12", how'd you figure that out so easy?! I was heading toward a 10" or so with the info another post suggested in stating twice as big as crank pulley. I have a habit over the years of jumping to fast to get these projects finished an onto the next, perhaps I better ask all the correct questions up front for a change!
Thanks you for you help and direction.
The formula for reducing / increasing speed (simple, no load factors) is based on the circumference, or distance around a pulley. If you have a circumference (distance) of 10", and you want to go slower, say twice as slow, you'd double the circumference and need 20" around the outside. Then for every turn of the drive pulley the belt would move 10", the driven pulley would go halfway around.
We don't use circumference, we measure diameter. To convert diameter to circumference we use pi. Pi is the ratio of the circumference to the radius of a circle. To go halfway around a circle it takes 3.1416 radians, or the radius x pi. As we want the entire distance the formula is 2r (pi) or 2 x radius x pi, or d x pi. I use the diameter (d) which is 2r, since we already know that. The thing is, it all works out to a constant. So you don't need pi, or rather because it is a ratio it all works out. I was just including the bs.
You figured it right, to halve the speed, double the pully width. I just used a different output speed. You intuitively knew how to get what you wanted.
To figure the speed difference you divide the input speed by the desired output speed. I chose 75, as the range while the motor is idling can vary. Best to be mid-range to start. If engine is fast, you can handle 33% over and still be within spec. For an un-governed motor, usual minimum is 20% over speed allowance, so it is cool.
So, 200 rpm ÷ 75 rpm = 2.67 2.67 x 4.5 = 12.015, or a 12" pulley.
