We propose output shafts with a preload generation mechanism to improve the output torque and thrust force of the rotary-linear ultrasonic motor. The stator is comprised of a single metallic cube with a throughhole, and the output shafts are inserted into the hole to generate motion in both its circumferential and axial directions arbitrarily. In this paper, two design concepts for optimizing the preload using the output shafts are examined. The first involves a cylinder shaft with micron-order accuracy diameter realization. The cylinder shaft makes contact with the entire inner surface of the stator and generates a preload between the stator and shaft. The second concept employs a spring shaft having a slightly larger diameter than the stator hole, which expands in the radial direction and generates the preload. Experiments show that these design concepts improve the output torque and thrust force.
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