TRANSIENT MECHANICS OF V-BELT VARIATORS

The steady theory of V-belt mechanics is well established [1, 2, 3]. Owing to the side compression of the pulley walls, the belt penetrates into the groove a depth variable along the arc of contact and is subject to radial components of the friction forces. Nevertheless, the operative conditions of mechanical variators are seldom steady (see cars and motorcycles), because alternating phases of shift up and down of the speed ratio are continuously requested by the driver. Very few theoretical approaches to this topic are reported in the literature. They generally assume an ideal inextensible-incompressible belt and find out the path of each belt element by simple geometrical analyses. The "rigid belt " hypothesis is certainly acceptable for metal V-belts (e. g. Van Doorne) and rapid ratio changes, but become inappropriate for rubber V-belt or slow shifts. In the rubber belt case, the longitudinal elongation and the radial penetration influence the sliding direction of the single belt elements in conjunction with the overall inside/outside motion due to the opening/closing of the pulley. In the metal belt case, the plate bending is generally more significant than the belt deformation and may affect the mechanical behaviour of the drive during the ratio change for moderate shift rate [4]