A planetary gear unit inside a case includes coaxial sun and ring gears, a planetary pinion gear that meshes with the sun and ring gears, and a carrier that holds the planetary pinion gear, the ring gear being fixed with respect to the case, and a flange disposed inward of the ring gear. The moisture content is at most 25% by mass, and the viscosity at a shear velocity of 10 s\u22121 is at most 2000 mPa\xb7s. For example, the lock-off mechanism may include a lower jaw mounted to the base, and an upper jaw movable relative to the lower jaw between an open position and a closed position.