WO2018121884A1

WO2018121884A1 - A drive belt for a continuously variable transmission with transverse segments and a ring stack

Info

Publication number: WO2018121884A1
Application number: PCT/EP2017/025371
Authority: WO
Inventors: Cornelis Johannes Van Der Meer
Original assignee: Robert Bosch Gmbh
Priority date: 2016-12-27
Filing date: 2017-12-27
Publication date: 2018-07-05
Also published as: CN110114591B; CN110114591A; NL1042198B1; KR20190104553A; JP2020503484A; JP7009502B2

Abstract

The present invention concerns a drive belt (6) for a belt-and-pulley-type continuously variable transmission comprising a row of transverse segments (10) mounted on a stack (9) of several, mutually nested rings. The transverse segments (10) are provided with a projection (40) that protrudes from a front surface (11) thereof and with a corresponding hole (41) that is provided in a back surface (12) thereof. An offset is provided between the projection (40) and the hole (41) in the radial direction (R) of the drive belt (6), such that in the row of transverse segments (10) in the drive belt (6) these will be tilted forward by the forced insertion of the projection (40) into the hole (41).

Description

A DRIVE BELT FOR A CONTINUOUSLY VARIABLE TRANSMISSION WITH TRANSVERSE SEGMENTS AND A RING STACK

This disclosure relates to a drive belt for a continuously variable transmission with two pulleys and the drive belt. Such a drive belt is known from the international patent application publication WO2015/063132-A1 and comprises a row of transverse segments mounted on a stack of several, mutually nested continuous bands, i.e. flat and thin rings, each. The transverse segments define a slot for accommodating and confining a respective circumference section of the ring stack, while allowing the transverse segments to move along the circumference of the ring stacks. This particular type of drive belt is also referred to as a push-type drive belt or pushbelt.

In the following description the axial, radial and circumference directions are defined relative to the drive belt when placed in a circular posture. Furthermore, a thickness dimension of the transverse segments is defined in the circumference direction of the drive belt, a height dimension of the transverse segment is defined in the said radial direction and a width dimension of the transverse segment is defined in the said axial direction.

The known transverse segments each comprise a base portion, a middle portion and a top portion. The middle portion of the transverse segments extends in radial direction interconnecting the said base and top portions thereof. On either side of the middle portion the transverse segments define a slot between the base portion and the top portion thereof for accommodating a respective ring stack of the drive belt. At each slot, a radially outward facing bottom surface thereof contacts and supports the ring stack in radial outward direction. These bottom surfaces of the slots that are associated with the base portion of the transverse segments are denoted carrying surfaces hereinafter.

In the row of transverse segments of the drive belt, at least a part of a front main body surface of the transverse segment abuts against at least a part of the back main body surface of a respectively preceding transverse segment in the said row, whereas at least a part of the back main body surface of the transverse segment abuts against at least a part of the front main body surface of a respectively succeeding transverse segment. At least one of these front and back main body surfaces of the transverse segment, for example the front main body surface includes an axially extending, convexly curved surface part. This curved surface part divides the front main body surface into a radially outer and a radially inner surface parts that are oriented at an angle relative to one other. Abutting transverse segments in the drive belt are able to tilt relative to one another, while remaining in mutual contact at and through such curved surface part that is therefore denoted tilting edge hereinafter. The tilting edge allows the row of the transverse segments of the drive belt to follow a local curving of the ring stacks imposed by the transmission pulleys.

The transverse segment is further provided with a projection that protrudes from its front main body surface and with a corresponding hole that is provided in its back main body surface. In the row of transverse segments of the drive belt, the projection of the said succeeding transverse segment is at least partially located in the hole of the said preceding transverse segment, such that a mutual displacement of the abutting transverse segments in a plane perpendicular to the circumference direction of the drive belt is prevented or, at least, limited. Typically, the projection and the hole are of a similar overall shape, e.g. predominantly cylindrical or slightly conical.

As mentioned hereinabove, in the drive belt the transverse segments can move relative to the ring stacks along the circumference thereof. This has the advantage that during operation of the drive belt the ring stack is tensioned to a relatively low level in relation to a torque transmitted by the drive belt between the pulleys, at least compared to other types of drive belt. However, on the other hand, such a sliding movement or slip between the transverse segments and the ring stack is known to bring about a small, but notional friction loss. It is known that such sliding movement can be favourably minimised by arranging the tilting edge of the transverse segments as close to the radial inside of the ring stack as possible in the height direction. In theory, in this respect, the tilting edge is preferably arranged to coincide with the carrying surfaces of the transverse segment in question.

According to the present disclosure, however, locating the tilting edge close to the carrying surface brings about a problem or disadvantage that may be understood as follows. The closer the tilting edge is to the carrying surface, the sharper a transition edge there between must be. In turn, a sharper transition edge results in a higher contact stress in the ring stack. In fact, it may even occur that the yield stress of the radially innermost ring of the ring package is exceeded during operation of the drive belt because of the said high contact stress, compromising the service life of that innermost ring.

According to the present disclosure such disadvantage can, surprisingly, be mitigated by including an offset in vertical direction between the projection and the hole of the individual transverse segment, in particular by locating the hole somewhat lower, i.e. more towards the radial inside of the drive belt, than the projection. By this measure, even when otherwise traveling in a straight line between the pulleys, the transverse segments will be tilted forward in the row of the drive belt, because of the forced insertion of the projection into the (lower lying) hole. Hereby, in particular, a contact between the radial inside of the ring stack and the transition edge between the carrying surface and the tilting edge is favourably avoided or, at least, reduced in its intensity .

In hindsight, it can be observed that in the conventional drive belt said measure will not have a considerable effect, because the said transition edge is smoothly rounded at a relatively large radius of curvature, made possible by the relatively large separation between the carrying surfaces and the tilting edge. For example, in the conventional drive belt, the tilting edge is separated from the carrying surfaces by around 1 mm, such that a radius of curvature of the transition edge can be 0.5 mm or more.

In particular, the offset 0 in radial inward direction of the hole relative to the projection of the individual transverse segment in accordance with the present disclosure can be quantified geometrically as follows:

0_min = Rr_min - (Rr_min^A2 - ¾^«D^A2) (1), with 0_min representing a minimal value for the said offset 0, with Rr_min representing a minimum radius of longitudinal curvature of the drive belt, in particular at the transmission pulleys, and with D representing the thickness of the transverse segment.

By the minimum offset 0_min according to equation (1), the transverse segments in the drive belt are tilted forward in the drive belt to such an extent that the transition edge lies to the radial inside of a virtual circle of radius Rr_min and intersecting an edge between the carrying surface and the back main body surface .

For example, for a typical drive belt with an Rr_min value of 30 mm and a D value of 1.6 mm, a minim offset 0_min of 11 micrometre is calculated with equation (1) . A practical design value for the said offset 0 that allows for production spread and other uncertainties some is then 1.5 up to 5 times 0_min or between 15 and 75 micron. Preferably according to the present disclosure, such actually applied offset 0 and thus the said forward tilting of the transverse segment imposed thereby, is limited to five times, more preferably to three times such minimally required value 0_min. Otherwise, the alignment forces between the projection become unnecessarily high and/or the ring stack is forced to contact a back edge of the carrying surface instead.

The above-described novel drive belt will now be explained further with reference to the drawing, in which equal reference signs indicate equal or similar parts and in which:

- figure 1 provides a schematic perspective view of a continuously variable transmission with a drive belt running over two pulleys;

- figure 2 provides a schematic cross section of the known drive belt oriented in the circumference direction thereof;

- figure 3 provides a schematic width-wise oriented view of a transverse segment of the known drive belt;

- figure 4 is an enlargement of a part of the known transverse segment depicted in figure 3;

- figure 5 is an enlargement of a part of a novel transverse segment; and

- figure 6 schematically illustrates a straight trajectory part of the drive belt incorporating the novel transverse segment. Figure 1 schematically shows a continuously variable transmission, such as for utilization in a motor vehicle between the prime mover and the drive wheels thereof. The continuously variable transmission is indicated in general by the reference sign 1. The continuously variable transmission 1 comprises two pulleys 2, 3 and a drive belt 6 that is provided in a closed loop around the pulleys 2, 3. The pulleys 2, 3 are each provided with a pulley shaft 4 and with two pulley sheaves 7, 8, whereof a first pulley sheave 7 is fixed to the pulley shaft 4 of the respective pulley 2, 3 and whereof a second pulley sheave 8 is axially displaceable relative to such pulley shaft 4, while being fixed thereto in rotational direction. During operation of the transmission 1, the drive belt 6 is clamped at a running radius Rr at each pulley 2, 3 by and between the respective pulley sheaves 7, 8 thereof, which running radii Rr can be varied to vary the speed ratio of the transmission by moving the pulley sheaves 7, 8 of the pulleys 2, 3 towards, respectively away from each other.

The drive belt 6 comprises two sets of mutually radially stacked continuous bands or rings, denoted ring stacks 9 hereinafter. Transverse segments 10 of the drive belt 6 are arranged on the ring stacks 9 forming an essentially contiguous row along the entire circumference thereof. For the sake of simplicity, only a part of these transverse segments 10 are shown in figure 1.

The transverse segments 10 are provided movable with respect to the ring stacks 9, at least along the circumference thereof. As a result, a torque can be transmitted between the transmission pulleys 2, 3 by means of friction and by the transverse segments 10 pressing against one another and pushing each other forward along the circumference of the ring stacks 9 in a direction of rotation of the pulleys 2, 3. The transverse segments 10 and the (rings of the) ring stacks 9 of the drive belt 6 are typically made of steel. This particular type of transmission 1 and its principal operation are well-known per se.

In figure 2, an exemplary embodiment of the drive belt 6 is shown in cross section oriented in length or circumference direction C thereof, i.e. perpendicular to the width or axial direction A and the height or radial direction R of the drive belt 6. In figure 3, only the transverse segment 10 of figure 2 is shown in a side elevation in the axial direction A. In figure 2, the ring stacks 9 are shown in cross-section and one transverse segments 10 of the drive belt 6 is shown in a front elevation. The ring stacks 9 are in this case composed of five individual flat, thin and flexible endless rings 5 each, which endless rings 5 are mutually concentrically stacked in the radial direction R to form the respective ring stack 9. In practice, however, these ring stacks 9 often comprise more than five endless rings 5, e.g. nine or twelve or possibly even more.

In figures 2 and 3, the transverse segment 10 are shown to successively comprise in the radial direction R a base portion 13 of predominantly trapezoidal shape, a relatively narrow middle portion 14 and a top portion 15 of predominantly triangular shape. On either side of the middle portion 14 slots 33 are defined between the base portion 13 and the top portion 15, wherein the ring stacks 9 are accommodated. At each slot 33, a radially outward facing carrying surface 42 of the base portion 13 contacts the radial inside of a respective ring stack 9 during operation.

A front surface of the transverse segment 10 is indicated in general by the reference sign 11, whereas a back surface of the transverse segment 10 is indicated in general by the reference sign 12. In the following, the front surface 11 and the back surface 12 are generally indicated as main body surfaces 11, 12. In the drive belt 6, at least a part of the front surface 11 of the transverse segment 10 abuts against at least a part of the back surface 12 of a succeeding transverse segment 10, whereas at least a part of the back surface 12 of the transverse segment 10 abuts against at least a part of the front surface 11 of a preceding transverse segment 10.

The transverse segment 10 takes-up a clamping force exerted between the discs 7, 8 of each pulley 2, 3 via contact faces 37 thereof, one such contact face 37 being provided at each axial side of the transverse segment 10. These contact faces 37 are mutually diverging in radial outward direction such that an acute angle is defined there between that is denoted the belt angle Φ and that closely matches a pulley angle Θ defined between the pulley sheaves 7, 8 of the pulleys 2, 3.

The transverse segment 10 is provided with a projection 40 that protrudes from its front surface 11 and with a corresponding hole 41 that is provided in its back surface 12. In the drive belt 3, the projection 40 of the trailing transverse segment 10 is at least partially located in the hole 41 of the leading transverse segment 10, such that mutual displacement of these adjacent transverse segments 10 in a plane perpendicular to the circumference direction C of the drive belt 3 is prevented or, at least, limited. Typically, a nominal clearance of between 10 and 50 micron is provided between an outer circumference of the projection 40 and an inner circumference of the hole 41, i.e. the projection/hole-clearanee.

At the front surface 11 in the base portion 13 of the transverse segment 10, a rocking edge 18 is defined. The rocking edge 18 is represented by a convexly curved area of the front surface 11, which area separates two sections of the said front surface 11 in the radial direction R, which two sections are oriented at an angle relative to one other. An important function of the rocking edge 18 is to provide the mutual pushing contact between the adjacent transverse segments 10 when these are in a slightly rotated, i.e. tilted position relative to one another at the pulleys 2, 3. In order to favourable realise a minimal contact stress in the said pushing contact, as well as for the stability of such contact, the rocking edge 18 preferably extends along the full local width of the transverse segments 10. The rocking edge 18 is preferably located close to the carrying surfaces 42, i.e. at minimal distance Drc radial inward thereof. However, the smaller such distance Drc is, the sharper a transition edge 50 between the front surface 11 and the carrying surfaces 42 of the transverse segment 10 will be. This latter aspect of the design of the transverse segment 10 is illustrated in figure 4 in an enlargement of the area E of figure 3 indicted by the dotted circle. On the left side of figure 4, a relatively large rocking edge -to-carrying surface distance Drc is illustrated, allowing the transition edge 50 to be provided with a relatively large radius of curvature Rte, at least in comparison with the design of the transverse segment 10 on the right side of figure 4 with a relatively small rocking edge - to-carrying surface distance Drc. In practice and as illustrated in figure 4, the radius of curvature Rte is somewhat smaller than the rocking edge-to-carrying surface distance Drc in order to reliably ensure in mass manufacture that the rocking edge 18 does not overlap with the transition edge 50. In figure 4, the transition edge 50 is depicted as a circular arc of radius Rte . In practice, however, the transition edge 50 may not be so uniformly shaped, in which case its contour is approximated by a (closest fit of a) circular arc of radius Rte, at least within the context of the present disclosure. As such, the transition edge radius Rte between the carrying surfaces 42 and the front surface 11 might seem unimportant. However, this transition edge 50 does in practice arrive in contact with the radial inside of a respective ring stack 9 raising the overall stress level thereof. More in particular in this latter respect, a substantial stress raising effect was found to occur when the radius Rte of the transition edge 50 becomes less than 0.5 mm, in particular less than 0.3 mm.

According to the present disclosure such contact between the transition edge 50 and the ring stack 9 can favourably be avoided, or at least reduced in intensity by providing an offset 0 between the radial position of the projection 40 and the radial position of the hole 41 of the transverse segment 10. By thus lowering the overall stress level of the ring stack 9, the load carrying capacity and/or the longevity of the drive belt 3 can be increased.

This novel design of the transverse segment 10 is schematically illustrated in figure 5 in an enlargement of a part thereof, which part corresponds to area F indicated in figure 3 by the dotted oval in relation to the transverse segment 10. In figure 5 the central axis of the cylindrical projection 40 is indicated by the solid line CA40 and the central axis of the hole 41 is indicated by the dashed line CA41. The said offset 0 thus corresponds to the separation between the central axis CA40 of the cylindrical projection 40 and the central axis CA41 of the hole 41. According to the present disclosure such offset 0 amounts between 15 and 75 micrometre or so for the typical thickness of the transverse segment 10 of between 1.4 and 1.8 mm. So even on the scale of figure 5, the offset 0 has been exaggerated therein.

If the said offset 0 incorporated in the novel transverse segment 10 exceeds the nominal projection/hole-clearance, then the transverse segments 10 are forced to tilt forward relative to the ring stacks 9 by the forced insertion of the projection 40 of a first transverse segment 10 into the (lower lying) hole 41 of an adjacent transverse segment 10 as these are pressed together in the row of the drive belt 3. Hereby, the contact between the radial inside of the respective ring stack 9 and the transition edge 50 of the transverse segments 10 can be favourably avoided in a straight section of the drive belt 3 crossing between the transmission pulleys 2, 3, as is schematically illustrated in figure 6. Furthermore, the transverse segments 10 also enter between the two pulley sheaves 7, 8 in such tilted position relative to the ring stacks 9, whereby a radial position of the transition edge 50 of a respective transverse segment 10, clamped between pulley sheaves 7, 8, is somewhat smaller than a radial positon of an opposite edge of the carrying surfaces 42 on the side of the back surface 12 of the transverse segment 10. Hereby, the contact between the radial inside of the respective ring stack 9 and the transition edge 50 between the carrying surface and the tilting edge is at least favourably reduced in its intensity.

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

1. A transverse segment (10) for a drive belt (6) with a ring stack (9) and with a number of consecutive transverse segments (10) that are movably arranged on the ring stack (9), which transverse segment (10) is provided with an opening (33) for receiving the ring stack (9) of the drive belt (6) , which opening (33) , at a bottom side thereof, is bounded by a carrying surface (42) at an upper side of a base portion (13) of the transverse segment (10), which base portion (13) is also provided with a rocking edge (18) in the form of a convexly curved area of a front surface (11) of the transverse segment (10) extending over the width of the base portion (13) of the transverse segment (1) and with a likewise convexly curved transition edge (50) between the carrying surface (42) and the front surface (11) of the transverse segment (10), which transverse segment (10) is further provided with a projection (40) on the front surface (11) thereof and with a hole (41) in a back surface (12) thereof located opposite the said front surface (11), characterized in that, the projection (40) and the hole (41) are shaped predominantly conical, or at least cylindrical and in that an offset (0) is provided between a central axis (CA40) of the projection (40) and a central axis (CA41) of the hole (41) .

2. The transverse segment (10) according to claim 1, characterized in that, a radius of curvature (Rte) of the curved transition edge (50) is smaller than 0.5 mm, in particular is smaller than 0.3 mm.

3. The transverse segment (10) according to claim 1 or 2, characterised in that, the rocking edge (18) is located at less than 0.9 mm below the carrying surface (42), in particular is located at less than 0.7 mm below it, more in particular is located at less than 0.6 mm below it.

4. The transverse segment (10) according to claim 1, 2 or 3, characterised in that, the offset (0) amounts to between 15 and 75 micrometer .

5. The transverse segment (10) according to a preceding claim, characterised in that, a clearance is defined between an outer circumference of the projection (40) and an inner circumference of the hole (41), which clearance amounts to between 10 and 50 micrometer.

6. The transverse segment (10) according to a preceding claim, characterised in that, the offset (0) is larger than halve the difference between the diameter of an outer circumference of the projection (40) and the diameter of an inner circumference of the hole (41) .

7. A drive belt (6) provided with a number of transverse segments (10) according to a preceding claim, characterized in that the said offset (0) applied in these transverse segments (10) is larger than a minimal value (0_min) therefor according to the equation::

0_min > Rr_min - (Rr_min^A2 - ¾^«D^A2) with Rr_min representing a minimum radius of longitudinal curvature of the drive belt (6) and with D representing the maximum thickness of the transverse segments (10) as measured between the front surface (11) and the back surface (12) thereof .

8. The drive belt (6) according to claim 7, characterized in that the said offset (0) applied in these transverse segments (10) is smaller than 5 times and preferably smaller than 3 times the said minimal value (0 min) therefor.