CN113227609B - Method for producing a drive belt for a continuously variable transmission and drive belt produced thereby - Google Patents

Abstract

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

Description

Method for producing a drive belt for a continuously variable transmission and drive belt produced thereby

Technical Field

The present disclosure relates to a method for manufacturing a drive belt of a continuously variable transmission and a drive belt manufactured thereby. Such a drive belt is known, for example, from GB1286777 (a) and more recently from international publication WO2015/177372 (A1). Such known drive belts comprise a plurality of mutually nested endless flexible metal bands or rings (i.e. they are stacked concentrically to each other in a set of rings or rings), and a plurality of metal transverse segments arranged in substantially continuous rows along the circumference of such rings. Each transverse segment defines a central opening defined by and between a base of the transverse segment and two cylindrical portions extending radially outwardly from respective axial sides of the base, respectively, in which respective circumferential segments of the ring set are received 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 a radially outward direction by a respective axial extension of at least one or possibly both cylindrical portions. In particular, such axial extension of the respective cylindrical portion extends partially towards the other, axially opposite cylindrical portion of the transverse segment above the ring set and is hereinafter denoted as hook of the cylindrical portion. It is noted that alternative measures and/or means for accommodating the ring sets in the central opening of the transverse segment, i.e. for example accommodating rings (see e.g. US 5123880) and closure pins (see e.g. EP 0122064) instead of 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 belt when placed in a circular posture. The transverse segments have a thickness direction and a thickness dimension defined in a circumferential direction of the drive belt, a height direction and a height dimension defined in a radial direction of the drive belt, and a width direction and a width dimension defined in an axial direction of the drive belt. The thickness direction and thickness dimension of the ring set and its individual rings are defined in the radial direction of the drive belt, the width direction and 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 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 variable width V-groove, with a respective portion of the drive belt being retained in the pulley V-groove at a variable radius. By varying such a belt radius at the drive pulley, the speed ratio of the transmission can be varied. Transmissions of this type are well known and are commonly used in drive trains for passenger and other motor vehicles.

The above-mentioned drive belt is distinguished from another known design in which each transverse segment defines two lateral openings at a respective one of the sides of the central portion of the transverse segment or the neck portion between and connecting the bottom or body portion of the transverse segment and the top of the head portion. This type of belt comprises two sets of nested loops, each set being received in a respective lateral opening of the 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 above-mentioned drive belt. For a given application of the continuously variable transmission, the width of a single ring set of so-called single ring set belts and its constituent rings is typically about twice the ring width of either of the two ring sets of so-called double ring set belts, at least when these ring sets individually contain the same number of rings. In general, the loop width of the double loop group of tapes can be up to 12mm, with typical values of about 10mm, whereas the loop width of the single loop group of tapes exceeds 14mm, with typical values in the range between 16mm and 20 mm.

As part of the overall manufacturing process of the known drive belt, the rings are subjected to a rolling process step in which the rings are reduced in thickness and increased in diameter, i.e. circumferential length, by rotating them in their circumferential direction while being compressed between a pair of rollers. For example, the thickness of the semifinished annular product before rolling is 0.4 mm, and then the thickness is reduced in rolling to between 200 and 150 μm. Such ring rolling process steps are 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 the desired cross-sectional shape and/or the desired surface relief, both of which are also mentioned in WO 2004/050270.

Disclosure of Invention

While the basic set-up of the overall manufacturing process of the known drive belt is well known, long-standing and satisfactory, it can still be improved in accordance with the present disclosure, especially in terms of reliability in mass production of the drive belt. Also in accordance with the present disclosure, the performance of the drive belt in the transmission may be enhanced by such improvements 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 selected rings (i.e., a portion of successive rolled rings) relative to another portion of the rings (i.e., the remaining rings) before the rings are nested within one another to construct a ring set.

It is noted that in the context of the present disclosure, turning means turning around it radially one half turn (i.e. 180 degrees), while turning inside out means 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 corresponding opposite side of the ring via the radially outer side of the ring. In either case, the axial sides of the ring are swapped, i.e., the positions are switched.

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

Drawings

The method of manufacturing a drive belt 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;

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

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

FIG. 4 is a schematic view of a rolling apparatus for rolling metal rings as part of the overall manufacturing process of a drive belt;

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

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

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

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

fig. 9 shows new process steps in the overall manufacturing process of the drive belt according to the present disclosure.

Detailed Description

Fig. 1 shows the core components of a known continuously variable transmission or CVT, which is commonly used in a transmission system between the engine and the driving 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 pulley shafts 6 or 7, a generally V-shaped circumferential pulley groove being defined between the pulley discs 4, 5. 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 the pulley grooves for transmitting rotational movement and 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 such that the drive belt 3 is clamped between each pair of such pulley discs 4, 5. These clamping forces not only determine the frictional forces that can be applied to the greatest extent 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 well known per se.

In fig. 2, two known examples of the drive belt 3 are schematically shown with their cross section facing in the circumferential direction thereof. In both examples, the drive belt 3 comprises transverse segments 32, the transverse segments 32 being arranged in a row along the circumference of an endless 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. consists of a plurality of mutually nested, flat, thin and flexible individual rings 41. The thickness of the transverse segments 32 is small relative to the circumferential length of the ring set 31, in particular such that hundreds of transverse segments 32 are included in the rows thereof.

Although in the figures the ring set 31 is shown as consisting of 5 nested rings 41, in practice most cases 6, 9, 10 or 12 rings 41 are applied in such ring sets 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, comprising two such ring sets 31, each of which is accommodated in a respective lateral oriented groove of the transverse segment 32, which lateral oriented groove is open towards the respective, i.e. left and right, axial sides. Such lateral openings are defined between the body portion 33 and the head 35 of the transverse segment 32 on either side of a relatively narrow neck 34, said neck 34 being arranged between the body portion 33 and the head 35 and interconnecting them.

On the right side of fig. 2, an embodiment of the drive belt 3 comprising only a single ring set 31 is shown. In this case, the ring set 31 is accommodated in a centrally located groove of the transverse segment 32, which groove faces radially outwards of the drive belt 3. Such a central opening is defined between a base 39 of the transverse segment 32 and two cylindrical portions 36, which extend in a radially outward direction from one axial side of the base 39, respectively. 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 sections 32 of both drive belts 3 are provided for connection with the beltThe contact surfaces 38 of the discs 4, 5 in frictional contact. The contact surface 38 of each transverse segment 32 is angled

Oriented with respect to each other, which substantially matches the angle of the V-shaped pulley groove. The transverse segment 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. However, the generated ring tension is not constant 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, maraging steel is used as a base material for the ring 41, which steel can be hardened by precipitation (ageing) to increase its overall strength, and additionally case hardened by nitriding (gas soft nitriding) to increase wear resistance, in particular fatigue strength.

Fig. 3 shows a relevant part of a known manufacturing method of a ring set 31, which is commonly used in the art for producing a metal drive belt 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 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 encountered 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 oven 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 larger diameter rings 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 allowing the ring material to recover and recrystallize in the furnace chamber 50 at a temperature significantly higher than 600 ℃, for example about 800 ℃. At such high temperatures, the microstructure of the ring material is entirely composed of austenite-type crystals. However, when the temperature of the ring 41 drops again to room temperature, this microstructure will desirably transform back to martensite.

After annealing VI, the ring 41 is calibrated in a seventh process step VII by being mounted around two rotating calibration rolls and stretched to a predetermined circumferential length by forcing the rolls apart. In a seventh process step VII of ring alignment, the ring 41 also typically has a slight lateral curvature, i.e. convexity (crown), and an internal stress is exerted on the 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 includes maintaining ring 41 in furnace chamber 50 that contains a process atmosphere comprised of ammonia, nitrogen, and hydrogen. In the furnace chamber, the ammonia molecules decompose 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 (e.g. Mo 2N). These interstitials and precipitates are known to significantly increase the wear resistance and fatigue fracture resistance of ring 41. It is particularly noted that this combined heat treatment may alternatively be performed after or before the ageing treatment (without simultaneous nitriding), 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 simultaneously.

A plurality of so-machined rings 41 are assembled in a ninth process step IX to form the ring set 31 by radial nesting (i.e., concentric stacking of selected rings 41) to achieve a minimum radial play or clearance 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 nitriding.

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 support rolls 8, 9, a rotatable roll 10, a pair of rotatable support rolls 11 and a rotatable press roll 12. The press roll 12 acts on the support roll 11, which support roll 11 in turn acts on the first support roll 8 of the two support rolls 8, 9. The first support roll 8 is placed in the centre of the rolling device and the other second support roll 9 is movably accommodated in the rolling device such that it can be moved away from (and back towards) the first support roll 8 to exert a pulling force FI on the ring 41 around and mounted on the two support rolls 8, 9. Also, the pressing roller 12 is movably housed in the rolling device so that it can move toward (and away from) the support roller 11 to exert a thrust force Fs on the inside of the ring 41 via the support roller 11 and the first support roller 8. The thrust Fs is balanced by the reaction force Fr exerted by the roller 10 on the outer surface of the ring 41 opposite to 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 surrounds and is rotated by the two support rolls 8, 9 in the direction indicated by the arrow RD in fig. 4, while being compressed by the pushing force Fs and stretched by the pulling force FI between the first support roll 8 and the roll 10.

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 greater 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). Note that in fig. 5, the thickness dimensions Tm, ts of the ring and the barrel shape thereof are exaggerated for illustration 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 side as viewed in the rolling direction RD and radially outward with respect to the ring 41) may be slightly smaller than the thickness Tsl at the other axial side thereof (in fig. 6: left side) and/or the thickest part Tmax of the ring 41 may not appear in the middle thereof, but somewhere between the middle thereof and one of the axial sides thereof (i.e., tm < Tmax).

After being rolled and without the barrel shape, this defect in the cross-sectional shape of the ring 41 is exaggerated in fig. 7 and not shown to scale. In fig. 7, the ring 41 is shown in cross section showing (only, i.e. without the barrel shape) the wedge or taper in its width direction resulting from the thickness difference between its axial sides. When a plurality of such rings 41 are nested with each other to form the ring set 31, the thickness difference of the individual rings 31 disadvantageously accumulates in the thickness direction of the ring set 31. For example, with a thickness difference of 8 microns, a ring set 31 with 12 rings may exhibit an overall taper of almost 100 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 levels experienced by the rings during operation of the drive belt 3 will be much higher than would be the case without such taper.

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

Due to the barrel-shaped alternating taper and/or barrel-shaped 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, i.e. alternately to the left and right in fig. 2, during operation of the drive belt 3. In this way, the overall width of the ring set 31 will be slightly greater than the width of a single ring 41. In the case of an embodiment of the drive belt 3 with a single ring set 31 (shown on the right in fig. 2), this axial expansion of the individual rings 41 of the ring set 31 (with respect 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 lateral segments 32 are thereby increasingly prevented from being unintentionally separated from the ring set 31 during operation.

Theoretically (i.e. in case the ring set 31 has an even number of rings 41, each ring has 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 the ring set 31 made in accordance with the present disclosure is significantly reduced relative to the prior art ring set 31.

Instead of this turning, 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 turning of some rings 41b, e.g. every other ring 41b, relative to other rings 41a according to the present disclosure is performed after the fifth process step V of ring rolling and before the ninth process step IX of ring set assembly or finally as part of assembly. Preferably and as schematically shown in fig. 9, such additional process step APS is performed before the rolled ring 41 is advanced to ring calibration (process step VII). In so doing, the convexity radius and internal stress exerted on the rings 41 in said seventh process step VII of ring calibration is exerted in a corresponding way (e.g. in terms of calibration rotation direction, convexity transverse symmetry, internal stress distribution, etc.) to all rings 41, i.e. whether they are turned (ring 41 b) or not turned (ring 41 a) according to the present disclosure. Furthermore, the additional process step APS is performed after the ring 41 has undergone a ring anneal (process step VI). In so doing, the ring 41 is more compliant and less fragile when processed in said additional process step APS, at least compared to immediately processing after ring rolling (process step V), because the work hardening effect of ring rolling is removed by recrystallization and normalizing in ring annealing (process step VI).

In the context of the present disclosure, said turning of the ring 41 entails turning the ring 41 180 degrees (i.e. half a turn) about its radial axis RA (as indicated by the arrow marked (1) in fig. 9), so that its axial side switches positions, i.e. is exchanged. Turning 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 noted that in the latter case, i.e. by turning the ring 41 inside out, not only its axial sides are interchanged, but also its radially inner and outer sides, i.e. the radially inner surface of the ring 41 becomes its radially outwardly facing surface, and vice versa. It should be noted, however, that such an inside-out turning of the ring 41 may not be preferred over the turning described, because in practice one radial side of the ring 41, typically the radial inner side, is provided with a surface relief or increased roughness in the ring rolling, while the other radial side is not and relatively smooth. Preferably, the latter radial side abuts against an opposite radial side of the adjacent rings 41 in the ring set 41, which is relatively smooth and/or free of surface relief structures, which is no longer possible if one of these adjacent rings 41 is turned inside out.

In addition to all details of the foregoing description and the accompanying drawings, this disclosure also relates to and includes all features of the appended claims. Parentheses in the claims are not limiting the scope thereof but are provided merely as non-limiting examples of the corresponding features. The claimed features may be applied singly in any given product or in any 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 as far as the skilled person is concerned.

Claims (6)

1. Method for manufacturing a ring set of mutually nested metal rings (41) for a drive belt (3), wherein the rings (41) of the ring set (31) are rolled (V) successively in their radial or thickness direction, after which the ring set (31) is assembled (IX) from the mutually nested plurality of rolled rings (41), wherein the method comprises A Process Step (APS) of turning or turning inside out a part of the rings (41 b) of the successively rolled rings (41) of the ring set (31) in relation to another part of the rings (41 a) which are not turned or turned inside out before the mutually nested rings are assembled.

2. Method according to claim 1, characterized in that the annealing (VI) and the calibrating (VII) are performed on the successively rolled rings (41) of the ring set (31), and that the part of the rings (41 b) of the successively rolled rings (41) of the ring set (31) that is turned or turned inside out after the annealing (VI) of the rings and before the calibrating (VII) of the rings.

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

4. Method according to claim 1 or 2, characterized in that the successively rolled rings (41) of the ring set (31) have an axial or width dimension greater than 14mm and have 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 ring set (31) of mutually nested metal rings (41) manufactured according to the method of any one of claims 1-4 and a plurality of transverse segments (32) arranged in a row around the circumference of the ring set (31), wherein the cross section of a single ring (41) is wedge-shaped in 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 thereof, characterized in that there are two axial orientations of the wedge-shaped cross section of a single 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).

Images