WO2009056169A1

WO2009056169A1 - Drive belt ring component and manufacturing method therefor

Info

Publication number: WO2009056169A1
Application number: PCT/EP2007/061721
Authority: WO
Inventors: Bert Pennings
Original assignee: Robert Bosch Gmbh
Priority date: 2007-10-31
Filing date: 2007-10-31
Publication date: 2009-05-07
Also published as: CN101842502A; JP5528347B2; JP2011502214A; CN101842502B

Abstract

The invention concerns a method for manufacturing an endless metal ring (32) for a drive belt (1) suitable for power transmission in a continuously variable transmission including at least a process step (IX) of nitriding or case hardening the ring (32) in an atmosphere comprising at least ammonia gas at elevated temperature, wherein, after the process step (IX) of case hardening has been completed, the ring (32) is placed in a reducing atmosphere at elevated temperature in a further or post-treatment process step (IX-A; VIII) of the manufacturing method.

Description

DRIVE BELT RING COMPONENT AND MANUFACTURING METHOD THEREFOR

The present invention relates to a manufacturing method for an endless, thin and flexible metal band, which band is typically incorporated in a drive belt for power transmission between two adjustable pulleys of the well-known continuously variable transmission or CVT applied in motor vehicles. The invention further relates to the product obtained by the said method.

At least in relation to its application in the drive belt such band is also referred to as the ring component of the drive belt Such a drive belt and its ring component are generally known, e.g. from EP-A-1 403 551. In this known type of drive belt, which is usually referred to as a pushbelt, a number of such rings are incorporated in at least one, but typically two laminated, i.e. mutually radially nested sets thereof. The known pushbelt further comprises a number of transverse metal elements that are slidably mounted on such band set or sets. In the pushbelt application thereof, the state of the art rings are produced from maraging steel, which type of steel combines, amongst others, 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 and thus wear resistant outer surface layer of the ring material. The said hard surface layer is provided with a maximum thickness 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. Of course, especially this latter feature is very significant in the drive belt application of the rings, because of the numerous number of load and bending cycles it is subjected to during the service life span thereof. To achieve the aforementioned desired material characteristic the known manufacturing method includes the two heat-treatments of ageing, i.e. bulk precipitation hardening, and of case hardening, more in particular nitriding, typically gas soft nitriding, of the rings. The process steps of the general manufacturing method for such belts, at least for the ring component thereof, as applied by Applicant for several decades now, have in the mean time become well known in the art and or for instance described in the European patent application EP-A-1 055 738. A problem with the known ring manufacturing method is that during the gas soft nitriding a layer of iron-nitrides may form on the ring surface, which layer is referred to as compound layer and which layer influences surface hardness and ductility. As a result the mechanical properties of the nitrided ring component will not be as uniform as desired and, moreover, the ring surface layer may locally even have become too brittle for its proper application in the pushbelt.

The thickness of such compound layer heavily depends not only on the maraging steel alloy composition, but also on the process settings of the gas soft nitriding process, such as the processing temperature, time and atmosphere, i.e. the composition of the nitriding gas. However, it tends to from more quickly and more extensively as the process settings are optimised towards the efficiency of the nitriding as such, e.g. when reducing the processing time by increasing the processing temperature and/or the (gas) nitriding potential (as determined by the respective ammonia and hydrogen gas concentrations). It is an object of the present invention to be able to realise the desired mechanical properties for the pushbelt ring component, without having to put special constraints on the settings of the ring nitriding process to avoid the said compound layer from being formed.

According to the invention, such object may be realised with the specific measure that is indicated in the characterising portion of the present claim 1. According to this claim, following the process step of nitriding, a further, post- treatment process step is performed, wherein the rings are heat-treated in a reducing atmosphere that is essentially free from ammonia and oxygen and preferably is mainly composed of nitrogen, preferably containing several (e.g. up to 15) volume-% of hydrogen. During such post-treatment the iron-nitrides dissolve and the compound layer are effectively removed from the ring surface. The presence of hydrogen can significantly speed up this process of compound removal. Such ring post-treatment can be arranged to take a relatively short period of time when compared to the period of time that is typically or at least conventionally required for the ring nitriding and ageing processes. The temperature, however, should preferably have a setting that is comparable to the temperature setting of the preceding nitriding process step, more preferably having a value in the range between 400 and 500 degrees Celsius.

With such a set-up of the ring manufacturing method in particular the ring nitriding process can be performed a time and cost efficient way, at least without having to avoid the formation of a iron-nitride compound layer that is, after all removed, afterwards in the said ring post-treatment.

According to the invention, the said post-treatment can be set up such that bulk precipitation hardening, i.e. ageing of the ring material occurs simultaneously. Indeed, according to the invention it has been found that when the proper process settings are applied in such post-treatment, the ageing process can be completed during said post-treatment and the conventional process step of ageing prior to nitriding can thus be favourably skipped. In this latter arrangement of the ring manufacturing method the known set-up of the ring heat-treatment by ageing before nitriding is effectively reversed to fist nitriding, followed by ageing. Hereby, a so- called over-ageing of the rings in the post-treatment that could occur in the conventional manufacturing method is prevented.

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 that is provided with a drive belt incorporating a ring component.

Figure 2 is a section of the belt shown in perspective.

Figure 3 figuratively represents an overview of a part of the known manufacturing method for a metal ring component of such belt. Figures 4 and figure 5 each figuratively represent a part of the manufacturing method modified in accordance with the present 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 1 , 2, each provided with two conical pulley discs 4, 5, where between a predominantly V- shaped pulley groove is defined and whereof one disc 4 is axially moveable along a respective pulley shaft 6, 7 over which it is placed. A drive belt 3 is wrapped around the pulleys 1 , 2 for transmitting a rotational movement ω and an accompanying torque T from the one pulley 1 , 2 to the other 2, 1. The transmission generally also comprises activation means that impose on the said at least one disc 4 an axially oriented clamping force Fax directed towards the respective other pulley disc 5 such that the belt 3 is clamped there between. Also, a speed ratio of the transmission is thereby determined, which hereinafter is defined as the ratio between the rotational speed of the driven pulley 2 and the rotational speed of the driving pulley 1. An example of a known drive belt 3 is shown in more detail figure 2 in a section thereof, which belt 3 incorporates an endless tensile means 31 . In this particular example, the endless tensile means 31 is composed of two sets of flat, i.e. band-like, flexible metal rings 32. The belt 3 further comprises a multitude of plate-like transverse elements 33 that are in contact with and held together by the tensile means 31. The elements 33 take-up the said clamping force Fax, such when an input torque Tin 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. During operation in the CVT the belt 3 and in particular its ring component(s) 32 is (are) subjected to a cyclically varying tensile and bending stresses, i.e. a fatigue load. Typically the resistance against fatiguing or fatigue strength of the ring component 32 thus determines the functional life span of the drive belt 3 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 minimum combined material and processing cost.

Figure 3 illustrates the presently relevant part of the known manufacturing method for the drive belt ring component 32 as is practised since the early years of drive belt production, wherein the separate process steps are indicated by way of Roman numerals. In a first process step I a sheet 1 1 of base material is bent into a cylindrical shape, whereby the sheet ends 12 that meet each other are welded together in a second process step Il to form a tube 13. In a third step III of the process the tube 13 is annealed. Thereafter, in a fourth process step IV the tube 13 is cut into a number of hoops 14, which are subsequently -process step five V- rolled and elongated to a required thickness between 0.1 and 0.2 mm in the end product, typically about 185 μm. After rolling the hoops 14 are usually referred to as rings 32.

The rings 32 are subjected to a further annealing process step Vl to remove the internal stresses introduced during rolling (step five V). Thereafter, in a seventh process step VII, the rings 32 are calibrated, i.e. they are mounted around two rotating rollers and stretched to a predefined circumference length. In this seventh process step VII, also an internal stress distribution is imposed on the rings 32. There after, the rings 32 are heat-treated in two separate 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. More in particular, both such heat-treatments involve heating the rings 32 in an industrial oven or furnace containing a controlled gas atmosphere that typically is composed of nitrogen and some, i.e. typically around 5 volume-% hydrogen gas for ring ageing and of nitrogen and ammonia for ring nitriding. Both heat-treatments typically occur within the temperature range from 400 to 500 degree Celsius and can last for about 45 to over 120 minutes in dependence on the base material (maraging steel alloy composition) for the rings 32, as well as on the mechanical properties desired for the rings 32.

Finally, a set 31 of thus processed rings 32 is formed by radially stacking, i.e. nesting, a number of purposely selected rings 32, as is further indicated in figure 3. To obtain the required number of rings 32 suited for forming one set 31 , the circumference length (or other suitable parameter such as the diameter) of the rings 32 is measured in a tenth process step X and in the last depicted eleventh process step Xl the set 31 of rings 32 is assembled by mutually nesting the required number of suitably dimensioned rings 32.

In the nitriding process (step nine IX in figure 3) ammonia molecules present in the oven atmosphere dissociate on the surface of a ring 32, whereby nitrogen atoms are formed that are absorbed by (i.e. diffuse into) into the steel matrix of the ring 32. Hereby additional hardness in a surface layer of the ring 32 is created by nitrogen- interstitials and nitrogen containing precipitates. However, inevitably, a part of the thus formed nitrogen atoms reacts with the iron atoms of the maraging steel already on the surface of the ring 32, locally forming a layer of iron-nitrides that is referred to as a compound layer. Depending on time, temperature and process atmosphere settings of the nitriding process step IX, these iron-nitrides are formed to a smaller or larger extent. As a result the mechanical properties of the ring 32 will not be as uniform as desired and, moreover, the ring surface layer may locally even have become too brittle for its proper application in the drive belt 3. Therefore, it is to be principally preferred to prevent the formation of such compound layer by optimising the process settings of the ring nitriding in that respect. However, in doing so, the efficiency of the ring nitriding process step IX will normally be detrimentally affected.

According to the present invention, however, it is not necessary to prevent the compound layer from being formed in the ring nitriding process step IX, because it was found that such layer can be removed reliably thereafter in a further, post- treatment process step IX-A that is performed after the process step IX of ring nitriding has been completed, as is schematically illustrated in figure 4. With such a set-up of the ring manufacturing method in particular the nitriding process step IX can be performed a time and cost efficient way, at least without having to avoid the formation of a iron-nitride compound layer on the ring 32 that is, after all removed, afterwards in the said post-treatment process step IX-A.

In such post-treatment step IX-A the ring 32 is heated to between 400 and 500 degrees Celsius in a reducing atmosphere that is essentially free from ammonia and oxygen and preferably is mainly composed of nitrogen, more preferably containing several volume-% of hydrogen. In this latter heat-treatment step IX-A, the iron- nitrides are decomposed and the compound layer formed during nitriding is thus effectively and favourably removed from the ring surface. The presence of hydrogen gas can significantly speed up this process of compound removal. According to the invention the hydrogen concentration preferably amounts to between 5 and 15 volume-% of the process atmosphere, more preferably is around 10 volume-%.

According to the invention, the above post-treatment of the ring 32 for removing the compound layer can be favourably set up such that bulk precipitation hardening, i.e. ageing of the ring material occurs simultaneously. Indeed, it has been found that when the process settings of the conventional ageing process step VIII are applied in the said post-treatment, the bulk precipitation hardening of the ring material can even be completed entirely during said post-treatment. Accordingly, a favourably efficient and effective manner for implementing the present invention into the overall ring manufacturing method is, to simply reverse the conventionally applied order of the ring heat-treatments of ageing and nitriding, i.e. to perform the known process step of ring ageing VIII only after the known process step of ring nitriding IX has been completed. In this arrangement of the ring manufacturing method, the process step of ring ageing VIII thus simultaneously serves as the post-treatment according to the present invention, i.e. serves to remove the compound layer that may be formed during ring nitriding from the ring surface. This latter arrangement of the ring manufacturing method according to the invention is schematically illustrated in figure 5. Hereby, it is avoided that the said post-treatment would require an additional process step vis-a-vis the known ring manufacturing method as represented by figure 3. Moreover, a so-called over-ageing and consequent reduction of the core hardness of the rings 32, which could occur if the said post-treatment would be added as an additional process step to such known ring manufacturing method, will be prevented.

The invention will now be defined further along a set of claims and, apart from the preceding description, also relates to all details therein, and to all details and aspects in the discussed drawing which are directly and unambiguously derivable there from by the one skilled in the art.

Claims

1 . Method for manufacturing an endless metal ring (32) for a drive belt (1 ) suitable for power transmission in a continuously variable transmission including at least a process step (IX) of nitriding or case hardening the ring (32) in an atmosphere comprising at least ammonia gas at elevated temperature, characterised in that, after the process step (IX) of case hardening has been completed, the ring (32) is placed in a reducing atmosphere at elevated temperature in a further or post-treatment process step (IX-A; VIII) of the manufacturing method.

2. Method according to claim 1 , wherein the said process steps (IX, IX-A; VIII) are performed in immediate succession, possibly even in a single chamber of an industrial oven by flushing the nitriding atmosphere from such chamber after case hardening has been completed by pumping in the said reducing atmosphere.

3. Method according to claim 1 or 2, characterised in that the said reducing atmosphere is predominantly composed of nitrogen gas.

4. Method according to claim 3, characterised in that the said reducing atmosphere further comprises hydrogen gas.

5. Method according to claim 4, characterised in that the hydrogen gas amounts to between 5 and 15 volume-% of the said reducing atmosphere, preferably between 9 and 1 1 volume-%.

6. Method according to any one of the preceding claims, characterised in that in post-treatment process step (IX-A; VIII) the said reducing atmosphere is kept at a temperature of between 400 and 500 degrees Celsius.

7. Method according to any one of the preceding claims, characterised in that the post-treatment process step (IX-A; VIII) lasts for between 45 and 120 minutes.

8. Method for manufacturing an endless metal ring (32), preferably according to any one of the preceding claims, including a process step (IX) of nitriding or case hardening the ring (32) in an atmosphere comprising at least ammonia gas at elevated temperature and a single process step (VIII) of ageing or bulk precipitation hardening the ring (32) in an atmosphere predominantly comprised of nitrogen gas at elevated temperature, characterised in that, the said single process step (VIII) of ageing is performed after the said process step (IX) of nitriding has been completed.

9. Method according to any one of the preceding claims, characterised in that, the ring (32) is made from a maraging steel.

10. Endless metal ring (32) component for a drive belt (1 ) suitable for power transmission in a continuously variable transmission and produced in a manufacturing method according to one or more of the preceding claims.