WO2014102272A1

WO2014102272A1 - Continuously variable transmission equipped with variabele pulleys and a drive belt having mutually contacting transverse segments

Info

Publication number: WO2014102272A1
Application number: PCT/EP2013/077979
Authority: WO
Inventors: Joost Johannes Cornelis Jonkers; Lucas Hendricus Robertus Maria Prinsen; Paulus Adrianus Josephus Maria Faes
Original assignee: Robert Bosch Gmbh
Priority date: 2012-12-28
Filing date: 2013-12-24
Publication date: 2014-07-03
Also published as: CN104884837A; NL1039979C2

Abstract

A continuously variable transmission is provided with two variable pulleys (1, 2), each pulley (1, 2) defining a range of possible running radii (Rr) for a drive belt (3) of the transmission that rotationally connects these pulleys (1, 2), wherein the smallest possible running radius (Rr-min1) at one pulley (1) is smaller than that at the other one pulley (2). The drive belt (3) includes transverse segments (5) that can tilt relative to one another while remaining in mutual contact through a convexly curved tilting edge thereof. The radius of convex curvature of such tilting edge in a part thereof that provides such contact in a range of running radii (Rr) provided by the other one pulley (2) is substantially smaller than in a part thereof that provides the said contact in a running radius (Rr) in-between the said smallest possible running radii (Rr-min1).

Description

CONTINUOUSLY VARIABLE TRANSMISSION EQUIPPED WITH VARIABELE PULLEYS AND A DRIVE BELT HAVING MUTUALLY CONTACTING TRANSVERSE SEGMENTS The present invention relates to a continuously variable transmission according to the preamble of claim 1. Such a transmission is generally known, for example from the European patent publication EP-A-1 221 564.

The known transmission includes two variable pulleys with a drive belt trained around the pulleys, while being clamped between a pair of pulley discs of each transmission pulley. By the simultaneous adjustment of the transmission pulleys in a mutually coordinated manner, in particular of the separation of the pulley discs thereof, a radius at which the drive belt runs at each such pulley can be changed, whereby the ratio of the rotational speeds of the transmission pulleys, i.e. the transmission ratio is changed as well. This type of transmission is typically applied in the drive line of motor vehicles, in particular passenger cars.

The known drive belt incorporates two basic components, namely a number of endless bands and a number of transverse segments. The endless bands are provided in two laminated sets, each such band set being composed of a number of concentrically arranged, i.e. nested endless bands. In the drive belt the band sets are arranged in parallel, mutually separated in the width, i.e. axial direction by a first part of the transverse segment. The transverse segments are arranged in mutual succession forming a contiguous row along essentially the entire the circumference of the band sets. Apart from in-between the band sets, the transverse segments also extend below, i.e. on the radial inside of the band sets, as well as above, i.e. on the radial outside of the band sets. As a result, a movement of the transverse segments relative to the band sets is restricted both in the axial direction and in the radial direction. However, the transverse segments can in principle move along the circumference of the band sets. With the present type of the drive belt, the transverse segments thereof push each other forward from one pulley to the other, while being contained and guided by the band sets.

In order for the drive belt to be able to follow a curved trajectory, in particular in between the pulley discs of the transmission pulleys, a bottom, i.e. radially inner part of the transverse segments radially inward from tapers in radially inward direction. This bottom part includes a so-called tilting edge in the form of a convexly curved part of a main body surface thereof, which tilting edge extends in the width direction of the transverse segment and which tilting edge forms a smooth transition between the bottom part and a top, i.e. radially outer part of the transverse segment that is mostly provided with a largely constant thickness at least in comparison with the tapered bottom part. In the said curved trajectory a pair of adjacent transverse segments in the drive belt are tilted slightly relative to each other, whereby a contact there between occurs (only) at the tilting edge of one transverse segment of such pair, in principle along an axial line of contact. As a tilting angle between the adjacent transverse segments increases from zero, when the segments are aligned in parallel, to a maximum value of a few degrees, when the trajectory of the drive belt is most tightly curved, the said axial line of contact displaces in radial inward direction.

According to EP-A-1 221 564 the tilting edge is convexly curved according to a relatively large radius of curvature, as a result whereof a total radial inward displacement of the axial line of contact, between a largest running radius and a smallest running radius of the drive belt between the pairs of pulley discs of the transmission pulleys, is considerable. In this manner, according to EP-A-1 221 564, a range of transmission ratios provided by the transmission is favourably extended relative to a transmission whereof the drive belt is provided with transverse segments having a sharply curved tilting edge. This is because the transmission ratio is equal to the quotient between the radial positions of the axial lines of contact on the tilting edge at the two pulleys. By applying a larger radius of curvature to the tilting edge, the difference and, hence, the quotient between the radial positions of the axial lines of contact on the tilting edge can be increased without having to change the size or shape of the transmission pulleys. EP-A-1 221 564 gives as an example that by increasing the radius of curvature of the tilting edge from 6 mm to 60 mm, the most extreme ratio of the transmission (that is given by the quotient between the largest radial position of the said axial line of contact at one transmission pulley and the smallest radial position of the axial line of contact at the other one transmission pulley) increases by more than 3%.

The design of the drive belt according to EP-A-1 221 564, i.e. the increase in the transmission ratio range of the transmission resulting there from, is known to come with a disadvantage. Namely, during rotation of the drive belt in the transmission, friction losses occur between the transverse segments and the band sets, which friction losses are proportional to a rotational speed difference there between that occurs during operation of the transmission and that is proportional to a (radial) distance between the said radial position of the axial line of contact on the tilting edge and (a radial inside surface of) the band sets. Thus, when applying the large radius of curvature of the tilting edge according to EP-A-1 221 564, for obtaining a favourably large range of transmission ratios, a lesser transmission efficiency must be accepted as a consequence.

The present disclosure takes a closer look at the above phenomenon, in particular with the aim of minimising the loss of transmission efficiency, while still realising the extended transmission ratio range.

An important insight underlying the present disclosure solution is that the transmission can be designed with a minimum running radius for the drive belt that is different for the two transmission pulley, which is in practice often the case already. In particular, the minimum running radius provided by a driving pulley (that is connected to the vehicle engine) is in practice mostly smaller than the minimum running radius provided by a driven pulley (that is connected to the driven wheels of the vehicle). This known asymmetric design of the transmission, i.e. of the pulleys thereof, is closely related to the main application of the present transmission in motor vehicles.

In the motor vehicle application thereof, the drive belt is at the minimum running radius of the driving pulley at take-off of the vehicle, whereas it is at the minimum running radius of the driven pulley, providing the so-called Overdrive transmission ratio, at a (medium to high) constant vehicle speed. As a result, the average speed of the drive belt when the transmission is in (vehicle) take-off mode is lower than in Overdrive. Furthermore, during normal operation the transmission will be operated in Overdrive ratio far longer than in take-off mode. Thus, if the transmission would be designed symmetrically, i.e. with the same minimum running radius for both transmission pulleys, a (fatigue) load exerted on the drive belt thereof will be far less in the take-off mode than in Overdrive. Then, by lowering reducing the minimum running radius of the driving pulley relative to the minimum running radius of the driven pulley, the said load is equalised, while at the same time improving the so-called launch performance of the vehicle at take-off.

Thus, in the known asymmetric design of the transmission the contact between the adjacent transverse segments in a bottom side of the tilting edge (as seen in the above-defined radial direction) occurs only at the one transmission pulley that provides the said smallest possible running radius for the drive belt. This is due to the above- explained circumstances that the said tilting angle between the adjacent transverse segments increases as the curvature of the trajectory of the drive belt increase, i.e. as its running radius decreases and that the axial line of contact on the tilting edge displaces in radial inward direction as the said tilting angle increases. Of course, the contact between the adjacent transverse segments in the radially outer or top side of the tilting edge will occur at both transmission pulleys.

According to the present disclosure, the asymmetric design of the known transmission opens up the possibility to fulfil the aim of an extended transmission ratio range without a substantial loss of transmission efficiency due to friction, by varying the design of the tilting edge between the said top and bottom sides thereof. In particular, the said bottom side of the tilting edge is provided with a section that extends in a straight line or that is convexly curved according to a comparatively large radius of curvature, at least in comparison with the radius or radii of curvature of the top side of the tilting edge. In this way, it is realised that the axial line of contact between the adjacent transverse segments is displaced in radial inward direction by a significant amount for increasing the transmission ratio range, at least when the drive belt runs on the smallest possible running radius at the driving pulley, whereas the axial line of contact between the adjacent transverse segments remains relatively close to the band sets for decreasing the friction loss between the transverse segments and the band sets, at least when the drive belt runs on the smallest possible running radius at the driven pulley. In a preferred embodiment, the radius of curvature of the tilting edge changes discontinuously between the said at least one section of the bottom side and the said top side thereof. In another preferred embodiment, the average radius of curvature of the bottom side of the tilting edge is larger than the average radius of curvature of the top side thereof. In either case, the radius of curvature of the bottom side of the tilting edge preferably increases by a factor of 10 or more relative to the radius of curvature of the bottom side thereof.

The above insights and the design principle based thereon are explained further with reference to the drawing, in which equal reference signs indicate equal or similar parts, and in which:

figure 1 provides a schematically representation of the known continuously variable transmission with two pulleys and a drive belt;

figure 2 illustrates a transverse segment of the drive belt according to figure 1 , both in a side and in a front elevation thereof;

figure 3 is a graph illustrating an operational feature of the known transmission; figure 4 illustrates two possible embodiments of a tilting edge of the transverse segment in accordance with the present disclosure; and

figure 5 is a graph illustrating an operational feature of the transmission according to the present disclosure in comparison with the known transmission.

Figure 1 is a schematic representation of the main components of the known continuously variable transmission. The transmission comprises a driving pulley 1 with a pair of pulley discs and a driven pulley 2 with a pair of pulley discs. A drive belt 3 is wrapped around the pulleys 1 , 2 while being clamped between the discs thereof, such that it drivingly connects the pulleys 1 , 2.

Between the pulley discs of the pulleys 1 , 2 a part of the drive belt 3 runs in a curved trajectory with radius of curvature Rr that is determined by the axial separation of the pulley discs of the respective pulley 1 , 2. At least one pulley disc of each pulley 1 , 2 is axially displaceable such that the said axial separation there between can be varied to vary the radii Rr at which the drive belt runs and, hence, to vary the (rotational) speed ratio provided by the transmission. The range of running radii Rr provided by the driving pulley is typically larger than the range of running radii Rr provided by the driven pulley. More in particular, the minimum running radius Rr-min1 provided by the driving pulley 1 can be somewhat smaller than the minimum running radius Rr-min2 provided by the driven pulley 2.

The drive belt 3 comprises a number of transverse segments 5 and two band sets 4, which belt 3 is shown in more detail in the figure 2. Figure 2 provides both a view in the longitudinal or circumferential direction of the drive belt 3 and in the transverse or axial direction thereof. The band sets 4 are shown to be composed of a number of mutually nested endless bands 6. The transverse segments 5 are shown to be provided with a stud 10 that protrudes from a front main body surface thereof, for interaction with a hole (not shown) provided in a back main body surface of the transverse segment 5, so as to mutually position the adjacent transverse segments 5 in the drive belt 3. It is further shown in figure 2 that the front main body surface includes a convexly curved part or tilting edge 12 that extends in the axial direction over the width of the transverse segment 5 such that the transverse segment 5 includes a radially outer, top part 13 of relatively constant thickness and a radially inner, bottom part 14 that is tapered in radially inward direction, at least effectively.

In the said curved trajectory of the drive belt 3 at the pulleys 1 , 2, the adjacent transverse segments 5 of the drive belt 3 are tilted relative to each other, whereby these segments 5 contact each other along an axial line of contact 7 on the tilting edge 12 of one of these segments 5. As also appears from figure 2, that the said axial line of contact 7 displaces in radial inward direction (7'→· 7"), as a tilting angle a between a pair of mutually adjacent transverse segments 5 increases from zero degrees, when these segments 5 are aligned in parallel, to a maximum value of a few degrees, when the trajectory of the drive belt is most tightly curved (Rr-min1 ; Rr-min2) due to the curvature R₁₂ of the tilting edge 12.

In the graph of figure 3 an example of the dependency between the said tilting angle a in degrees and the running radius Rr of the curved trajectory of the drive belt 3 at the pulleys 1 , 2 is given by the solid line. Furthermore, the dashed line the graph of figure 3 shows the radial inward displacement RID of the said axial line of contact 7 also in dependency on the said running radius Rr and relative to the mutual parallel alignment of the adjacent transverse segments 5. The graph of figure 3 is determined for a practical embodiment of the transmission, wherein the transverse segments have a thickness of 1 .8 mm and are provided with a tilting edge having a radius of curvature R₁₂ of 20 mm.

From the graph of figure 3 it appears that, by the relatively large radius R₁₂ of the tilting edge 12, a considerable radial inward displacement RID of the said axial line of contact 7 of about 1 .5 mm is achieved, whereby the transmission ratio range is extended relative to the running radii Rr provided by the transmission pulleys 1 , 2 as such. However, since such radial inward displacement RID occurs not only at the driving pulley 1 for improving the take-off performance of the vehicle, but also at the driven pulley 2, the efficiency of the transmission is adversely affected, because a speed difference between the transverse segments 5 and the band sets 4 and the friction losses associated with such speed difference are increased thereby.

In this latter respect, transverse segments 5 of the drive belt 3 incorporating a specific type of design of the tilting edge 12 in accordance with the present disclosure can represent a substantial improvement. This type of design of the tilting edge 12 is schematically illustrated in figure 4 by way of two possible embodiments thereof. It is noted that the figure 4 is not drawn to scale in order to more clearly elucidate the basic principle behind the novel embodiments of the tilting edge 12 in accordance with the present disclosure.

The said specific type of design of the tilting edge 12 in accordance with the present disclosure advantageously utilise the known feature of most contemporary transmissions designs that the minimum running radius Rr-min1 provided by the driving pulley 1 is somewhat smaller than the minimum running radius Rr-min2 provided by the driven pulley 2. This means that the contact between the adjacent transverse segments in a bottom side of the tilting edge occurs only at the driven transmission pulley 1 . This design feature is indicated in the graph of figure 3, indicating, by way of example, a value of 25,0 mm for the minimum running radius Rr- minl provided by the driving pulley 1 and a value of 27,5 mm for the minimum running radius Rr-min2 provided by the driving pulley 2. It then follows that in this specific case the bottom side of the tilting edge 12 extends from approximately 1 .30 to 1 .45 mm in terms of the radial inward displacement RI D.

In a first, novel embodiment of the tilting edge 12 that is illustrated in figure 4A the bottom side thereof is convexly curved according to a local radius of curvature R₁₂B that is considerably larger than such local radius of curvature Ri_2T of a top side thereof. For example, the top side of the tilting edge 12 may be circularly curved with a radius of curvature R₁₂T of 10 mm and the bottom side of the tilting edge 12 may be circularly curved with a radius of curvature R₁₂B of 120 mm.

In a second, novel embodiment of the tilting edge 12 of figure 4B, the bottom side thereof includes two sections 12', 12", namely a first section 12' that extends in an essentially straight line and a second section 12" that is again is convexly curved according to a local radius of curvature R_12B that, in this second embodiment, may be comparable to the local radius of curvature Ri_2T of the top side thereof. For example, both the top side of the tilting edge 12 and the second, curved section 12" of the bottom side of the tilting edge 12 may be circularly curved with a radius of curvature Ri2T, Ri2B of 6 mm. For the sake of completeness it is noted that in the second novel embodiment of the tilting edge 12, the said first section 12' of the bottom side of the tilting edge 12 may in fact be convexly curved, however, only according to a relatively large radius of curvature, in particular in comparison with the radius of curvature R_12B of the said second section 12".

In the graph of figure 5 the radial inward displacement RI D of the axial line of contact 7 between a pair of adjacent transverse segments 5 in the drive belt 3 is plotted in dependency on the running radius Rr of the drive belt 3 for both the novel designs of the tilting edge 12 thereof shown in figure 4. For comparison, such radial inward displacement RID has also been indicated for the known design of the tilting edge 12 in accordance with figures 2 and 3 by way of the dashed line.

From the graph of figure 5 it appears for the first, novel embodiment of the tilting edge 12 according to figure 4A, the radial inward displacement RID of the said axial line of contact 7 remains limited when the running radius Rr decreases until it reaches the minimum running radius Rr-min2 of the driven pulley 2, due to the comparatively small radius of curvature R_12T of the top side of the tilting edge 12. At this minimum running radius Rr-min2 of the driven pulley 2, the radial inward displacement RID favourably amounts to only 0.65 mm, instead of the 1 .30 mm that is indicated for the known design of the tilting edge 12. As the running radius Rr is decreased further, which is possible only for the driving pulley 1 until the particular minimum running radius Rr-min1 thereof, the radial inward displacement RID of the said axial line of contact 7 is much greater, due to the relatively large radius of curvature R₁₂B of the bottom side of the tilting edge 12. In fact, by a specific selection of this latter radius of curvature R₁₂B the radial inward displacement RID can favourably reach the value of 1 .45 mm that was also reached by the known design of the tilting edge 12.

The graph of figure 5 also indicates the radial inward displacement RID of the said axial line of contact 7 for the second, novel embodiment of the tilting edge 12 according to figure 4B. In this case too such radial inward displacement RI D remains limited when the running radius Rr decreases until it reaches the minimum running radius Rr-min2 of the driven pulley 2, due to the comparatively small radius of curvature R₁₂T of the top side of the tilting edge 12. At this minimum running radius Rr- min2 of the driven pulley 2, the radial inward displacement RID favourably amounts to less than 0.40 mm. Thereafter, i.e. as the running radius Rr is decreased further, the radial inward displacement RI D of the said axial line of contact 7 jumps more or less suddenly to 1 .40 mm, i.e. to the beginning of the said second section 12" of the bottom side of the tilting edge 12. This is because the straight first section 12' of the bottom side of the tilting edge 12 does of course not provide for any mutually tilting of the adjacent transverse segments 5. The second section 12" of the bottom side of the tilting edge 12 then accommodates the remaining radial inward displacement RID of the said axial line of contact 7 in dependency on the said tilting angle a, which remaining displacement RID is limited, due to the relatively small radius of curvature R₁₂B of the second section 12" of the bottom side of the tilting edge 12. In the present specific example, the said remaining displacement RI D amounts to only 0.05 mm. Thus, by a specific selection of the length of the straight, first section 12' of the bottom side of the tilting edge 12, the radial inward displacement RID can favourably reach the value of 1 .45 mm at the minimum running radius Rr-min1 of the driving pulley 1 , which value was also reached by the known design of the tilting edge 12.

In summary, a continuously variable transmission is provided with two variable pulleys 1 , 2, each pulley 1 , 2 defining a range of possible running radii Rr for a drive belt 3 of the transmission that rotationally connects these pulleys 1 , 2, wherein the smallest possible running radius Rr-min1 at one pulley 1 is smaller than a corresponding smallest possible running radius Rr-min2 at the other one pulley 2 and whereof the drive belt 3 includes transverse segments 5 that can tilt relative to one another while remaining in mutual contact through a convexly curved tilting edge 12 thereof. The radius of convex curvature of such tilting edge R₁₂T in a part thereof that provides the said contact in the range of running radii Rr of the said other one pulley 2 is substantially smaller than the radius of convex curvature of such tilting edge R₁₂B in a part thereof that provides the said contact in a running radius Rr of the drive belt 3 in-between the said smallest possible running radii Rr-min1 , Rr-min2, which latter running radius Rr occurs only at the said one pulley 1 .

Whilst the above has been described with reference to preferred embodiments and best possible modes, it will be understood that these embodiments are by no means to be construed as limiting examples of the continuously variable transmission concerned, because various modifications, features and combinations of features falling within the scope of the appended claims are now within reach of the person skilled in the relevant art.

Claims

1 . Continuously variable transmission with two variable pulleys (1 , 2) that are both provided with a pair of mutually displaceable pulley discs and with a drive belt (3) that is wrapped around such pulleys (1 , 2) in a curved trajectory between the pulley discs of each such pulley (1 , 2) and that is provided with a laminated set (4) of a number of endless bands (6) and a number of transverse segments (5) that are placed on the band set (4) oriented transversely to the circumference direction thereof, with the band set (4) being contained in a recess of the transverse segments (5) such that the transverse segments (5) can slide along the band set (4) in the circumference direction thereof, a part (14) of the transverse segments (5) is located on the radial inner side of the band set (4), a thickness of which part (14) decreases in radial inward direction and a surface of which part (4) is provided with a convexly curved section, i.e. tilting edge (12) that extends in transverse direction over the width of the transverse segments (5) and that allows neighboring transverse segments (5) located in a curved trajectory part of the drive belt (3) to tilt relative to one another, whereby a radial position (RID) of a line of contact (7) on the tilting edge (12) between these neighboring transverse segments (5) is determined by the amount of curvature, i.e. the radius of curvature (Rr) of the said curved trajectory of the drive belt (3) between the pulley discs of a respective pulley (1 , 2) of the transmission, in which transmission this latter radius of curvature (Rr) can assume a smaller value (Rr-min1 ) at a first one pulley (1 ) than such minimum value (Rr-min2) at the respective other one pulley (2) of the two pulleys (1 , 2), whereby a bottom side of the tilting edge (12), as defined by a range of radial positions (RID) of the said line of contact (7), is used for the said relative tilting of the neighboring transverse segments (5) only at said first one pulley (1 ), whereas a top side of the tilting edge (12) is used for that purpose at both pulleys (1 , 2), characterized in that the bottom side of the tilting edge (12) includes a section that either extends in radial direction in an essentially straight line, or that is convexly curved with a radius of curvature (Ri_2B) that, at least on average, is considerably larger than a radius of curvature (Ri_2T) of the top side of the tilting edge (12).

2. The transmission according to claim 1 , characterized in that the radius of curvature (Ri_2B) of the bottom side of the tilting edge (12) is, at least on average, at least a factor of 10 larger than the radius of curvature (Ri_2T) of the top side of the tilting edge (12).

3. The transmission according to the claim 1 or 2, characterized in that the radius of curvature (R₁₂; R12B, R12T) of the tilting edge (12) increases stepwise at the transition between the top side and the bottom side thereof.

4. The transmission according to the claim 3, characterized in that the radius of curvature (R₁₂; R12B, R12T) of the tilting edge (12) increases by at least a factor of 10 at the transition between the top side and the bottom side thereof.

5. The transmission according to the claim 1 , characterized in that the bottom side of the tilting edge (12) extends in a straight line in the said section and is further provided with a second section wherein the tilting edge (12) is convexly curved.

6. The transmission according to the claim 5, characterized in that a radius of curvature (Ri_2B) of the tilting edge (12) in the said second section of the bottom side thereof is essentially equal to the radius of curvature (Ri_2T) of the top side of the tilting edge (12).