1. Field of the Invention
The present invention relates to a rotary drive apparatus, particularly to a rotary drive apparatus suitable for use in an image formation apparatus such as a copier, a printer or the like, and to an optical scan apparatus and an image formation apparatus including the rotary drive apparatus.
2. Description of the Related Art
In order to form images at high speed or form high-precision images by an electrophotographic image formation apparatus, it is necessary to rotate a rotary body therein such as a polygon mirror at a speed of 30,000 rpm or more with high precision. However, there is a drawback in the high speed rotation of the rotary body that the rotary body may be unbalanced, causing a motor thereof to vibrate, or temperature rises in the rotary body due to heat emission from a rotating motor.
Although it is possible to correct and suppress the unbalance of the rotary body in a stationary state, the heat emission during the rotation of the motor may change the unbalance of the rotary body corrected in the stationary state.
The change of the balance of the rotary body due to the temperature increases occurs since components of the rotary body differ from each other in thermal expansion rate. In particular, one of the components, a rotor magnet with a large thermal expansion rate causes the change of the balance of the rotary body when it is fixed to the rotation axis of the rotary body by an adhesive. Since it is not possible to process the rotation axis of the rotary body to be in a perfect circular shape, it is unable to completely seal the entire circumference and surfaces of the axis of the rotary body with the rotor magnet by the adhesive. Accordingly, when the rotor magnet is fitted into the rotation axis, the axis is always inclined radially, and uneven contact with the rotor magnet occurs. At a high ambient temperature or during high-speed rotation of the rotary body, thermal stress occurs due to a difference in the thermal expansion rate between the uneven contact portions and non-contact portions.
The thermal stress is much larger than adhesive strength of the adhesive so that the fitted portions minutely moves. Especially, the above problems are conspicuous in the polygon mirror since it is rotated at the high speed of 30,000 rpm or more and exposed at the high temperature of 80 degrees or more.
In view of solving the above problems, Japanese Laid-Open Patent Application Publication No. 2003-70194 discloses a rotary drive apparatus with improved reliability which generates less vibration and less noise due to the vibration even at the high rotation speed of 30,000 rpm and at the high temperature.
Moreover, Japanese Laid-Open Patent Application Publication No. 2005-92129 discloses an optical deflector used in a color image formation apparatus and having a plurality of polygon mirrors which are superimposedly disposed in stages in a rotation axis direction and whose deflection mirrors in the respective stages are shifted by a predetermined angle in a rotation direction.
However, when the above optical deflector is rotated at a speed of 40,000 rpm or more for the purpose of image formation at a higher speed with higher precision, the rotor magnet may have a crack due to centrifugal force, causing the balance of the rotary body to change and the vibration thereof to increase.