Diameter (Phi) is fixed automatically during rotation, in the Alpha zone, as the belt (6) pushes the pad (2) on the rail (7, 12), compressing the spring (8).
Said diameter is maintained in the Beta zone because of the adhesion of the belt on the pads : In the overlay zone of two neighboring pads, the belt prevents the said pads to come closer one to another. So, they can't move towards the center as the two displacements are linked by the angle of their rails.
Radial position of the belt is set in the Alpha zone set the effective diameter of the pulley, between Phi1 and Phi2.
This position is set by the wheel (3) at the end of the rod (4).
Not easy to explain in few words though ;)
A complete file is available on request.