The present disclosure relates to a method for manufacturing transverse elements made from steel, in particular to a method for the batch wise heat treatment thereof, which transverse elements are applied in a drive belt in an essentially continuous row filling the circumference of an endless, i.e. ring shaped carrier of the drive belt. These transverse elements and the drive belt, as well as the continuously variable transmission wherein there are typically applied are well known in the art, for example from the European patent publications EP-A-0 626 526 and EP-A-1 167 829 respectively.

In the transmission, the transverse elements of the drive belt arrive in friction contact with two pulleys and can transmit a driving force from the one transmission pulley by a first transverse element exerting a pushing force on a second, adjacent transverse element, which second transverse element exerts such pushing force onto a third transverse element and so on. The endless carrier of the drive belt mainly serves to constrain and guide the transverse elements in their trajectory around and in-between the said pulleys. Typically, the endless tensile element is composed of two sets of a number of mutually nested, i.e. radially stacked flexible metal rings. Typically also, the transverse elements each define two slots that respectively open towards a respective lateral side of the respective transverse elements and that respectively accommodate a part of a respective one of the two ring sets. Also on each lateral side thereof, the transverse elements are provided with a respective one of two contact faces thereof, which contact faces are mutually oriented at an angle that essentially matches an angle defined between two, mutually opposing conical discs of each transmission pulley.

The design of the known transverse elements is largely optimized in terms of the stress levels and stress amplitudes that occur during operation, i.e. rotation of the drive belt , e.g. due to the intermittent axial compression of the transverse elements by and between the discs of the pulleys, as well as the varying pushing force between adjacent transverse elements. Also, the manufacturing process of the transverse elements is largely optimized for this purpose. For example, the transverse elements are hardened by the heat treatment of quench hardening that includes the three well-known stages of austenitizing in an endothermic process gas ("endogas"), quenching in oil and tempering in air. Because a single drive belt already includes several hundred transverse elements, the most practically applicable and/or commercially effective arrangements of the quench hardening heat treatment must provide for the processing of transverse elements in bulk. In practice, several hundred to several thousand transverse elements are heat treated together, i.e. batch-wise, which batch is contained in a heat treatment basket in a multilayer stack or pile thereof, as discussed in the European patent publication EP-A-1531284.

A limitation of this latter type of processing, in particular in comparison with the individual handling thereof, is that some transverse elements experience different heat treatment conditions as others depending on their respective positions relative to the other transverse segment in the batch. For example, when a pile of transverse elements is placed in an oven chamber, those elements making up the outside of the pile will heat up far more quickly than those in the middle of the pile. As a result, some properties of the transverse elements, such as the exact angle between the contact faces thereof, their flatness or their exact microstructure may vary slightly between them after quench hardening. Although such geometric and metallurgic variations are minimal only and are in fact mostly acceptable within the present technical context, it is principally preferable to maximize process and product consistency from a general process control and product quality point of view.

According to the present disclosure, such process consistency and the resulting product consistency of the quench hardening heat treatment of transverse elements can be favorably improved by the modification of the known quench hardening heat treatment that is defined in claim 1 hereinafter. In particular according to the present disclosure the austenitizing stage is carried out in the oven chamber at reduced pressure relative to ambient pressure. In particular, a pressure of only 10 mbar or less is applied in the austenitizing stage in accordance with the present disclosure.

Preferably, the remaining process atmosphere in such low-pressure austenitizing is created by a supply of hydrocarbon gas, such as natural gas, propane or acetylene, that cracks at the temperature applied in the austenitizing and that thus provides a source of carbon to favorably enrich a surface layer of the transverse elements with dissolved carbon. Hereby, a mild carburization of a surface layer of the transverse elements is realized, resulting in an improvement of some of the mechanical properties of the transverse element in view of the drive belt application thereof, such as surface hardness and/or wear resistance, in particular as compared to the conventionally applied process gas mixture of nitrogen, hydrogen and natural gas at atmospheric pressure. Particularly good results in this respective have been obtained with acetylene.

In a further, more detailed embodiment of the quench hardening heat treatment according to the present disclosure, a flow of process gas is admitted to the oven chamber during the austenitizing stage. At the same time, process gas can be pumped from the oven chamber to maintain the said reduced pressure therein. According to the present disclosure, by such flow thereof, it is realized that the process atmosphere favorably mixes throughout the austenitizing oven chamber and/or penetrates the stack of transverse elements particularly well, at least to an improved extent than without such flow. Further according to the present disclosure, the process gas may be admitted to the oven chamber intermittedly, e.g. in short pulses at 100 mbar each, rather than continuously at 10 mbar or less, whereby the said penetration can be enhanced further and/or can be optimally controlled in view of the desired carbon enrichment of the surface of the transverse elements.

The above-described setup of the austenitizing stage of the quench hardening heat treatment in accordance with the present disclosure is ideally combined with gas quenching, i.e. with cooling the austenitized transverse elements by means of a gas flow. Gas quenching is, as such, well-known in industry, however, only for cooling/quenching individual, or at least individually arranged work pieces. According to the present disclosure, however, gas quenching has now been successfully applied in combination with mutually stacked transverse segments as well. In particular according to the present disclosure, the transverse elements are hereto stacked in and transported by a basket and a flow of quenching gas is effected through the basket holes and (subsequently) through the stack of transverse elements in upward direction relative to the direction of gravity. More in particular, the flow of quenching gas is regulated such that individual transverse elements are lifted thereby, however, only partly and/or only instantaneously without being blown out of the basket. In this manner, a stack of transverse elements with a (stack) height of 10 times the thickness of an individual transverse element has been successfully treated, i.e. quenched, but a further increase, e.g. doubling of such stack height appears feasible.

The above discussed principles and features of the novel transverse element and its proposed manufacturing method will now be elucidated further along a drawing in which:

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 steel transverse elements and a tensile element;

Figure 3 schematically indicates the three stages of a conventional quench hardening heat treatment that is applied as part of the overall manufacturing method of the transverse element; and

Figure 4 schematically indicates the three stages of a conventional quench hardening heat treatment in accordance with the present disclosure.

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 set of rings 44 is received in a respective recess or slot 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 slots 33 of the transverse element 32 are located in- between a bottom part 34 and a top part 36 of the transverse element 32, as seen in radial direction relative to the drive belt 3 as a whole.

On the axial sides of the said bottom 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 clamping obviously results in a compression of the bottom part 34 of the transverse elements 32, tensile forces are generated therein as well, in particular in a transition region between the bottom part 34 and the central part 35 thereof. Thus, the transverse elements 32 are not only subjected to wear, but due the said intermittent clamping thereof also to metal fatigue loading.

It is well-known and generally applied to manufacture the transverse elements 32 from steel, such as 75Cr1 (DIN 1 .2003) steel, and to quench harden the steel as part of the overall production process of the drive belt 3. The conventional process step of quench hardening comprises three stages I, II and III that are schematically illustrated in figure 3. In stage I a batch of the (possibly only partially) pre-cut transverse elements 32 are heated in an oven chamber 60 to a temperature substantially above the austenitizing temperature of the steel in question in order to provide these with a crystalline structure of austenite. In this stage I, the transverse elements 32 are typically placed in a carbon containing gas atmosphere, for example in the form of nitrogen mixed with methane, in order to prevent a depletion of carbon from a surface layer of the transverse elements 32 to the gas atmosphere. In stage II the batch of transverse elements 32 are quenched, i.e. are rapidly cooled from the austenitizing temperature to a quenching temperature, to form a (meta-stable) microstructure largely composed of supersaturated martensitic crystals. In this stage II, the cooling of the transverse elements 32 is typically realized by immersing these in an oil bath 70 that is typically maintained at temperature of between 80 and 120 degrees Celsius. Thereafter, in stage III, the batch of transverse elements 32 are re-heated in an oven chamber 80, to increase the ductility and toughness thereof. The temperature applied in this stage III is much lower than that in stage I, e.g. is around 200 degrees Celsius, such that it can take place without any protective atmosphere, i.e. in air.

According to the present disclosure the above, conventional quench hardening heat treatment can be improved in terms of the consistency of the geometric and metallurgic properties of the transverse segments 32. An exemplary embodiment of such novel quench hardening heat treatment in accordance with the present disclosure is illustrated in figure 4. The austenitizing stage I of the novel heat treatment of the transverse segments 32 is carried out at a reduced pressure of less than 10 mbar. For creating such low-pressure process atmosphere, a pump 62 is connected to austenitizing oven chamber 60 for drawing gas therefrom. Even at such low pressure, it is highly preferably to perform the austenite phase transformation not in air but, endogas. Therefore, a controllable supply 61 of hydrocarbon gas, here: acetylene gas, is connected to the oven chamber 60 as well. Furthermore, such process atmosphere is preferably not only created at the start of the austenitizing stage I, but is also partly refreshed during the progression of the austenitizing stage I by the controlled, preferably intermittent, supply of acetylene.

The quenching stage II of the novel heat treatment entails the immediate cooling of the austenitized transverse segments 32 by means of a flow of nitrogen gas 71 that is preferably chilled relative to room temperature and that is forced through the stack 72 of transverse elements 32 contained in a heat treatment basket 73. Preferably, the quenching gas flow is directed through the stack 72 of transverse elements 32 in upward direction relative to the direction of gravity.

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.