Single-loop mechanisms can realize various kinds of motion and have wide applications. Closed chains are often put in the open-chain mechanisms as partial structure to improve the dynamic performance and increase the structure stiffness, which are widely used in engineering practice. A spatial single-loop RPSC mechanism which can realize vertical Darboux motion is introduced and its constraint and motion properties are analyzed based on screw theory. A special class of actuation singularity can be found by ...