Abstract Compliant constant-force mechanisms (CCFMs) are critical for precision operations, but traditional planar configurations suffer from undesirable parasitic displacements under external disturbances due to inadequate out-of-plane constraints, leading to significant performance degradation. To address this limitation, this paper proposes a spatial compliant constant-force mechanism (SCCFM) synthesized via degree-of-freedom (DOF) analysis integrated with the freedom and constrained topology (FACT) method. Composed of parallel flexible units, the mechanism achieves a single translational DOF and inherently exhibits enhanced constraint stability, effectively suppressing parasitic displacements. A systematic design approach is implemented, including theoretical modeling, parameter sensitivity analysis, and multi-objective structural optimization, to realize ideal constant-force characteristics. Experimental validation of the fabricated prototype demonstrates that the output force remains constant at 8.9 N within a stroke of 1.91 mm. Comparative tests under lateral disturbance demonstrate that the spatial configuration reduces out-of-plane deviations by approximately 80% compared to traditional planar designs, owing to its significantly enhanced out-of-plane stiffness. These results validate the proposed spatial design paradigm as a robust solution for applications requiring high precision and stable constant-force output.
Tang et al. (2026) studied this question.