The invention relates to a continuously variable transmission for transmitting torque from an engine to wheels of a vehicle, and to a method therefor.

BACKGROUND

A continuously variable transmission for vehicles is well known.

Such a continuously variable transmission typically comprises a primary shaft with a primary pulley, a secondary shaft with a secondary pulley, wherein the primary shaft and the secondary shaft are connected with each other by a flexible member between the primary pulley and the secondary pulley.

Thus, the continuously variable transmission shows in driving mode a torque path from the primary shaft and primary pulley, via a flexible member, to the secondary pulley and secondary shaft.

Known continuously variable transmissions are adapted to vary the speed ratio of the pulleys continuously in order to reach an optimum torque output at the wheels of the vehicle by varying an effective diameter position of the flexible member. The diameter position of the flexible member is varied by actuating the primary pulley and/or the secondary pulley.

Thereto, the primary and secondary pulleys may each be provided with a movable and a non-movable sheave to clamp the flexible member inbetween both sheaves. The movable sheave can be actuated to move towards or away in respect to the non-movable sheave along the

longitudinal direction of the respective rotating shaft. The flexible member usually is a belt or a chain. The secondary shaft is provided with a gear adapted to transfer the torque further from the secondary shaft to the differential and further to the wheels of the vehicle.

During operation, high forces and bending moments are exerted on the pulleys, shafts, flexible member etc. of the transmission as high torques are to be transferred from the engine to the wheels of the vehicle. In particular, the secondary shaft is subject to relatively high bending moments. These bending moments typically result from axial forces exerted by the gear and clamping forces exerted by the primary pulley and the secondary pulley on the flexible member. These forces and moments during operation provide for a rather complex deforming behavior of the

transmission. For example, in forward drive the bending moments on the secondary shaft are higher than the bending moments on the secondary shaft in reverse drive. This is not beneficial as transmissions normally operate more often and/or longer in forward drive than in reverse drive. This may reduce the lifetime of the secondary shaft and/or may impact design parameters of the secondary shaft, e.g. with respect to thickness, material etc. A drawback of the prior art transmission may be e.g. high costs and/or heavy construction, which may result in reduction of vehicle efficiency and/or increase in maintenance of the transmission. Also, relatively high bending moments may result in relatively high shaft deflection. This may lead to reduced belt contact between the belt and the pulleys and/or to reduced bearing lifetime and/or to increased gear noise and/or to decreased gear lifetime and/or possibly increased fuel

consumption. On the other hand, a sufficiently stiff shaft resisting the relatively high bending moments may result in a more heavy construction which may be disadvantageous in terms of costs and fuel consumption.

There is thus a need for an improved transmission system. SUMMARY OF THE INVENTION

An object of the invention is to provide a continuously variable transmission that overcomes at least one of above described disadvantages.

Thereto the invention provides for a continuously variable transmission comprising a primary shaft comprising a primary pulley, a secondary shaft comprising a secondary pulley, wherein the primary pulley and the secondary pulley are connectable with each other via a flexible member to transmit torque from the primary pulley to the secondary pulley, wherein the secondary shaft comprises a secondary gear for further coupling towards an output shaft of the transmission, characterized in that the secondary gear is provided with a helical winding having an angle smaller than 90 degrees with respect to the secondary shaft when viewed in rotational direction of the secondary shaft in forward drive operation of the transmission.

By providing the secondary gear with a helical winding with an angle smaller than 90 degrees with respect to the secondary shaft when viewed in rotational direction of the secondary shaft in forward drive operation, the secondary gear has a so-called 'reversed helical winding'. Typically, prior art secondary gears have their helical winding in the opposite direction. By providing this reversed helical winding, the axial force during forward drive operation may mainly be oriented away from the secondary pulley, as such partially compensating the bending moment caused by the pulley-belt operation.

The secondary gear having the reversed helical winding may reduce the bending moment and/or shaft deflection in forward drive of the secondary shaft which may lead to better fuel efficiency. The belt-pulley contact and/or gear contact may be improved. The secondary shaft can be of a lighter construction, which can be advantageous in terms of weight and/or efficiency. Further advantages may be a reduction of operational costs, and/or reduced maintenance require and/or, increased bearing lifetime and/or decreased transmission noise and/or increased gear lifetime and/or decreased fuel consumption.

Advantageously, the helical winding has an angle between 20 and 35 degrees with respect to the secondary shaft when viewed in rotational direction of the secondary shaft in forward drive operation of the

transmission. This helical configuration may provide for an optimal load path, thereby minimizing deflecting forces and/or moments. The inventors found that an angle between 20 and 35 degrees provided optimal results in terms of reducing the bending moment on the secondary shaft while optimizing the load path or torque transmission from the primary pulley to the secondary pulley.

By positioning the secondary gear positioned at a side of the secondary pulley having the movable sheave, an optimal axial force path on the secondary shaft may be obtained. Thus, deflection and/or misalignment of bearings and/or gears may be reduced and/or belt contact in forward drive may be improved which may result in improved torque transmission.

The secondary pulley typically comprises a movable sheave and a non-movable sheave, wherein the movable sheave is displaceable with respect to the non-movable sheave along an axial direction of the shaft. By adjusting the distance between the sheaves, the clamping force on the belt may be adjusted and thus the torque transmission can be varied. The transmission is typically arranged in a transmission housing.

Advantageously, a locking plate is provided to connect the secondary shaft to the transmission housing, wherein the locking plate is arranged at a side of the secondary pulley having the non-movable sheave and is arranged between the non-movable pulley and an end bearing of the secondary shaft, wherein the locking plate is fixedly mounted to the transmission. By providing the locking plate, the forces from the secondary shaft can be transmitted to the housing, while sparing the end bearing of the secondary shaft. This may keep the bearing more in position in forward drive and may contribute to the lifetime of the bearing. The locking plate can be dimensioned as to transmit the relatively high forces and/or bending moments in forward drive from the secondary shaft to the transmission housing.

In a typical embodiment, the primary and/or secondary shaft are provided with at least two bearings, there is at least one bearing provided per end of the shaft. The bearings are advantageously positioned on a shaft end, but can also be positioned elsewhere, depending on requirements, space available and/or loads. The bearings are preferably, but not limited to roller and/or ball bearings. Bearings can be positioned between the pulley and the transmission housing and/or between the secondary gear and the secondary pulley and/or the secondary gear and the transmission housing.

Advantageously, the secondary shaft is provided with a roller bearing between the secondary gear and the transmission housing. A ball bearing can be provided on the opposite side of the secondary shaft between the pulley and the transmission housing, wherein the locking plate is provided between the pulley and the transmission housing to keep the ball bearing in position in forward drive and to transmit forces to the

transmission housing.

In a further aspect, the invention relates to a method to transfer a torque from an engine via a continuous variable transmission to wheels of a vehicle wherein a primary shaft with a primary pulley transfers engine torque via a flexible member to the secondary shaft with a secondary pulley thereon wherein a force exerting element in forward drive exerts an axial force on the secondary shaft away from a secondary pulley such that the bending moment on the secondary shaft is reduced.

By providing a force exerting element exerting an axial force, the bending moment on the secondary shaft may partially be compensated and/or reduced. This may result in reduced shaft deflection in forward drive which may lead to better fuel efficiency. Also, belt-pulley contact may improve and/or contact between the secondary gear and a further gear may improve. The secondary shaft may be of a lighter construction, maintenance may be reduced, the bearing lifetime may be increased, the transmission noise may be reduced, the gear lifetime may be increased and/or the fuel consumption may be reduced, which may result in lower operational and/or manufacturing costs.

The force exerting element may be a mechanical, hydraulic, electrical or pneumatic arrangement, e.g. a mechanical gear, or a hydraulic cylinder, a gear reduction, switchable gear reduction, electric system or hydraulic system etc.

Advantageously, the force exerting element is a gear with a helical winding having an angle between 0 and 90 degrees, preferably between 20 and 35 degrees with respect to the secondary shaft when viewed in rotational direction of the secondary shaft in forward drive operation of the transmission. Such a helical configuration, a so-called reversed helical configuration, may result in an optimal load path with minimal losses in the torque transmission e.g. due to shaft deflection.

In an embodiment, a further switchable gear can be provided that can be configured to exerting axial forces directed away from the secondary pulley towards the secondary gear both in forward and in reverse driving mode. In an embodiment, the further gear can be embodied as the

switchable gear, in another embodiment, the further gear can be engaged to the switchable gear in reverse driving mode and disengaged to the switchable gear in forward driving mode. The advantage of this

configuration is that the axial force on the secondary shaft is optimal in forward as well in reverse drive.

Further advantageous embodiments are represented in the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS

The invention will further be elucidated on the basis of exemplary embodiments which are represented in a drawing. The exemplary drawing is given by way of non-limitative illustration of embodiments of the invention.

In the drawing shows

Figure la a schematical section view according to a first

embodiment of the invention in forward drive;

Figure lb a schematical section view according to the first embodiment of the invention in reverse drive;

Figure 2 a schematical section view of a second embodiment of the invention;

Figure 3 a schematical section view of the secondary pulley with the secondary shaft according the first embodiment of the invention;

Figure 4 a schematical three-dimensional view of the first embodiment the invention.

It is noted that the figures are only schematic representations of embodiments of the invention that are given by way of non-limiting example. In the figures, the same or corresponding parts are designated with the same reference numerals.

A schematic section view of a continuously variable transmission 1, according to a first embodiment of the present invention, in forward drive operation, is shown in figure la. The continuously variable transmission 1 comprises a primary shaft 2 comprising a primary pulley 3 and a secondary shaft 4 comprising a secondary pulley 5. The primary pulley 3 and

secondary pulley 5 each comprises two sheaves 14, 16. At least one sheave of the secondary pulley 3 and least one sheave of the secondary pulley 5 is movable in axial direction of the respective primary shaft or secondary shaft, with respect to the other sheave on the primary pulley 3 and on the secondary pulley 5 respectively, wherein the actuation of the sheaves can be achieved hydraulically, mechanically or electrically or by other means. The primary pulley 3 and the secondary pulley 5 are connectable with each other via a flexible member 6, such as a belt or chain, to transmit torque from the primary pulley 3 to the secondary pulley 5. The diameter position of the flexible member 6 is varied by actuating the sheaves towards or away from each other in axial direction A, of their respective shafts 2, 4, in order to reach an optimum torque output at the wheels 13.

The secondary shaft 4 comprises a force exerting element 7, such as but not limited to a mechanical gear, a hydraulic cylinder, a gear reduction, a switchable gear reduction, an electric system or an hydraulic system etc, for further coupling towards an output shaft 8 of the transmission 1. The force exerting element 7 is in this embodiment positioned at the side of a movable sheave 16 of the secondary pulley 5, wherein the force exerting element 7 is embodied as a mechanical gear, a so-called secondary gear 7. The secondary gear 7 is provided with a helical winding 9 having an angle a smaller than 90 degrees and preferably between 20 and 35 degrees with respect to the secondary shaft 4 when viewed in rotational direction Vd of the secondary shaft 4 in forward drive operation of the transmission 1.

The secondary gear 7 in the first embodiment has a so-called 'reversed helical winding'. Typically, prior art secondary gears have their helical winding 9 in the opposite direction. By providing this reversed helical winding 9, the axial force Fforward during forward drive operation may mainly be oriented away from the secondary pulley 5, thus partially compensating the bending moment caused by the pulley-belt operation.

The exerted force Fforward of the secondary gear 7 in forward drive partially compensates the bending moments and shaft deflection on the secondary shaft as result of pulley clamping forces Fpl and Fp2 and belt forces Fb.

In reverse driving mode and forward driving mode the force exerting element 7 transfers a torque from the secondary shaft 4 to the output shaft 8. A further coupling 17 transfers torque from the output shaft 8 to the final drive 10, the final drive 10 transfers the torque to the differential 11 and the differential lltransfers the torque to the wheel shafts 12 and eventually to the wheels 13. The further coupling 17 is coupled to the gear 7 and is here embodied as a further gear. The further coupling 17 can also be embodied differently, such as but not limited to a mechanical gear, a hydraulic cylinder, a gear reduction, switchable gear reduction, electric system or hydraulic system. The final drive 10 is here embodied as cooperating gears, but various embodiments are possible and well known. One of the cooperating gears is on the same shaft of the further gear 7 and is coupled to another one of the cooperating gears of the final drive which then couples to the differential.

A schematic section view of a continuously variable transmission 1, according to the first embodiment of the present invention, in reverse drive operation, is shown in figure lb. The axial force exerted by the secondary gear 7 may now mainly be oriented away from the secondary pulley 5. This may result in higher bending moments and shaft deflection on the

secondary shaft 4. However, a continuous variable transmission 1 according to the present invention arranged in a vehicle, such as but not limited to a car, truck, motorcycle, trike, three-wheeled tilting vehicle, operates most of the time in forward drive. Reverse driving operation is normally used during short time intervals, e.g. for reverse parking manoeuvres, also reverse driving operations typically may be performed during limited time when compared to forward drive operations. Also, the torque to be transmitted in reverse drive may be less with respect to torque transmission in forward drive. Shaft deflection and bending moments in reverse drive operation may have thus a limited impact on the continuous variable transmission 1 as result of clamping forces Fpl, Fp2 and belt forces Fbl, Fb2 in combination with the exerted force Freverse of the secondary gear 7. A configuration wherein the force exerting element 7 as well in forward and reverse drive operation exerts forces Fforward, Freverse away from the secondary pulley 5, may result in a more complex construction and/or additional components, such as e.g. with a switchable gear.

In another embodiment, the further coupling 17 can be embodied as a switchable gear (this embodiment is not shown), such as but not limited to an electric motor, two or more mechanical engagable/disengable gears or as a planetary gear system.. The switchable gear can for example be engaged in reverse drive mode and disengaged in forward drive mode, or vice versa. By providing a switchable gear, it may be possible that the force exerted by switchable gear may be both in reverse drive and in forward drive optimally oriented as to minimize or at least reduce bending moments on the secondary shaft.

The switchable gear may be integrated in the final drive 17, arranged on the output shaft 8 between the further coupling 17 and final drive 10 or arranged between the further coupling 17 and the transmission housing 19. The switchable gear may further be present on an additional shaft (not shown).

Operational modes may be optimized in forward drive operation as well in reverse drive operation. The force exerting element 7 exerts an axial force Fforward in forward drive operation and Freverse in reverse drive operation on the secondary shaft 4, wherein the axial force Fforward, Freverse both are directed away from the secondary pulley 5 such that a bending moment on the secondary shaft 4 is at least partially compensated.

When the switchable gear is embodied as two or more gears on the output shaft 8 or on an additional shaft, the switchable gear may be engaged or disengaged to the force exerting element 7, further coupling 17 or final drive 10. This configuration may have at least two operational modes, such as but not limited to forward drive and reverse drive operation. Additional operational modes may be provided such as but not limited to a park mode, neutral mode or one or more extra gear ratios. The force exerting element 7, further coupling 17 or final drive 10 may be embodied as two or more mechanical gears, whereby each gear of the force exerting element 7 may cooperate with one gear of the switchable gear.

Synchronizer(s) may be provided to actuated the gears in a specific operational mode.

The switchable gear may also be embodied as a planetary gear comprising at least three rotational members. A first rotational member may cooperate with the force exerting element 7 or further coupling 17, a second rotational member may cooperate with the output shaft 8 or the additional shaft and a third rotational member may cooperate with another reverse gear and/or electric motor/generator, a brake, fixed world or transmission housing 19. This configuration may provide at least two operational modes, such as but not limited to forward drive and reverse drive operation. Other operational modes may be provided, such as but not limited to a neutral mode, a park mode or an extra gear ratio. A clutch may be provided between the third rotational member and the brake or electric motor or fixed world. A clutch may be further provided between the second rotational member and the wheels 13.

The switchable gear may alternatively be embodied as an electric motor/generator. This configuration may provide at least two operational modes, such as but not limited to forward drive and reverse drive mode. Other operational modes may be park mode, neutral mode, an extra electric gear ratio(s) or energy storage/release mode. The electric motor/generator may be provided on the output shaft (8) or additional shaft and may be engageable to the force exerting element 7, further coupling 17 or final drive 10. One or more clutches may be provided between the secondary shaft 4 and the electric motor/generator or between the electric motor/generator and the wheels (13). A battery may be coupled to the motor/generator which may enable to store braking energy in a battery and to release electric energy when required. A schematic section view of a continuously variable transmission 1, according to second embodiment of the present invention, is shown in figure 2. The force exerting element 7 is here positioned at the side of a non- movable sheave 14 of the secondary pulley 5. In figure 2, the force exerting element 7 is a gear provided with a helical winding 9 having an angle a smaller than 90 degrees and preferably between 20 and 35 degrees with respect to the secondary shaft 4 when viewed in rotational direction Vd of the secondary shaft 4 in forward drive operation of the transmission 1.

Figure 3 shows a schematical section view of the first embodiment according to the invention. The secondary pulley 5 is mounted on the secondary shaft 4 which is mounted to the transmission housing 19. The secondary pulley 5 comprises a movable sheave 16 and a non-movable sheave 14, wherein the movable sheave 16 is displaceable with respect to the non-movable 14 sheave in axial direction A of the secondary shaft 4. By adjusting the distance between the sheaves 14 and 16, the diameter position of the flexible member may be adjusted and thus the torque transmission may be varied.

The force exerting element 7 is provided on the side of the movable sheave 16, wherein the force exerting element 7 comprises here a

mechanical gear, a so-called secondary gear, which is provided with a helical winding 9 having an angle a smaller than 90 degrees and preferably between 20 and 35 degrees with respect to the secondary shaft 4 when viewed in rotational direction Vd of the secondary shaft 4 in forward drive operation of the transmission 1.

A locking plate 21 is provided to connect the secondary shaft 4 to the transmission housing 19, wherein the locking plate 21 is arranged between the secondary pulley 5 and the transmission housing 19 and arranged between the non-movable sheave 14 and an end bearing 18 of the secondary shaft 4. The locking plate 21 is fixedly mounted to the

transmission. The locking plate 21 transfers axial forces of the secondary shaft 4 in forward and reverse drive operation to the transmission housing 19. Especially when axial forces directed away from the secondary pulley, the locking plate 21 keeps the bearing 18 in position and prevents peek forces on the transmission housing 19. The transmission housing may be of two or more housing parts and locking plate 19 may keep different parts in position. The locking plate may be of different materials, such as but not limited to metal, plastic, composites or other reinforced materials. In this embodiment, the locking plate 21 also keeps the bearing 18 in position with respect to the transmission housing.

The primary and/or secondary shaft 2, 4, are provided with two bearings 18 and 20, wherein at least one bearing is provided per end of the shaft 2, 4. The bearings can also be positioned elsewhere, depending on requirements, space available and/or loads. A roller bearing 20 is in figure 3 positioned between the force exerting element 7 and the transmission housing 19 and a ball bearing 18 is provided on the opposite side of the secondary shaft 4 between the secondary pulley 5 and the transmission housing 19. The locking plate 21 is here provided between the secondary pulley 5 and the transmission housing 19 to keep the ball bearing 18 in position in forward drive operation and to transmit forces to the

transmission housing 19.

Figure 4 shows a schematical three dimensional view of an embodiment of the present invention, comprising a continuously variable transmission 1, with a primary shaft 2, comprising a primary pulley 3 and a secondary shaft 4, comprising a secondary pulley 5. The primary pulley 3 and the secondary pulley 5 are connectable with each other via a flexible member 6 to transmit torque from the primary pulley 3 to the secondary pulley 5. The secondary shaft 4 comprises a force exerting element 7, such as but not limited to a mechanical gear, a hydraulic cylinder, a gear reduction, switchable gear reduction, electric system or hydraulic system, for further coupling towards an output shaft 8 of the transmission. The force exerting element 7 in figure 4 is embodied as a so-called secondary gear 7 and provided with a helical winding having an angle a smaller than 90 degrees and preferably between 20 and 35 degrees with respect to the secondary shaft 4 when viewed in rotational direction of the secondary shaft 4 in forward drive operation of the transmission 1. In forward drive the axial force Fforward, exerted by the secondary gear 7 on the secondary shaft 4 is directed away from the secondary pulley 5 such that a bending moment on the secondary shaft 4 is reduced. In reverse drive operation the axial force Freverse, exerted by the secondary gear 7 on the secondary shaft 4 is directed towards the secondary pulley 5. For the purpose of clarity and a concise description features are described herein as part of the same or separate embodiments, however, it will be appreciated that the scope of the invention may include embodiments having combinations of all or some of the features described. It may be understood that the embodiments shown have the same or similar components, apart from where they are described as being different.

Many variants will be apparent to the person skilled in the art. All variants are understood to be comprised within the scope of the invention defined in the following claims.