Cross-Axis Flexural Pivots in Mechatronic Applications: Stress-Based Design for Combined Tension and Bending

Cross-axis flexural pivots (x-pivots) hold immense promise as precise, frictionless bearing elements in mechatronic systems. In real-world settings, where bearings are called upon to bear nontrivial loads orthogonal to the axis of rotation, kinematic and stiffness-based design approaches are insufficient to ensure longevity. Stress-based design, which is the norm in conventional rolling- or sliding-contact bearing selection, allows for direct calculation of expected fatigue lifetime, as well as performance in acute overload scenarios. However, the principles that guide stress-based design for flexural bearings are distinct from those that govern contact bearings, and have not yet been clearly described. In this article, we present three physical principles that came to light as we applied stress-oriented finite element analysis to design x-pivots for large angular deformation and heavy tensile loads. Specifically, we describe cross-blade anticlastic effects, loading scenarios that can lead to buckling when the mechanism is in tension, and nonlinear stress effects that emerge in combined tension and bending. These principles have an outsized impact on the mechanism's stress profile, and are not well represented in existing x-pivot models. We also discuss ways to leverage gross mechanism geometry and blade profile to mitigate or avoid these effects. We expect that this work will help facilitate the design of x-pivots for applications in real-world mechatronic systems.