WO2009132689A1

WO2009132689A1 - Manufacturing method for a drive belt ring component

Info

Publication number: WO2009132689A1
Application number: PCT/EP2008/055166
Authority: WO
Inventors: Bert Pennings; Cornelia Adriana Elizabeth Crebolder; Hendrikus Van Den Boomen
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2008-04-28
Filing date: 2008-04-28
Publication date: 2009-11-05
Anticipated expiration: 2010-10-28
Also published as: JP2011518672A; CN102015148B; CN102015148A

Abstract

The invention concerns a method of manufacturing a metal ring (32) for use in a drive belt (3) for a continuously variable transmission including the process steps of: reducing the thickness of an annular metal hoop (14) while increasing its circumference length thus forming the ring (32) by plastic deformation and of heat treating the ring (32) to harden the material thereof, wherein after the process step of ring rolling has been completed the temperature of the ring (32) is kept below 600 degree Celsius for the remaining process step or steps of the manufacturing method, i.e. the cold work hardening of the material of the ring (32) resulting from the plastic deformation thereof is not removed prior to the said heat treatment thereof.

Description

MANUFACTURING METHOD FOR A DRIVE BELT RING COMPONENT

The present invention relates to a manufacturing method for an endless, thin and flexible metal, which band is typically incorporated in a drive belt for power transmission between two adjustable pulleys of the well-known continuously variable transmission or CVT applied in motor vehicles. At least in relation to its application in the drive belt such band is also referred to as the ring component thereof. The invention further relates to the product obtained by the said method.

In a particular type of drive belt, which is known as pushbelt, a number of such rings are incorporated in at least one, but typically two laminated, i.e. mutually radially nested sets thereof. The known pushbelt further comprises a number of transverse metal elements that are slidably mounted on such band set or sets. In the pushbelt application thereof, the state of the art rings are produced from maraging steel, which type of steel combines, amongst others, a comparatively favourable possibility to weld and plastically deform the material with the characteristics of great tensile strength and good resistance against both abrasive wear and bending and/or tensile stress fatigue, at least after the appropriate heat treatment thereof.

The known rings are provided with a fair hardness of the core material for realising the properties of good tensile, yield and bending strength combined with a high resistance against metal fatigue, which ring core is encased in a substantially harder and thus wear resistant outer surface layer of the ring material. The said hard surface layer is provided with a maximum thickness to limit the internal ring stress and to provide the ring with a sufficient elasticity to allow longitudinal bending as well as resistance against fatigue fracture. Of course, especially this latter feature is very significant in the pushbelt application of the rings, because of the numerous number of load and bending cycles it is subjected to during the service life span thereof.

To achieve the aforementioned desired material characteristic, the known manufacturing method includes at least the following process steps. Starting from a plate of base material, this plate is bend into a cylindrical shape, where after the now adjoining plate ends are welded together to form a tube, which tube is subsequently annealed to homogenize the material structure and remove internal stresses. Thereafter, the tube is cut into a number of annular hoops that are rolled to a desired thickness, i.e. as required for the end-product. After rolling the now thin and flexible hoops are referred to as rings. The rings are subjected to a further annealing, i.e. ring annealing process step to remove the work or strain hardening and ductility reducing effect of the previous process step of cold rolling. Thereafter, the rings are calibrated towards a desired circumference length, i.e. as required for the end-product, by mounting each ring around two or more rotating rollers and forcing said rollers apart, whereby the ring is stretched. During this process step of ring calibration also a specifically desired internal stress distribution is created in the rings, as has been described in detail in EP-A-1273824. After calibration the rings are subjected to the heat- treatments of ageing or bulk precipitation hardening and of nitriding or case hardening. Finally, the laminated set of rings is formed by radially stacking, i.e. nesting, a number of thus processed rings.

Even though several modifications of the above manufacturing method for the pushbelt ring component have been proposed over time and occasionally also have been applied in practice, the basic process steps thereof, namely of hoop forming, hoop-to-ring rolling, ring annealing, ring calibration and of ring hardening, have always remained fundamental thereto and, as such, are applied universally in pushbelt production. However, recent studies including extensive component testing, as performed by Applicant in spite of the prevailing technical knowledge and conviction, surprisingly showed hardly any difference between the fatigue strength of work hardened and subsequently aged and nitrided ring material and that of work ring material that has been annealed after work hardening, i.e. before being aged and nitrided. This observation has led applicant to the insight that the compulsory inclusion of the process step of ring annealing in the pushbelt ring component manufacturing process relies on a technical prejudice. Accordingly, it is presently proposed to omit the process step of ring annealing from the overall manufacturing method, thus favourably reducing at least the complexity thereof.

The above modified manufacturing method provides the possibility to further simplify the known manufacturing method, namely by performing the process step of ring calibration on the machine that is used for hoop-to-ring rolling. Such a rolling machine is described in detail in EP-A-1569764 and is known to include two so- called bearing rollers, around which bearing rollers the annular hoop is entrained in a tensioned state, which state is realised and maintained by at least one of said rollers being forced away from the other one roller in radial direction. During rolling at least one bearing roller is rotationally driven to rotate the tensioned hoop and at least one so-called rolling roller is pressed against the hoop to effect the plastic deformation thereof, more in particular to effect a flow of material from the radial or thickness dimension of the hoop towards the axial or width and the tangential or circumference length dimensions of the hoop. According to the invention, potentially, the process step of ring calibration can be omitted from the ring manufacturing method altogether, however, it could also be performed on the rolling machine after the process step of hoop-to-ring rolling has been completed, i.e. after a ring of predetermined desired thickness has been obtained. The latter improvement being realised by forcing the bearing rollers of the rolling machine apart even further, while continuing to rotate the ring, but without the rolling roller exerting a noticeable (pressing) force on the ring.

For the above considered dual use of the rolling machine in accordance with the present invention it is favoured to provide at least one bearing roller with a cylindrical barrel shape, i.e. with an at least slightly convexly curved cylinder surface.

Applicant also studied the influence of several relevant process and product parameters on the resulting mechanical properties of the end-product rings obtained with the presently considered manufacturing method. Firstly, it is considered favourable to limit the thickness reduction, i.e. to limit the amount of (cold) work hardening during hoop-to-ring rolling, so as to limit the loss of ductility and fatigue strength of the ring material. Where maraging steel principally allows a cold work ratio, i.e. a thickness reduction of considerably more than 50%, in the present manufacturing method this should preferably be limited to 50% or less, more preferably have a value in the range from 20% to 40%.

Further, it was found that the previous (cold) work hardening of the ring material greatly enhances the effectiveness of the subsequent nitriding process step. Apparently, either the dissociation of the ammonia gas on the ring surface and/or the absorption and inward diffusion of nitrogen atoms into the steel lattice of the ring is catalysed thereby. The present invention thus allows the processing time that is required for the nitriding heat treatment to be favourably shortened, the nitriding temperature to be lowered, or the ammonia concentration to be reduced.

Alternatively, the observed phenomenon of enhanced nitriding renders the present invention remarkably suitable for the so-called ring-set nitriding process that is discussed in EP-A-1815160, i.e. a method for manufacturing a laminated set of rings wherein such set is (pre-)assembled prior to the process steps of ageing and of nitriding that are thus performed on the laminated set of rings as a whole rather than on the individual rings thereof. It being known from that EP-A-1815160 such ring-set nitriding process suffers from a reduced effectiveness of the ring nitriding process at least for the centrally located rings of the said laminated set. The basic principle of the invention will now be elucidated by way of example, along a drawing in which:

Figure 1 provides a schematically depicted example of the well-known continuously variable transmission that is provided with a drive belt incorporating a ring component,

Figure 2 is a section of the belt shown in perspective,

Figure 3 figuratively represents the presently relevant part of the known manufacturing method of the drive belt ring component,

Figure 4 is a diagram providing experimental results of a fatigue strength test performed on two types of test pieces, and in which

Figure 5 figuratively represents the part of the manufacturing method of the drive belt ring component according to the present invention.

Figure 1 shows the central parts of a known continuously variable transmission or CVT that is commonly applied in the drive line of motor vehicles between the engine and the drive wheels thereof. The transmission comprises two pulleys 1 , 2, each provided with two conical pulley discs 4, 5, where between a predominantly V-shaped pulley groove is defined and whereof one disc 4 is axially moveable along a respective pulley shaft 6, 7 over which it is placed. A drive belt 3 is wrapped around the pulleys 1 , 2 for transmitting a rotational movement ω and an accompanying torque T from the one pulley 1 , 2 to the other 2, 1.

The transmission generally also comprises activation means that impose on the said at least one disc 4 an axially oriented clamping force Fax directed towards the respective other pulley disc 5 such that the belt 3 is clamped there between. Also, a speed ratio of the transmission is determined thereby, which hereinafter is defined as the ratio between the rotational speed of the driven pulley 2 and the rotational speed of the driving pulley 1 .

An example of a known drive belt 3 is shown in more detail figure 2 in a section thereof, which belt 3 incorporates an endless tensile means 31. The tensile means 31 is shown only in part and -in this example- is composed of two sets of thin and flat, i.e. band-like, flexible metal rings 32. The belt 3 further comprises a multitude of plate-like transverse elements 33 that are in contact with and held together by the tensile means 31 . The elements 33 take-up the said clamping force

Fax, such when an input torque Tin is exerted on the so-called driving pulley 1 , friction between the discs 4, 5 and the belt 3, causes a rotation of the driving pulley 1 to be transferred to the so-called driven pulley 2 via the likewise rotating drive belt 3. During operation in the CVT the belt 3 and in particular its ring component(s) 32 is (are) subjected to a cyclically varying tensile and bending stresses, i.e. a fatigue load. Typically the resistance against fatiguing or fatigue strength of the ring component 32 thus determines the functional life span of the drive belt 3 at a given torque T to be transmitted thereby. Therefore, it has been a long standing general aim in the development of the drive belt manufacturing method to realise a required ring fatigue strength at minimum combined material and processing cost.

Figure 3 illustrates the presently relevant part of the known manufacturing method for the drive belt ring component 32 as is practised ever since the early days of drive belt production, wherein the separate process steps are indicated by way of Roman numerals. In a first process step I a thin sheet or plate 1 1 of base material that typically has a thickness in the range between 0.4 mm and 0.5 mm is bend into a cylindrical shape and the meeting plate ends 12 are welded together in a second process step Il to form a open, hollow cylinder or tube 13. In a third 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 subsequently -process step five V- rolled to reduced the thickness thereof to less than 0.250 mm, typically around 185 μm, while being elongated. After rolling the hoops 14 are usually referred to as rings 32. The rings 32 are then subjected to a further or ring annealing process step Vl for removing the work hardening effect of the previous rolling process (i.e. step five V) by recovery and recristalisation of the ring material at a temperature considerably above 600 degree Celsius, e.g. around 800 degree Celsius. Thereafter, in a seventh process step VII, the rings 32 are calibrated, i.e. they are mounted around two rotating rollers and stretched to a predefined circumference length by forcing the said rollers apart. In this seventh process step VII, also an internal stress distribution is imposed on the rings 32. Thereafter, the rings 32 are heat-treated in two separate process steps, namely an eighth process step VIII of ageing or bulk precipitation hardening and a ninth process step IX of nitriding or case hardening. More in particular, both such heat-treatments involve heating the rings 32 in an industrial oven or furnace containing a controlled gas atmosphere that typically is composed of nitrogen and some, e.g. around 5 volume-% of hydrogen for ring ageing and of nitrogen and ammonia for ring nitriding. Both heat-treatments typically occur within the temperature range from 400 degrees Celsius to 500 degrees Celsius and can each last for about 45 to over 120 minutes in dependence on the base material (maraging steel alloy composition) for the rings 32, as well as on the mechanical properties desired for the rings 32. In this latter respect it is remarked that, typically, it is aimed at a core hardness value of 520 HV1 .0 or more, a surface hardness value of 875 HVO.1 or more and at a thickness of the nitrided surface layer, alternatively denoted nitrogen diffusion zone, in the range from 20 to 40 μm.

Finally, a laminated set 31 of thus processed rings 32 is formed by radially stacking, i.e. nesting, a number of rings 32, as is further indicated in figure 3 in the last depicted tenth process step X. Obviously, the rings 32 of the laminated set 31 have to be suitably dimensioned therefor, e.g. have to differ slightly in circumference length to allow the rings 31 to be fitted one around the other. To this end the rings 32 of the laminated set 31 are typically purposively selected from a stock of rings 32.

Figure 4 represents a graph depicting the results of a selection of a number of fatigue tests that underlie the present invention. The tests concerned were performed on two different types of test pieces. The fatigue test is as such well-known and concerns subjecting the test pieces to a tensile stress that varies cyclically, in particular sinusoidally, between a minimum value σ_MiN and a maximum value σ_MAx until fracture. This fatigue test is characterised and defined by the stress ratio (i.e. G_MINAJ_MAX) and the stress amplitude (i.e. [σ_MAχ-σ_MiN]/2) of the said minimum and maximum stresses applied in the test. The number of stress cycles until fracture represents the fatigue strength of the test piece, which number is plotted on a logarithmic scale against the said stress amplitude in figure 4. Because of the typical and inherent spread in the measurement result of such a fatigue test, each test is normally (and has presently been) repeated several times with corresponding test pieces and at the same stress ratio and stress amplitude test settings. Each point in the graph of figure 4 thus represents a fatigue test result obtained by the above method, while the said stress ratio is kept constant between all tests performed. A linear fit through the plotted test results obtained from one type of test piece in figure 4 would represent (a part of) the commonly known Wδhler-curve.

In figure 4, the test results obtained from the said two different types of test pieces A and B are respectively represented by the crosses and the solid circles. In the context of the present invention, it may be clear that the said two types of test pieces A and B are characterised solely by whether the material of the test pieces, after it has been subjected to the same cold plastic deformation process such as a rolling process (e.g. step V hereinabove) and before it is subjected to the said heat- treatments of nitriding and ageing, has been annealed, i.e. type A, or not, i.e. type B. The basic material composition of the two types of test pieces A and B is identical and corresponds with the maraging steel alloy composition that is currently applied in the commercially available -and presently illustrated- drive belt 3 for automotive CVT applications. It follows clearly from figure 4 that both types of test pieces A and B show identical fatigue strength, even though the material structure of the test pieces A and B differs considerably. For example, the core hardness of the work hardened test piece B amounted to around 625 HV1.0, whereas the core hardness of the annealed test piece A amounted only to around 550 HV1.0. Accordingly, it was concluded that the existing practice of annealing the rings after hoop-to-ring rolling in actuality relies on a technical prejudice. Accordingly, it is presently proposed to omit the process step Vl of ring annealing from the overall manufacturing method, thus favourably reducing at least the complexity thereof. The relevant part of the thus simplified manufacturing method is shown in figure 5 hereof. In figure 5 it is shown to exclude the said sixth process step Vl from the ring manufacturing method in accordance with the present invention, such that the process step five V of (hoop-to-ring) rolling of the ring 32 is immediately followed by the process step seven VII of calibrating the ring 32. Thus the cold work hardening of the material of the ring 32 that resulted from the plastic deformation thereof during rolling (step V) is not removed prior to the said heat treatment (steps VIII; IX) thereof.

Indeed, the new, simplified manufacturing method provides the possibility to simplify the known manufacturing method even further, namely by performing the process step of ring calibration (process step VII) on the same machine that is used for hoop-to-ring rolling (process step V). As is schematically indicated in figure 5, the known rolling machine includes two so-called bearing rollers 50 and 51 , around which bearing rollers 50, 51 the said annular hoop 14 is entrained prior to the actual rolling and brought in a tensioned state by at least one of said rollers 50 being forced away from the other one roller 51 in radial direction. During the actual rolling at least one bearing roller 50, 51 is rotationally driven to rotate the tensioned hoop 14 and at least one further, so-called rolling roller 52 is pressed against the hoop to effect plastic deformation thereof, more in particular to effect a flow of material from the radial or thickness dimension of the hoop 14 towards the axial or width and the tangential or circumference length dimensions thereof. Thereafter, i.e. after he process step V of hoop-to-ring rolling has been completed, i.e. after a ring 32 of desired thickness has been obtained, the ring 32 is calibrated by forcing the said bearing rollers 50, 51 of the rolling machine even further apart, while rotating the ring 32, but without the said rolling roller 52 exerting a noticeable (pressing) force thereon.

According to the invention the process step VII of ring calibration may even be omitted altogether provided that the rolling machine and the process settings of the rolling process (step V), such as the forces exerted on the hoop 14, are setup in such manner that the rolled rings 32 exhibit the required properties for the use in the drive belt, in particular in terms of the ultimately realised circumference length of the rings 32. This latter setup of the overall ring manufacturing method in accordance with the present invention is illustrated in figure 6. Additionally in figure 6 yet a further simplification of the overall manufacturing method is illustrated, which entails the use of base material in the form of a strip section 10 rather than the said plate 1 1 , which strip section 10 will typically have been cut-off from a coil of such strip material. Such strip section 10 may already be provided with a limited thickness, e.g. in the range between 0.25 mm and 0.35 mm compared to the thickness of the known plate-shaped base material that is minimally required for bending such into an even cylindrical shape. In this manner the cold work ratio, i.e. thickness reduction, in the hoop-to-ring rolling process (step V) can be favourably limited. As illustrated in figure 6, the strip section 10 is bent into an annular shape in the first step Ia and the meeting strip ends 9 are connected, e.g. welded together in a second process step Na, whereby the annular hoop is directly formed 14. Thereafter, the hoop 14 is annealed in the third step III of the process and subsequently -process step five V- rolled to reduce the thickness thereof. Thus apart from the possibility to favourably limit the said cold work ratio, in this particular embodiment of the invention yet a further process step may be omitted from the overall manufacturing method, namely that of cutting the tube 13 into annular hoops 14 (process step IV).

Finally it is remarked, even though in figure 3 the process step VIII of ageing and the process step IX of nitriding are depicted as being performed subsequently and on individual rings 32, such is not at all required in the context of the present invention. In fact, in addition to omitting the process step Vl of ring annealing from the overall ring manufacturing method in accordance with the present invention, the said process steps VIII, IX of ageing and nitriding may just as well, or even preferably, be performed simultaneously and/or on a pre-assembled laminated set 31 of rings 32 as shown in figure 6.

Claims

1 . Method of manufacturing a metal ring (32) for use in a drive belt (3) for a continuously variable transmission including the process steps of:

- reducing the thickness of an annular metal hoop (14) while increasing its circumference length thus forming the ring (32) by plastic deformation and of

- heat treating the ring (32) to harden the material thereof, characterised in that after the process step of rolling has been completed the temperature of the ring (32) is kept below 600 degree Celsius for the remaining process step or steps of the manufacturing method.

2. Method of manufacturing a metal ring (32) for use in a drive belt (3) for a continuously variable transmission including the process steps of:

- heat treating the ring (32) to harden the material thereof, characterised in that in between the said process steps of rolling and of hardening the material of the ring (32) is not recristalised.

3. Manufacturing method according to claim 1 or 2, characterised in that the thickness reduction of the annular hoop (14) is 50 % or less and preferably has a value in the range from 20 to 40 %.

4. Manufacturing method according to claim 1 , 2 or 3, characterised in that, prior to the process step of heat treating the ring (32), a number of rings (32) are mutually nested in radial direction to form a laminated set (31 ) of rings (32) that is subsequently heat treated as a whole.

5. Manufacturing method according to any one of the preceding claims, characterised in that the rolling machine is used to provide the ring (32) both with a desired thickness and with a desired circumference length.

6. Manufacturing method according to any one of the preceding claims, characterised in that, in the process step of heat treating the ring (32), the ring (32) is placed in a controlled gas atmosphere comprising ammonia at a temperature within the range from 400 to 500 degrees Celsius for less than 45 minutes.

7. Manufacturing method according to any one of the preceding claims, characterised in that, prior to the process step of reducing the thickness of the hoop (14), the hoop (14) is formed by first bending a strip section (10) into an annular shape and then connecting the meeting strip ends (9).

8. Manufacturing method according to claim 7, characterised in that the strip section (10) has a thickness in the range between 0.25 and 0.35 mm.

9. Drive belt (1 ) that comprises at least one or a laminated set (31 ) of metal rings (32) with a multitude of transverse metal elements (40), whereof at least one ring component (32) is produced in accordance with the manufacturing method according to any one of the preceding claims.

10. Drive belt (1 ) that comprises at least one flat and thin metal ring (32) that has a core hardness value of more than 600 HV1 .0, preferably of around 625 HV1.0.