A revolute pair with a flexible translational constraint on a plane is proposed as simple mechanism for safe robots. The mechanism is composed of two pairing elements, one with a circular and one with a cam profile that are connected by a linear spring. Flexible translational constraint is generated by spring forces and the reaction force between the two pairing elements. Two methods for designing the cam profile are proposed in order to implement the specified non-linear stiffness in the flexible constraint. Design examples with various stiffness characteristics are shown. Some prototypes are fabricated, and it is confirmed that they perform as designed. As an application, a flexible, underactuated link mechanism with the proposed pairs is synthesized, and its flexibility and kinematic performance are investigated.

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