Development of Traction Fluid Property Tables for a Toroidal CVT Multi-Body Simulation

A toroidal variator is the core part of an advanced Continuously Variable Transmission (CVT) design. Knowing its behavior and internal forces is key to defining the operational conditions of the transmission. To maintain a steady-state speed ratio, or to accurately and efficiently move between speed ratios, optimal trunnion control force is required. The unique design of the toroidal CVT makes the design very sensitive to trunnion positioning and force transients. Analytical understanding of the mechanism response is critical to toroidal variator controller design. A critical feature of the toroidal CVT simulation is representation of the friction forces in the disk-roller contact. This effect is important to the mechanism torque capacity and efficiency. This article describes a new toroidal CVT multi-body simulation which includes a detailed representation of the disk-roller friction, including contact patch geometry based on Hertzian formulation; subdivision of the contact patch to calculate accurate pressure, slip, and friction force at all locations on the patch; application of table-based traction fluid friction properties at each sub-patch; and summing of the total friction force and torque at the patch. To support the contact patch formulation, a separate virtual test rig simulation was created to reproduce testing used to characterize the traction fluid properties, and to reproduce physical test rig results. In support of this test rig simulation, a spreadsheet-based solution was used to extract localized traction fluid properties based on assumed friction coefficient curve fits. With these techniques, the multi-body simulation was used to perform both steady-state speed ratio and ratio change simulations, to predict system dynamics, system performance, and controllability.