a housing;

a motor comprising a stator casing, a stator, an output shaft, a rotor, an annular plate, and a rear end cap; the stator casing is fixed in the housing and is restricted from rotating along an output shaft, the stator is embedded into the stator casing; a circumference of the stator has a plurality of through holes; the output shaft passes through a middle of the stator casing; the rotor is engaged with a portion of the output shaft where is surrounded by the stator; the annular plate abuts against an end surface of the stator casing and an end surface of the stator, and the annular plate has a plurality of screw holes; the rear end cap has a plurality of perforations and abuts against another end surface of the stator casing and another end surface of the stator; each of the perforations on the rear end cap is inserted by a screw, and each of the screws passes through one of the through holes and is screwed into corresponding one of the screw holes on the annular plate; the rear end cap is disposed with a rear bearing, and the output shaft passes through the rear bearing; and

a motor fixing frame engaged with the end surface of the housing and having a front bearing, wherein the output shaft passes through the front bearing, and the motor fixing frame is spaced from the motor.