The Rotary-Prismatic-Rotary (RPR, working title) is a novel, extremely torque dense transmission type that uses a two-stage rotary to prismatic conversion. It is limited by ROM, but this is not a problem for many applications in prosthetics and humanoids.
Humanoids, quadrupeds and powered prosthetics mostly use strain wave generators, planetary gearboxes, or cycloidal gearboxes. These are all effective transmissions that suffer from various drawbacks. Strain wave generators have poor starting backdrive torque and highly non-linear friction and cannot be easily made in ratios below 50:1. Cycloids have rolling friction and are very sensitive to tolerances. Planetary gearboxes are not as torque dense as other options, and for reductions larger than 6:1, a two-stage planetary is generally needed.
We propose a new type of gearbox that addresses some limitations of other designs. The RPR actuator uses a ball-screw or roller screw first stage, which prismatically actuates a large-diameter, high helix angle threaded shaft, which is converted back to rotation at the output with an internally threaded tube.
