Heretofore, disc drive devices have been constructed e.g. as shown in FIG. 1. Such a disc drive unit is typically used for a spindle motor.
More specifically, the disc drive device has a rotational assembly 10 on which a disc (not shown) can be mounted, a shaft 1 on which the rotational assembly 10 is rotatably supported by means of two ball bearings 23, and a base 7 to which the lower end of the shaft 1 is secured The rotational assembly 10 and the disc mounted thereon are rotatably driven by a motor M which is disposed between the base 7 and the rotational assembly 10. The motor M has a rotor 8 secured to the rotational assembly 10, and a stator 9 secured to the base 7 with the rotor 8 and stator 9 radially opposed to each other.
The rotational assembly 10 comprises a hub in tie case of the spindle motor.
With the magnetic disc units, however, due to the development of high recording densities, it is desirable that the deviation of non-repetitive runout (NRRO) in the disc drive device used in the magnetic disc units be small. The term "non-repetitive runout" is also referred to as "non-repeatable runout".
With the conventional disc drive device, as shown in FIG. 1, the deviation of non-repetitive runout (NRRO) needs to be small with the ball bearings 23. However, since the ball bearings 23 have vibration due to rolling of the balls and vibration due to errors in the shape of the bearing members, even with improved manufacturing accuracy, it is difficult to keep the deviation of non-repetable runout below a specified value.
FIG. 2 shows another example of the conventional disc drive devices, that is a spindle motor, which uses hydrodynamic fluid bearings with small deviation of non-repetitive runout, for both a radial bearing and a thrust bearing. In this example, the rotational assembly includes a shaft 1 and a hub 40.
A sleeve 6 is fitted into the base 7 to which the motor M is mounted, and provided with a cylindrical bole, the inner peripheral surface of which is formed with a radial bearing face 6a. The radial bearing face 6a of the sleeve 6 and the outer peripheral surface of the shaft 1 cooperate to form a radial hydrodynamic fluid bearing. On the other hand, the lower end face 1a of the shaft 1 and the upper surface 58a of the thrust bearing 58 cooperate to form a thrust hydrodynamic fluid bearing to support the thrust load.
Although such hydrodynamic fluid bearings make small the deviation of non-repetitive runout, there is a drawback that the shaft 1 with the hub 40 can easily come off the sleeve 6 if it receives shock or load from outside in the direction where the shaft comes off Therefore, in the example of FIG. 2, a stopper 60 is mounted to the hub 40 in an interference relation with a flanged portion 6b of the sleeve 6. However, it is troublesome to provide such a stopper 60. And during transportation, the stopper 60 sometimes bumps the axially opposing flange portion 6b of the sleeve 6 causing abrasion debris.