Electric motor and electric power steering device using same

An electric power steering device for a vehicle comprising:

• an electric motor which includes (a) a cylindrical casing which has a first end and a second end opposed to the first end, (b) a first frame which is made of metallic material containing aluminum and covers the first end of the cylindrical casing, the first frame having a first recess and a first hole formed therein, the first recess being formed in a middle portion of the first frame which faces the cylindrical casing, the first hole communicating between one of surfaces of the first frame which is farther away from the cylindrical casing and a first bottom wall that is a bottom of the first recess, (c) a second frame which is made of metallic material containing aluminum and covers the second end of the cylindrical casing to define a storage chamber between itself and the first frame in the cylindrical casing, the second frame having a second recess formed in a middle portion thereof which faces the cylindrical casing;
• (d) an annular stator which is disposed in the storage chamber coaxially with the cylindrical casing, the annular stator being held from rotating relative to the cylindrical casing;
• (e) a winding wound around the stator;
• (f) a first bearing which is mounted in the first recess with an outer wall thereof fit in a first inner wall that is a cylindrical inner wall of the first recess;
• (g) a second bearing which is mounted in the second recess with an outer wall thereof fit in a second inner wall that is a cylindrical inner wall of the second recess;
• (h) a shaft which has a given length with a first end portion and a second end portion and is retained by the first frame and the second frame to be rotatable, the first end portion passing through the first hole to connect with a target and having an outer wall fit in an inner wall of the first bearing so that the first end portion is retained by the first bearing, the second end portion having an outer wall fit in an inner wall of the second bearing, so that the second end portion is retained by the second bearing;
• (i) a rotor which has an outer wall facing an inner wall of the stator and an inner wall fit on an outer wall of the shaft, so that the rotor is rotatable together with the shaft;
• (j) a biasing member which is made of austenite stainless steel and disposed between the first bottom wall and the first bearing or between a second bottom wall that is a bottom wall of the second recess and the second bearing, the biasing member urging the shaft through the first bearing or the second bearing in an axial direction of the shaft;
• and (k) a coating which is disposed on an outer surface of the first frame or the second frame, the outer surface of the first frame or the second frame including a coating-occupied area where there is the coating and a coating-unoccupied area where there is not the coating, the coating-unoccupied area being provided on at least a portion of the outer surface of the first frame or the second frame which faces the biasing member;
• and the target object which is connected to an end of the shaft and driven by the electric motor to produce assist torque which assists a steering wheel of a vehicle in steering the vehicle.