Abstract
Belt type CVTs consist of two pairs of discs on two shafts, one disc on each shaft is movable in axial direction. By moving these discs, the belt or chain change to a different radius on the discs and the transmission ratio, input-and output-torque increase or decrease according to the new radii.
In most cases, the clamping force which is needed to transmit torque, derives from a hydraulic system. Due to the fact that they are not ideally rigid bodies, the pulleys, the shafts and of cause the belt or chain will assume a state of elastic deformation until a balance of force is given. The clearance in the guidance of the movable disc leads to a tilting movement of the disc. Due to these deformations, the belt or chain will run in a spiral path along the arc of contact with sliding motion between the discs and the belt. The described sliding motion causes losses and war in the transmission. By calculating the deflection of the parts as well as the sliding motion between disc and belt, the efficiency of the variator in steady state motion can be predicted.
Most of the time the CVT does not run at a constant ratio. Taking the additional decrease in efficiency when the ratio is changed into consideration is therefore of great importance. The losses induced due to ratio change are experimentally measured on a very precise variator test rig. The influence of the speed of ration change on the additional power losses in comparison to steady state motion is shown.
Keywords: CVT, losses, ratio change, efficiency, V-belt-theory