A Comparison Criterion for the Flexure Shape in Constant–Torque Compliant Joints

Constant-torque compliant joints are able to maintain a quasi-constant torque over a defined range of rotations, while minimizing backlash, friction, and wear. However, comparing different design solutions remains a challenging task, due to variety of materials, dimensions, and fabrication technologies. In this paper, a comparison criterion based on the maximum stress value that occurs in the flexure during the deflections is presented. The maximum stress criterion provides a standardized framework for evaluating the performance of different joints, in terms of maximum output torque, regardless of their material properties, footprint, and flexural rigidity. The criterion highlights the shapes that generate the lowest stress values and that can be exploited to produce the highest torque outputs. The criterion is elucidated considering different flexure designs and used to compare the compliant joints presented in Literature.