Recently, robots with increased wor

Recently, robots with increased workspaces have been developed, and these robots work in the vicinity of (and together with) humans. Such robots are required to have high compliance for its joints, to avoid unexpected collisions [1]. Moreover, to perform tasks cooperatively with humans, a high compliance is essential for the robots. To perform compliance control, backdrivable mechanisms are typically used and the electric current of the motor is controlled. Some studies insist that electrical control lacks reliability. To achieve better human safety, a study developed a velocity- and contact force-based mechanical safety device [2]. Some studies introduced elastic elements between the actuator and the output joint to achieve better compliance [3]. Pneumatic actuators are also used for improving the compliance of the mechanism [4]. Moreover, soft robotics, in which the whole body of a robot is made from soft materials, is widely studied [5].A worm gear has the advantages of a large reduction ratio and being able to hold the output position without energy consumption because of its non-backdrivability [6]. However, mechanisms that use a worm gear do not have compliance, because a worm gear does not have backdrivability. Moreover, because of the non-backdrivability of the worm gear, it requires complicated control methods [7, 8]. If a worm gear that can switch its backdrivability is developed, it can be used for a robot that requires compliance. We have utilized the non-backdrivability of the worm gear to develop some high-speed/ high-torque coexistence devices [9, 10]. If such a backdrivability switchable worm gear were created, we could achieve high compliance on our high-speed/high-torque coexistence devices.