NL1040568C2 - Method for measuring a fatigue property of a source material for an endless, flexible metal band of a drive belt. - Google Patents

Abstract

The present disclosure concerns a method for measuring a fatigue property of a metal test piece (53). The test piece (53) is provided in a ring shape and is coupled to a spring (54). The system of the test piece (53) and the spring (54) is cyclically compressed and stretched at a frequency that matches a resonance frequency of such test system (53, 54).

Description

METHOD FOR MEASURING A FATIGUE PROPERTY OF A SOURCE MATERIAL FOR AN ENDLESS, FLEXIBLE METAL BAND OF A DRIVE BELT

The present disclosure relates to a method for measuring a fatigue property of a source material for an endless, flexible metal band. Such a metal band is known from its application in a drive belt for power transmission between two adjustable pulleys of the well-known continuously variable transmission that is mainly applied in motor vehicles. This type of drive belt is well-known, for example from EP 1815160 Al, and is composed of a multitude of plate-like, transverse elements that are slideably incorporated in the drive belt on at least one laminated endless tensile means of the drive belt. The laminated endless tensile means is composed of a set of the endless, flexible metal bands that are mutually nested.

During operation in the said transmission the drive belt is rotated by and around the pulleys of the transmission. As a consequence, the metal bands thereof are alternatingly bend and stretched and are typically also tensioned to a varying extend in numerous stress cycles during the service life of the drive belt in the transmission. Hence, the resistance against fatigue fracture, i.e. the fatigue strength, of the metal bands is an important property thereof. Typically, the metal bands are thus manufactured from a maraging steel source material that favourably combines a/o the property of a high fatigue strength with a relatively favourable possibility to process the source material towards the end-product endless, flexible metal band, including the process steps of precipitation hardening and surface nitriding.

As part of the manufacturing process of the drive belt, i.e. as part of the drive belt quality assurance, at least the bending fatigue strength of the said source material is regularly measured and compared against a predetermined requirement therefor. Furthermore, such bending fatigue testing plays an important role in the development of the drive belt, in particular in the selection and optimisation of a (new) source material for the metal band component thereof and/or of the process steps employed in its manufacture.

Several tests methods are available in the known art for measuring, i.e. for quantifying the bending fatigue strength of metals. Typically in each such test method, a strip-shaped, i.e. rectangular test piece is subjected to a cyclically varying bending stress and the number of stress cycles until breakage by fatigue fracture is determined in dependence on the mean value and the amplitude of the applied bending stress. However, the known test methods differ in the way the bending stress is introduced to the test piece. In one of such known test methods, the stripshaped test piece is bent along its length and a first one distal end of such bend test piece is fixed, whereas the other one distal end thereof is oscillatingly moved towards and away from the said first one distal end by means of a linear actuator. This latter test method is preferable, because the maximum bending stress (and hence the ultimate fatigue fracture of the test piece) occurs in the well-defined fulcrum point of the bend applied to the test piece. Furthermore, in this latter test method the said maximum bending stress is directly related to the magnitude of the displacement imposed on the said other one distal end of the test piece by the linear actuator. A typically occurring limitation of this latter and other known test methods for measuring the bending fatigue strength is that the frequency of the stress oscillation obtained therein is about 50 Hz or so, which stress oscillation frequency is rather limited in view of the number of stress cycles of the bending stress that are, at least typically in industry, required until fracture. In particular, when the mean value and the amplitude of the bending stress that are applied to the test piece are set to correspond to those occurring in the presently considered drive belt application of the source material, even a single measurement of the bending fatigue strength thereof can easily take several weeks to complete. Furthermore, during such measurement a considerable electric power is consumed by the linear actuator and a total power consumption of 250 kWh for completing it is not uncommon.

The present disclosure aims to improve upon, in particular, the said latter known test method, at least in terms of a reduction of the said total power consumption, but preferably also in terms of an increase of the said frequency of the stress oscillation that is attainable therewith. Furthermore, the test piece is preferably subjected to the same (test) stresses, such that measurement results can continue to be correlated to another.

In accordance with the present disclosure, a novel method for measuring a fatigue property of the source material for a flexible metal band of a drive belt ring includes providing a test piece of the source material in an endless, i.e. circular or ring shape and a spring is attached to such test ring and this test system of the test ring and the spring is cyclically compressed and stretched, i.e. is excited, at a cycle time or frequency that essentially matches a resonance frequency of the system to conduct the fatigue strength measurement, e.g. until fatigue fracture of the test ring occurs. By exciting such test system at its resonance frequency, the power that is required for the actuation of the bending fatigue strength, i.e. for the oscillation of the bending stress in the test ring is favourably reduced relative to, in particular, the said latter known test method.

The spring of the test system favourably allows the said resonance frequency to be influenced simply by adapting the spring (force) constant. Furthermore, the spring allows the said mean value of the bending stress to be influenced simply by pre-stressing the system of the spring and the test piece in advance of the oscillation thereof. Additionally, in this novel fatigue test method the same or similar linear actuator can be used as before in the said latter known test method.

In a first elaboration of the above novel fatigue test method, an additional mass is included in the test system separately from and, preferably, in between the test ring and the spring thereof. In practice, such mass can be in the form of a connector that is provided between and connects the test ring and the spring, such as a metal clamp. By applying the mass, the amplitude of the bending stress that is applied to the test piece can be increased. On the other hand, the frequency of the resonant oscillation of the test piece is reduced by adding mass to the test system, which effect may, however, be counteracted by applying a stiffer spring and/or increasing the stiffness of the test piece.

In a second elaboration of the above novel fatigue test method, the spring is ring-shaped. This setup of the novel fatigue test method favourably allows two identical test pieces to be used, i.e. to be tested, in a single fatigue strength measurement, such that when one of the test pieces fractures, the other one test piece has been subject to the same number (and size) of oscillations of the bending stress, such that a second duplication or confirmation measurement with such other one test piece can normally be rapidly completed. Alternatively, the size of the ring-shaped spring, i.e. in terms of its thickness, width and/or diameter, can be adapted with relative ease to influence the said resonance frequency of the test system to coincide with a preferred value there for.

The above-described basic features of the novel fatigue test method according to the present disclosure will now be elucidated by way of example with reference to the accompanying figures.

Figure 1 is a schematic representation in a perspective view of a known drive belt and of a transmission incorporating such known belt.

Figure 2 is a schematic representation of a part of the known drive belt, which includes two sets of a number of flexible metal bands, as well as a plurality of transverse members.

Figure 3 figuratively illustrates a known manufacturing method of the drive belt flexible metal band.

Figure 4 schematically illustrates a known method and setup for measuring a bending fatigue strength of a source material of the drive belt flexible metal band.

Figure 5 schematically illustrates a novel method and setup for measuring a bending fatigue strength of a source material of the of the drive belt flexible metal band in a first embodiment thereof.

Figure 6 schematically illustrates the novel method and setup for measuring a bending fatigue strength of a source material of the of the drive belt flexible metal band in a second embodiment thereof.

Figure 7 contains two photographic representations of the setup for measuring a bending fatigue strength of figure 6.

Figure 1 shows the central parts of a known continuously variable transmission 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 pulley discs 4, 5, between which a push-type driving belt 3 is present for transmitting a rotational movement M and an accompanying torque from one pulley 1, 2 to the other 2, 1. The pulley discs 4, 5 are shaped generally conical and at least one pulley disc 4 is incorporated in the transmission axially moveable along a respective pulley shaft 6, 7 on which both discs 4, 5 are placed. The transmission normally also comprises activation means that impose on the said at least one movable disc 4 an axially oriented clamping force directed towards the respective other pulley disc 5 such that the push belt 3 is clamped there between.

The push belt 3 comprises an endless tensile means 31 and a multitude of relatively thin transverse elements 32 that are provided on the tensile means 31 movable along the longitudinal direction thereof and oriented predominantly transversely thereto. The elements 32 take-up the said clamping force, such that at rotation of a driving pulley 1, friction between the discs 4, 5 and the push belt 3, causes the elements 32 to be thrust in the said direction of movement M from the said driving pulley 1 to a driven pulley 2 and back again, thereby being guided and supported by the tensile means 31. A geometric transmission ratio of the transmission is determined by the quotient of an effective contact radius R2 of the push belt 3 at the driven pulley 2 and an effective contact radius R1 of the push belt 3 at the driving pulley 1, which may be varied continuously in range of values.

Figure 2 provides a cross section of the known push belt, which cross section is oriented in the longitudinal direction of the belt 3. In figure 2 the transverse element component 32 of the push belt 3 is shown in front elevation. The transverse element 32 is incorporated in the push belt 3 mounted on the endless tensile means 31 thereof, which, at least in this example, consists of two laminated parts, each part being formed by a number of endless, flexible metal bands 30 that are relatively thin and flat and that are nested one around the other in the radial direction and each part being accommodated in a respective opening 33 of the transverse element 32. Further, a projection 39 is provided on a front main face of the transverse element 32, which projection 39 is accommodated in a hole (not visible) provided in a rear main face of an adjacent transverse element 32. The projection 39 and hole are provided to mutually connect and align adjacent transverse elements 32 of the push belt 3 during operation in the transmission. An underside of the transverse elements 32 below a so-called tilting edge 36 is wedge-shaped, so as to allow adjacent elements 32 to mutually tilt about the axial direction when clamped between the discs 4, 5 of the pulleys 1, 2.

Figure 3 illustrates a relevant part of the known manufacturing method for the drive belt's flexible metal band component 30, as it is commonly applied in the art for the production of metal drive belts 3 for automotive application. The separate process steps of the known manufacturing method are indicated by way of Roman numerals.

In a first process step I a thin sheet or plate 11 of a maraging steel basic material having a thickness of around 0.4 mm is bend into a cylindrical shape and the meeting plate ends 12 are welded together in a second process step II to form a hollow cylinder or 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 annular, i.e. endless rings 14, which are subsequently -process step five V- rolled to reduce the thickness thereof to, typically, around 0.2 mm, while being elongated. After rolling, the rings 14 are here referred to as metal bands 30.

The metal bands 30 are subjected to a further annealing process step VI for removing the work hardening effect of the previous rolling process step by recovery and re-crystallization of the ring material at a temperature considerably above 600 degrees Celsius, e.g. about 800°C. Thereafter, in a seventh process step VII, the metal band 30 is calibrated by mounting it around two rotating rollers and stretching it to a predefined circumference length by forcing the said rollers apart. In this seventh process step VII of metal band calibration, also internal stresses are imposed on the metal band 30.

Thereafter, the metal band 30 is heat-treated in an eighth process step VIII of combined ageing or bulk precipitation hardening and nitriding or case hardening. More in particular, such combined heat-treatment involves keeping the metal band 30 in an oven chamber containing a controlled gas atmosphere that comprises ammonia, nitrogen and hydrogen gas. In the oven chamber, i.e. in the process atmosphere, the ammonia molecules decompose at the surface of the metal band 30 into hydrogen gas and nitrogen atoms that can enter into the metal lattice of the metal band 30. By these interstitial nitrogen atoms the resistance against wear as well as against fatigue fracture is known to be increased remarkably. Typically, the eighth process step VIII of combined ageing and nitriding is carried out until a nitrided layer or nitrogen diffusion zone formed at the outer surface of the metal band 30 is between 25 and 35 micron thick. A number of the thus processed endless and flexible metal band 30 are assembled into an endless tensile means 31 by radially stacking, i.e. concentrically nesting these metal bands 30 with a minimal radial play or clearance between each pair of adjoining metal bands 30.

Inter alia it is noted that it is also known in the art to instead assemble the tensile means 31 from a number of individual metal bands 30 immediately following the seventh process step VII of metal band calibration, i.e. in advance of the eighth process step VIII of combined ageing and nitriding the metal bands 30. Furthermore it is known in the art that this latter combined heat-treatment (process step VIII) can alternatively be carried out in two separate and subsequent stages of ageing and nitriding.

As part of the overall manufacturing method for metal drive belts 3, the quality of the source material for the metal bands 30 is typically tested randomly. In particular, it is practiced to take a test piece in the form of a strip 50 of the maraging steel plate 11 and to measure a bending fatigue strength thereof. Figure 4 schematically illustrates a known setup for such purpose of measuring a bending fatigue strength of the source material of the drive belt's flexible metal band component 30.

In figure 4 the test strip 50 is arranged lengthwise between a linear actuator 51 and a fixed surface 52. For the actual testing of the bending fatigue strength of the test strip 50, the linear actuator 51 is oscillatingly moved in the length direction of the test strip 50, as is indicated in figure 4 by the arrow O, until a predetermined number of oscillations of the linear actuator 51 and test strip 50 have occurred or until the test strip 50 fractures. In this known test setup, the linear actuator 51 takes up a considerably amount of electrical power in forcing the test sample to bend and relax, i.e. to generate the required oscillations of the bending stress therein.

In an effort to improve upon such known bending fatigue strength test method, in particular in terms of the power consumption thereof, a novel test method has been conceived, which novel test method is schematically illustrated in figure 5.

In the novel test method of figure 5 the test piece is provided in the form of a ring 53 and, furthermore, a spring 54 is attached to such test ring 53. The linear actuator 50 is then connected to either the test ring 53 or the spring 54 (as depicted in figure 5) to oscillate the whole test system 53, 54 of such test ring 53 and spring 54 at a resonance frequency thereof. By exciting the test system 53, 54 at its resonance frequency, the power that is required for the actuation of the bending fatigue strength, i.e. for the oscillation of the bending stress in the test ring 53, could indeed be significantly reduced relative to the known test method of figure 4. By providing the spring 54 with a specific stiffness, the resonance frequency of the novel test system 53, 54 can be influenced with relative ease. A further advantage of the novel test method in accordance with the present disclosure is that the test ring 53 that is applied therein need not be separately manufactured, but can simply and favorably be taken from the above-described, commonly practiced manufacturing method for the drive belt's flexible metal band component 30 in the form of the endless ring 14 that is produced therein as a semifinished product.

Figure 6 illustrates an alternative embodiment of the novel test method in accordance with the present disclosure. In this alternative embodiment, the spring 54 is provided in the form of a metal ring, which can even correspond in terms of the shape, size and material thereof with the test ring 53 (14) and which can thus also be taken from the said known manufacturing method. Additionally, in the embodiment of figure 6 a separate mass 55 is included in the test system 53, 54, 55 in between the test ring 53 (14) and the spring 54 (14). By applying the separate mass 55, the amplitude of the bending stress that is applied to the test piece 53 (14) can be increased. On the other hand, the frequency of the resonant oscillation of the whole test system 53, 54, 55 is reduced by adding the separate mass 55 thereto, which effect may, however, be counteracted by applying a stiffer spring 54 (14) anchor increasing the stiffness of the test piece itself 53 (14).

Figure 7 is a photographic representation of a physical embodiment of the novel test method according to figure 6. In this physical embodiment of the novel test method, both the test ring 53 (14) and the spring 54 (14) are represented by the said intermediate ring 14 product taken the commonly practiced manufacturing method for the endless, flexible metal band 14. These two rings 14 are held in place relative to, respectively, to the linear actuator 51 and to the fixed surface 52 by means of a respective magnet 56. Furthermore, the two rings 14 are connected together by means of a nut-and-bolt connecter 57 that also provides an additional mass to the test system, i.e. that serves as the said separate mass 55 of the test system 53, 54, 55 (14,14, 57).

In fact, in figure 7, the test system 53, 54, 55 (14, 14, 57) is illustrated during its operation, i.e. at two instances during the oscillation thereof at its resonance frequency by means of the linear actuator 51. From figure 7 it can be taken that due to the resonant oscillation of the test system 53, 54, 55 (14,14, 57) a displacement of the separate mass 57 (55) that is representative of a (bending) deformation of the test ring 53, is much larger than a displacement of the linear actuator 51. To the contrary, in the known setup for measuring a bending fatigue strength according to figure 4, the (bending) deformation of the test ring 53 corresponds to the displacement of the linear actuator 51. As a result, in the novel setup for measuring the bending fatigue strength, the linear actuator 51 takes up considerably less power.

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.

Claims (8)

A method for measuring a fatigue characteristic of a metal test part (53), wherein the test part (53) is provided in ring form and is coupled to a spring (54) and in which the whole of the test part (53) and the spring ( 54) is cyclically depressed and stretched, or vibrated, with a cycle time, or frequency, corresponding to a resonance frequency of a test system (53, 54, 55) comprising at least the test part (53) and the spring (54) . The method according to claim 1, characterized in that the test system (53, 54, 55) also comprises a separate mass (55), which mass (55) is preferably between the test part (53) and the spring (54) ) is installed. The method according to claim 2, characterized in that the test system (53, 54, 55) also connects the individual mass (55) to the test part (53) and the spring (54) and / or with respect to fixes each other. The method according to claim 1, 2 or 3, characterized in that the spring (54) is annular and is made of metal and preferably corresponds to the test part (53) in shape, size and composition. The method according to one or more of the preceding claims, characterized in that the test system (53, 54, 55) is vibrated with the aid of a linear actuator. The method according to one or more of the preceding claims, characterized in that it forms part, at least incidentally eq / or randomly, in a method for manufacturing a drive belt (3) for a continuously variable transmission for motor vehicles, which drive belt (3) comprises at least one endless, flexible metal band (30) made from the same metal as the test part (53). The method according to claim 6, characterized in that the test ring (53) is a semi-finished product in the manufacture of the drive belt (3), in particular the endless, flexible metal band (30) thereof. The method according to one or more of the preceding claims, characterized in that said fatigue property relates to a flexural fatigue strength.