The present invention relates to a drive belt for use in the well-known pulley- type continuously variable transmission or CVT that is typically applied in motor vehicles. The drive belt is described in detail in EP 2 115 318 A and includes a multitude of plastic transverse segments that are each provided with at least one, but typically two recesses, each such recess accommodating only a single, flexible steel ring of the drive belt. In the art, the transverse segments are also referred as transverse elements. In the present type drive belt the transverse elements are either not fixed to the ring, in which case they can slide along the circumference thereof during operation in the CVT, or they are fixed to the ring by means of a elastomeric spacer provided there between, which spacer is typically in the form of a profiled rubber covering layer of the ring.

For the present drive belt application thereof, a ring is produced from maraging steel, which type of steel combines a comparatively favourable possibility to weld and plastically deform the material with the characteristics of great tensile strength and good resistance against both abrasive wear and bending and/or tensile stress fatigue, at least after the appropriate heat treatment thereof. The known rings are provided with a fair hardness of the core material for realising the properties of good tensile, yield and bending strength combined with a high resistance against metal fatigue, which ring core is encased in a substantially harder, typically gas soft nitrided, and thus compressively stressed and hence fatigue-fracture resistant outer surface layer of the ring material. This ring, its material specifications and its manufacturing method are well-known in relation to the application thereof in a particular type of drive belt that is known as the (Van Doorne) push belt that includes steel transverse elements and a number of said rings that are in this case mutually concentrically. Indeed, many patent (application) documents on this topic exist, including for example EP-A-1 055 738, EP-A-1 176 224 A, JP-A-2000-337453 and the non pre-published international patent application PCT/NL2008/050259.

From these documents the generally accepted teaching emerges that a thickness and hardness of the nitrided surface layer is to be carefully controlled in manufacturing to limit the internal ring stress and to provide the ring with a sufficient elasticity to allow longitudinal bending as well as resistance against fatigue fracture. Especially this latter feature is known to be a determining factor in the drive belt application of the rings, because of the numerous load and bending cycles it is subjected to during its service life. More in particular, it is universally taught in theory and applied in practice to provide the ring with a nitrided surface layer having a thickness of at least 20 % of the total ring thickness, as measured in the radial direction of the ring, and with a surface hardness of at least 800 HV0.1 (Vickers hardness with 100 gr. weight applied). It is noted that, in the context of the present application, the thickness of the nitride layer is determined optically (with the aid of a microscope) on a polished and suitably etched cross-sectional surface of the ring.

The present invention departs from the above-described state of the art technology and aims to further increase the service life of the drive belt and/or to reduce the manufacturing cost thereof at least in combination with the present, specific type of drive belt. According to the invention such aim is realised by the drive belt incorporating the features of Claim 1 hereinafter. It was found that the manufacturing cost could be reduced significantly in this way, favourably without a detrimental effect on the fatigue strength of the drive belt. More in particular, it was found that the previously prescribed minimum nitrided surface layer thickness is not so much required for realising the desired fatigue strength of the ring as a separate component, but rather for the ring as incorporated in the drive belt, i.e. for limiting ring wear and thus the realising the desired fatigue strength of the drive belt as whole. Apparently, in the push belt, it is the contact of the ring with the steel transverse elements and/or with an adjacent ring that requires the known minimum nitride layer thickness and ring surface hardness. Obviously, a thin nitride layer can be provided to the ring more rapidly and/or with a relative lowered ammonia partial pressure in the processing gas, as compared to a thicker nitride layer.

Specifically if no nitrided surface layer is applied to the ring or rings of the present drive belt at all, alternative processes for introducing a compressive stress in such surface layer, such as a shot-peening process or a stone tumbling process, can be favourably applied instead.

Specifically in relation to a maraging steel alloy taken form the range of alloys having a basic composition with 17 to 19 mass-% nickel, 4 to 6 mass-% molybdenum, 8 to 18 mass-% cobalt and with balance iron, possibly with some, i.e. less than 1 mass-%, titanium added and with less than 1 mass-% of inevitable contaminations, the optimum thickness of the nitride layer has been investigated. According to the invention in this case a 200 pm thick ring is optimally provided with a nitride layer of between 5 and 15 pm thick, i.e. between 2.5 and 7.5% of the total, radial ring thickness, preferably combined with a surface hardness value of between 550 and 800 HV0.1. The lower end of this range, i.e. between 550 and 700 HV0.1 surface hardness, being preferred to minimise the wear of the plastic transverse elements in the frictional contact with the ring. The hardness of the ring core material may be even less, i.e. below 500 HV1.0 (Vickers hardness with 1000 gr. weight applied).

The basic principle of the invention will now be elucidated by way of example, along a drawing in which:

Figure 1 provides a schematically depicted example of the well-known continuously variable transmission provided with a drive belt,

Figure 2 is a cross-section of the known drive belt,

Figure 3 figuratively represents the presently relevant part of the known manufacturing method of the ring component of the drive belt,

Figure 4 is a schematic cross-section of the endless ring component of the drive belt in accordance with the invention.

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 drive wheels thereof. The transmission comprises two pulleys 11 , 12, each provided with two conical pulley discs 15, where between a predominantly V-shaped pulley groove is defined and whereof one disc 15 is axially moveable relative to the other disc 15 of a respective pulley 11 , 12. A drive belt 10 is wrapped around the pulleys 11 , 12 for transmitting a rotational movement and an accompanying torque from the one pulley 1 1 , 12 to the other 12, 11. The transmission generally also comprises activation means that impose on the said at least one disc 15 an axially oriented clamping force Fax directed towards the respective other pulley disc 15 such that the drive belt 10 is clamped there between. Also, a (speed) ratio of the transmission between the rotational speed of a driven pulley 12 and the rotational speed of a driving pulley 11 is determined thereby.

An example of a known drive belt 10 is shown in more detail figure 2 in a cross-section thereof, which drive belt 10 is shown to incorporate two endless rings 14. The belt 10 further comprises a number of transverse segments or elements 13 (see also figure 1) that are held together by the rings 14 that are each located in a respective recess provided in the transverse elements 13. The transverse elements 13 take-up the said clamping force exerted between the pulley discs 15, such when an input torque is exerted on a so-called driving pulley 11 , friction between the discs 15 and the drive belt 10, causes a rotation of that driving pulley 11 to be transferred to a so-called driven pulley 12 via the likewise rotating drive belt 10.

During operation in the CVT the drive belt 10 and in particular the rings 14 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 14 thus determines the functional life span of the drive belt 10 at a given torque T to be transmitted thereby. Therefore, it has been a long standing general aim in the development of the drive belt manufacturing method to realise a required ring fatigue strength 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 14, wherein the separate process steps are indicated by way of Roman numerals. In a first process step I a thin sheet or plate 51 of base material that typically has a thickness in the range between 0.4 mm and 0.5 mm is bend into a cylindrical shape and the meeting plate ends 52 are welded together in a second process step II to form an open, hollow cylinder or tube 53. In a third step III of the process the tube 53 is annealed. Thereafter, in a fourth process step IV the tube 53 is cut into a number of annular hoops 54, which are subsequently -process step five V- rolled to reduce the thickness thereof to less than 0.250 mm, typically about 185 μιη, while being elongated. After rolling the hoops 54 are usually referred to as rings 14. The rings 14 are then subjected to a further or ring annealing process step VI for removing the work hardening effect of the previous rolling process (i.e. step five V) by recovery and recristalisation of the ring material at a temperature considerably above 600 degree Celsius, e.g. about 800 degree Celsius. Thereafter, in a seventh process step VII, the rings 14 are calibrated, i.e. they are mounted around two rotating rollers and stretched to a predefined circumference length by forcing the said rollers apart. In this seventh process step VII, also an internal stress distribution is imposed on the rings 14. Thereafter, the rings 14 are heat-treated in two consecutive process steps, namely an eighth process step VIII of ageing or bulk precipitation hardening and a ninth process step IX of nitriding or case hardening, which eight and ninth process steps VIII, IX may be combined into one, i.e. may be performed simultaneously. More in particular, both such heat-treatments involve heating the rings 14 in an industrial oven or furnace containing a controlled gas atmosphere that typically is composed of nitrogen and some, e.g. about 5 volume-% of hydrogen for ring ageing and of nitrogen and ammonia for ring nitriding. Both heat- treatments typically occur within the temperature range from 400 degrees Celsius to 500 degrees Celsius and can each last for about 45 to over 120 minutes in dependence on the base material (maraging steel alloy composition) for the rings 14, as well as on the mechanical properties desired for the rings 14. In this latter respect it is remarked that, typically in the known art, it is aimed at a core hardness value of 550 HV1.0 or more, a surface hardness value of 800 HV0.1 or more and at a thickness of the nitrided surface layer, alternatively denoted nitrogen diffusion zone, in the range from 19 to 37 μιτι or 10% to 20% of the overall thickness of the ring 14 as measured in its radial direction.

Figure 4 represents a greatly enlarged, schematically drawn cross-section of the known ring 14 including an indication therein of a case hardened, in this case nitrided surface layer 20 in accordance with the present invention. Figure 4 illustrates the feature of the nitrided surface layer 20 (by the darker grey tone) of the rings 14 thereof and its thickness relative to the total radial thickness of the rings 14. In figure 4 the thickness T of the nitrided surface layer 20 of the ring 14 amounts to 7.5% of its total thickness D, whereby the optimum fatigue properties for the ring 14 are achieved at least for its application in the drive belt 10 comprising plastic transverse elements 13 with one or more recesses wherein only one such ring 14 is located each time.