WO2017108205A1

WO2017108205A1 - Transverse element provided with a nanocrystalline surface layer for a drive belt for a continuously variable transmission and method for producing it

Info

Publication number: WO2017108205A1
Application number: PCT/EP2016/025188
Authority: WO
Inventors: Bert Pennings
Original assignee: Robert Bosch Gmbh
Priority date: 2015-12-22
Filing date: 2016-12-22
Publication date: 2017-06-29
Also published as: JP2019506575A; CN108474447A; NL1041641B1; JP6890595B2

Abstract

The invention relates to a steel transverse element (32) component of a drive belt (3) for a continuously variable transmission, which drive belt (3) comprises a number of such transverse elements (32) and one or more endless tensile elements (31). Each transverse element (32) defines at least one recess (33) accommodating the one or more endless carriers (31) and bounded by a support surface (41), a side face (43) and by a concavely curved transition surface (42) that is provided there between. According to the invention the transverse element (32) is provided with a nanocrystalline surface layer at the location of a concavely curved transition surface (42).

Description

TRANSVERSE ELEMENT PROVIDED WITH A NANOCRYSTALLINE SURFACE LAYER FOR A DRIVE BELT FOR A CONTINUOUSLY VARIABLE TRANSMISSION AND METHOD FOR PRODUCING IT The present disclosure relates both to a transverse element according to the preamble of claim 1 hereinafter and to a method for producing such a transverse element. This type of transverse element 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 composed of a row of a number of such transverse elements that are arranged in mutual succession along the circumference of at least one set of mutually concentrically arranged or nested metal rings. Typically, the transverse members are provided with, or at least define, one recess for each set of rings of the drive belt, wherein -for each transverse member- a circumference part of such ring set is accommodated.

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.

The said transfer of the driving power between each pulley and the drive belt takes place by means of friction, whereto the transverse elements on either lateral, i.e. axial side thereof are provided with a contact face for arriving into contact with a pulley disc. The contact faces of a transverse element are mutually oriented at an angle that essentially matches the angle of the V-shaped groove defined by and between the conical discs of each pulley. Typically, the contact faces are provided with low lying areas, such as grooves or holes for receiving a lubricating and/or cooling fluid that is typically applied in the known transmission and that is forced out from in- between the (higher) parts of the contact faces and the pulley discs that arrive in physical friction contact during operation of the transmission.

In the below, a vertically upwards direction is defined in relation to the transverse element to correspond with the direction of divergence of the contact surfaces thereof, a thickness direction of the transverse element is defined in the circumference direction of the ring set and a width direction is oriented at right angles to both said height direction and said thickness direction The known transverse element is further provided with two main body surfaces, namely a front surface and a back surface that extend substantially in parallel with one another, substantially at right angles to the thickness direction. The recess of the transverse element extends between these two main body surfaces. A bottom-side of the recess is defined by a support surface of the transverse element that supports the ring set in the drive belt and that adjoins a width-wise oriented side face of the transverse element that defines a side-wall of the recess via an at least partly concavely curved transition surface thereof .

In order to cope with the forces that occur during operation of the drive belt in the automotive application thereof, at least the transverse elements of the drive belt are made from metal, typically steel. In particular, the transverse elements are made from steel with between 0.6 and 1.2 mass-% carbon that can be quenched hardened after the transverse elements have been cut from such basic material. After such quench hardening, the transverse elements are deburred, preferably in a (stone) tumbling process, wherein burrs are removed from the cut edges of the transverse elements, but that also imparts a compressive residual stress in a surface layer thereof. By such compressive residual stress, the tensile stress levels during operation are reduced and the initiation and/or growth of micro cracks from surface imperfections, i.e. fatiguing, is suppressed. It remains, however, an ever present desire in the art to further increase the fatigue strength of the transverse elements. On the one hand, the service life of the transmission as a whole may be improved thereby, on the other hand the driving power to be transferred by the transmission may be improved and/or the transmission may be miniaturized.

According to the present disclosure, the fatigue strength of the transverse element can be improved by providing it with a so-called nanocrystalline surface layer at the location of the said transition surface thereof. Such nanocrystalline surface layer being a relatively thin layer forming the outer surface where the metal crystals, i.e. the grains are particularly small. It was found that, by such feature, the initiation of a fatigue fracture at the transition surface is deferred, i.e. occurs only at a considerably higher stress level and/or only after more stress cycles have occurred. This can be explained by the circumstance that the stones applied in stone tumbling are generally too big for easily entering inside the recess of the transverse elements. Therefore, these stones do not reach, i.e. do not impact the transition surface, such that locally the compressive residual stress level, and consequently the resistance against fatigue crack initiation, is less than at other surface parts of the transverse element.

Although the above-identified limitation of the stone tumbling process in principle pertains to the complete boundary surface of the recess, it is most noticeable at the said transition surface because here the highest tensile stress level occurs during operation.

A drive belt with thus defined transverse elements then has the advantage that, on the one hand, it can have a longer operational (fatigue) service life, or that, on the other hand, it can transmit more power between the pulleys while retaining its service life.

Particularly good results are obtained with a crystal or grain size in the nanocrystalline surface layer at the said transition surface that fits within a (virtual) sphere of 0.1 micrometre in diameter. This grain size of less than 0.1 μηι, for example of about 50 nm, is typically 2 orders of magnitude (i.e. 100 times) smaller than the grain size beyond the nanocrystalline surface layer or at other (surface) parts of the transverse element. A highly suitable thickness range for the nanocrystalline surface layer at the said transition surface of the transverse element is between 1 and 10 lm.

In a more detailed embodiment of the transverse element according to the present disclosure, not only the transition surface of the transverse element, but also the said support surface thereof is provided with a nano- crystalline surface layer. Although this surface is in practice not prone to fatigue crack initiation, it is loaded quite intensively by the continuous friction contact with the ring set. According to the present disclosure, not only the local resistance against fatigue crack initiation, but also the resistance against wear is favourably improved by the presence of the nanocrystalline surface layer.

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, plastic deformation thereof. Within the context of the present disclosure, a plastic deformation process must be provided that is able to enter inside the recess of the transverse element and to reach the said transition surface thereof, while deforming only a thin surface layer and leaving other surface parts of the transverse element untouched. According to the present disclosure, the shot-peening process that is known as such can be appropriately set up for this purpose conveniently. Furthermore, other possible 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.

The above discussed principles and features of the novel transverse element and its proposed manufacturing method will now be elucidated further by way of example with reference to the accompanying figures.

Figure 1 provides a schematically depicted example of the well-known continuously variable transmission provided with two pulleys and a drive belt.

Figure 2 provides a schematically depicted cross- section of the known drive belt incorporating transverse elements and a ring set.

Figure 3 is a schematic representation of a metal crystal structure including a nanocrystalline surface layer .

Figure 4 is an enlargement of a real cross-section of the transverse element revealing the crystal structure thereof with a nanocrystalline surface layer.

Figure 5 is a schematic illustration of a process for providing the transverse element with the nanocrystalline surface layer.

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 driven wheels thereof. The transmission comprises two pulleys 1, 2 that are each provided with a pair of conical pulley discs 4, 5 mounted on a pulley shaft 6 or 7, between which pulley discs 4, 5 a predominantly V-shaped circumferential pulley groove is defined. At least one pulley disc 4 of each pair of pulley discs 4, 5, i.e. of each pulley 1, 2, is axially moveable along the pulley shaft 6, 7 of the respective pulley 1, 2. A drive belt 3 is wrapped around the pulleys 1, 2, located in the pulley grooves thereof for transmitting a rotational movement and an accompanying torque between the pulley shafts 6, 7.

The transmission generally also comprises activation means that -during operation- impose on the said axially moveable pulley disc 4 of each pulley 1, 2 an axially oriented clamping force that is directed towards the respective other pulley disc 5 of that pulley 1, 2, such that the drive belt 3 is clamped between these discs 4, 5 of the pulleys 1, 2. These clamping forces not only determine a friction force between the drive belt 3 and the respective pulleys 1, 2, but also a radial position R of the drive belt 3 at each pulley 1, 2 between the pulley discs 4, 5 thereof, which radial position (s) R determine a speed ratio of the transmission between the pulley shafts 6, 7 thereof.

An example of a known drive belt 3 is shown in more detail in figure 2, in a cross-section thereof facing in its circumference direction. The drive belt 3 incorporates an endless tensile element 31 in the form of two sets of flat and thin, i.e. of ribbon-like, flexible metal rings 44. The drive belt 3 further comprises a number of transverse elements 32 that are mounted on the tensile element 31 along the circumference thereof. In this particular example, each ring set 31 is received in a respective recess 33 defined by the transverse elements 32 on either lateral side thereof, i.e. on either axial side of a central part 35 of the transverse elements 32. The recesses 33 of the transverse element are located in- between an inner part 34 and an outer part 36 of the transverse element 32, as seen in radial direction relative to the drive belt 3 as a whole. A bottom-side of the recesses 33 is defined by support surfaces 41 of the inner part 34 of the transverse element 32 and a side wall of the recesses 33 is defined by side faces 43 of the central part 35 of the transverse element 32.

On the axial sides of the said inner part 34 thereof, the transverse elements 32 are provided with contact faces 37 for arriving in friction contact with the pulley discs 4, 5. The contact faces 37 of each transverse element 32 are mutually oriented at an angle φ that essentially matches an angle of the V-shaped pulley grooves. Thus, the transverse elements 32 take-up the said clamping force, such that when an input torque 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 or vice versa.

During operation in the CVT the transverse element 32 components of the drive belt 3 are intermittently clamped between the respective pairs of pulley discs 4, 5 of the pulleys 1, 2. Although such intermitted clamping obviously results in an intermittent compression of the inner part 34 of the transverse elements 32, a varying tensile stress is generated therein as well a/o because of the contact faces 37 on either side thereof being oriented at an angle and because of the contact with adjacent transverse elements 32 in the drive belt 3. This varying tensile stress occurs (also) at a transition region between the inner part 34 and the central part 35 of the transverse elements 32. It has been observed that, as a result, a fatigue crack will typically initiate at the concavely curved transition surfaces 42 provided in-between and interconnecting the support surfaces 41 and the side faces 43. According to the present disclosure, such fatigue crack initiation can be avoided, or at least postponed, by providing the transverse element 32 with a so-called nano- crystalline surface layer NSL at the location of the said transition surfaces 42.

In figure 3 the presently desired, crystal structure is schematically indicated with smaller sized grains Gs near the outer surface of a work piece 50, as compared to those grains Gb that are located more towards the bulk material thereof. The smaller sized grains Gs at the surface define the nanocrystalline surface layer NSL.

In figure 4 such nanocrystalline surface layer NSL is illustrated further by way of a photograph made by scanning electron microscopy (SEM) . The smallest grains in the nanocrystalline surface layer NSL are more than 100 times smaller than the largest grains Gb in the bulk material of the work piece 50. In particular, in figure 4, the nanocrystalline surface layer NSL is approximately 5 pm thick and the grain size in the nanocrystalline surface layer NSL is about 50 nm on average, whereas in the bulk material of the transverse element 32 the crystal grains are between 2 and 20 pm wide in any direction. The aforementioned values are typically suitable within the context of the present disclosure, i.e. for being applied at the location of the transition surfaces 42 of the transverse elements 32 of the drive belt 3.

The above nanocrystalline surface layer NSL can be applied to the transverse element 32 by means of a shot- peening process, which process is schematically illustra^¬ ted in figure 5. In the shot peening process, small particles 60, such as miniscule glass beads, are ejected at speed from one or more nozzles 61, for example by being carried in a flow of air. The nozzle 61 is both shaped and oriented such that the said particles 60 impact the desired surface part of the transverse element 32, i.e. at least the said concavely curved transition surfaces 42 thereof. By these impacts of shot peening particles 60, a minimal plastic deformation is effected in a surface layer of the transverse element 32, resulting in the local refinement of the crystal structure thereof.

In addition to the said concavely curved transition surfaces 42, also the support surfaces 41 may be treated, i.e. shot peened, according to preference. However, other surface parts of the transverse element 32, such as its front and rear main body surfaces and its (pulley) contact faces 37, are preferably left untreated by such shot- peening process to avoid (further) increasing the compressive residual stress near such other surface parts that results from the known deburring process. Furthermore, in accordance with the present disclosure such shot- peening process is carried out after the known quench- hardening process to prevent that the nanocrystalline surface layer NSL is removed by crystal growth and recrystallization in the austenitizing heat treatment that is part of quench-hardening.

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 method, 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 perturbations, amendments, modifications and practical applications thereof, in particular those that lie within reach of the person skilled in the relevant art

Claims

1. A transverse element (32) for a drive belt (3) with an endless tensile element (31) and with number of the transverse element (32) provided slideably thereon for transmitting a drive power (32) between two pulleys (1, 2), which transverse element (32) is made from steel and is provided with at least one recess (33) for taking-up a part of the tensile element (31), which recess (33) is, amongst others, limited by a support surface (41) that, in the drive belt (3), is located on the radial inside of the tensile element (31), a side face (43) that, in the drive belt (3), is located opposite an axial side of the tensile element (31) and a transition surface (42) that is at least partly concavely curved and that is provided between and that interconnects the support surface (41) and the side face (43), characterized in that a surface layer of the transverse element (32) at the location of the transition surface (42) and/or of the support surface (41) is provided with a nanocrystalline microstructure.

2. The transverse element (32) according to the claim 1, characterized in that the transverse element (32) is exclusively provided with the nanocrystalline surface layer (NSL) at the location of the transition surface (42) and/or of the support surface (41) .

3. The transverse element (32 ) according to the claim 1 or 2, characterized in that a grain size of metal crystals of the transverse element (32) in the nanocrystalline surface layer (NSL) amounts to 0.1 micrometre at most and preferably amounts to around 50 nanometre.

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

5. The transverse element (32) according to a preceding claim, characterized in that a thickness of the nanocrystalline surface layer (NSL) amounts to between 1 and 10 micrometre.

6. A method for the manufacture of a transverse element (32) for a drive belt (3) with an endless tensile element (31) and with number of the transverse element (32) provided slideably thereon for transmitting a drive power (32) between two pulleys (1, 2), which transverse element (32) is made from steel and is provided with at least one recess (33) for taking-up a part of the tensile element (31), which recess (33) is, amongst others, limited by a support surface (41) that, in the drive belt (3), is located on the radial inside of the tensile element (31), a side face (43) that, in the drive belt (3), is located opposite an axial side of the tensile element (31) and a transition surface (42) that is at least partly concavely curved and that is provided between and that interconnects the support surfaces (41) and the side face (43), characterized in that the transverse element (32) is subjected to a plastic deformation of only a surface layer thereof at the location of the transition surface (42) and/or of the support surface (41) whereby such surface layer is provided with a nanocrystalline microstructure.

7. The method for the manufacture of a transverse element (32) according to claim 6, characterized in that the transverse element (32) is subjected to a plastic deformation of only a surface layer thereof exclusively at the location of the transition surface (42) and/or of the support surface (41).

8. The method for the manufacture of a transverse element (32) according to claim 6 or 7, characterized in that a grain size of metal crystals of the transverse element (32) in the nanocrystalline surface layer (NSL) formed therein amounts to 0.1 micrometre at most and preferably amounts to around 50 nanometre.

9. The method for the manufacture of a transverse element (32) according to claim 8, characterized in that an average grain size of metal crystals of the transverse element (32) in the nanocrystalline surface layer (NSL) formed therein is two orders of magnitude smaller than an average grain size of metal crystals outside such nanocrystalline surface layer (NSL) .

10. The method for the manufacture of a transverse element (32) according to claim 8 or 9, characterized in that a thickness of the nanocrystalline surface layer (NSL) formed therein amounts to between 1 and 10 micrometre .