CVT EVOLUTION

We describe now a known CVT, whose working principle will be now briefly remembered. In the figure below an inclined plane P is shown in contact with a roller R, which rests on an horizontal plane PO. The inclined plane P and the plane PO form an angle (called containment angle) designed such that the vincular reactions of the static and dynamic friction between the planes P and PO and the roller R always maintain the roller R in contact with said planes without slipping or departing.

-----------------------

By exerting an orthogonal force F on the inclined plane P, the roller R is forced to roll towards the apex of plane P and to push said plane PO with a force F2. Called m the friction coefficient between the plane PO and the roller R, it must hold F2<=m*F for the non-slipping condition of the roller R. Called S1 the lowering of the plane P and S2 the lateral displacement of the plane PO, considering the system as being lossless, from an energy balance we find S1*F=S2*F2, from which S2=S1/m. These data are approximate for designing purposes of the system, and, if the conditions are met, mechanical power from plane P to plane PO can be efficiently transferred. EP 1 688 645 is the application of the abovementioned concept, only repeated n times for n rollers R along a circumference.

----------

There are two coaxial rotors R1, R2, one inside the other, between which some rollers R are interposed. The inner rotor R1 is provided with an superficial inclined-plane toothing which implements the ramp of the plane P in the figure for every roller R. Altogether the rotors R1, R2 and the rollers R constitute a free wheel. By offsetting the rotors R1, R2 (i.e. by increasing their reciprocal eccentricity), the toothing of the rotor R1 in its 360 degrees of rotation cyclically nears to, and depart from, the rotor R2, thereby compressing and releasing the rollers R, which in the phase of compression (about 180°) transfer, time after time (passing the baton in the thrust action), rotary motion from the rotor R1 to the rotor R2 with an increased rpm, because the transferred motion is that of the rotor R1 summed to that of the roller R.

Notwithstanding the optimal efficiency of this transmission, improvements may be brought in.

There are indeed two conflicting needs: increasing the TR of a stage and assuring a fluid torque transfer between rotors R1, R2. The TR between the rotors R1, R2 is higher the greater the length of the inclined planes provided on the rotor R1 (namely, the longer the travel of the roller R on them). Also, the total torque transferred to the rotor R2 is the "averaged" sum of the impulsive contributions of each roller R when it is in the phase of thrust/grip.

Thus, while it would be desired to lower the number of inclined planes on rotor R1 in order to increase the length thereof, such number should be increased to have a steady torque transmission. Actually, the compromise is six inclined planes. Another aspect is the cyclic variation of the contact angle between a roller R with the relative inclined plane on the rotor R1 and the same roller R with the rotor R2. Since the containment angle varies dynamically, the transmission is to be designed such that it always meets the numerical relations described above, i.e. in the worst case. The lesser is the amplitude of the containment angle, the bigger are the compression stresses on the roller R, which determine dilatations and expansions and, therefore, a rise in losses and a drop in efficiency.

The need to modify the transmission to achieve a better efficiency is the reason for the quest of new topologies.

The problem is solved by a CVT comprising rolling members with peculiar shape: with a contact profile having a variable distance from their rotation axis.

This way the containment angle can be kept substantially constant or at a desired value for the whole duration of the rolling in the thrust phase (called active phase) of each rolling member. Let us consider the following figure which illustrates the working principle.

--------

Instead of the assembly [inclined plane + roller] used in the prior art, these two elements are combined in a rolling member or a cam C, whose profile has a progressively longer distance from its rotation centre CR, here constituted by a pin PR arranged on a support P. In practice, the ramp of the inclined plane P has been transferred directly on the rolling member, thereby transforming the roller R in the cam C. The profile of the cam C is in contact at point CT with a horizontal plane PO, and the profile of the cam C is such to assure that the contact angle (defined as the angle between the plane PO and the tangent to the same profile of the cam C at the contact point CT) be (preferably) substantially constant.

By applying an orthogonal force F to the support P, and hence to the pin PR and to the cam C, the cam C nears to the plane PO and is obliged to roll thereupon in the verse indicated by the arrow. The non-slipping conditions are the same as those set forth for the first figure, and also the kinematic result is the same: the cam C while rotating pushes with a force F2 the plane PO, which consequently moves laterally. But this time the containment angle does not vary ( or varies little), thereby eliminating the drawbacks described above.

The new CVTs are a generalization of the concept applied circularly to rotating members (rotors), see following and side figure.

------------------------------

This way the optimal friction between cams and rotors is guaranteed. It might be useful to carry out cams which offer, during their rolling, a contact angle with a slightly increasing or decreasing pattern/trend. This to obtain a desired distribution of the thrust pressures on the ( inner and/or outer) rotors. A cunning of this type has the advantage of designing the specific pressures that every part exerts, thereby increasing the efficiency thereof.