AUFBAU EINES ROTIERENDEN POLYGONSPIEGELS

A rotation supporting device of a polygon mirror (33) comprising a stationary shaft provided with a shaft sleeve (27) around its circumference, a rotary assembly (R) rotatably mounted around the circumference of the shaft sleeve and having a radial circumferential surface of a polygon made to serve as mirror surfaces (M) and having a magnet member (47), thrust plates (29/30) for suppressing the motion of a rotary assembly in the axial direction. The rotary assembly comprises a first rotor (28) rotatably mounted at the outer circumference of the shaft sleeve (27), a second rotor (33) having a circumferential surface of a polygon made to serve as mirror surfaces and being disposed at the outer circumference of the first rotor, and a yoke (31) secured on the outer circumference of the first rotor. The first rotor is made of a ceramic material, the second rotor is made of aluminum and the yoke is made of steel. Radial dynamic pressure bearing is included between the outer circumference of the shaft sleeve and the rotary assembly. Dynamic thrust pressure bearings are included between the end surfaces of a rotary assembly and the thrust plates.