogen diffusion zone formed at the outer surface 41o of the rings 41 reaches a desired thickness of, for example, 25 micrometer.

Inter alia it is noted that such combined heat treatment can alternatively be followed or preceded by an aging heat treatment, i.e. without simultaneous nitriding, in a processing gas that is free from ammonia. Such separate aging heat treatment is applied when the duration of the nitriding heat 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 stack 8 by the radially stacking, i.e. concentrically nesting of selected rings 41 to realize a minimal radial play or clearance between each pair of adjoining rings 41. Inter alia it is noted that it also known in the art to instead assemble the ring stack 8 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.

It is well-known that, during operation in the transmission, individual rings 41 of the ring stack 8 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 50 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.

In this latter respect, the present invention relies on the observation that, in testing, an initiation of the ultimate fatigue of the ring 41 can occur on the outer surface 41o of the radially innermost ring 41 of the ring stack 8 near the middle thereof. This initiation position was linked to the contact between the outer surface 41o of the innermost ring 41 and the inner surface 41i of the subsequent ring 41 of the ring stack 8. In such contact, the surface profile that is provided to the inner surface 41i of the subsequent ring 41 was found to interact with protuberances on the outer surface 41o of the innermost ring 41, whereby stresses are locally raised. These protuberances on the outer surface 41o of the ring 41 are formed in the process step V of ring rolling, wherein the surface profile that is impressed on its inner surface 41i results in a deformation of its outer surface 41o as well. In figure 5, a surface contour plot is included both of a typical inner surface 41i of the ring 41 that is intentionally provided with a surface profile, in the form of a cross-hatch pattern of grooves in ring rolling, and of a resulting outer surface 41o of that ring 41 that is unintentionally provided with a surface profile as well. In figure 5 the surface profile of the inner surface 41i has a maximum height difference between the lightest and the darkest regions of around 12 micron, whereas such maximum height difference is around 4 micron for the surface profile of the outer surface 41i. In order to improve the fatigue strength of, in particular, the radially innermost ring 41 of the ring stack 8, the present invention proposes in general to manufacture the outer surface 41o thereof as smooth as possible, by not applying the said surface profile to a central part CP of the inner surface 41i thereof, as illustrated in figure 6. After all, this central part CP of the inner surface 41i corresponds to the observed location of the fatigue fracture initiation on the outer surface 41o. In other words, only the side edges SE of the inner surface 41i of the ring 41 are provided with the surface profile according to the present invention. Moreover, from a manufacturing perspective, it may be convenient to apply the same surface profile to the side edges SE with a smooth central part CP on the inner surfaces 41i of the other rings 41 as well.

Additionally or alternatively to the above design of the ring 41, however for the same purpose, the fifth process step V of ring rolling can be optimised according to the present invention as illustrated in figure 7. In figure 7 the compressive rolling force Fr that is applied to the ring 41 between the rolling rolls is plotted as a function of time t for the duration of the rolling process. Figure 7 illustrates that the novel ring rolling process includes both a main rolling phase MRP and a smoothing rolling phase SRP. In the main rolling phase MRP a relatively high rolling force Fr is applied in order to reduce the thickness of the ring 41. At the end of the main rolling phase MRP the rolling force Fr is reduced, however not to zero, but rather the rolling process is continued with the said smoothing rolling phase SRP, while applying a reduced rolling force Fr of (in this example) ±40% of the (maximum) rolling force Fr applied in the main rolling phase MRP. At the end of the smoothing rolling phase SRP the rolling force Fr is reduced to zero to complete this fifth process step V of ring rolling. In particular, the rolling force Fr that is applied in the smoothing rolling phase SRP is chosen so low that the effect of the surface profile of the rolling roll acting on the inner surface 41i of the ring 41 does not extend to the outer surface 41o. At the same time, that outer surface 41o is still being processed, i.e. plastically deformed, by the other rolling roll having a smooth rolling surface. Thus, in the smoothing rolling phase SRP, the unevenness of the outer surface 41o of the ring 41 resulting from the main rolling phase MRP is reduced again. Preferably, in such smoothing rolling phase SRP, also a supply of lubricant to the contact between the ring and the rolling rolls is reduced relative to such lubricant supply in the main rolling phase.

The present invention, 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, as the case may be, but it is also possible to apply any combination of two or more of such features therein.

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