The stiffness characteristics of flexure strips in the constrained directions are an important attribute of their behavior when serving as a constituent of flexure mechanisms. The decrease in support stiffness that accompanies movement in the intended degrees of freedom limits the performance of mechanisms comprised of such strips. This paper presents a closed-form nonlinear model that describes the support stiffness in 3-D under arbitrary end-load for the elementary flexure strip. The formulation takes into account geometrical nonlinearities by means of finite strain relations and deformed-configuration equilibrium equations. By distinguishing the low-stiffness large-deflection motion (the degrees of freedom) from the high-stiffness small-deflection motion (the constrained motion) with the appropriate simplification of limited twist, a closed-form stiffness model is obtained for dimension and load ranges of practical interest.

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