A motor assembly may comprise: a motor rotor shaft; and a flange attached to the motor rotor shaft, at least a part of an inner surface of the flange contacted with a first part of a circumferential surface of the motor rotor shaft, wherein the motor rotor shaft comprises: a toothed pulley directly machined on a second part of the circumferential surface of the motor rotor shaft to be coupled with a drive belt, the toothed pulley of the motor rotor shaft having a diameter smaller than the first part of the motor rotor shaft contacting the flange; and a step part disposed between the first and second parts of the motor rotor shaft, the step part of the motor rotor shaft having a diameter gradually increasing from the toothed pulley to the first part of the motor rotor shaft contacting the flange.