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2D fully compliant mechanisms can typically only have rockers, and not cranks, as a crank would destroy the flexural element, twisting it until it is broken. This proves a major challenge with almost all conventional mechanisms. One way of addressing this could be to use a material that only bends in one direction to allow the foot to drag in one direction and grip in the other, to use a redirecting element to convert linear motion to circular (note that this does introduce a bearing element, however it would be low speed and possibly self clearing), or to implement a crank by having a compliant linkage that is into the plane (essentially spins around like a noodle).
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