Modelling Slip- and Creepmode Shift Speed Characteristics of a Pushbelt Type Continuously Variable Transmission

Historically slip in a CVT was regarded as destructive. The reason for this was that slip was not controllable and since it is unstable always resulted in damage to the variator. Recent publications suggest that limited amounts of slip in a pushbelt type variator can be allowed [6]. This opens the door to other strategies for lowering the powerconsumption of CVT's. Not only can slip be used for optimizing variator efficiency [1], actuation efficiency can also be greatly improved. If the safety margin is eliminated, the clamping force can be reduced by more than 25%. This can be directly translated into a 25% decrease in actuation power. Shifting behaviour is also influenced by slip [3]. This effect can be used to greatly reduce the power needed for fast shifting during emergency stopping, tip-shifting and kickdown actions. Using this strategy the force needed for shifting is reduced, and with it the power needed from the actuation system is reduced.

This has not only effects on the power consumed by the actuation system of the CVT, which is by itself a significant factor in the variator efficiency, but also has some implications on the design of the CVT. If actuation forces are smaller, the actuation system can be smaller and cheaper, and the CVT itself will be lighter. Furthermore, other actuation systems than hydraulics can be considered, for example electromechanical actuation, to further enhance the controllability and efficiency of the actuation system. In this paper measurements are shown for shifting behavior of the CVT and a relation will be given with slip in the system. The results are used to model the transient behavior of the CVT.