A motor assembly for use in a surgical tool, has a motor and a gear train. The gear train has an output shaft, a fixed ring gear, a movable ring gear, a cover, and a multi-stage planetary gear set. Each gear stage has a sun gear, a carrier, and planet gears mounted on one side of the carrier. The planet gears are in mesh with a sun gear and either the fixed ring gear or the movable ring gear. The movable ring gear has inner teeth formed on an inner peripheral surface and one of the carriers has outer teeth formed on an outer peripheral surface. The movable ring gear is movable between a first position where the movable ring gear is engaged with and non-rotatable relative to the cover and is disengaged from said one carrier, giving the gear train a higher gear reduction ratio, and a second position where the movable ring gear is disengaged from the cover and is engaged with and rotatable with said one carrier and the planet gears adjacent to the other side of said one carrier giving the gear train a lower gear reduction ratio.