| 摘要 |
PURPOSE: To prevent the drop of the dynamic pressure of a bearing at the time of high speed rotation and to realize stable rotation by covering the outer periphery part of a rotor shaft in a bearing area with material having a smaller thermal expansion coefficient than that of the rotor shaft and a fixed shaft. CONSTITUTION: A magnet 6 is fixed on the outer periphery part of the rotor shaft 3 through a cylindrical outer periphery ring 5. The ring 5 is made of the material having the smaller thermal expansion coefficient than that of the rotor shaft 3 and the fixed shaft 4. The ring 5 is provided in a range covering all over the bearing surface. However, the ring 5 is not provided on the mount bearing surface of a polygon mirror 1 because squareness with the bearing surface is required. Thus, the spread of a bearing gap caused by the difference of temperature rise between the rotor shaft 3 and the fixed shaft 4 made in a high speed rotation area is restrained, so that the large drop of the dynamic pressure of the bearing is prevented, and vibration caused by unstableness is prevented. The same effect is obtained by making the rotor shaft 3 of the material having the smaller thermal expansion coefficient than that of the fixed shaft 4. |
