JP7179615B2 - Method for austenitizing and/or carburizing steel transverse elements for drive belts for continuously variable transmissions - Google Patents

Description

TECHNICAL FIELD The present disclosure relates to a method for manufacturing transverse elements made of steel, and in particular to a batch heat treatment method thereof, the transverse elements being essentially continuous for filling around an endless carrier, i.e. a ring-shaped carrier, of a drive belt. applied to the drive belt as a series of rows. These commonly applied transverse elements and drive belts and continuously variable transmissions are well known in the art, for example from EP-A-0626526 and EP-A-1167829, respectively. It is

In the transmission, the transverse elements of the drive belt are in frictional contact with two pulleys, a first transverse element exerting a pushing force on a second adjacent transverse element, which in turn contacts a third transverse element. Driving force can be transmitted from one transmission pulley by applying such a pushing force (the same applies hereinafter). The endless carrier of the drive belt serves primarily to constrain and guide the transverse elements in their tracks around and between said pulleys. An endless tensile element generally consists of a number of mutually nested, ie, radially stacked, pairs of flexible metal rings. Generally each transverse element also defines two slots, each opening towards a respective lateral portion of the respective transverse element, each accommodating a portion of a respective one of the two sets of rings. On each of its lateral sides, too, the transverse element is provided with a respective one of its two contact surfaces, which contact surfaces define the angle defined between the two mutually opposite conical discs of each transmission pulley. are oriented to each other at an angle substantially coinciding with

Known transverse element designs are subject to intermittent axial compression of the transverse element, e.g. Highly optimized with respect to resulting stress levels and stress amplitudes. The manufacturing process of the transverse elements is also greatly optimized for this purpose. For example, the transverse elements are hardened by a heat treatment of quench hardening, which involves the three well-known stages of austenitizing in an endothermic process gas (“endogas”), quenching in oil, and tempering in air. Since a single drive belt already contains hundreds of transverse elements, the most practically applicable and/or commercially effective arrangement of quench hardening heat treatments must be provided for bulk processing of the transverse elements. not. In practice, as described in EP 1 531 284 A1, hundreds to thousands of transverse elements are heat treated together, i.e. batchwise, and the batches are heat treated baskets in multi-layer stacks or piles. are housed in

A limitation of this latter type of processing is that some transverse elements are different from others, depending on their respective positions relative to other transverse segments in the batch, especially compared to their individual handling. It is subject to different heat treatment conditions. For example, if a pile of transverse elements is placed in an oven chamber, those elements that make up the outside of the pile will heat up much more quickly than the elements in the middle of the pile. As a result, some properties of the transverse elements, such as the exact angle between their contact surfaces, their flatness or their exact microstructure, may change slightly between them after quench hardening. Although such geometric and metallurgical variations are minimal and, in practice, acceptable in the current state of technology, process and product consistency are subject to general process control. and is primarily preferred from a product quality standpoint.

According to the present disclosure, such process consistency and resulting product consistency of the transverse element quench hardening heat treatment is advantageously achieved by improvements to known quench hardening heat treatments as defined in claim 1 below. can be improved. Specifically, according to the present disclosure, the austenitizing step is performed at reduced pressure relative to ambient pressure in an oven chamber. In particular according to the present disclosure pressures of no more than 10 mbar are applied in the austenitizing stage.

Preferably, the remaining process atmosphere in such low pressure austenitization is created by feeding a hydrocarbon gas such as natural gas, propane or acetylene, which decomposes at the temperature applied for austenitization and thus becomes a carbon source. to advantageously enrich the surface layer of the transverse elements with dissolved carbon. This results in a mild carburization of the surface layer of the transverse elements, and some of the mechanical properties of the transverse elements when applied to drive belts, such as surface hardness and/or wear resistance, are improved, especially under atmospheric pressure. is improved compared to the customarily applied process gas mixtures of nitrogen, hydrogen and natural gas. Particularly good results in this regard 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 into the oven chamber during the austenitizing step. At the same time, process gas can be pumped out of the oven chamber to maintain the reduced pressure therein. According to the present disclosure, such a flow of process gas provides good mixing of the process atmosphere throughout the austenitizing oven chamber and/or at least such flow is particularly satisfactory for the stack of transverse elements. An improved degree of penetration is achieved than without it. Further, according to the present disclosure, the process gas may enter the oven chamber intermittently, e.g., at 100 mbar each in short pulses rather than continuously at 10 mbar or less, to further enhance said penetration. and/or can be optimally controlled in view of the desired carbon enrichment of the surface of the transverse elements.

The above configuration of the austenitizing stage of the quench hardening heat treatment according to the present disclosure is ideally combined with gas quenching, ie cooling of the austenitizing transverse elements by gas flow. Gas quenching per se is well known in the industry, but only for cooling/quenching individual or at least individually arranged workpieces. However, according to the present disclosure, gas quenching has also been successfully applied in combination with mutually stacked transverse segments. In particular, according to the present disclosure, in this regard the transverse elements are stacked and transported in baskets, the flow of quenching gas being directed through the stack of transverse elements through the holes in the basket (and thereafter) against the direction of gravity. turned upwards. More specifically, the flow of quenching gas causes the individual transverse element to be lifted by it, but thereby only partially and /or instantaneously, without the transverse element being blown out of the basket. It is controlled so that it can only be lifted by In this way stacks of transverse elements having a (stack) height of ten times the thickness of the individual transverse elements are successfully treated, i.e. quenched, but further increasing such stack height It seems feasible to do so, for example to double.

The above-mentioned principles and features of the novel transverse element and its proposed manufacturing method will now be explained with reference to the drawings.

1 schematically shows an example of a known continuously variable transmission with two pulleys and one drive belt; FIG. 1 schematically shows a cross-section of a known drive belt incorporating steel transverse and tensile elements; FIG. FIG. 2 schematically illustrates a conventional three-step hardening heat treatment applied as part of the overall manufacturing process for transverse elements; 1 schematically illustrates a three stage conventional quench hardening heat treatment according to the present disclosure;

FIG. 1 shows the core of a known continuously variable transmission, or CVT, commonly applied in the driveline of motor vehicles between an engine and its driven wheels. The transmission has two pulleys 1,2 each provided with a pair of conical pulley discs 4,5 mounted on pulley shafts 6 or 7, said pulley discs 4 , 5 define mainly V-shaped circumferential pulley grooves. At least one pulley disk 4 of each pair of pulley disks 4,5, ie of each pulley 1,2, is axially movable along the pulley shafts 6,7 of the respective pulley 1,2. A drive belt 3 is wound around the pulleys 1,2 while being positioned in pulley grooves for transmitting rotational motion and associated torque between the pulley shafts 6,7.

The transmission generally provides (during operation) the axially displaceable pulley disc 4 of each pulley 1, 2 with an axial It also includes actuation means by which the drive belt 3 is clamped between these discs of the pulleys 1,2 by applying a clamping force directed to . These clamping forces not only determine the frictional forces between the drive belt 3 and the respective pulleys 1,2, but also the radial position of the drive belt 3 on each pulley 1,2 between its pulley discs 4,5. R is also determined and this radial position (R) determines the speed ratio of the transmission between its pulley shafts 6,7.

An example of a known drive belt 3 is shown in more detail in FIG. 2 in its circumferentially facing cross-sectional view. The drive belt 3 incorporates endless tensile elements 31 in the form of two sets of flat thin or ribbon-like flexible metal rings 44 . The drive belt 3 also has a number of transverse elements 32 attached to the tension elements 31 along its circumference. In this particular example, each set of rings 44 is received in a respective recess or slot 33 defined by transverse element 32 at its lateral portion, i.e. at the axial side of central portion 35 of transverse element 32. . The slot 33 of the transverse element 32 is located between the bottom 34 and the top 36 of the transverse element 32, viewed radially with respect to the drive belt 3 as a whole.

The transverse element 32 is provided on its axial side of said bottom 34 with a contact surface 37 for frictional contact with the pulley discs 4,5. The contact surfaces 37 of each transverse element 32 face each other at an angle φ that essentially coincides with the angle of the V-shaped pulley groove. The transverse element 32 thus absorbs said clamping force and when an input torque is applied to the so-called drive pulley 1, the friction between the discs 4, 5 and the belt 3 causes the rotation of the drive pulley 1 to cause the so-called drive pulley 2 to rotate. is likewise transmitted via the rotary drive belt 3 or vice versa.

During operation in the CVT, the components of the transverse element 32 of the drive belt 3 are intermittently clamped between the respective pulley disc pairs 4,5 of the pulleys 1,2. Such a clamp obviously results in a compression of the bottom portion 34 of the transverse element 32, but tensile forces are generated therein as well, especially in the transition region between its bottom portion 34 and the central portion 35 thereof. The transverse element 32 is therefore not only exposed to wear, but also to metal fatigue loads due to said intermittent clamping.

It is well known and commonly applied to manufacture the transverse elements 32 of steel, such as 75Cr1 (DIN 1.2003) steel, and to harden the steel as part of the overall manufacturing process of the drive belt 3. . The conventional process steps of quench hardening include three stages I, II and III shown schematically in FIG. In Stage I, a batch of pre-cut transverse elements 32 are heated (only as partially as possible) to a temperature substantially above the austenitizing temperature of the steel in question in an oven chamber 60, thereby austenizing them. is given the crystal structure of In this stage I, the transverse elements 32 are generally placed in a carbon-containing gas atmosphere, for example in the form of nitrogen mixed with methane, to prevent depletion of carbon from the surface layer of the transverse elements 32 into the gas atmosphere. . In stage II, the batch of transverse elements 32 is quenched, i.e. rapidly cooled from the austenitizing temperature to the quenching temperature to form a (metastable) microstructure consisting mainly of supersaturated martensite crystals. In this phase II cooling of the transverse elements 32 is typically accomplished by immersing them in an oil bath 70 which is typically maintained at a temperature between 80°C and 120°C. Thereafter, in Stage III, the batch of transverse elements 32 is reheated in oven chamber 80 to increase its ductility and toughness. The temperature applied in this stage III is much lower than that in stage I, for example about 200° C., so that it can be done without a protective atmosphere, ie in air.

According to the present disclosure, the conventional quench hardening heat treatments described above may be improved with respect to the consistency of the geometric and metallurgical properties of transverse segments 32 . An exemplary embodiment of such a novel hardening heat treatment according to the present disclosure is shown in FIG. The austenitizing stage I of the novel heat treatment of the transverse segments 32 is carried out under reduced pressure of less than 10 mbar. To create such a low pressure process atmosphere, austenitizing oven chamber 60 is connected to pump 62 from which gas is drawn. Even at such low pressures, it is highly desirable to carry out the austenitic phase transformation in end gas rather than in air. Accordingly, a controllable source 61 of hydrocarbon gas (here acetylene gas) is also connected to the oven chamber 60 . Moreover, such a process atmosphere is preferably not only created at the beginning of the austenitization stage I, but also partially refreshed during the course of the austenitization stage I by a controlled, preferably intermittent supply of acetylene. be.

The quenching stage II of the novel heat treatment comprises the austenitizing transverse segments 32 preferably cooled to room temperature and forced through a stack 72 of transverse elements 32 housed in a heat treatment basket 73 by a flow of nitrogen gas 71 . Accompanied by immediate cooling. Preferably, the flow of quenching gas is directed upwardly with respect to the direction of gravity through the stack 72 of transverse elements 32 .

The present disclosure relates to and includes all details of the entire foregoing description and of the accompanying drawings, as well as all features of the appended claims. Parenthesized references in the claims do not limit the scope but are provided merely as non-limiting examples of the respective features. Claimed features may be applied separately in a given product or process, as the case may be, although any combination of two or more of such features may be applied therein. be.

The invention represented by this disclosure is not limited to the embodiments and/or examples explicitly referred to herein, but encompasses modifications, improvements and practical applications thereof, in particular the relevant It is within the reach of those skilled in the art.

1,2 pulleys 3 drive belt 4,5 pulley discs 6,7 pulley shaft R radial position 31 endless tensile element 32 transverse element 33 slot 34 bottom of transverse element 35 middle part of transverse element 36 top of transverse element 37 contact surface 44 ring 60 oven chamber 62 pump 70 oil bath 71 nitrogen gas 72 stack 73 heat treatment basket 80 oven chamber I, II, III quench hardening heat treatment

Claims (2)

Steel transverse element (32) for a drive belt (3) comprising an endless tension element (31) and a number of transverse elements (32) slidably mounted on said endless tension element (31) A manufacturing method comprising:
Hundreds to thousands of said transverse elements (32) are arranged in a heat treatment basket (73), and said hundreds to thousands of said transverse elements (32) are heated from the ferrite/austenite transformation temperature (Tp) of the steel in question. quench hardening said transverse elements (32) by heating to a higher temperature to austenitize and subsequently quenching said hundreds to thousands of said transverse elements (32),
austenitizing of said transverse element (32) is carried out in an oven chamber (60) in a non-oxidizing atmosphere at low pressure with a value in the range of 1-10 mbar,
Said transverse elements (32) are stacked and housed in said heat treatment basket (73) and during said quenching stage said transverse elements (32) cooled by a stream of nitrogen gas forced to the top from
Said flow of nitrogen gas lifts each of said transverse elements (32) , thereby removing said transverse elements (32) without blowing said transverse elements (32) out of said heat treatment basket (73). controlled to lift only part and /or momentarily of the
A method of manufacturing a transverse element (32), characterized in that: The method of claim 1, wherein the atmosphere in the oven chamber (60) comprises acetylene gas.

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