CN113227609A - Method for manufacturing drive belt for continuously variable transmission and drive belt manufactured thereby - Google Patents

Abstract

The invention relates to a method for producing a metal ring (41) of a ring set of a drive belt for a continuously variable transmission, wherein the metal ring (41) extends solely in the circumferential direction thereof, while the thickness thereof is reduced in a rolling process step. The rolled metal rings (41) are further processed, a plurality of the so processed metal rings (41) being nested one within the other to form a ring set. According to the invention, after the plurality of metal rings (41) have been rolled, but before these are nested to form a ring group, some of the rings (41; 41b) are turned inside out or rotated half a turn around their radial direction in a new Additional Process Step (APS), while others of the rings (41; 41a) are not.

Description

Method for manufacturing drive belt for continuously variable transmission and drive belt manufactured thereby

Technical Field

The present disclosure relates to a method for manufacturing a transmission belt for a continuously variable transmission and a transmission belt manufactured thereby. Such a drive belt is known, for example, from british patent GB1286777(a) and more recently international patent publication WO2015/177372(a 1). This known drive belt comprises a plurality of mutually nested endless flexible metal bands or rings (i.e. they are stacked concentrically on top of each other in a set or ring group), and a plurality of metal transverse segments arranged in substantially continuous rows along the circumference of such a ring group. Each transverse segment defines a central opening defined by and between the base of the transverse segment and two cylindrical portions extending respectively radially outwardly from respective axial sides of the base, the respective circumferential segments of the ring set being received in the central opening while allowing the transverse segments to move, i.e. slide along the circumference of the ring set. In order to contain the ring set in the central opening, the central opening is partially closed in the radially outward direction by respective axial extensions of at least one cylindrical portion or possibly of two cylindrical portions. In particular, such axial extension of the respective cylindrical portion extends partially above the ring set towards the other, axially opposite cylindrical portion of the transverse segment, and is denoted hereinafter as hook of cylindrical portion. It is noted that alternative measures and/or means for receiving a ring set in the central opening of a transverse segment, i.e. for example receiving rings (see e.g. US5123880) and closing pins (see e.g. EP0122064) replacing such hooks, are known in the art.

Background

In the above and in the following description, the axial, radial and circumferential directions are defined with respect to the drive belt when placed in a circular posture. The transverse segments have thickness and thickness dimensions defined in the circumferential direction of the drive belt, height and height dimensions defined in the radial direction of the drive belt, and width dimensions defined in the axial direction of the drive belt. The thickness direction and the thickness dimension of the ring set and its individual rings are defined in the radial direction of the drive belt, the width direction and the width dimension of the ring set and its individual rings are defined in the axial direction of the drive belt, and the length direction and the length dimension of the ring set and its individual rings are defined in the circumferential direction of the drive belt. The up-down direction and the up-down position are defined with respect to the radial or height direction.

In a continuously variable transmission, a drive belt is wrapped around and in frictional contact with two pulleys, each defining a V-shaped groove of variable width, with a respective portion of the drive belt being retained within the pulley V-groove at a variable radius. By changing this belt radius at the drive pulley, the speed ratio of the transmission can be changed. Transmissions of this type are well known and are commonly used in the drive train of passenger cars and other motor vehicles.

The above-described drive belt is distinguished from another known design in which each transverse segment defines two lateral openings, one at each side of a central portion of the transverse segment or of a neck portion between and connecting the bottom or body portion of the transverse segment and the top of the head portion. This type of drive belt comprises two sets of nested rings, each set being received in a respective lateral opening of a transverse segment. In the latter known design, which is known for example from WO2015/097293, the two ring sets are each much narrower than the single ring set of the drive belt described above. For a given application of a continuously variable transmission, the width of a single ring set of a so-called single ring set belt and its constituent rings is typically about twice the ring width of either of the two ring sets of a so-called double ring set belt, at least when these ring sets individually contain the same number of rings. Generally, the loop width of the double loop set of bands is up to 12mm, typically around 10mm, whereas the loop width of the single loop set of bands exceeds 14mm, typically in the range between 16mm and 20 mm.

As part of the known overall manufacturing process of the drive belt, the rings are subjected to a rolling process step in which their thickness is reduced and their diameter, i.e. circumferential length, is increased by rotating the rings in their circumferential direction while compressing them between a pair of rollers. For example, the thickness of the semi-finished annular product before rolling is 0.4 mm, and then the thickness is reduced to between 200 and 150 microns in rolling. Such a ring rolling process step is described in detail in WO 2004/050270. In addition to providing the ring with the desired thickness and diameter, ring rolling also provides the ring with a desired cross-sectional shape and/or a desired surface relief structure, both of which are also mentioned in WO 2004/050270.

Disclosure of Invention

While the basic set of known overall manufacturing processes for drive belts is well known, long-standing and satisfactory, improvements can still be made thereto according to the present disclosure, particularly in terms of reliability in the mass production of drive belts. Also in accordance with the present disclosure, the performance of the drive belt in the transmission can be improved by such an improvement in its manufacturing process.

According to the present disclosure, an additional process step is included in the overall manufacturing process, namely, turning or inside-out the selected ring (i.e., a portion of the successive rolled rings) relative to another portion of the rings (i.e., the remaining rings) before the rings are nested within each other to build the ring set.

It is noted that in the context of the present disclosure, turn-around refers to turning it radially one-half turn (i.e. 180 degrees) around, while inside-out-turn refers to pushing one axial side of the ring to the opposite axial side of the ring via the radially inner side of the ring, while pulling the other axial side of the ring to the respective opposite side of the ring via the radially outer side of the ring. In either case, the axial sides of the rings are swapped, i.e., switched positions.

The present disclosure is based on the finding that, at least in mass production, the ring is provided with a minimal but consistent tangentially tapered cross-section, which means that the thickness of the rolled ring at its axial side may be smaller or larger than at its other axial side. Such systematic deviations in ring thickness in the axial direction are believed to be due to misalignment between the respective axes of rotation of the pair of rolls applied in ring rolling. In this case, the amount of such hoop taper is directly proportional to the width of the hoop. Furthermore, the system ring thickness variations add up disadvantageously when the rings are nested after rolling to build the ring set. Thus, even with a small ring taper, the stress level to which the ring is subjected during operation of the drive belt can be significantly and disadvantageously higher than without such taper. In these cases, the system ring thickness variation is advantageously compensated for, rather than accumulated, at least in part between rings of a ring set by turning or flipping some rings in and out relative to others. In this way, the ring stress level during operation is advantageously reduced. Additionally or alternatively, the required ring rolling process accuracy may be advantageously relaxed. Preferably, according to the present disclosure, every other ring in the ring set is turned. In this case, the system ring thickness deviations are optimally compensated within the ring set.

Drawings

The drive belt manufacturing method according to the present disclosure will now be further explained with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a known transmission including two variable pulleys and a drive belt;

figure 2 shows, in schematic cross-section, two known drive belt types, each provided with a set of nested flexible metal rings and a plurality of metal transverse segments slidably mounted on such ring set along the circumference thereof;

FIG. 3 provides a schematic illustration of a currently relevant portion of a known overall manufacturing process for the drive belt;

figure 4 a schematic view of a rolling device for rolling a metal ring as part of the overall process for manufacturing the drive belt;

fig. 5 is a cross-section of a metal ring, schematically illustrating its desired rolled geometry.

Fig. 6 is a cross-section of a metal ring, schematically showing its actual rolled geometry.

FIG. 7 is a cross-section of a ring set schematically illustrating problems associated with the actual ring geometry shown in FIG. 6;

FIG. 8 is a cross-section of the novel ring set; and

figure 9 illustrates a new process step in the overall manufacturing process of the power transmission belt according to the present disclosure.

Detailed Description

Fig. 1 shows the core components of a known continuously variable transmission or CVT which is typically applied in a transmission system between an engine and drive wheels of a motor vehicle. The transmission comprises two pulleys 1, 2 each provided with a pair of conical pulley discs 4, 5 mounted on a pulley shaft 6 or 7, defining between the pulley discs 4, 5 a substantially V-shaped circumferential pulley groove. At least one pulley disc 4 of each pair of pulley discs 4, 5, i.e. of each pulley 1, 2, is axially movable along the pulley shaft 6, 7 of the respective pulley 1, 2. A drive belt 3 is wound around the pulleys 1, 2 and is located in a pulley groove for transmitting a rotational movement and an accompanying torque between the pulley shafts 6, 7.

The transmission typically further comprises an activation device (not shown) which, at least during operation, exerts an axially directed clamping force on said axially movable pulley discs 4 of each pulley 1, 2, which clamping force is directed towards the other respective pulley disc 5 of the pulley 1, 2, so that the drive belt 3 is clamped between each pair of such pulley discs 4, 5. These clamping forces determine not only the frictional forces that can be maximally exerted between the drive belt 3 and the respective pulley 1, 2 for transmitting said torque, but also the radial position R of the drive belt 3 in the pulley groove. These radial positions R determine the speed ratio of the transmission. Transmissions of this type and their operation are known per se.

In fig. 2, two known examples of the drive belt 3 are schematically shown in their cross-sections facing in the circumferential direction thereof. In both examples, the drive belt 3 comprises transverse segments 32, which transverse segments 32 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. composed of a plurality of mutually nested, flat, thin and flexible individual rings 41. The thickness of the transverse segments 32 is small with respect to the circumferential length of the ring set 31, in particular so that hundreds of transverse segments 32 are included in the rows thereof.

Although in the figures the ring set 31 is shown as being made up of 5 nested rings 41, in practice, in most cases 6, 9, 10 or 12 rings 41 are applied in such a ring set 31, each ring having a nominal thickness of 185 microns.

On the left side of fig. 2, an embodiment of the drive belt 3 is shown, which comprises two such ring sets 31, each of which is accommodated in a respective laterally oriented groove of the transverse section 32, which laterally oriented grooves are open towards the respective, i.e. left and right, axial side. Such a lateral opening is defined between the body portion 33 and the head portion 35 of the transverse section 32 on either side of a relatively narrow neck portion 34, said neck portion 34 being disposed between and interconnecting the body portion 33 and the head portion 35.

On the right side of fig. 2, an embodiment of the drive belt 3 is shown which contains only a single ring set 31. In this case, the ring set 31 is accommodated in a centrally located groove of the transverse section 32, which groove faces radially outwards of the drive belt 3. Such a central opening is defined between a base 39 of the transverse section 32 and two cylindrical portions 36, each extending in a radially outward direction from one axial side of the base 39. In this radially outward direction, the central opening is partially closed by a correspondingly axially extending hook 37 of the cylindrical portion 36.

On either side of the transverse section 32 of both drive belts 3, contact surfaces 38 are provided for frictional contact with the pulley discs 4, 5. The contact surface 38 of each transverse segment 32 is angled

Oriented relative to each other, the angle substantially matching the angle of the V-pulley groove. The transverse section 32 is also typically made of metal.

As is well known, during operation of the transmission, the individual rings 41 of the drive belt 3 are tensioned by the radially directed reaction force of said clamping force. The generated ring tension is not constant, however, and varies not only according to the torque to be transmitted by the transmission, but also according to the rotation of the drive belt 3 in the transmission. Thus, in addition to the yield strength and wear resistance of the ring 41, fatigue strength is also an important characteristic and design parameter thereof. Thus, as the base material of the ring 41, a maraging steel is used, which can be hardened by precipitation (ageing) to increase its overall strength, and additionally case hardened by nitriding (gas soft nitriding) to increase the wear resistance, in particular the fatigue strength.

Figure 3 shows the relevant parts of a known manufacturing method of a ring set 31, which is commonly used in the art for producing metal drive belts 3 for automotive applications. The individual process steps of the known manufacturing method are indicated by roman numerals.

In a first process step I a sheet or plate 20 of a maraging steel substrate having a thickness of about 0.4 mm is bent into a cylindrical shape and in a second process step II the meeting plate ends 21 are welded together to form a hollow cylinder or tube 22. In a third step III of the process, the tube 22 is annealed in the furnace chamber 50. Thereafter, in a fourth process step IV, the tube 22 is cut into a plurality of rings 41, which are subsequently rolled into a larger diameter ring in a fifth process step V, while reducing its thickness to typically about 0.2 mm. The ring 41 thus rolled is subjected to a further, ring annealing process step VI to remove the work hardening effect of the previous rolling process step V by recovering and re-crystallizing the ring material in the furnace chamber 50 at a temperature significantly above 600 ℃, e.g. about 800 ℃. At such high temperatures, the microstructure of the ring material is composed entirely of austenite crystals. However, when the temperature of the ring 41 is again lowered to room temperature, this microstructure may transform back to martensite as desired.

After annealing VI, the ring 41 is calibrated in a seventh process step VII by being mounted around two rotating calibration rollers and stretched to a predetermined circumferential length by forcing the rollers apart. In a seventh process step VII of ring calibration, ring 41 also typically has a slight lateral curvature, i.e., convexity (crown), and internal stresses are applied to ring 41. Thereafter, the ring 41 is heat treated in an eighth process step VIII of combined ageing (i.e. bulk precipitation hardening) and nitriding (i.e. case hardening). In particular, this combined heat treatment comprises maintaining the ring 41 in a furnace chamber 50 containing a process atmosphere consisting of ammonia, nitrogen and hydrogen. In the furnace chamber, ammonia molecules are decomposed at the surface of the ring 41 into hydrogen and nitrogen atoms, which can enter the microstructure of the ring 41. These nitrogen atoms remain partly in the microstructure as interstitial atoms, partly in combination with certain alloying elements of the maraging steel, such as in particular molybdenum, to form intermetallic precipitates (for example Mo 2N). These fillers and precipitates are known to significantly increase the wear resistance and fatigue fracture resistance of the ring 41. It is especially noted that this combined heat treatment may alternatively be performed after or before the aging treatment (without simultaneous nitridation), i.e. in an ammonia-free process gas. This separate aging treatment is applied when the duration of the nitriding treatment is too short to complete the precipitation hardening process at the same time.

A plurality of rings 41 so machined are assembled in a ninth process step IX to form a ring set 31 by radial nesting (i.e. a concentric stack of selected rings 41) to achieve a minimum radial play or gap between each pair of adjacent rings 41. It is noted that it is also known in the art to assemble the ring set 31 immediately after the seventh process step VII of ring calibration, i.e. before the eighth process step VIII of ring aging and ring nitridation.

The process step V of rolling the ring 41 is shown in more detail in fig. 4, fig. 4 depicting a known ring rolling device comprising two rotatable back-up rolls 8, 9, a rotatable roll 10, a pair of rotatable back-up rolls 11 and a rotatable roll 12. The pressure roller 12 acts on a support roller 11, which support roller 11 in turn acts on the first support roller 8 of the two support rollers 8, 9. The first back-up roll 8 is placed in the centre of the rolling device, while the other second back-up roll 9 is movably received in the rolling device such that it can be moved away from (and back towards) the first back-up roll 8 to apply a pulling force FI to a ring 41 surrounding and mounted on the two back-up rolls 8, 9. Also, the pressure roll 12 is movably housed in the rolling device so that it can move towards (and away from) the support roll 11 to exert a thrust Fs on the inner side of the ring 41 via the support roll 11 and the first support roll 8. Said thrust Fs is balanced by the reaction force Fr exerted by the roller 10 on the outer surface of the ring 41 opposite the first support roller 8. Other embodiments of ring rolling apparatus are also known. During the actual rolling of the ring 41, the ring 41 is rotated around and by the two back-up rolls 8, 9 in the direction indicated by the arrow RD in fig. 4, while being compressed between the first back-up roll 8 and the roll 10 by a pushing force Fs and stretched by a pulling force FI.

The ring rolling process (step V) is primarily intended to achieve the desired cross-sectional shape and circumferential length of the ring 41. An example of such a desired cross-sectional shape of the ring 41 is schematically not shown to scale in fig. 5. As shown in fig. 5, the ring 41 has a generally symmetrical so-called barrel shape, wherein the thickness Tm in the middle of the ring 41 is larger than the thickness Ts at or near its axial sides (especially when measured within 1mm of the respective axially oriented side of the ring 41, e.g. at a distance of 0.5mm from the respective side). It is noted that in fig. 5, the thickness dimensions Tm, Ts of the ring and the barrel shape thereof are 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 its axial symmetry, as schematically shown in fig. 6. In particular, the thickness Tsr at one axial side of the ring 41 (in FIG. 6: right-hand side, as viewed in the rolling direction RD and with respect to the radially outer side of the ring 41) may be slightly smaller than the thickness Tsl at its other axial side (in FIG. 6: left-hand side) and/or the thickest part Tmax of the ring 41 may not occur in the middle thereof, but somewhere between the middle thereof and one of its axial sides (i.e., Tm < Tmax).

This defect in the cross-sectional shape of the ring 41 after being rolled without the barrel shape is exaggerated and not shown to scale in fig. 7. In fig. 7, the ring 41 is shown in cross-section showing (i.e. without the barrel shape only) its widthwise wedge or taper resulting from the thickness difference between its axial sides. When a plurality of such rings 41 are nested with one another to form the ring group 31, the thickness difference of the individual rings 31 disadvantageously accumulates in the thickness direction of the ring group 31. For example, a ring set 31 having 12 rings may exhibit an overall taper of almost 100 microns with a thickness differential of 8 microns, relative to a nominal ring thickness of 185 microns. Thus, even if the individual rings 41 of the ring set 31 have a relatively minimal taper, the stress level experienced by the rings during operation of the drive belt 3 will be much higher than without such a taper.

According to the present disclosure, prior to or as part of the assembly of the ring set 31 in said ninth process step IX, the overall taper of the ring set 31 may advantageously and economically be minimized by turning every other ring 41b of the rings 41a, 41b of the ring set 31 relative to the remaining rings 41a of the ring set 31, as schematically illustrated in fig. 8. Thus, after being turned, every other ring 41b is thinnest on its left axial side, while the remaining rings 41a are thinner on their right axial side. In particular, in this way, for each individual ring 41 according to the present disclosure, a thickness difference of at least 5 microns, possibly in the range between 10 and 25 microns, may be allowed.

Due to the barrel-alternating taper and/or barrel-alternating asymmetry of adjacent rings 41 in the ring set 31, these rings 41 will have a tendency to shift slightly in opposite axial directions during operation of the drive belt 3, i.e. to shift alternately to the left and to the right in figure 2. Thus, the overall width of the ring set 31 will be slightly greater than the width of the individual rings 41. In the case of the embodiment of the drive belt 3 with a single ring set 31 (shown on the right in fig. 2), such an axial expansion of the individual rings 41 of the ring set 31 (relative to their perfect axial alignment shown in fig. 2) advantageously increases the overlap with the hooks 37 in the axial direction and thus supports the reliability of the drive belt 3, since the transverse segments 32 are thereby increasingly prevented from unintentionally separating from the ring set 31 during operation.

Theoretically (i.e. in the case of a ring set 31 with an even number of rings 41, each ring having the same taper), the final 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 on average exhibit a taper of the same order of magnitude as its individual rings 41. In any event, the overall taper of a ring set 31 made in accordance with the present disclosure is significantly reduced relative to prior art ring sets 31.

Instead of such a turn, every other ring 41b may be turned inside out for the same effect.

As an additional process step APS in the overall manufacturing process, the turning or inside-out flipping of some rings 41b, e.g. every other ring 41b, relative to other rings 41a according to the present disclosure is performed after said fifth process step V of ring rolling and before or finally as part of the assembly before said ninth process step IX of ring set assembly. Preferably and as schematically shown in fig. 9, such an additional process step APS is carried out before the rolled ring 41 is advanced to the ring calibration (process step VII). In so doing, the radius of convexity and the internal stress exerted on the ring 41 in said seventh process step VII of ring calibration are applied in a corresponding manner (e.g. in terms of calibration rotation direction, convexity lateral symmetry, internal stress distribution, etc.) to all rings 41, i.e. whether it is turned (ring 41b) or not (ring 41a) according to the present disclosure. Furthermore, the additional process step APS is carried out after ring 41 has undergone a ring anneal (process step VI). In so doing, the ring 41 is more compliant and less vulnerable to damage when handled in said additional process step APS, at least compared to the immediate handling after ring rolling (process step V), since the work hardening effect of ring rolling is removed by recrystallization and normalizing in the ring annealing (process step VI).

In the context of the present disclosure, said turning of the ring 41 requires turning the ring 41 around its radial axis RA (as indicated by the arrow marked (r) in fig. 9) by 180 degrees (i.e. half a turn) so that its axial side switches position, i.e. is swapped. Flipping the ring 41 inside out requires pushing the left axial side of the ring 41 to the right of the ring 41 via the radially inner side of the ring 41, while pulling the right axial side of the ring 41 to the left of the ring 41 via the radially outer side of the ring 41 (as indicated by the arrow labeled ② in fig. 9), and vice versa. It is to be noted that in the latter case, i.e. by turning the ring 41 inside out, not only the axial sides thereof are interchanged, but also the radial inside and outside thereof, i.e. the radially inward surface of the ring 41 becomes the radially outward surface thereof, and vice versa. It should be noted, however, that such inside-outside turning of the ring 41 may not be as preferred as said turning, since in practice one radial side of the ring 41, usually the radially inner side, is provided with a surface relief or increased roughness in the ring rolling, while the other radial side is free and relatively smooth. Preferably, the latter radial side abuts in the ring set 41 against the opposite radial side of the adjacent ring 41, which is relatively smooth and/or free of surface irregularities, which is no longer possible if one of these adjacent rings 41 is turned inside out.

In addition to all of the details of the above description and all of the accompanying drawings, the present disclosure also relates to and includes all of the features of the appended claims. The parenthetical references in the claims do not limit their scope, but are provided merely as non-limiting examples of the corresponding features. The claimed features may be applied individually in a given product or in a given process or method, as the case may be, but any combination of two or more such features may also be applied therein.

The invention represented by the present disclosure is not limited to the embodiments and/or examples explicitly mentioned herein, but also includes modifications, variations and practical applications thereof, especially within the reach of a person skilled in the art.

Claims (9)

1. Method for manufacturing a drive belt (3), in particular for manufacturing a ring set (31) thereof, having a set (31) of a plurality of metal rings (41) nested into each other, wherein the rings (41) are rolled (V) in their radial or thickness direction, respectively, and the ring set (31) is assembled (IX) from the plurality of rolled rings (41), characterized in that a part of the rolled rings (41) is turned or turned inside out in relation to another part of the rolled rings (41) which is not turned or turned inside out.

2. Method for manufacturing a drive belt (3) according to claim 1, characterized in that the rolled ring (41) is annealed (VI) and calibrated (VII) and that the part of the rolled ring (41) that is turned or inside-outside-flipped after annealing (VI) and before calibrating (VII) the ring.

3. Method for manufacturing a drive belt (3) according to claim 1 or 2, characterized in that the ring set (31) is assembled from rolled rings (41) that are turned or turned inside out and rolled rings (41) that are not turned or turned inside out alternately.

4. Method for manufacturing a drive belt (3) according to claim 1 or 2, characterized in that the rolled ring (41) has an axial or width dimension greater than 14mm and has a radial or thickness dimension in the range of 0.15mm to 0.20 mm.

5. Drive belt (3) for a continuously variable transmission, provided with at least one set (31) of a plurality of metal rings (41) nested into each other and a plurality of transverse segments (32) arranged in a row around the circumference of the ring set (31), wherein the cross section of an individual ring (41) is wedge-shaped in the axial or width direction, i.e. the thickness dimension near one of its two axial sides is smaller or larger than the thickness dimension near the other axial side, characterized in that there are two axial orientations of the wedge-shaped cross section of the individual ring (41) in the ring set (31).

6. Drive belt (3) according to claim 5, characterized in that the two axial orientations of the wedge-shaped cross section of the individual rings (41) alternate in the ring set (31).

7. Drive belt (3) for a continuously variable transmission, provided with at least one set (31) of a plurality of metal rings (41) nested into each other and a plurality of transverse segments (32) arranged in a row around the circumference of the ring set (31), wherein an individual ring (41) has a substantially barrel-shaped cross-section with asymmetry, wherein the maximum thickness (Tmax) of the respective ring (41) is located on one axial side of its middle, characterized in that there are two axial orientations of the asymmetric barrel-shaped cross-section of its individual ring (41) in the ring set (31).

8. Drive belt (3) according to claim 5, 6 or 7, characterized in that it is provided with a single ring set (31), the axial or width dimension of the single rings (41) of which is 14mm or more.

9. Drive belt (3) according to claim 8, characterized in that the single ring (41) has a radial or thickness dimension in the range between 0.15mm and 0.20mm, the thickness dimension varying in the width direction of the ring (41) in the range of 5 to 25 micrometers.

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