If you had a 3" shaft welded to a 9" dia wheel. what is the rpm's if the 3" shaft is at 15rpm?
If the shaft is perpendicular to the wheel (like an axle), then the 9" wheel will also be rotating at 15 rpm. If you were to mark a point on the shaft, then mark the same point on the wheel, they would stay in the same relative positions to each other. The speed of the point on the wheel would be going faster than the point on the shaft, but the point on the wheel also has to travel farther, making the time for one rotation equal for both. Make sense?