Reverse-twisting of helicoidal shells to obtain neutrally stable linkage mechanisms

Mechanisms that consist of many elements and are potentially small sized, benefit from kinematic elementary units like revolute joints that are compliant and monolithic, and therefore could be produced without need for assembly. We present a novel concept of a compliant revolute joint that features low axis drift, high support stiffness and a large range of motion. The concept is based on a helicoidal shell of which a portion reverses its twist direction upon application of a rotation. The reversed region increases gradually, resulting in a constant reaction moment. Analytical, numerical, and experimental analyses are presented to reveal and quantify the constant-moment behaviour. Prototypes of the concept are employed in exemplary linkages to demonstrate the ability to create a large variety of neutrally stable compliant linkages, which require extremely low actuation forces and exhibit large ranges of motion.