JP2011518672A - Method for manufacturing ring component of drive belt - Google Patents

Abstract

  A method of manufacturing a metal ring (32) for use in a drive belt (3) for a continuously variable transmission, comprising increasing the circumferential length of an annular metal hoop (14). The rolling step is completed in a method that includes reducing the thickness and forming the ring (32) by plastic deformation and heat treating the ring (32) to cure the material of the ring (32). After that, the temperature of the ring (32) is kept below 600 ° C. for one or more of the remaining steps of the manufacturing method, ie the material of the ring (32) caused by plastic deformation of the ring This cold working effect is not removed before the heat treatment.

Description

  The present invention relates to a manufacturing method for an endless, thin, flexible metal, which band is generally between two adjustable pulleys of a known continuously variable transmission or CVT used in automobiles. It is provided in the drive belt for power transmission. Such a band is also called a ring component, at least for applications in the drive belt. Furthermore, the present invention relates to a product obtained by said method.

  In a particular type of drive belt known as a push belt, a large number of such rings are incorporated as at least one, but usually two, stacked or radially interdigitated sets. Yes. Furthermore, the known push belts have a number of transverse metal elements slidably attached to one or more such band sets. In push belt applications, conventional rings are formed from maraging steels, and this type of steel has a particularly advantageous potential for welding and plastic deformation of materials, and at least suitable heat treatment. Later, it has the characteristics of high tensile strength and excellent resistance to wear and bending and / or tensile stress fatigue.

  Known rings are provided with a hard core material to obtain excellent tensile strength, yield strength, bending strength and high resistance to metal fatigue, which ring core is substantially the same as that of the ring material. It is housed in an outer layer which is harder and thus more wear resistant. This outer layer is given a maximum thickness in order to limit the stress inside the ring and give the ring enough elasticity to allow resistance to longitudinal bending and fatigue failure. Of course, this last feature is particularly important when the ring is applied as a push belt. This is because the ring is exposed to numerous loads and bending cycles during its useful life.

  In order to obtain the desired material properties, the known production process has at least the following steps. Starting with a plate of base material, this plate is bent into a cylindrical shape and then the butted plate ends are welded to form a tube, which is then annealed to homogenize the material structure and internal stress Remove. After this, the tube is cut into a number of annular hoops, which are rolled to the desired thickness, i.e. as required for the final product. After rolling, the thin and flexible hoop is called a ring.

  The ring is subjected to a further annealing step, i.e. a ring annealing step to eliminate work hardening or strain hardening and ductile reduction hardening of the previous cold rolling process. Thereafter, by attaching each ring around two or more rotating rollers and separating the rollers, the rings are calibrated to the desired circumferential length, i.e. as required by the final product, thereby The ring is stretched. During this ring calibration process, the ring has a particularly desired internal stress distribution, as described in EP 1273824. After calibration, the ring is subjected to aging or bulk precipitation hardening and nitriding or surface hardening heat treatments. Finally, a set of stacked rings is formed by radially overlapping, i.e. fitting, a number of rings thus treated.

  Several variations of the above manufacturing method for ring components of push belts have been proposed in the past and are sometimes used in practice, but the basic steps are: hoop formation, hoop to ring rolling, ring The basic steps of annealing, ring calibration, and ring hardening have always remained essential for the manufacturing process and are therefore widely used in push belt manufacturing. However, recent work, including extensive component testing, conducted by the Applicant, despite widespread technical knowledge and judgment, has surprisingly found that fatigue of aging and nitrided ring materials after work hardening It has been shown that there is little difference between strength and fatigue strength of the work ring material annealed after work hardening, ie before aging and nitriding. As a result of this observation, the Applicant has recognized that the essential implementation of the ring annealing process in the push belt ring component manufacturing process is based on a technical preconception. Accordingly, it is proposed in the present invention to omit the ring annealing step from the entire manufacturing process, thereby advantageously reducing at least the complexity of the manufacturing process.

  The modified manufacturing method described above can further simplify the known manufacturing method by performing a ring calibration step in the equipment used for rolling from the hoop to the ring. Such a rolling device is described in detail in European Patent Application No. 1569644, and is known to have so-called two bearing rollers, around which an annular hoop is pulled. This state is realized and maintained by moving at least one of the rollers radially away from the other roller. During rolling, at least one bearing roller is rotationally driven to rotate the pulled hoop, and at least one so-called rolling roller is pressed against the hoop to effect plastic deformation of the hoop, in particular the hoop radius. The material flows from the direction or thickness dimension to the axial or width dimension or tangential or circumferential length dimension of the hoop. According to the present invention, the ring calibration step can potentially be omitted from the entire ring manufacturing method, but the ring calibration step can also be performed in the rolling device after the step of rolling from the hoop to the ring is completed. . The latter improvement is realized by further separating the rolling device bearing rollers while the ring continues to rotate, but without the rolling ring exerting a significant (pressing) force on the ring.

  For the dual use considered above of the rolling device according to the invention, it is preferred to provide at least one bearing roller with a cylindrical barrel shape, ie a cylindrical surface curved at least slightly convex.

  Applicants have also studied the impact of several related methods and manufacturing parameters on the resulting mechanical properties of the final product ring obtained by the currently considered manufacturing method. First, in order to limit the loss of ductility and fatigue strength of the ring material, it is preferable to limit the thickness reduction, ie to limit the degree of (cold) work hardening during rolling from hoop to ring. . If the maraging steel primarily allows a cold work ratio significantly greater than 50% in the current production process, ie a reduction in thickness, this cold work ratio is preferably limited to 50% or less. And more preferably has a value in the range of 20% to 40%.

  Furthermore, it has been found that prior (cold) work hardening of the ring material significantly enhances the effect of the subsequent nitriding step. Clearly, the dissociation of ammonia gas at the ring surface and / or the absorption and inward diffusion of nitrogen atoms into the steel crystal lattice of the ring is thereby catalyzed. That is, the present invention suitably shortens the processing time required for the nitriding heat treatment, lowers the nitriding temperature, or lowers the ammonia concentration.

  Alternatively, the observed phenomenon of improved nitriding is remarkably suitable for the so-called ring set nitriding described in EP-A-1815160, ie a method for producing a set of stacked rings. Make it. In this method, such a set is assembled (pre-) before the aging and nitriding steps, so that the aging and nitriding steps are performed on the entire set of stacked rings, not individual rings. It is known from EP 1815160 that such a ring set nitridation has a reduced effect of ring nitridation on at least the rings arranged in the center of the stacked set.

  The basic principle of the invention will now be described by way of example with reference to the drawings.

1 schematically shows an example of a known continuously variable transmission provided with a drive belt having a ring component. FIG. It is a perspective view which shows a part of belt. FIG. 2 symbolically shows a part relevant to the present invention of a known method of manufacturing a ring component of a drive belt. It is a figure which shows the experimental result of the fatigue strength test done with respect to two types of test pieces. FIG. 2 is a symbolic view of part of a method for manufacturing a drive belt ring component according to the present invention.

  FIG. 1 shows the central portion of a known continuously variable transmission or CVT commonly used in the driveline of an automobile between the engine and the drive wheels. The transmission has two pulleys 1 and 2. Each pulley is provided with two conical pulley disks 4 and 5, and a substantially V-shaped pulley groove is formed between these pulley disks. One of the disks 4 is movable in the axial direction along individual pulley shafts 6 and 7 on which the disks are arranged. A drive belt 3 is wound around the pulleys 1 and 2 in order to transmit the rotational motion ω and the accompanying torque T from one pulley 1 and 2 to the other pulley 2 and 1.

  The transmission generally also has actuating means for applying an axial clamping force Fax to the at least one disk 4 respectively towards the other pulley disk 5, so that the belt is clamped between the two disks. It is done. Also, the transmission speed ratio is determined thereby, which is defined below as the ratio of the rotational speed of the driven pulley 2 and the rotational speed of the drive pulley 1.

  A part of an example of a known drive belt 3 is shown in more detail in FIG. 2, which belt 3 has endless tension means 31. The pulling means 31 is shown only partially, and in this embodiment consists of two sets of thin, flat, ie strip-like, flexible metal rings 32. Furthermore, the belt 3 has a large number of plate-like transverse elements 33 which are in contact with the tension means 31 and are held by the tension means 31. In the element 33, the input torque Tin is applied to the so-called driving pulley 1, and the rotation of the driving pulley 1 is transmitted to the so-called driven pulley 2 via the driving belt 3 that rotates in the same manner due to friction between the disks 4, 5 and the belt 3. The clamping force Fax is received when

  During operation in CVT, the belt 3, and in particular the belt ring component 32, is subjected to periodically changing tensile and bending stresses, ie fatigue loads. Normally, the resistance or fatigue strength of the ring component 32 to fatigue thus determines the service life of the drive belt 3 at a given torque T transmitted by the ring component. Therefore, achieving the required ring fatigue strength with a combination of minimal material costs and processing costs has been a conventional general goal in the development of drive belt manufacturing methods.

  FIG. 3 shows the relevant part of the present invention of a known manufacturing method for the drive belt ring component 32 that has been put into practical use since the early days of drive belt manufacture, where the separate steps are represented by Roman numerals. It is shown. In a first step I, a thin sheet or plate 11 of base material, usually having a thickness in the range of 0.4 mm to 0.5 mm, is bent into a cylindrical shape and the abutted plate end 12 is a second step II. Is welded to form an open hollow cylinder or tube 13. In the third step III of the method, the tube 13 is annealed. Thereafter, in a fourth step IV, the tube 13 is cut into a number of annular hoops 14, which are then rolled and stretched in a fifth step V, with a thickness of less than 0.250 mm, usually It is reduced to about 185 μm. After rolling, the hoop 14 is usually referred to as the ring 32.

  The ring 32 is then removed to remove the work hardening effect of the previous rolling step (ie, the fifth step V) by recovery and recrystallization of the ring material at a temperature significantly higher than 600 ° C., eg about 800 ° C. A further or ring annealing step VI is performed. Thereafter, in a seventh step VII, the ring 32 is calibrated. That is, the ring 32 is attached around two rotating rollers, and is extended to a predetermined circumferential length by separating the rollers from each other. In this seventh step VII, internal stress distribution is also provided to the ring 32. The ring 32 is then heat treated in two separate steps: an aging or bulk precipitation hardening eighth step VIII and a nitriding or surface hardening ninth step IX. In particular, both such heat treatments typically involve a controlled gas atmosphere consisting of nitrogen for ring aging and a small amount of hydrogen, for example about 5% by volume, and nitrogen and ammonia for ring nitriding. Heating the ring 32 in an oven or furnace. Both heat treatments are typically performed at a temperature in the range of 400 ° C. to 500 ° C., depending on the base material (maraging steel alloy composition) for the ring 32 and the desired mechanical properties for the ring 32, Each can last about 45-120 minutes. For this latter, a core hardness value of 520 HV 1.0 or higher, a surface hardness value of 875 HV 0.1 or higher, and a thickness of a nitrided surface layer, alternatively referred to as a nitrogen diffusion zone, of 20-40 μm. Is the target.

  Finally, the stacked set 31 of rings 32 thus treated is further coupled to a number of rings 32 in the radial direction, as shown in FIG. 3 and in the tenth step X shown last. It is formed by stacking, that is, by fitting. Of course, the rings 32 of the stacked set 31 must be appropriately dimensioned for that purpose, for example the circumferential lengths should be slightly different so that the rings 31 can be fitted together. Don't be. For this purpose, the rings 32 of the stacked set 31 are usually selected from the stock of rings 32 according to purpose.

FIG. 4 shows a graph showing the results of selection among a number of fatigue tests on which the present invention is based. Related tests were performed on two different types of specimens. Fatigue tests are known per se and relate to exposing the specimen to breakage between a minimum value σ _MIN and a maximum value σ _MAX periodically, in particular sinusoidal, until fracture. This fatigue test is characterized and defined by the stress ratio (ie, σ _MIN / σ _MAX ) and stress amplitude (ie, [σ _MAX −σ _MIN ] / 2) applied in the test. The number of stress cycles to failure represents the fatigue strength of the specimen, and this number is plotted in a logarithmic scale against the stress amplitude in FIG. Because of the typical inherent diffusion in such fatigue test measurements, each test is usually repeated multiple times using the corresponding specimen and the same stress ratio and stress amplitude test settings, again multiple times. Repeated times. That is, each point in the graph of FIG. 4 represents a fatigue test result obtained by the above method, while the stress ratio is kept constant during all tests performed. The linear fit through the plotted test results obtained from one type of specimen in FIG. 4 represents (part of) a commonly known Wehrer curve.

  In FIG. 4, the test results obtained from the two different types of test specimens A and B are indicated by crosses and black dots, respectively. In the context of the present invention, the two types of specimens A and B are merely used after the specimen material has undergone the same cold plastic deformation, such as a rolling process (eg, step V above), and nitriding and aging. It is only characterized by being annealed, i.e. type A, or not annealed, i.e. type B, prior to undergoing said heat treatment. The basic material composition of the two types of test specimens A and B is the same, and the maraging alloy composition currently used in the drive belt 3 exemplified herein, which is commercially available for automotive CVT applications. And corresponding.

  From FIG. 4, it can be clearly seen that both types of test pieces A and B exhibit the same fatigue strength, although the material structures of the test pieces A and B are significantly different. For example, the work-cured specimen B had a core hardness of about 625 HV 1.0, whereas the annealed specimen A had a core hardness of only about 550 HV 1.0. Therefore, it was concluded that the current practice of annealing after rolling from hoop to ring is actually based on technical preconceptions. Accordingly, it is proposed in the present invention to omit the ring annealing step VI from the overall production process, thereby advantageously reducing at least the complexity of the production process. The relevant part of the manufacturing method thus simplified is shown in FIG.

  FIG. 5 shows the elimination of the sixth step VI from the ring manufacturing method according to the invention, so that after the fifth step V of the rolling of the ring 32 (hoop to ring). A seventh step VII is performed in which the ring 32 is immediately calibrated. In other words, the cold working effect of the material of the ring 32 caused by plastic deformation of the ring 32 during rolling (process V) is not removed before the heat treatment (process VIII; IX).

  In fact, the new, simplified manufacturing method is based on the known manufacturing method by performing the ring calibration (step VII) step in the same equipment used for the hoop-to-ring rolling (step V). It can be further simplified. As schematically shown in FIG. 5, the known rolling mill has two so-called bearing rollers 50 and 51, and the annular hoop 14 is actually provided around these bearing rollers 50 and 51. Wrapped before rolling, at least one of the rollers 50 is pulled away from the other roller 51 in the radial direction. During actual rolling, at least one bearing roller 50, 51 is rotationally driven to rotate the pulled hoop 14, and at least one other so-called rolling roller 52 is pressed against the hoop, thereby , Causing plastic deformation of the hoop, and in particular, causing material flow from the radial dimension or thickness dimension of the hoop 14 to the axial dimension or width and tangential length or circumferential length dimension of the hoop. Thereafter, that is, after the step V of rolling from the hoop to the ring is completed, that is, after the ring 32 having a desired thickness is obtained, the ring 32 is rotated while the rolling roller 52 is turned into the ring. It is calibrated by further separating the bearing rollers 50, 51 of the rolling device without applying a significant (pressing) force.

  According to the present invention, the processing conditions of the rolling device and the rolling process (process V), for example, the force applied to the hoop 14 is set, so that the rolled ring can be turned into the ring 32, particularly the finally realized circumference. The ring calibration step VII may be omitted entirely if it has the required characteristics for use in the drive belt with respect to the directional length. This latter aspect of the overall ring manufacturing process according to the invention is shown in FIG.

  Furthermore, FIG. 6 shows another simplification of the overall manufacturing process which requires the use of a base material in the form of a strip section 10 rather than the plate 11, which is usually such a strip. Cut from the coil of material. Such a strip section 10 already has a limited thickness compared to the known plate-like material thickness required to bend the plate-like material into a uniform cylindrical shape, e.g. A thickness in the range of 0.35 mm is provided. In this embodiment, the cold working ratio, i.e. thickness reduction, in the rolling process from hoop to ring (process V) can be advantageously limited. As shown in FIG. 6, the strip section 10 is bent annularly in a first step Ia, and the butted strip ends 9 are joined, eg welded, in a second step IIa, so that an annular hoop is obtained. 14 is formed directly. Thereafter, the hoop 14 is annealed in the third step III of the method and then rolled in step V to reduce the thickness. Thus, not only can the cold work ratio be advantageously limited, but in this particular embodiment of the invention, another step from the overall manufacturing process, ie, cutting the tube 13 into the annular hoop 14. (Step IV) may be omitted.

  Finally, FIG. 3 shows that the aging step VIII and the nitridation step IX continue to be performed on the individual rings 32, but such steps are not absolutely necessary in connection with the present invention. Absent. Indeed, in addition to omitting the ring annealing step VI from the overall ring manufacturing method according to the present invention, the aging and nitriding steps VIII, IX may be performed simultaneously or preferably simultaneously and / or 6 may be performed on a pre-assembled stacked set 31 of rings 32.

Claims (10)

A method of manufacturing a metal ring (32) for use in a drive belt (3) for a continuously variable transmission comprising:
Reducing the thickness of the hoop while increasing the circumferential length of the annular metal hoop (14) and forming the ring (32) by plastic deformation;
Heat treating the ring (32) to cure the material of the ring (32).
For a continuously variable transmission, characterized in that the temperature of the ring (32) is kept below 600 ° C. for one or more remaining steps of the manufacturing method after the rolling process is completed Of manufacturing a metal ring (32) for use in a drive belt (3) of A method of manufacturing a metal ring (32) for use in a drive belt (3) for a continuously variable transmission comprising:
Reducing the thickness of the hoop while increasing the circumferential length of the annular metal hoop (14) and forming the ring (32) by plastic deformation;
Heat treating the ring (32) to cure the material of the ring (32).
A metal ring (32) for use in a drive belt (3) for a continuously variable transmission, characterized in that the material of the ring (32) is not recrystallized between the rolling and hardening steps. How to manufacture.   3. A method according to claim 1 or 2, wherein the thickness reduction of the annular hoop (14) is not more than 50%, preferably having a value in the range of 20-40%.   Prior to the step of heat treating the ring (32), a number of rings (32) are radially fitted together to form a stacked set (31) of rings (32), which is then heat treated as a whole. The method of manufacturing according to any one of claims 1 to 3, wherein:   The method according to any one of claims 1 to 4, wherein a rolling device is used to provide the ring (32) with a desired thickness and a desired circumferential length.   6. In the step of heat treating the ring (32), the ring (32) is placed in a controlled gas atmosphere containing ammonia at a temperature in the range of 400-500 [deg.] C. for less than 45 minutes. The manufacturing method of any one of these.   Prior to the step of reducing the thickness of the hoop (14), the hoop (14) is formed by first bending the strip section (10) annularly and then joining the butted strip ends (9). Item 7. The manufacturing method according to any one of Items 1 to 6.   The method of manufacturing according to claim 7, wherein the strip section (10) has a thickness in the range of 0.25 to 0.35 mm.   9. At least one metal ring or metal comprising a plurality of transverse metal elements (40), wherein at least one ring component (32) is manufactured according to the manufacturing method according to any one of claims 1-8. Drive belt (1) with a stacked set (31) of rings (32).   A drive belt (1) having at least one flat thin metal ring (32) having a core hardness value greater than 600HV1.0, preferably about 625HV1.0.

Images