Helical Shift Mechanics of Rubber V-Belt Variators

A very common configuration of motorcycle V-belt variators allows for the correction of the belt tensioning in dependence on the resistant torque by a suitable helical shape of the tracks that enable the closure/opening of the driven half-pulleys. The theoretical model for the belt-pulley coupling is just complex for this arrangement, where one half-pulley may run in advance and the other is slower than the belt, and requires the repeated numerical solution of a strongly non-linear differential system by a sort of shooting technique, until all boundary conditions are fulfilled (angular contact extent, torque, axial force). After solving the full equations, the present study develops closed-form approximations characterized by an excellent fit with the numerical plots and proposes a simple and practical formulary for the axial thrust in dependence on the torque and the tension level. Then, results of a theoretical-experimental comparison are also reported, revealing a very good agreement of the model with the real operation.