A planetary gear assembly includes a gear housing, a carrier including a carrier flange positioned inside the gear housing, and a carrier shaft extending in an axial direction from the carrier flange toward a front side of the gear housing and coupled to a drum shaft, a sun gear including a front end that is coupled to the carrier while spaced apart from a back end of the drum shaft, and a back end that protrudes toward a back side of the gear housing and is coupled to a rotor frame of a motor, a key bolt including a bolt head coupled to the front end of the sun gear and a bolt shaft extending backward along the axial direction and penetrating the sun gear, pinion gears coupled to the carrier flange and tooth-coupled to the sun gear, and a ring gear tooth-coupled to the pinion gears.