| 摘要 |
PROBLEM TO BE SOLVED: To reduce vibrations and noise of a planetary gear unit while suppressing increase of manufacturing cost and change of gear ratio, strength and the like. SOLUTION: The planetary gear unit 7 has a plurality of planet gears 11, 13, 15, 17 arranged around a sun gear 9 at a central portion thereof, and an internal gear 19 arranged outside the planet gears 11, 13, 15, 17. Any of the number of teeth of the sun gear 9 and the internal gear 19 is a multiple of the number of the planet gears 11, 13, 15, 17, that is, four in this case. The planet gear 11 among these four planet gears 11, 13, 15, 17 has a different tooth flank shape from those of other planet gears 13, 15, 17. Concretely speaking, three planet gears 13, 15, 17 are helical gears having the helical angle from 10 deg. to 30 deg., while the planet gear 11 is modified so as to have a helical angle larger than that of the planet gears 13, 15, 17.
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