Calibration process for circumferential length of rings in assembly method of ring set for drive belt

Abstract

A method for assembling a set (31) of flexible rings (32) arranged one on top of the other, comprising a process step (VII), in which each flexible ring (32) is calibrated by stretching the ring (32) to a respective predetermined diameter or circumferential length corresponding to the intended radial position of the ring (32) within the ring set (31). According to the invention, said predetermined diameter or circumferential length of the flexible ring (32) is determined in relation to a measured thickness of the flexible ring (32).

Description

[Technical field]

[0001] The present disclosure relates to a method for assembling a ring set for a drive belt, in particular to a process for calibrating the circumferential length of the rings in the assembly method. Drive belts are primarily used as a means for power transmission between two adjustable pulleys in well-known continuously variable transmissions, primarily applied in motor vehicles.

[0002] Drive belts of this kind are generally known and consist of a number of relatively thin transverse elements slidably assembled in one or more ring sets, each of which consists of a number of flexible rings arranged one on top of the other. In this type of drive belt, the transverse elements are not connected to the ring sets and can slide along the circumference of the ring sets, in particular during operation of the transmission, and the individual rings of the ring sets can slide relative to one another.

[0003] The rings of the drive belt, alternatively called hoops, loops or endless bands, are manufactured from steel, in particular from maraging steel, which combines, inter alia, high mechanical properties of tensile strength and (bending) fatigue strength with relatively favorable possibilities of processing the steel from a sheet-like substrate towards the desired shape and desired material properties of the final ring, which should ideally not vary along the circumference of the ring. These desired material properties comprise a considerable hardness of the core material of the ring, which combines properties of high tensile strength with properties of sufficient elasticity and ductility to allow bending of the ring in the longitudinal direction, and a significantly harder outer or surface layer to provide the ring with wear resistance. Also, a considerable residual compressive stress is applied to the surface layer of the ring, providing a high resistance to metal fatigue. This latter feature is particularly important, since the rings are subjected to a large number of (tensile) loading and bending cycles during the service life of the transmission drive belt.

[0004] The overall manufacturing methods for such drive belts are well known in the art. Specifically, with regard to the ring set components of the drive belt, for example, EP 1815160 A1 discloses a manufacturing method in which a plate-like substrate is bent and welded into a cylindrical shape, i.e., a tube. The tube is annealed to restore its original material properties, i.e., to substantially remove internal / residual stresses and / or inhomogeneous microstructures introduced by the bending of the plate and welding of the tube.

[0005] The annealed tube is cut into a number of hoops, for example in a slitting process or a laser cutting process. The edges of the hoops (between the radially oriented main surface of the hoop and the axially oriented side surface of the hoop) are typically treated to remove burrs from the edges and / or to round off the burrs, i.e., in a barrel polishing process or a laser melting process. The hoops are rolled to a desired thickness to lengthen them, which is typically about 0.185 mm in the final product. After rolling, the hoops are flexible in the circumferential direction and are called rings or bands, and the hoops are annealed to restore their original material properties, i.e., to remove much of the internal / residual stresses and / or inhomogeneous microstructures introduced by the rolling of the hoops.

[0006] The annealed rings are calibrated, in a process step of calibration, the rings are individually wound around two rollers and stretched by forcing the calibration rollers apart from each other during rotation. In particular, each ring is stretched to a respective predefined diameter or circumferential length that corresponds to the intended radial position of the ring in the ring set. In this process step of ring calibration, a residual stress distribution is also imposed on the rings, which defines the so-called curl radius of the ring, as explained in detail in EP 1 403 551 A1. In particular, the diameter of the calibration rollers largely determines such residual stress distribution after calibration. In fact, such a diameter is carefully selected in order to achieve that the maximum stress level occurs substantially equally on the radial inner surface of the ring (at the straightened section of the ring) as on the radial outer surface of the ring (at the maximally bent section of the ring) during operation of the drive belt of the transmission, whereby the maximum stress level of the ring is minimized.

[0007] After the rings are calibrated, the ring set is assembled by placing the rings on top of each other, typically from radially outside to inside, i.e., in order of decreasing circumferential length of the rings. Only a small gap, or play, is permitted between each pair of radially adjacent rings of the ring set, i.e., between the outer radius of each inner ring and the inner radius of each outer ring of each pair, to maximize the overall fatigue strength of the ring set in its drive belt application. Typically, such radial play of the rings varies between a few microns of negative play to about 10 microns or so of positive play.

[0008] After assembly, the ring set is heat treated by precipitation hardening to increase the toughness of the rings and by (surface) nitriding to introduce residual compressive stresses in the outer surface layers of the rings.

[0009] Precipitation hardening, also known as aging, is achieved by heating the rings to temperatures in excess of 400 degrees Celsius (°C). At this temperature, fine metallic precipitates form and grow in random locations throughout the ring blank. As the precipitates grow, the hardness of the ring material generally increases until it reaches a maximum hardness value, after which the hardness of the ring material typically begins to decrease again (so-called overaging). To prevent (intense) oxidation of the ring surface, precipitation hardening is usually performed in an inert or reducing process atmosphere, usually nitrogen gas or nitrogen gas mixed with hydrogen gas.

[0010] Nitriding provides an additively hardened and compressively stressed surface layer. In nitriding, at least in the typically applied variant of gas nitrocarburizing, the ring is kept at a temperature above 400° C. in a process atmosphere containing ammonia gas (NH3). At such temperatures, ammonia molecules dissociate at the surface of the ring, forming hydrogen gas and nitrogen atoms, the latter of which enter the crystal lattice of the ring material. As the nitriding process continues, the nitrogen atoms migrate from the surface into the ring material by diffusion, thus providing a ring with a nitrided surface layer of increased thickness. The thickness of the nitrided layer and closely related material properties such as hardness and residual compressive stress at the ring surface are obtained by the nitriding process and thus depend on the composition of the nitriding process atmosphere, in particular the ammonia concentration in the atmosphere, and the temperature and duration of the nitriding process.

[0011] When the drive belt is actually used in a given transmission application, the thickness of the nitride layer largely determines the mechanical performance and service life of the drive belt in the transmission. In particular, if the nitride layer is too thin, the wear and fatigue characteristics of the ring will not be optimal, or if the nitride layer is too thick, the ring material will be too brittle and the stress levels of the ring during operation may exceed the elastic limit. In either case, the ring and therefore the entire drive belt may not be able to achieve its full potential or may even fail prematurely. Therefore, once a target value for the nitride layer thickness has been determined, it is highly desirable that such target thickness be accurately and consistently achieved in the (mass) manufacturing of drive belts.

[0012] When the ring set is assembled after the calibration of the rings, the radially oriented surfaces of the rings located inside the ring set must be close to each other with only a small radial gap between the surfaces, so that the desired small radial ring play is achieved in the final product after nitriding. Thus, significantly less ammonia gas can be supplied and / or utilized inside the ring set compared to the outer surface of the ring set. The outer surface of the ring set is constituted by the axially oriented side surfaces of all the rings, the radially inner surface of the radially innermost ring of the ring set, and the radially outer surface of the radially outermost ring of the ring set. As a result, the thickness of the nitride layer on the inner surface of the ring set (i.e. the radially oriented surfaces of the rings facing each other between adjacent pairs of rings of the ring set) is smaller than the thickness of the nitride layer on said outer surface of the ring set. However, this particular aspect of the nitriding of the ring set is not problematic. This is because these inner surfaces of the ring sets do not abut against the transverse elements or the transmission pulleys, i.e. are subjected to less contact stresses than the outer surfaces of the ring sets.

[0013] In other words, the target thickness of the nitride layer may be less on the inner surface of the ring set than on the outer surface. Nevertheless, in (mass) manufacturing of drive belts, it is highly desirable that such a target thickness value of the nitride layer is realized as accurately and consistently as possible. However, in practice, it has been observed that the nitride layer on the inner surface of the ring set is not formed as accurately and / or consistently as on the outer surface of the ring set. The present invention aims to improve known ring set manufacturing methods in this respect, namely to improve the accuracy and / or consistency in forming the nitride layer on the interior of the ring set, i.e. on the radially oriented surfaces of the rings facing each other between adjacent pairs of rings of the ring set.

[0014] According to the present invention, such an improvement is found in a ring set assembly method as set forth in claim 1 below. This novel assembly method allows a more accurate and more consistent radial gap between adjacent rings of a ring set when the ring set is assembled. As a result, process gas can flow substantially evenly between all pairs of adjacent rings of the ring set during nitriding, so that the nitride layer is more uniformly and favorably formed between the individual rings of the ring set. Alternatively, different gaps may be provided between different pairs of adjacent rings of the ring set. This latter aspect of the invention may be desirable to affect the tensile stresses experienced by the individual rings of the ring set during operation of the drive belt and / or to differentiate the thickness of the nitride layer of different pairs of rings. This latter aspect of the invention may also be applied to affect the small but significant circumferential growth of the rings that typically occurs in nitriding, i.e. ring elongation.

[0015] In particular, according to the invention, the thickness of the rings is measured as part of the process step of ring calibration. This measured ring thickness is taken into account when determining the desired (inner or outer) diameter or (inner or outer) circumferential length in the ring calibration, i.e. when determining the final maximum distance to be provided (i.e. forced) between the calibration rollers in the ring calibration. The invention therefore takes into account the ring thickness variations between the individual rings after the rings are rolled. Although these latter thickness variations are small compared to the overall ring thickness, these latter thickness variations are in the same order of magnitude as the radial gaps required between the rings in the nitriding process to realize said radial ring play in the final product, i.e. in the micrometer range. Such thickness measurements are preferably performed non-contact, e.g. optically, and can be performed before the ring stretching in the ring calibration, but preferably during the ring stretching.

[0016] Further, in accordance with the present invention, in the calibration of the rings, the maximum distance S provided between the centerlines of the calibration rollers is increased or decreased in steps between successively calibrated rings of the ring set. For example, S[n-1]-S[n]=-π·(D[n-1]+G[n-1]) (1o) or S[n+1]-S[n]=π·(D[n]+G[n]) (1i) Where: - [x] is an index representing the position of a ring in a ring set ranging from ring number [1] corresponding to the radially innermost ring in the ring set to ring number [m] corresponding to the radially outermost ring in a ring set consisting of m rings; - D[x] represents the measured thickness of the xth ring, - G[x] represents the gap provided radially between the xth and (x+1)th ring pairs of a ring set during calibration and may have the same value for all ring pairs in a ring set.

[0017] Equation (1o) applies when the rings of the ring set are calibrated radially inwards starting from the radially outermost ring (index [m]), whereas equation (1i) applies when the ring set is assembled by stacking the rings one on top of the other from the inside to the outside. Of course, when determining the absolute value of said maximum distance S provided between the centerlines of the calibration rollers, at least the elastic deformations of the rings must also be taken into account by known methods, i.e. by calculation / estimation or measurement.

[0018] From the above formulas (1o;1i), it is clear that the thickness (index [m]) of the outermost ring is not necessary, at least within the context of the present invention, and the measurement of the thickness of the outermost ring can potentially be omitted. Furthermore, when the ring set is assembled from the outside to the inside, formula (1o) suggests that the desired diameter or circumferential length of the rings (excluding the outermost ring) is determined in part by the (measured) thickness of each ring itself, which therefore needs to be measured when the ring is stretched to such desired diameter or circumferential length. However, when the ring set is assembled from the inside to the outside, formula (1i) suggests that the desired diameter or circumferential length of the rings (excluding the outermost ring) is determined in part by the (measured) thickness of the adjacent rings located inside each ring of the ring set. In this case, the measurement of the thickness of the rings may also be performed after the rings are stretched.

[0019] Preferably, the thickness of the ring is measured while the ring is tensioned by a calibration roller, more preferably said measured ring thickness is determined as an average of a number of measurements along the circumference of the ring, wherein the ring may be rotated by a calibration roller in between or even during said thickness measurements.

[0020] The novel assembly method according to the present invention and its technical background will be described below with reference to the accompanying drawings. [Brief description of the drawings]

[0021] [Figure 1] FIG. 1 provides a schematic illustration of a known continuously variable transmission with a drive belt. [Diagram 2] FIG. 2 is a perspective view of a cross section of a drive belt. [Diagram 3] 1 is a schematic diagram showing currently relevant portions of a known overall manufacturing method for a ring set component of a drive belt; FIG. [Figure 4] FIG. 4 provides a diagrammatic representation of the gas nitrocarburizing heat treatment in the manufacturing method according to FIG. [Diagram 5] 5A-5D show schematic process steps for calibration of a ring according to the invention;

[0022] FIG. 1 shows the central part of a known continuously variable transmission or CVT, generally applied in the drive train between the engine and the drive wheels of a motor vehicle. The transmission comprises two pulleys 1,2, each of which is provided with two conical pulley discs 4,5, between which a mainly V-shaped groove is defined, one disc 4 being axially movable along the respective pulley shaft 6,7 on which it is located. A drive belt 3 is wrapped around the pulleys 1,2, and transmits a rotational movement ω and therefore a torque T from one pulley 1,2 to the other pulley 2,1. The transmission also generally comprises actuation means for exerting an axial clamping force Fax on said at least one disc 4 towards the respective other pulley disc 5, so that the belt 3 is clamped between them. This also determines the transmission (speed) ratio between the rotational speed of the driven pulley 2 and the rotational speed of the drive pulley 1.

[0023] An example of a known drive belt 3 is shown in detail in its cross section in Fig. 2, the belt 3 incorporating two ring sets 31, each consisting of a set of, in this example, six thin, flat, i.e. strip-like, flexible rings 32. The belt 3 further comprises a number of plate-like metallic transverse elements 33, which are held together by the ring sets 31, each located in a recess of each of the transverse elements 33. The transverse elements 33 receive said clamping force Fax, and when an input torque Tin is exerted on the so-called drive pulley 1, the friction between the discs 4, 5 and the belt 3 transmits the rotation of the drive pulley 1 to the so-called driven pulley 2 via the drive belt 3, which also rotates.

[0024] During operation of the CVT, the drive belt 3, and in particular its rings 32, are subjected to cyclically varying tensile and bending stresses, i.e., fatigue loads. Thus, the resistance to metal fatigue, i.e., the fatigue strength of the rings 32, typically determines the useful life of the drive belt 3 at a given torque T transmitted by the drive belt 3. Thus, a general goal over the years in the development of ring set manufacturing methods has been to achieve the required ring fatigue strength and wear resistance in combination with minimal material and processing costs.

[0025] FIG. 3 shows the currently relevant part of the overall manufacturing method of the known drive belt 3, i.e. the currently relevant part of the manufacturing of its ring set 31, the separate process steps being indicated with Roman numerals.

[0026] In a first process step I, a thin sheet or plate of substrate 11, typically having a thickness in the range 0.3 mm to 0.6 mm, is bent into a cylindrical shape, and in a second process step II, the butted plate ends 12 are welded together to form an open, hollow cylinder or tube 13. In a third process step III of the process, the tube 13 is annealed. Thereafter, in a fourth process step IV, the tube 13 is cut into a number of annular hoops 14, which are then rolled and stretched in process step 5V while their thickness is reduced to a value between 0.100 and 0.250 mm, typically around 185 microns.

[0027] After rolling, the hoop 14 is referred to as a ring 32. The ring 32 is subjected to a further, i.e., ring annealing process step VI to remove the work hardening effects of the previous rolling process (i.e., step 5V) by recovery and recrystallization of the ring material at a temperature significantly higher than 600 degrees Celsius ("°C"), e.g., about 800°C.

[0028] Then, in a seventh process step VII, the ring 32 is calibrated, i.e. it is stretched to a predetermined circumferential length by wrapping it individually around two rollers and forcing the rollers apart while rotating, in which an internal stress distribution is also imposed on the ring 32.

[0029] Thereafter, in an eighth process step VIII, a ring set 31 is assembled from a number of rings 32, each having a suitable circumferential length that is compatible with one another, by stacking the rings 32 on top of one another. Although in FIG. 3 (and FIG. 4), for simplicity, the ring set 31 is shown with only two rings 32, the ring set 31 typically consists of 6 to 12 rings.

[0030] Finally, the ring set 31 is heat treated in a ninth process step IX of precipitation hardening or ageing IX-A and gas soft nitriding IX-N. More specifically, ageing and nitriding involve heating the ring set 31 to a temperature of 400-550 °C in a furnace containing a controlled gas atmosphere consisting of nitrogen, hydrogen and ammonia gas. The exact process settings of the heat treatment are selected depending on the base material of the rings 32 (i.e. the alloy composition of the maraging steel) and on the mechanical properties desired for the rings 32. In this latter respect, it is pointed out that typically a core hardness of at least 500 HV1.0, a surface hardness of at least 800 HV0.1 and a thickness of the nitrided surface layer, alternatively denoted as nitrogen diffusion zone, of 25-35 microns is aimed for. In this case, the duration of the above mentioned heat treatment is the result of these mechanical properties, the process temperature and the process atmosphere composition and in practice has a value usually ranging from 30 to 90 minutes.

[0031] In FIG. 4, the ninth process step IX of nitridation is shown diagrammatically in cross section BB of the ring set 31 shown in FIG. 3. In FIG. 4, the gaps 34 between the pairs of adjacent rings 32 shown are greatly exaggerated (i.e., in reality, such gaps 34 are significantly smaller than the thickness of the rings 32, i.e., much smaller than 10%). To provide the rings 32 with a nitrided surface layer, the ring set 31 is immersed in a process atmosphere containing gaseous ammonia molecules, which are diagrammatically represented in FIG. 4 by four circles, a large circle representing nitrogen atoms and three small circles representing hydrogen atoms of the ammonia molecules. At least a portion of the ammonia molecules dissociate at the surface of the rings 32, thereby releasing three hydrogen atoms and allowing one nitrogen atom to enter the crystal lattice of the rings 32. This ammonia dissociation reaction is diagrammatically represented inside the dashed oval in FIG. 4. As part of the ammonia dissociation reaction, the released hydrogen atoms combine to form hydrogen gas. The ammonia dissociation reaction is therefore represented by the following equation: 2NH3→2[N]+3H2(2) It can be expressed as:

[0032] It is well known that the rate of this ammonia dissociation reaction (2) is proportional to the process temperature and the ammonia concentration in the process atmosphere, and inversely proportional to the hydrogen concentration in the process atmosphere. However, inside the gap 34 between each pair of adjacent rings 32 in the ring set 31, the ammonia concentration is typically lower than outside the ring set 31. After all, inside this gap 34, ammonia is mainly supplied by (gas) diffusion, while outside the gap 34, forced circulation of the process atmosphere is also available for supplying ammonia (and removing hydrogen). Moreover, the path length of ammonia diffusion inside the gap 34 is much longer than that to the outside of the ring set 31. This means that the ammonia concentration inside the gap 34 depends greatly on the size of the gap 34 as well as the thickness of the resulting nitride layer.

[0033] Thus, in conclusion, in said eighth process step VIII of the assembly of the ring set 31, the precision and / or consistency of the gaps 34 between each adjacent pair of rings 32 of the ring set 31 is improved, in order to advantageously minimize the variations in the thickness of the nitride layer of the rings 32. According to the invention, such improvement is realised by determining the diameter or (inner) circumferential length of the rings 32 obtained in the process step VII of the calibration of the rings not only in relation to the radial position of that respective ring 32 within the ring set 31, but also in relation to the actual thickness of the rings 32, the thickness of such rings being measured as part of the process step VII of the calibration of the rings.

[0034] Preferably, according to the invention, as shown diagrammatically in Fig. 5, process step VII of the calibration of the ring is now carried out in four stages. In a first stage (1) of the calibration of the ring, the ring 32 is placed around the two calibration rollers 15. In a second stage (2) of the calibration of the ring, the two calibration rollers 15 are moved away from each other until the ring 32 extends straight between them, i.e. until it is stretched between them with minimal elastic elongation. In a third stage (3) of the calibration of the ring, the thickness D of the ring 32 is measured at several points along the circumference of the ring 32 while the ring 32 is rotated by (rotating) driving one or both of the calibration rollers 15, and an average value of such thicknesses D is determined. Preferably, the thickness D of the ring is measured continuously along the circumference of the ring and averaged. Finally, in a fourth stage (4) of the ring calibration, the two calibration rollers 15 are moved further away from each other, increasing the distance S between the centerlines of the two calibration rollers 15, thereby causing the ring 32 to plastically deform, and subsequently reach the desired diameter or (inner) circumferential length taking into account the measured (average) thickness D of the ring 32. Of course, as already known and practiced in the art, the elastic springback of the ring 32 when releasing the tension thereon and possibly other influencing factors must be taken into account as well.

[0035] The present invention relates to and includes all the features of the appended claims as well as the entire foregoing description and all the details of the accompanying drawings. References in parentheses in the claims do not limit the scope thereof but are provided merely as non-binding examples of the respective features. The claimed features may be applied individually to a given product or a given process, as the case may be, but any combination of two or more of such features may also be applied.

[0036] The invention represented by this disclosure is not limited to the embodiments and / or examples expressly mentioned herein, but encompasses modifications, variations and practical applications thereof that are within the scope of those skilled in the relevant art.

Claims

1. A method for assembling a ring set (31) consisting of a number of flexible rings (32) arranged one on top of the other, comprising the steps of:

1. A method of assembling a ring set (31), comprising a calibration process step (VII) of individually stretching the rings (32) of the ring set (31) to a predetermined circumferential length or diameter, the predetermined length or diameter for each ring (32) being determined at least in part by a position of each of the rings (32) of the ring set (31) relative to other rings (32) of the ring set (31), measuring the thickness of at least each of said rings (32) of said set of rings (31), except for the outermost ring (32), in said calibration process step (VII); The predetermined length or diameter for each ring (32) is additionally determined in relation to the measured thickness of such ring (32) itself or to the measured thickness of each adjacent ring (32) of the ring set (31), in particular of each inner adjacent ring (32). A method for assembling a ring set (31), comprising:

2. 2. A method for assembling a ring set (31) according to claim 1, characterized in that each ring (32) is placed around two rollers (15) before measuring the thickness of said ring (32), then the rollers (15) are moved away from each other to apply tension to each of said rings (32), and then the thickness of each of said rings (32) is measured.

3. 3. The method of assembling a ring set (31) according to claim 2, characterized in that in the calibration process step (VII), the rollers (15) are rotated simultaneously and the rollers (15) are moved further away from each other to stretch each of the rings (32) to the predetermined circumferential length or diameter.

4. 2. The method of assembling a ring set (31) according to claim 1, characterized in that after the calibration process step (VII), the rings (32) are placed one on top of the other (VIII) to form the ring set (31), and then the entire ring set (31) is subjected to a nitriding heat treatment (IX).

5. 2. The method of assembling a ring set (31) according to claim 1, characterized in that between each pair of adjacent rings (32) of the ring set (31), the predetermined circumferential length or diameter of each ring (32) is changed in a step size determined by both the measured thickness of at least one of the adjacent rings (32) and the desired dimension of the gap (34) to be provided between the adjacent rings (32).

6. 2. The method for assembling a ring set (31) according to claim 1, characterized in that the rings (32) of the ring set (31) are subjected to the calibration process step (VII) in the same order as the rings (32) are subsequently placed one on top of the other (VIII) to form the ring set (31).

7. 7. A method for assembling a ring set (31) according to any one of claims 4 to 6, characterized in that the length or diameter in a predetermined circumferential direction of at least each ring (32) of the ring set (31), except for the outermost ring (32), is determined by the measured thickness of each ring (32) itself, and the ring set (31) is assembled by arranging the rings (32) in successively overlapping positions inward from the outermost ring (32) of the ring set (31).

8. 7. A method for assembling a ring set (31) according to any one of claims 4 to 6, characterized in that the length or diameter in a predetermined circumferential direction of at least each ring (32) of the ring set (31), except for the innermost ring (32), is determined by the measured thickness of each of the innermost adjacent rings (32), and the ring set (31) is assembled by arranging the rings (32) in a successive overlapping manner from the innermost ring (32) of the ring set (31) outward.

Images