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Calculate the circumference of the driving pully and the driven pulley. The ratio of these two measurements gives you the reduction factor for the speed.Jo
No need to calculate the circumference ratio. Use the diameter ratio; the missing factor of pi cancels out.
The idea with an additional Countershaft is you only put the belts round it when you want to use the slow speeds, otherwise you leave it out . Don't forget one additional belt will reverse the direction of the headstock (hence the term "counter"shaft as it counter rotates ). Jo