Patent ID: 7651436

Claim:
A planetary gear device comprising a sun gear having a sun spur gear, an internal gear having an internal spur gear, and a planetary gear having a planetary spur gear meshed between the spur gears of the sun gear and internal gear, wherein the sun gear coaxially has a sun screw-shaped gear rotating integrally with the sun spur gear, the internal gear coaxially has a female screw-shaped gear rotating integrally with the internal spur gear, the planetary gear coaxially has a planetary screw-shaped gear rotating integrally with the planetary spur gear, the sun screw-shaped gear and the planetary screw-shaped gear are formed in screw shapes being in reverse directions to each other, equal in lead angle, and equal in screw pitch and are meshed with each other, the female screw-shaped gear of the internal gear and the planetary screw-shaped gear are formed in screw shapes equal in lead angle and equal in screw pitch to each other and are meshed with each other, the planetary spur gear is meshed with the sun spur gear and is also meshed with the internal spur gear, a ratio of effective screw diameters of the meshing sun screw-shaped gear and planetary screw-shaped gear is coincident with a ratio of screw thread numbers, and a ratio of effective screw diameters of the meshing female-screw-shaped gear and planetary screw-shaped gear is coincident with a ratio of screw thread numbers, and wherein the planetary gears are supported about axes at equiangular intervals around the sun gear, and the number of the planetary gears is equal to a sum of a thread number of the sun screw-shaped gear of the sun gear and a thread number of the female screw-shaped gear of the internal gear or a divisor of the sum of these threads numbers.