CONTINUOUSLY VARIABLE TRANSMISSION INCORPORATING A DRIVE BELT, METHOD FOR OPERATING IT AND METHOD FOR MANUFACTURING THE DRIVE BELT

The present invention relates to the continuously variable transmission according to the preamble of the following claim 1 , which transmission is well known in the art, for example from the European patent EP-A- 1 167 829.

Such a transmission typically comprises a drive belt in the form of a so-called pushbelt that is known per se, for example from the European patent EP-A-O 626 526. The known pushbelt comprises steel transverse elements that are freely slidingly included in the belt relative to the circumference of an endless tensile means thereof that primarily has the function of confining and guiding the transverse elements. Typically, the endless tensile means is composed of two sets of a number a mutually nested, i.e. radially stacked flat and thin metal rings. In such transmission, a torque is transferred from one pulley to the other by clamping the belt between conical steel discs of the respective pulley under the influence of a clamping force, for which purpose at least one disc of the pulley is included axially moveable by means of an actuator. The said transfer of torque is realized by frictionally transferring a rotational movement of a driving pulley to the belt, of which the elements are provided with axial contact areas for contacting the discs of the pulley. At the other, i.e. driven pulley the force underlying said rotational movement of the driving pulley is conversely and also frictionally transferred from said belt to the discs of the driven pulley. The radius at which the belt runs in each pulley can be changed by changing the clamping force at each pulley in relation to one another, hereby a mathematical quotient between such radii represents a so- called geometric ratio of the transmission, whereas a ratio between the rotational speeds of the pulleys represents the actual speed ratio of the transmission.

It is imperative to cool the belt and the pulley components of the transmission during operation of the transmission, to remove the heat that is unavoidably generated in frictional contact there between. In this respect, the transverse elements of the belt are normally the most critical components, since the steel thereof has been hardened by heating (austenizing), quenching and subsequent tempering in order to be able to reliably cope with the high level of the said clamping force typically applied in contemporary transmission designs. If heated above a certain critical temperature, the tempering process would continue and the transverse elements

would wear-out rapidly or even break under the influence of the clamping force. Therefore, the known transmission further includes a cooling device for supplying a cooling agent, such as a synthetic lubrication oil to the belt and subsequently re- circulating it via a reservoir by means of a pump, which known cooling device further includes a component for cooling the cooling agent itself, i.e. a heat exchange device such as an oil cooler.

Hereby, the flow and temperature of the cooling agent supplied to the belt largely determine the heat that is removed thereby and thus ultimately the maximum temperature that the belt reaches during operation of the transmission. In practice it is true that the higher the supply flow and/or the lower the supply temperature of the cooling agent is, the lower the overall efficiency of the transmission becomes, because of the increasing amount of power consumed by the cooling device. Therefore, it has been a general development aim to increase the transmission efficiency by lowering the supply flow, for example by controlling it in accordance with the relevant transmission conditions and/or by directing the supply flow firstly to the hottest part of the transmission. Some examples of these known efforts are provided by the patent publications JP-A-09/05371 1 , EP-A-O 688 980 and US-A-7, 125,355.

It is an object of the present invention to improve the known transmission, in particular its operating efficiency by reducing the amount of power required for the operation of the cooling device.

According to the invention the above object is realized by a transmission incorporating the features of the characterizing portion of claim 1 . The present invention has found a solution not in optimizing the cooling device it self, but surprisingly in modifying the design of the transmission in such a manner that the cooling effort required thereby is reduced in the first place. The invention departs from the notion that the material hardness specified for the transverse elements is actually required only as surface hardness for coping with the wear and tear of the frictional contact with the pulley discs. In principle a considerably lower core hardness value could be allowed for the transverse elements, while still providing a sufficient strength. Provided that the transverse elements are additionally hardened by a surface, i.e. case hardening treatment, the quench hardened core material of the element can thus be allowed to be tempered to a considerably lower core hardness value, possibly even during operation of the transmission. Accordingly, a thus designed transmission can be operated at a considerably higher temperature so that the cooling effort required thereby is reduced. In practice this means that the

minimally required supply flow of the cooling agent is favorably reduced and/or that the maximally allowed supply temperature of the cooling agent is favorably increased.

Thus, in its most preferred embodiment, the invention relates to a continuously variable transmission incorporating a cooling device that is, however, not equipped with a heat exchange device such as an oil cooler. This measure significantly reduces the cooling effort, in particular a pumping effort required for circulating the cooling agent is reduced thereby. In this respect it is noted that conventionally the supply temperature of the cooling agent during operation of the transmission is limited to about 80-90 ⁰ C, whereas in the transmission according to the present invention a supply temperature of more than 100 ⁰ C can be easily allowed, potentially over 1 1 O ⁰ C and up to 125 ⁰ C, possibly even slightly beyond, e.g. up to 130 ⁰ C. Such not merely incidentally, but also for a prolonged period of time, i.e. a substantial part of the overall operation or service time of the transmission. The invention also relates to a method for operating a continuously variable transmission provided with a cooling device for cooling a belt of the transmission by continuously supplying a flow of a cooling agent to the belt. In the method according to the invention the supply flow of the cooling agent is regulated such that its temperature reaches 100 ⁰ C or more during operation. In a thus controlled transmission, either the said supply flow can be substantially reduced relative to the conventionally applied flow and/or the cooling capacity of the conventionally applied heat exchange device can be reduced in size, or potentially can even be omitted from the transmission altogether.

According to the invention, the additional surface hardness of the transverse elements may be realized by incorporating one of several well known case hardening treatments that do not rely on the phase transformation of the steel matrix (from austenite to martensite), such as coating with a suitable coating material, e.g. titanium nitride, shot-peening and or (carbo-)nitriding, in the manufacturing of the drive belt. More in particular, the method for manufacturing the drive belt preferably includes the combined case and core hardening the transverse elements by the said steps of austenizing and quenching the elements and subsequently subjecting the elements to the process step of simultaneous tempering and nitriding in a process atmosphere containing, for example, ammonia. Alternatively or additionally, in the said process step of austenizing the transverse elements are simultaneously

carburized, i.e. the austenizing is performed in a carburizing process atmosphere. This latter carburizing process may, however, also be performed prior to the process step of core hardening the transverse elements in a hot and carbon-rich environment, for example by first immersing the elements in a molten salt. In a preferred embodiment of the present invention the ultimate core hardness of the transverse elements has a value between 54 and 64 HRC (Hardness on Rockwell C-scale), more preferably between 56 and 60 HRC, whereas the surface hardness of the transverse elements amounts to more than 60 HRC, more preferably over 64 HRC. Further according to the present invention, also the pulley discs are additionally hardened by a case hardening treatment, preferably reaching a surface hardness that is comparable to, i.e. has a value that deviates less than 4 HRC-points, more preferably less than 5% from the surface hardness value of the transverse elements. By such equalization of the respective surface hardness values, the combined mechanical wear of the contact surfaces in the said frictional contact is favorably minimized. Preferably, also the core material of the pulley discs softer and thus more ductile than its surface. Preferably, the pulley discs are provided with a core hardness having a value between 50 and 60 HRC.

The invention is particularly suited for being applied in a transmission having convexly curved pulley discs, such as is known from, for example, WO-A- 2006/062400. In such a transmission the difference between the respectively required element's surface and core hardness values is especially significant, because of the highly concentrated frictional contact between the pulley discs and the belt's transverse elements thereof.