A gearbox includes two axially spaced epicyclic gear trains, each having a sun gear, axially spaced planet gears carried by a planet carrier, and a ring gear fixed to the gear box. One of the planet gears engages with the fixed ring gear, and another of the planet gears engages with an output ring gear. The sun gears of the two gear trains are mounted on a single shaft. The planet carriers of the two gear trains are mounted on a shaft so that they can freely rotate but have limited axial motion along the shaft. The two output ring gears are helical gears of opposite handedness.