CVT AUTOMOTIVE

We have seen in another chapter how by applying an orthogonal force F to a support P carrying a cam C, the latter gets nearer the underlying plane PO and is forced to roll thereupon with a precise verse.

We have seen also how good it is that the contact angle between the cam and the plane being pushed does not vary (or varies little), thereby eliminating a lot of drawbacks. Girotto's CVTs are a generalization of the described concept circularly applied to rolling elements (rotors), so as to guarantee a cyclicity of the working phases of the rolling element, see figure below.

Preferably the cam C is produced to have a profile such as to keep constant its contact angle with the plane PO, or more generally with a rotating part to which mechanical power and rotary motion is to be transferred. This way the optimal friction between cams and rotors is guaranteed, the friction being determined by the pressure on the same cam C and by the mutual friction coefficient.

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The constancy of the contact angle is however only a preferred embodiment. 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.

The Figure above show an application scheme of the concept just set out, repeated for five cams CD along a circumference to constitute a CVT according to Girotto's concept. There is a central cylindrical rotor (or shaft or core) POc1 and an annular outer rotor (or disc) Pc2, arranged one inside the other. The rotor Pc rotatably supports the five cams CD (the cams CD are pivoted on the rotor Pc by rolling bearings), all with their profile in contact with the rotor POc. It should be noted that the overall structure is that of a free wheel. When the outer rotor POc rotates, it drags with it the inner rotor Pc because of the binding/sticking of all the cams CD. Here the transmission ratio (TR) between the two rotors R1, R2 amounts to 1. By offsetting the rotor POc with respect of the rotor Pc (i.e. increasing the mutual eccentricity, as in the figure), the distance between the center of rotation CR of a cam CD and the rotor POc varies cyclically during the 360° of rotation of the rotor Pc. Every cam CD nears and departs from the rotor POc, with alternate motion. The equivalent effect of cyclically nearing and lifting the cam CD with respect with the plane PO is obtained ( see chapter CVT EVOLUTION and below): during the phase of nearing the cam CD pushes the plane PO, while during that of lifting the cam CD slips on it, without imparting force.

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Therefore, cyclically each of the cams CD of the CVT in turn pushes the rotor POc, rotating about it for roughly 180° of "satellite" rotation and at the same time rotating on itself, again for roughly 180° (non-slipping condition). The two rotary motions combined vectorially determine a TR > 1. One can observe that as a cam CD rolls without slipping on the rotor POc pushing it, the mutual point of contact CT progressively travels the profile of the cam CD, but the contact angle at the point CT remains constant (with the exceptions seen before).

We shown here on the side an animation for the CVT and below an exploded view of a possible CVT embodiment

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Hereunder instead we see a cross-section of a CVT transmission with parallel input and output shafts.

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