SPINDLE MOTOR

PURPOSE:To reduce the amplitude of vibration of a spindle motor, the rotary section of which is supported by a plurality of antifriction bearings, caused by damages, etc., due to deformation caused by manufacturing accuracy or external forces by changing the numbers of balls or rollers of bearings from each other. CONSTITUTION:When an impact force P is impressed upon the motor hub 7 of a spindle motor from the outside while the motor is operated or stopped, nearly the same portions of ball bearings 9a and 9b are deformed or flawed Q. When the inner races 10 of the bearings 9 are rotated in the directions indicated by arrows 14 together with the main shaft 8 of the motor in such a condition, vibrations are generated when each ball 12 of the bearings 9 passes through the positions of the flaws Q, because the balls 12 roll in the directions indicated by arrows 15. The time interval of the vibrations varies depending upon T/n, where the T and n respectively represent the cycle of revolution of the balls 12 and the number of balls. Since the numbers (n) of balls of the bearing 9a and 9b are different from each other, the oscillation frequencies of the bearings 9a and 9b differ from each other. Therefore, in the domain of oscillation frequencies higher than the revolutional frequency 1/T of the bearings 9a and 9b, the vibrations of the bearings 9a and 9b are not combined and do not develop to large-amplitude vibrations.