GO BACK TO POST #46& #47 THE JOHN DEER pully. The center shaft is about 2". The little bracket on the side has a shaft that goes through the center with some 11/2" bearings on each side of the pullys.
You are right about a 1" shaft would be about right for 1/2" belts and about 3" pullys. Then machine in some small bearings say for 5/8" shaft that would go through the 1" shaft with clearance. You would still need a tension spring or spring loaded tensioner......Curt