Abstract: The invention is directed to a process for establishing a coupling with adjustable reduction ratio between a first driving rotation motion (r1) around a first fixed axis (10) and a receiving translation motion. The invention is also directed to a coupling device (1) that activates the receiving translation motion (f2) by mechanically coupling it with a second rotation motion (r2), called receiving motion, around a fixed axis (21) and establishing an adjustable mechanical correlation between the said driving (r1) and receiving (r2) rotation motions.The invention refers also to apparatus for the regulation of the amplitude of a parameter, and more particularly to valves (30) for the regulation of the flow rate of a fluid, whose nominal flow rate factor is steplessly adjustable, and which uses the abovedefined process and coupling device.