British YouTuber Robert Murray-Smith has presented a fully 3D-printed model of a continuously variable transmission (CVT) based on a mechanically variable planetary gearbox. Its design is based on the “Ratio Zero CVT”, patented in 2020, which is intended to enable infinitely variable transmission ratio adjustment exclusively via gears and lever mechanisms. Without friction elements such as belts or cones.
Conventional CVTs usually use friction-based approaches, such as with conical pulleys or belts, which results in an efficiency of around 70 to 75 percent. Gear drives, on the other hand, achieve up to 97 percent. This is exactly where the Ratio Zero approach comes in. The design uses a modified planetary gearbox in which lever arms control the position of the planetary gears and thus continuously adjust the transmission ratio.
The type of lever adjustment is special. Instead of complex articulated joints, Murray-Smith uses gear racks that can be moved via a guide system even when rotating. This means that the system remains variable during operation. A central control unit synchronizes the adjustment of all lever arms. The resulting change in transmission ratio is transmitted directly to the output shaft.
In the demonstration, Murray-Smith showed how the speed ratio can be changed with a simple slider – from almost zero to a ratio of 1:2 or more. The system also functions as a mechanical clutch, which opens up additional application possibilities. The entire mechanism was printed using the FDM process with “print-in-place” technology, which creates moving parts directly in the print.
The YouTuber makes the 3D model data available free of charge on the Thingiverse platform.
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