Abstract: The present invention covers a one-to-one ratio harmonic drive transmission for use between a pressure chamber and a vacuum chamber for power transmission therebetween, while also providing a pressure seal therebetween. The transmission includes a chamber wall dividing the vacuum chamber from a pressure chamber. An annular housing has a first end and a second end with an output shaft through the first end supported therein by a bearing supported arrangement. An input shaft is arranged through the wall and a generally cup-shaped flexspline is sealingly arranged about the input shaft. An outer wave generator and an inner wave generator are arranged with respect to the flexspline, to provide a one-to-one ratio of input to output, with the flexspline acting as a hermetic seal.