A fluid dynamic bearing device supports a shaft member in a freely rotating manner with an oil film formed in a bearing gap. The fluid dynamic bearing device has characteristics of high-speed rotation, high rotational accuracy, low noise, and the like, and is recently being widely used as a bearing for a spindle motor mounted on information equipment such as a magnetic disk device including an HDD and an FDD, an optical disk device including a CD-ROM, a CD-R/RW, and a DVD-ROM/RAM, a magneto optical disk device including MD and MO, for a fan motor mounted on a personal computer (PC), etc. to cool a heat generating source, while exploiting such characteristics.
For example, in a fluid dynamic bearing device incorporated in a spindle motor of a disk drive of HDD and the like, a radial bearing part for supporting a shaft member in a radial direction and a thrust bearing part for supporting in a thrust direction both may be constructed of a dynamic pressure bearing. For the radial bearing part in this type of fluid dynamic bearing device, for example, a fluid dynamic bearing device in which a dynamic pressure groove serving as a dynamic pressure generating part is formed in any one of an inner circumferential surface of a bearing sleeve or an outer circumferential surface of the shaft member opposed thereto, and a radial bearing gap is formed between such surfaces is known as described in JP 2003-239951 A (Patent document 1).
In information equipment incorporating the fluid dynamic bearing device having the above-mentioned configuration such as the disk drive of HDD, mounting of a plurality of disks is requested in an aim of increasing capacity, but in this case, a moment load acting on the bearing part that supports a spindle shaft in a freely rotating manner becomes large. In order to respond to such increase in the moment load, there is a need to arrange the radial bearing part at a plurality of locations spaced apart in an axial direction, and set a span between the radial bearing parts large to enhance moment rigidity. A configuration in which a plurality of radial bearing parts are arranged on the inner circumferential side of one bearing sleeve is widely adopted including Patent document 1, but there are demands for downsizing of the motor, and decrease in diameter of the spindle shaft and the bearing sleeve accompanied therewith, and hence it is sometimes difficult to manufacture the bearing sleeve that can respond to increase in span between the radial bearing parts.
As means for increasing the span between the radial bearing parts and enhancing the moment rigidity, and for facilitating the manufacturing of the bearing sleeve, a configuration of axially disposing the bearing sleeve at a plurality of locations (e.g., two locations) as described in JP 11-269475 A (Patent document 2) and JP 3602707 B (Patent document 3) has been considered. However, in the fluid dynamic bearing device of such configuration, the array pattern and the arrangement location of the dynamic pressure groove arranged on the inner circumferential surface etc. are different in each bearing sleeve in view of the rotating direction, but the axial dimension thereof is formed the same and the difference in appearance is extremely small. Thus, the assembling direction or the assembling position tends to be easily mistaken, and parts control becomes complicated. The function as the bearing device may not be fulfilled if the assembling direction etc. is mistaken, and hence special consideration needs to be made for assembling, which runs up the manufacturing cost of the bearing device.
As another means for enhancing the moment rigidity, a structure in which the bearing span of the thrust bearing part is enlarged can be adopted. For the fluid dynamic bearing device having such type of structure, a fluid dynamic bearing device in which the thrust bearing part is arranged on both end sides of the bearing sleeve as disclosed in JP 2005-321089 A (Patent document 4) is known. A structure combining the configurations of the above-mentioned Patent document 2 (or Patent document 3) and Patent document 4 may be adopted in an aim of further enhancing the moment rigidity. In the case of such configuration, the dynamic pressure generating means such as the dynamic pressure groove for generating the fluid dynamic pressure at the thrust bearing gap is often arranged at the end surface of the bearing sleeve made of sintered metal in view of formability, but each dynamic pressure groove needs to have different slope directions in view of the rotating direction. Therefore, while two types of bearing sleeves become necessary, they are formed into substantially the same shape of a level difficult to be distinguished with eyes. Accordingly, the assembling direction and the assembling position tend to be easily mistaken. Similarly to the above, the function as the bearing device may not be fulfilled if the assembling direction etc. is mistaken, and hence special consideration needs to be made for assembling, which runs up the manufacturing cost of the bearing device.
In the configuration of the above-mentioned Patent document 2 (or 3), the moment rigidity is enhanced by increasing the bearing span between the radial bearing parts and the manufacturing of the bearing sleeve is facilitated, but following problems arise when assembling the bearing device. In other words, in such configuration, even when the individual bearing sleeve is formed at high accuracy, core shift may occur when fixing each bearing sleeve to a housing with means such as adhesion and press-fitting. The lowering in coaxiality between both radial bearing surfaces caused by the core shift leads to variation in bearing rigidity, and hence lowering in bearing performance including moment rigidity becomes a concern.
The rotational performance of the fluid dynamic bearing device is determined by a width accuracy of the bearing gap (e.g., radial bearing gap) in the first place. Thus, efforts have been made to form the outer circumferential surface of the shaft member and the inner circumferential surface of the bearing sleeve (bearing member) forming the radial bearing gap at satisfactory accuracy. The gap width of the radial bearing gap is often formed evenly over the entire axial length as described in JP 2004-132402 A (Patent document 5).
In incorporation to the motor, various rotating bodies are assembled to the shaft member of the fluid dynamic bearing device, but the barycentric position of the rotating body differs in each case since the size, the weight, and the like of the rotating body to be assembled differ depending on the motor. Therefore, if the gap width of the radial bearing gap is evenly formed over the entire axial length as described above, the bearing rigidity including the moment rigidity lacks when vibration or impact is applied to the fluid dynamic bearing device, whereby the shift amount of the rotating body may increase or resonance phenomenon may occur.
Moreover, in addition to the enhancement of the moment rigidity, enhancement of rotational accuracy is desired in the fluid dynamic bearing device to be incorporated in the spindle motor. In order to respond to this, the inner circumferential surface of the bearing member and the outer circumferential surface of the shaft member forming the radial bearing gap need to be finished to a higher accuracy, but generally, finishing the inner circumferential surface at high accuracy is more difficult than finishing the outer circumferential surface at high accuracy, and there is a limit to improving the processing accuracy with general machining.    [Patent document 1] Japanese Laid-Open Patent Publication No. 2003-239951    [Patent document 2] Japanese Laid-Open Patent Publication No. 11-269475    [Patent document 3] Japanese Patent Publication No. 3602707    [Patent document 4] Japanese Laid-Open Patent Publication No. 2005-321089    [Patent document 5] Japanese Laid-Open Patent Publication No. 2004-132402