How big of a sheave do I need?

[prrriiide]

I have a motor spec'd at 1750 rpm with a ¾" drive arbor. How big of a sheave do I need to attach to my grain mill shaft to drop the rpm's down to the 200 - 300 range

 

[Billy-Klubb]

what kind of grain mill do you have? you might flocc it up with a belt drive. direct drive with a reducing gear box might be the way to go.

 

[day_trippr]

Pretty cool belt drive reduction calculator here.
It says to get 1750 rpm down to ~250 rpm one combination would be 2" and 14" sheaves...

Cheers!