"clifto" wrote in message
news
Roger Hamlett wrote:
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.

I could be missing something, but at first glance I read the radius as
the diameter of the 10mm ball (10mm) plus half the diameter of the 3mm
shaft (1.5mm) and get 23mm internal diameter, ratio of 23:3 or 7.66:1.

Just in case this turns out to be a corecktion I've included speling
erors to comply with tradishun.
Yes. Sorry, I just added up the internal diameter wrong....

It is fairly simple to visualise what is happening, if you realise that
the balls are rolling round the internal diameter of the casing, driven by
the small shaft at the centre, and for the assembly of three balls to
complete one complete turn, their surfaces, must travel the internal
diameter of the outer casing. Since the surfaces of the balls are driven
by the internal shaft, you get the ratio from the ratio of these two
diameters.

Best Wishes