A power transmission assembly includes a base, a rotating frame rotatably coupled to the base and configured to rotate about an axis, an output shaft coupled to the rotating frame, a weight selectively repositionable relative to the rotating frame, and a weight actuator configured to reposition the weight relative to the rotating frame to move the weight from a subtraction position located at a first height to an addition position located at a second height. The second height is greater than the first height such that a gravitational force on the weight drives the rotating frame to rotate about the axis and drive the output shaft.