Gorgulu, IbrahimcanDede, Mehmet Ismet CanKiper, Gokhan2025-04-252025-04-2520250094-114X1873-3999https://doi.org/10.1016/j.mechmachtheory.2025.106015https://hdl.handle.net/11147/15550This paper introduces a novel flexible joint design that enhances mechanical versatility. The design shows how to obtain bidirectional deflections from a unidirectional spring. It enables the parallel and serial connections of springs. It features multiple stiffness regions determined by applied load levels, eliminating the need for a clutch mechanism. Mechanical limits can be added to customize stiffness, offering more hardening or softening regions. The design also allows for connecting multiple flexible joint units in series. In a case study, the series flexible joint design is developed with two flexible joint units, providing a softening stiffness characteristic. The joint has rigid, stiff, and soft regions. When used as a series elastic actuator in a robot manipulator, the rigid region aims to conduct the tasks at low torque levels, i.e., trajectory tracking. The stiff region is preserved for collaborative tasks in human-robot interaction, while the soft region enhances the robot's safety in case of control failure or collision. Static and dynamic test results comply with the ideal model. This flexible joint design improves mechanical performance, safety, and adaptability.eninfo:eu-repo/semantics/closedAccessFlexible Joint DesignBidirectional Torsion DeflectionsMultiple Stiffness RegionsParallel/Serial ConnectionSoftening Stiffness CharacteristicArticle2-s2.0-10500199587710.1016/j.mechmachtheory.2025.106015