3" drive pulley and 6.5" main pulley at 1050RPM
That means:
6.5"/3" = 2.17:1
1050RPM /2.17 = 484RPM
1.5" Rollers = 1.5" X Pi = 4.71" per Rev.
4.71" x 484RPM = 2,281" per minute speed
2,281" / 60secs = 38" per second
38" speed / 12inchs = 3.17 FEET PER SECOND.
Let's just say that's....... a bit high.
Let's try and get it closer to 2.3-2.4 fps ok?
So we want to try and make the main shaft run at 360RPM
about 25% slower than before.
So you have three options,
1. Upgrade main pulley
2. Upgrade motor pulley
3. Upgrade both pulleys
1. If changing only the main pulley, you will need a 9" pulley on the mill. This with your 3" drive pulley gives you a reduction of right at 3:1 so 1050RPM / 3 = 350RPM.
2. If changing the drive pulley, you will need a 2.25" drive (motor) pulley to run your 6.5" main pulley at 2.88:1. So 1050RPM / 2.88 = 364.5RPM
3. Now, if you want to start from scratch, all you need to do is pick two pullesy that have a ratio as close to 2.91:1 as possible. Yes this can be daunting, but don't let a few points either way discract you, as you see above we wavered by a RPM or 15, this is no big deal. Unless you machine your own pulleys to excact dementions you will never get the 2.91:1 ratio. 3-1 is close enough.
Admittedly that motor is probably a little shy for what we need here, but even if you put a new motor on it, the upgrades we are talking now will sure beef the thing up. Now that said, if you are thinking of a future motor upgrade, remember that it will likely be a different RPM then this one. So we would need to change ratios again. Just keep that in mind. But never dump gain in without the motor running. I promise that one does not have the torque to start a hopper full of grain. (at correct adjustment) If you don't mind running the grain twice, then leave your gap alone. It would drive me nuts, but in crazy. It will however, if you leave it where it is, help with the motor torque issue.
I think that's all I have for now, you may find that once you do the upgrades you still might not have the second roller spinning all the time. I found this in my research prototypes, but a bump with a stick would always get it going. My new mill no longer needs this though.
Hope this helps, I'm always here if you need more answers.