A pivoting power transmission unit is provided for transmitting rotational drive from a power source to a rotor configured to revolve about a first axis and configured to swivel about a second axis perpendicular thereto. The transmission unit comprises: input pinion driven in rotation by the power source;...http://www.google.com.au/patents/US6676073?utm_source=gb-gplus-sharePatent US6676073 - Pivoting power transmission unit with peripheral face toothed gearwheels of the face gear type
