CONTINUOUSLY VARIABLE TRANSMISSION

The present invention relates to a continuously variable transmission incorporating a push-type drive belt wrapped around two adjustable pulleys of the transmission, each defining a tapered groove of variable width between its pulley discs, as defined in the preamble of claim 1 , as well as to a method for operating such transmission.

Such a transmission is generally known, e.g. from European patent application EP-A-1 427 953 in the name of Applicant. It is a general development aim to optimise the design of the known transmission in terms of the throughput of driving power per unit of mass and/or volume of the transmission for a given range of transmission ratios, i.e. the ratio coverage of the transmission. However, specifically in terms of the desired increase in power throughput, the performance of a transmission can in practice be limited, because of torque fluctuations that may occur in the transmission. These torque fluctuations can in certain operational conditions exceed a certain critical, or at least accepted level in terms of transmission durability, noise or even drive line vibrations as a result thereof.

The present invention aims to reduce such torque fluctuations in relation to the transmission power throughput, in order to improve the durability of the transmission and/or to allow a further increase in power throughput. Moreover, the inventions aims to accomplish this goal without a significant redesign of, in particular, the transmission drive belt component, i.e. to realise the same in a cost effective manner that, preferably, can be implemented quickly even in existing transmission designs.

In accordance with the invention, this goal is realised by applying the measures as provided by the characterising portion of claim 1 and involving guiding means for guiding the drive belt, in particular the transverse elements thereof. By these guiding means a lateral position, movement and/or rotation of the transverse elements is/are stabilised before entering the pulley groove of in particular the driving pulley. It was found that, with the presence of such guiding means, the said torque fluctuations were reduced significantly and even the efficiency of the power transmission could even be improved slightly.

Preferably the guiding means at least position the elements in axial direction, so as to axially align the transverse elements with the pulley groove of the driving pulley. Typically during operation of the transmission, the individual transverse elements arrive at the driving pulley that is axially offset relative to the pulley groove. This

offset is, however, automatically corrected when the transverse elements enter, i.e. are clamped between the pulley discs and was therefore previously not considered a problem as long as the said offset is within an acceptable limit, as for example described in EP-A-0291 129. Still, based on the measurement results shown hereinafter, it was unexpectedly found in accordance with the present invention that by pre-positioning the transverse elements in axial direction to coincide with a pulley groove, i.e. just before they are taken-up between the pulley discs, the above-mentioned torque fluctuations show a significant reduction. Apparently, the transverse elements are than more smoothly taken up between the pulley discs, which was found to be of particular importance at a high torque level and rotational speed and at the driving pulley.

At such high torque levels the pulley discs are strongly forced together to effect a friction with the belt that is sufficient for transferring such torque, but as a result whereof the pulley discs deform elastically in such a manner that the pulley groove closes to a certain extend at the location where the transverse elements are entering the groove. The elements are thus required to simultaneous align and deform to fit the pulley groove and as a result thereof experience a considerable and apparently fluctuating resistance at entering the groove. It is considered that such resistance is substantially reduced when the transverse elements are axially aligned with the pulley groove before entering it.

The invention will now by way of a physical embodiment be elucidated further along a drawing in which: figure 1 is a schematic representation of a continuously variable transmission including a drive belt to which the present invention relates; figure 2 indicates a section of the belt shown in perspective; figure 3 is a graph of measurement results of a transmission input rotational speed and input and output torque as a function of time; figure 4 is a schematic representation of an embodiment of a guiding means according to the invention shown in perspective; figure 5 illustrates the principle working of the guiding means according to the invention; and in which figure 6 is a graph similar to figure 3 of further measurement results.

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 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 push-type drive belt 3 is wrapped around the pulleys 1 , 2 for transmitting a rotational movement and an accompanying torque 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.

The push-type drive belt 3 comprises an endless tensile means 31 and a multitude of plate-like transverse elements 33 that are provided on the tensile means 31 movable along its longitudinal direction and oriented predominantly transversely thereto. The elements 33 take-up the 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 the elements 33 to be thrust from the driving pulley 1 to the so-called driven pulley 2, exerting an output torque Tout thereon. Hereby, the transverse elements 33 are being guided and supported by the tensile means 31. Figure 2, in a perspective view, shows a section of the belt 3 indicating a two part tensile means 31 and a number of transverse elements 33, which belt 3 is described in more detail in, for example, the said European patent publication EP-A-O 626 526. In this type of belt 3, the transverse elements define two predominantly transversely extending recesses 37 that open up in an axially direction towards the outside of the belt 3 and that are each associated with a bearing surface 19 of the element 33 for accommodating the tensile means 31 with some axial play there between.

The transverse elements 33 are provided with lateral side or pulley contact faces 35 that during operation are clamped between and arrive into frictional contact with the discs 4, 5 of the driving and driven pulleys 1 , 2 respectively. For allowing a mutual rocking of the elements 33 about a generally axially oriented line of contact between two adjacent elements 33 in a bent trajectory part of the belt 3, they are provided with a so-called rocking edge 34 on their respective front principle faces 38, which edge 34 defines an axially oriented, sharp or slightly rounded edge between two sections of the front principle face 38 that are mutually oriented at a small but

notional angle. The elements 33 are further provided with a projection 39 protruding from the front principle face 38 thereof, for interaction with a complementary hole provided in a back principle face thereof, and the elements 33. Figure 2 also shows that a top part 40 of the elements 33 tapers towards the radial outside of the belt 3. In the belt 3, adjacent transverse elements 33 are laterally moveable relative to one another, as allowed by a play being provided both between the projection 39 and hole of the respective elements 33. This feature being present, inter alia, for the drive belt 3 to be able to accommodate an axial off-set between the pulley grooves of the respective pulleys 1 , 2. Also, the adjacent transverse elements 33 can rotate, at least about their projection 39 around a virtual axis oriented in the belt's longitudinal direction.

When in operation in the transmission and at a comparatively high nominal input torque Tin considerable torque and rotational speed fluctuations are found to appear, as is illustrated in figure 3. Figure 3 provides test bench measurement results of the rotational speed ω2 of the driven pulley 1 , the transmission input torque Tin and the transmission output torque Tout as a function of time t, during which measurement the driving pulley rotational speed ω1 is maintained essentially constant, i.e. controlled at a steady rpm.

Also in the figure 3, a substantial reduction of the said torque and rotational speed fluctuations can be observed in the time range starting just after 6 seconds and ending just after 8 seconds. This range exactly corresponds with the time range wherein guiding means according to the present invention are activated, which means stabilise the transverse elements 33, by imposing a pre-defined position thereon, thus removing any deviating movement or rotation thereof. In this embodiment the guiding means additionally axially align the elements 33 relative to the pulley groove of the driving pulley 1.

Further in figure 3 it can be observed that in the time range of activation of the guiding means also the driven pulley rotational speed α>2 actually increases slightly on average, such that during that time the transmission favourably operates with a higher efficiency. An example of a practical embodiment of the guiding means according to the invention is illustrated in figure 4 in a perspective side elevation.

Figure 4 shows a possible embodiment of the guiding means 50 according to the invention in a perspective view. The means comprising a roller 51 showing an predominantly V-shaped circumferential groove 52. The roller 51 is rotatably mounted in a sub-frame 53, which is moveable relative to a mounting plate 54 of the

guiding means 50 by means of an air cylinder 55, while being guided along a centre pin 56 of the guiding means 50. The mounting plate 54 is be bolted to, for instance, a transmission housing, whereby the roller 51 s located at the radial outside of the belt 3 and whereby the V-shaped circumferential groove 52 of the roller 51 is axially aligned with the V-shaped pulley groove of a pulley 1 , 2 in at least one speed transmission ratio of the transmission, preferably the driving pulley 1 and preferably at least the speed ratio wherein a maximum (engine) power is transmitted by the transmission, the so-called TOP-ratio

In this embodiment, the guiding means 50 are activated only in the said TOP ratio, whereby the air cylinder 55 is pressurised so that the roller 51 extents towards and engages the drive belt 3, whereby the tapered top part 40 of the transverse elements 33 is taken up in the circumferential groove 51. The pressure in the air cylinder 55 determines the force that is exerted by the roller 51 onto the belt 3 in an essentially radial inward direction. A transverse element 33a that is entering the V- shaped circumferential groove 52 of the roller 51 and that is axially displaced relative thereto will be positioned by an interaction of its top part 40 with the roller 51 , such that an element 33b that is leaving this groove 52 is axially aligned therewith, as is illustrated in figure 5. Thus, by providing the V-shaped circumferential groove 52 of the roller 51 in line with the V-shaped groove of a respective pulley 1 , 2, the transverse element 33 is pre-aligned therewith in axial direction. According to the present invention, such measure favourably reduces the resistance experienced by the elements 33 at entering between the discs 4, 5 of the respective pulley 1 , 2, as a result whereof the above-mentioned torque fluctuations show a significant reduction.

It is remarked that the roller 51 may be actively driven, for example in order to minimise power losses due to friction between the roller 51 and the transverse elements 33. Preferably, the roller 51 is thereto rotationally connected to the respective pulley 1 , 2. Further, the guiding means 50 can be arranged in the transmission axially displaceable, whereby the axial position of the (V-shaped circumferential groove 52 of the) roller 51 is preferably linked to that of the axially moveable pulley disc 4 of the respective pulley 1 , 2. This feature allows the guiding means 50 to be active in any speed ratio of the transmission, i.e. to follow the axial displacement of the respective pulley groove.

Further, according to the invention, the force with which the roller 51 engages the belt 3, i.e. the pressure in the air cylinder 55 is controllable, at least in relation to its rotational speed. According to the invention the said forces should be controlled to

increase as the rotational speed of the belt 3 increases and vice versa. This particular feature of the present invention is based on experimental observations. On the one hand it has been observed that, if the said force is too low in relation to the belt speed, the said torque fluctuations persists or at least can be dampened further by increasing the said force. One the other hand it has also been observed that, if the said force is too high in relation to the belt speed, the driven pulley rotational speed co2 is decreased such that the transmission operates at a disadvantageously lowered efficiency, even though the said torque fluctuations remain effectively dampened.

This latter experimental observation is illustrated in figure 6, along a graph similar to that of figure 3. In this particular experiment the guiding means 50 were activated after about 9 seconds until about 13 seconds, however, whereby the force exerted by the roller 51 of the guiding means 50 in radially inward direction on the drive belt 3 was approximately tripled relative to an optimum level thereof determined in an earlier experiment. Preferably, according to the invention the said force is also controlled in proportion with the torque to be transmitted by the transmission, i.e. the input torque Tin.

The invention apart from the preceding description and all the details from the pertaining figures as may readily and unambiguously be derived by a person skilled in the art, further relates to all principals and details as provided by the definitions within the following set of claims.