EP3087293A1

EP3087293A1 - Flexible steel ring provided with a nanocrystalline surface layer for a drive belt for a continuously variable transmission and method for producing such ring

Info

Publication number: EP3087293A1
Application number: EP14830807.5A
Authority: EP
Inventors: Bert Pennings
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2013-12-24
Filing date: 2014-12-24
Publication date: 2016-11-02
Also published as: WO2015097277A1; CN105849436B; CN105849436A

Abstract

The invention relates to a flexible ring (32) made from steel for use in a drive belt (3)for a continuously variable transmission, which ring (32) is provided with a surface layer including only crystals with a grain size of less than 0.1 micron. By such nanocrystalline surface layer the fatigue strength of the conventional ring (32) could be improved upon.

Description

FLEXIBLE STEEL RING PROVIDED WITH A NANOCRYSTALLINE SURFACE LAYER FOR A DRIVE BELT FOR A CONTINUOUSLY VARIABLE TRANSMISSION AND METHOD FOR PRODUCING SUCH RING The present invention relates both to a flexible steel ring according to the preamble of claim 1 hereinafter and to a method for producing such a ring. This type of ring is used as a component of a drive belt for a continuously variable transmission for, in particular, automotive use such as in passenger motor cars. The drive belt is typically composed of two sets of mutually concentrically arranged rings that are inserted in a recess of transverse members of the drive belt. The drive belt comprises a plurality of these transverse members that are arranged in mutual succession along the circumference of such ring sets. In such drive belt application thereof, an individual ring normally has a thickness of only 0.2 mm or less, typically of about 0.18 mm.

In the transmission the drive belt is used for transmitting a driving power between two shafts, whereto the drive belt is passed around two rotatable pulleys, respectively associated with one such transmission shaft and provided with two conical discs defining a circumferential V-groove of the pulley wherein the drive belt is accommodated. By varying an axial separation between the respective discs of the two pulleys in a coordinated manner, the drive belt's radius at each pulley -and hence the rotational speed ratio between the transmission shafts- can be varied, while maintaining the drive belt in a tensioned state. This transmission and drive belt are generally known in the art and are, for example, described in the European patent publication EP- A-1243812.

It is further generally known in art that the performance of the drive belt is directly linked to not only the combined tensile strength of the ring sets, but to a large extent also the fatigue strength of the individual rings thereof. This is because, during rotation of the drive belt in the transmission, the tension and bending stress in the rings oscillate. In practice it is universally resorted to special steel compositions, in particular so-called maraging steels, as the basic material for the rings in order to realize the desired performance of the drive belt. Additionally to this end, the rings are core hardened and surface nitrided (i.e. gas-soft nitrided) in one or two heat treatment (s) that is/are part of the standing ring production process. More recently also stainless steels are being investigated as basic material for the manufacture of the ring.

A common, long standing desire and general aim in the further development and/or improvement of the drive belt ring component has been to increase the fatigue strength thereof. According to the present disclosure, in parti^¬ cular this parameter of the ring's performance can be improved upon by providing it with a so-called nano- crystalline surface layer, i.e. a relatively thin layer forming the outer surface of the ring wherein the metal crystals, i.e. the grains are particularly small. It was found that by such feature, in particular, the initiation of a fatigue fracture at the ring surface occurs only at considerably higher stress levels and/or only after more stress cycles have occurred.

Particularly good results are, in this respect, obtained with a crystal or grain size in the said nano- crystalline surface layer that fits within a (virtual) sphere of 0.1 micrometer in diameter. This grain size of less than 0.1 μιη, for example of about 50 nm, is typically 2 orders of magnitude (i.e. 100 times) smaller than the grain size to the outside of such surface layer, e.g. of the material constituting the core of the ring. A highly suitable thickness range for such nanocrystalline surface layer is between 1 and 10 μη.

It is noted that it is known in the art that such nanocrystalline surface layer can be formed on steel products of varying composition by the mechanical deforma^¬ tion thereof. For example by the plastic deformation processes of rolling, brushing and/or shot-peening the grain size of the steel is reduced, at least in the parts thereof that are plastically deformed in such process. To obtain only a relatively thin nanocrystalline surface layer, a plastic deformation process must be selected that deforms only such thin surface layer of the product. Obviously, in this latter respect, the known rolling process will normally be less suited. Instead, the known shot-peening process can be appropriately arranged more easily. Furthermore, other suitable plastic deformation processes use fluid or gas (incl. plasma) as the medium to realize the deformation required for forming the nanocrystalline surface layer, e.g. ultrasonic- or laser- based processes.

In accordance with the present disclosure, the said nanocrystalline surface layer of the ring is preferably formed prior the above-mentioned heat treatment of nitriding thereof. It was namely observed that with such nanocrystalline surface layer, the ring can be nitrided much faster than without. In other words, with nanocrystalline surface layer present, a desired thickness of the nitride layer of the ring is achieved in a favorably shorter (process) time and/or with a favorably lower concentration of ammonia gas in the process atmosphere.

The above-described basic features of the present disclosure will now be elucidated by way of example with reference to the accompanying figures.

Figure 1 is a schematic illustration of a known drive belt and of a transmission incorporating such known belt.

Figure 2 is a schematic illustration of a part of the known drive belt, which includes two sets of a number of flexible steel rings, as well as a plurality of transverse members .

Figure 3 figuratively represents a known manufacturing method of the drive belt ring component that includes a process step of nitriding.

Figure 4 is a schematic indication of a metal crystal structure including a nanocrystalline surface layer. Figure 5 is a photographic representation of a cross- section of the drive belt ring component revealing the crystal structure thereof including a nanocrystalline surface layer.

Figure 6 figuratively represents a modification of the manufacturing method of figure 3 in accordance with the present disclosure.

Figure 1 shows schematically a continuously variable transmission (CVT) with a drive belt 3 wrapped around two pulleys 1 and 2. Each pulley 1, 2 is provided with two conical pulley discs 4, 5, where between an annular, 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. Each pulley 1, 2 generally also comprises activation means that can impose on the said at least one disc 4 thereof an axially oriented clamping force directed towards the respective other pulley disc 5 thereof, such that the belt 3 can be clamped between these discs 4, 5. Also, a (speed) ratio of the CVT between the rotational speed of the driven pulley 2 and the rotational speed of the driving pulley 1 is determined thereby. This CVT is known per se.

An example of a known drive belt 3 is shown in more detail in figure 2 by way of a small part thereof. The drive belt 3 is made up of two sets 31 of mutually nested, flat and flexible steel rings 32 and of a plurality of transverse members 30. The transverse members 30 are arranged in mutual succession along the circumference of the ring sets 31, in such manner that they can slide relative to and in the circumference direction of the ring sets 31.

The transverse segments 30 take-up the said clamping force, such when an input torque T 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 drive belt 3 and in particular the rings 32 thereof are subjected to a cyclically varying tensile and bending stresses, i.e. a fatigue load. Typically the resistance against metal fatigue, i.e. the fatigue strength of the rings 32, thus determines the functional life span of the drive belt 3. Therefore, it has been a long standing and general development aim to optimize the fatigue strength of the ring 32 at a minimum combined material and processing cost Figure 3 illustrates a relevant part of the known manufacturing method for the drive belt ring component 32, as it is typically applied in the art for the production of metal drive belts 3 for automotive application. The separate process steps of the known manufacturing method are indicated by way of Roman numerals.

In a first process step I a thin sheet or plate 11 of a maraging steel basic material having a thickness of around 0.4 mm is bend into a cylindrical shape and the meeting plate ends 12 are welded together in a second process step II to form a 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 reduce the thickness thereof to, typically, around 0.2 mm, while being elongated. After rolling, the hoops 14 are usually referred to as rings 32.

The ring 32 is subjected to a further, i.e. ring annealing process step VI for removing the work hardening effect of the previous rolling process step by recovery and re-crystallization of the ring material at a temperature considerably above 600 degrees Celsius, e.g. about 800°C. Thereafter, in a seventh process step VII, the ring 32 is calibrated by mounting it around two rotating rollers and stretching it to a predefined circumference length by forcing the said rollers apart. In this seventh process step VII of ring calibration, also internal stresses are imposed on the ring 32.

Thereafter, the ring 32 is heat-treated in an eighth process step VIII of combined ageing or bulk precipitation hardening and nitriding or case hardening. More in particular, such combined heat-treatment involves keeping the ring 32 in an oven chamber containing a controlled gas atmosphere that comprises ammonia, nitrogen and hydrogen gas. In the oven chamber, i.e. in the process atmosphere, the ammonia molecules decompose at the surface of the ring 32 into hydrogen gas and nitrogen atoms that can enter into the metal lattice of the ring 32. By these interstitial nitrogen atoms the resistance against wear as well as against fatigue fracture is known to be increased remarkably. Inter alia it is noted that such combined heat-treatment can be carried out in the separate and subsequent stages of ageing and nitriding, which alternative process setup is known in the art.

Typically, the eighth process step VIII of combined ring ageing and nitriding is carried out until a nitrided layer or nitrogen diffusion zone formed at the outer surface of the ring 32 is between 25 and 35 micron thick.

A number of the thus processed rings 32 are assembled in to the ring set 31 by radially stacking, i.e. concentrically nesting these rings 32 with a minimal radial play or clearance between each pair of adjoining rings 32. Inter alia it is noted that it also known in the art to instead assemble the ring set 31 from a number of individual rings 32 immediately following the seventh process step VII of ring calibration, i.e. in advance of the eighth process step VIII of ring ageing and ring nitriding .

The thus processed rings 32 show more or less correspondingly sized crystals/grains throughout the whole metal lattice thereof, i.e. also near the outer surface thereof. A typical, average grain size for such rings 32 is around 5 micron or more. Indeed, after the rings have been re-crystallized in the ring annealing process step VI, only a limited amount of plastic deformation occurs in the seventh process step VII of ring calibration, which deformation is moreover spread more or less evenly throughout the cross-section of the rings 32. However, in accordance with the present disclosure the rings 32 are preferably provided with a surface layer of smaller sized, i.e. sub-micrometer sized grains, which feature was found to increase the fatigue strength of the rings 32 and which feature is schematically illustrated in figure 4.

In figure 4 the presently, theoretically desired crystal structure of the ring 32 is indicated with smaller sized grains Gs at the surface of the ring 32 as compared to those grains Gb that are located more towards the bulk material of the ring 32. The smaller sized grains Gs at the ring surface are shown to define a layer of nano- crystalline ring material.

Figure 5 illustrates the crystal structure of an actual ring 32 provided with a nanocrystalline surface layer NSL in accordance with the present disclosure. In figure 5 the crystals/grains of the ring 32 have been made visible by scanning electron microscopy (SEM) . In this practical example of the ring 32 with the nanocrystalline surface layer NSL the smallest grains at the ring surface are more than 100 times smaller than the largest grains in the bulk material of the ring 32. In particular, in this example, the grain size in the nanocrystalline surface layer NSL is about 50 nm on average, whereas in the bulk material of the ring the crystal grains are between 2 and 12 μιη wide in any direction. The nanocrystalline surface layer NSL is approximately 5 μιη thick.

The above nanocrystalline surface layer NSL is preferably created by subjecting the ring 32 to a shot- peening process. In particular, according to the present disclosure such shot-peening process is included in the above-described, known manufacturing method for the drive belt ring component 32 in between the seventh process step VII and the heat treatment of ring nitriding in the eighth process step VIII thereof, as is schematically illustrated in figure 6. In figure 6, the process step SP indicates the presently proposed shot-peening process, wherein the ring 32 is mounted and rotated around (for example) three mounting rollers and wherein two spray nozzles 70 spray (for example) miniscule glass beads onto at least the radially facing outer surfaces of the ring 32.

By providing with a nanocrystalline surface layer NSL to the rings 32 before these are nitrided, the nitriding process, as such, is enhanced. In particular, the nano- crystalline surface layer NSL favorably allows for a reduction of either one or both of the duration and/or of the ammonia-concentration in the process atmosphere of the nitriding process that is/are required to provide the ring 32 with a nitrided layer of the desired thickness.

The present disclosure, in addition to the entirety of the preceding description and all details of the accompanying figures, also concerns and includes all the features of the appended set of claims. Bracketed references in the claims do not limit the scope thereof, but are merely provided as non-binding examples of the respective features. The claimed features can be applied separately in a given product or a given process, as the case may be, but it is also possible to apply any combination of two or more of such features therein.

The invention (s) represented by the present disclosure is (are) not limited to the embodiments and/or the examples that are explicitly mentioned herein, but also encompasses amendments, modifications and practical applications thereof, in particular those that lie within reach of the person skilled in the relevant art.

Claims

1. Flexible steel ring (32) destined for use as, or at least in, a drive belt (3) for a continuously variable transmission with two pulleys (1, 2) and the drive belt (3) , characterized in that a surface layer (NSL) of the flexible steel ring (32) is provided with a nanocrystalline microstructure.

2. The flexible steel ring (32) according to claim 1, characterized in that a grain size of the metal crystals in the nanocrystalline surface layer (NSL) amounts to 0.1 ym at most and preferably amounts to approximately 50 nanometer .

3. The flexible steel ring (32) according to claim 1 or

2, characterized in that a thickness of the nanocrystal^¬ line surface layer (NSL) amounts to between 1 and 10 ym.

4. The flexible steel ring (32) according to claim 1, 2 or 3, characterized in that an average grain size of the metal crystals outside the nanocrystalline surface layer (NSL) is two orders of magnitude larger than an average grain size of the metal crystals in the nanocrystalline surface layer (NSL) .

5. A method for producing a flexible steel ring (32) destined for use as, or at least in, a drive belt (3) for a continuously variable transmission with two pulleys (1, 2) and the drive belt (3), wherein the flexible steel ring (32) is provided with a nanocrystalline surface layer (NSL) .

6. The method for producing a flexible steel ring (32) according to claim 5, wherein the nanocrystalline surface layer (NSL) that is provided therein is between 1 and 10 ym thick.

7. A method for producing a flexible steel ring (32) destined for use as, or at least in, a drive belt (3) for a continuously variable transmission with two pulleys (1, 2) and the drive belt (3), wherein the flexible steel ring (32) is subjected to a plastic deformation of a relatively thin surface layer of the flexible steel ring (32) only.

8. The method for producing a flexible steel ring (32) according to claim 7, wherein the said plastic deformation of the surface layer of the flexible steel ring (32) is realized by means of shot-peening .

9. The method for producing a flexible steel ring (32) according to claim 7 or 8, wherein a nanocrystalline surface layer (NSL) is formed with a thickness of between 1 and 10 ym.

10. The method for producing a flexible steel ring (32) according to claim 5, 6 or 9, wherein a grain size of the metal crystals in the nanocrystalline surface layer (NSL) amounts to 0.1 ym at most.

11. The method for producing a flexible steel ring (32) according to claim 10, wherein an average grain size of the metal crystals outside the nanocrystalline surface layer (NSL) is two orders of magnitude larger than an average grain size of the metal crystals in the nanocrystalline surface layer (NSL) .

12. The method for producing a flexible steel ring (32) according to one of the claims 5-11, wherein the flexible steel ring (32) is subjected to a nitriding process after the ring (32) has either been provided with the nanocrystalline surface layer (NSL), or has been subjected to a plastic deformation of a relatively thin surface layer of the flexible steel ring (32) only.