NuVinci® Continuously Variable Planetary (CVP)
The NuVinci continuously variable planetary (CVP) is based on a set of rotating, tilting planets fitted between two rings.
Figure 10 illustrates the basic components of the NuVinci CVP technology.
- an input disc, or ring, driven by the power source.
- an output disc, or ring, connected to the CVP output
- a set of balls (planets), each rotating on its own axle and fitted between the input ring, output ring and a central sun to help maintain the planets in position
Torque from an engine, motor or other input source is transmitted from the input ring to the planets through a thin layer of traction fluid. The torque is then transmitted from the planets to the output ring via another thin layer of traction fluid. The input ring and output ring are “clamped” onto the planets tightly so that the requisite amount of clamping force is provided for the amount of torque being transmitted. However, as indicated above, torque is transmitted through the traction fluid, which prevents destructive metal to metal contact between the planets and ring while providing traction for the planets and rings and lubrication for bearings and other components.
The speed of the output ring compared to the speed of the input ring, or speed ratio, is controlled by the angle of the planet axles relative to the axis of the transmission. Tilting the planet axles shifts the transmission from low to high (as shown left to right), or from high to low, or to any ratio in between. The number of planets used depends on several factors including torque and speed requirements, operational requirements and space considerations. This design is scalable from small application such as a bicycle transmission up to the size of a wind turbine or commercial vehicle transmission. The animation in Figure 11, provides a visualization of how this design functions.
Figure 10– NuVinci CVP uses planets to transfer torque between an input ring and output ring. Tilting of the planet axis changes the ratio by varying the contact radii (click image to enlarge).
Figure 11– Animation of NuVinci CVP.
This design is analogous to a planetary gear set. Because this design is a continuously variable planetary drive, power can be input or output on either ring, the sun, or the carrier, just as with a planetary gear train. This allows the CVP to be used to sum power from multiple sources (such as an engine and motor in a hybrid vehicle) or split power to drive multiple devices (such as the drive wheels and a generator for battery charging). This is done through the selection of which elements are driven, fixed, or free. Compared to a toroidal drive, the NuVinci CVP provides the benefit of a simpler geometry, reducing cost of manufacturing, and lower clamp forces.
Fallbrook Technologies is the inventor of the NuVinci CVP technology. The NuVinci CVP is currently commercially available as a bicycle transmission, providing a continuously variable ratio solution that replaces the traditional fixed ratio derailleurs. The NuVinci design is also currently being developed for use as a primary transmission in automobiles (by Dana Incorporated), commercial vehicles (by Allison Transmission) and light utility and recreational vehicles (by TEAM Industries, Inc.).