NuVinci Continuously Variable Transmission

The NuVinci Continuously Variable Planetary Transmission is a type of roller-based continuously variable transmission (CVT) manufactured and marketed by the American company Fallbrook Technologies Inc. The design saw its initial market application as bicycle gearing system, first available in December 2006 in the Netherlands and United States. NuVinci CVP technology is also currently under development for other applications, including wind turbines, light electric vehicles, outdoor power equipment, and automotive front-end accessory drives.

Mechanical variators have existed since the 1800s, and they have been used successfully in machinery, particularly the Kopp tilting ball variator. Various attempts have been made to implement them in vehicle transmissions, but commercial success has been elusive.

The NuVinci CVT gear system uses a set of rotating and tilting balls positioned between the input and output discs of a transmission. Tilting the balls changes their contact diameters and varies the speed ratio. As a result, the NuVinci CVT system offers seamless and continuous transition to any ratio within its range. The gear ratio is shifted by tilting the axles of the spheres in a continuous fashion, to provide different contact radii, which in turn drive input and output discs. The system has multiple "planets" to transfer torque through multiple fluid patches. The spheres are placed in a circular array around a central idler (sun) and contact separate input and output traction discs. This configuration allows input and output to be concentric and compact. The result is the ability to sweep the transmission through the entire ratio range smoothly, while in motion, under load, or stopped.

Two factors allow the NuVinci CVT to provide a continuously variable ratio range in a compact package:

The NuVinci CVT system has a small number of parts.

Since any CVT may allow a power plant, human or motorized, to operate at (or closer to) the speed at which its efficiency, output torque or output power is optimal, the NuVinci CVT may improve a system's overall efficiency or performance compared to if a 'conventional' geared transmission were used, but only if such gain in the efficiency or performance of the power plant exceeded any loss in efficiency or performance that may be introduced by replacing the conventional transmission with the NuVinci CVT. For example, if the NuVinci CVT were only 90% as efficient as the conventional transmission it replaced, the gain in operating efficiency from the power plant would have to be 11% (i.e., 1/0.90 - 1 = 0.11 or 11%) before the overall efficiency improved. The company does not provide any numbers relating to efficiency of their transmissions, and has declined to do so when requested.

...
Wikipedia