Experimental and computational characterization of additively manufactured rotational compliant joints: a stacking strategy for adjustment of their equivalent stiffness

Compliant joints could be advantageous when used in mechanisms as they transmit motion due the deformation of flexure hinges while avoiding multiple mechanical components. However, compliant joints have limitations due to their reduced rotational displacement. In order to expand the range of applications for compliant joints, this work presents a stacking strategy for rotational stiffness reduction while increasing their rotational displacement range. The strategy assembles multiple iterations of compliant joints to achieve the displacement increment. This is studied in three different designs of compliant joints, i.e., multi-leaf, DR3 and semicircular compliant joints. The characterization was done in three experimental trials. The first one calculates the rotational stiffness of single joints with both computational and laboratory experiments. The second one stacks up to four iterations of each joint and analyzes their stiffness reduction. The third one uses finite element models to characterize the effect of weight on up to seven iterations stacks. Our results show that the strategy was effective for rotational stiffness reduction and increasing the rotational displacement. The equivalent stiffness of the mechanism was calculated as the stiffness of a single joint divided by the number of iterations with an average error of 4.35%. The limitations for the application of our strategy were vertical space and gravity. Because multiple iterations would require a larger workspace and accumulated weight of the iterations can displace the compliant joints in z-axis and alter its rotational stiffness up to 4.78%