The ratio, is determined by the shaft diameter, to the internal diameter
of the casing in which the balls sit.
If (for instance), you have a 3mm shaft, and 10mm balls, the casing has to
have an internal diameter of 26mm, and the ratio is 26/3 = 8.66:1. The
drive can be made remarkably good (the same system is used on some micro
focussers for telescopes, which carry significantly more torque than
needed for a pot). Units like the 'Williams optics feathertouch focusser',
use exactly this drive.
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Sorry ,but I don't understand the above .
If the shaft has a diameter of 3mm and the balls a diameter of 10mm, the
inner diameter of the casing should be 10 + 10 + 3 equals 23 mm

When the shaft makes 1 revolution the balls will make 3/10 revolution .
When the balls make 1 revolution the casing makes 10/23 revolution .
Hence when the shaft makes 1 revolution the casing will make
3/10 * 10/23 equals 3/23 equals 0.13 revolution , hence reduction factor
is 7.7
Please correct me if I made an error.

Frank GM0CSZ / KN6WH