Shaft couplings

A variety of mechanisms are provided for coupling the rotation of shafts or other mechanical elements having parallel but offset axes of rotation. The coupling mechanisms can couple such input and output elements via pins that are in contact with the input and output elements. The pins, upon rotation of the input and output elements, can engage in substantially pure roiling motion with respect to respective contact surfaces on the input and output members. Accordingly, the pins can reduce the number of bearings and increase efficiency relative to alternative coupling mechanisms. Such couplings may be employed in a variety of applications, including the coupling of rotations between elements of a cycloidal drive or a transmission that includes a member that undergoes cycloidal motion. For example, this coupling could replace the cage gear of a cycloidal drive in coupling the output to the cycloidal disc of a cycloidal drive.