Bearings – Rotary bearing – Fluid bearing
Reexamination Certificate
2000-02-24
2002-06-11
Hannon, Thomas R. (Department: 3682)
Bearings
Rotary bearing
Fluid bearing
C384S907100
Reexamination Certificate
active
06402383
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of hydrodynamic bearing assemblies which provide for support and rotation of a high-speed spindle. More specifically, the present invention relates to improved methods for the optimum design of high-speed spindle motors.
BACKGROUND OF THE INVENTION
Disc drive memory systems have been used in computers for many years for storage of digital information. Information is recorded on concentric memory tracks of a magnetic disc medium, the actual information being stored in the form of magnetic transitions within the medium. The discs themselves are rotatably mounted on a spindle, the information being accessed by means of transducers located on a pivoting arm or arms which move radially over the surface of the disc. The read/write heads or transducers must be accurately aligned with the storage tracks on the disc to ensure proper reading and writing of information; thus the discs must be rotationally stable.
During operation, the discs are rotated at very high speeds within an enclosed housing by means of an electric motor which is generally located inside the hub or below the discs. One type of motor in common use is known as an in-hub or in-spindle motor. Such in-spindle motors typically have a spindle mounted by means of two ball bearing systems to a motor shaft disposed in the center of the hub. One of the bearings is typically located near the top of the spindle, and the other near the bottom. These bearings allow for rotational movement between the shaft and hub, while maintaining accurate alignment of the spindle to the shaft. The bearings themselves are normally lubricated by grease or oil.
The conventional bearing system described above, however, is prone to several shortcomings. First is the problem of vibration generated by the balls rolling on the raceways. Ball bearings used in hard disc drive spindles run under conditions that generally guarantee physical contact between raceway and ball, in spite of the lubrication layer provided by the bearing oil or grease. Hence, bearing balls running on the generally smooth but microscopically uneven and rough raceways, transmit this surface structure as well as their imperfection in sphericity in the form of vibration to the rotating disc. This vibration results in misalignment between the data tracks and the read/write transducer, limiting the data track density and the overall performance of the disc drive system.
Another problem is related to the application of hard disc drives in portable computer equipment and resulting requirements in shock resistance. Shocks create relative acceleration between the discs and the drive casting which in turn show up as a force across the bearing system. Since the contact surfaces in ball bearings are very small, the resulting contact pressures may exceed the yield strength of the bearing material, and leave long term deformation and damage to the raceway and the balls of the ball bearing.
Moreover, mechanical bearings are not easily scaleable to smaller dimensions. This is a significant drawback since the tendency in the disc drive industry has been to continually shrink the physical dimensions of the disc drive unit.
As an alternative to conventional ball bearing spindle systems, researchers have concentrated much of their efforts on developing a hydrodynamic bearing. In these types of systems, lubricating fluid—either gas or liquid—functions as the actual bearing surface between a stationary base or housing in the rotating spindle or rotating hub of the motor. For example, liquid lubricants comprising oil, more complex ferro-magnetic fluids or even air have been utilized in hydrodynamic bearing systems. The reason for the popularity of the use of air is the importance of avoiding the outgassing of contaminants into the sealed area of the head/disc housing. However, air does not provide the lubricating qualities of oil. The relative high viscosity of oil allows for larger bearing gaps and therefore greater tolerances to achieve similar dynamic performance.
Recent trends in high performance disc drives are toward higher speeds and lower rotating mass in both 2.5″ and 3.5″ disc drives. This leads a designer in the direction of adopting a smaller shaft diameter while maintaining conventional bearing gaps for manufacturability reasons, but conventional steel will quickly become the limiting factor in such designs.
SUMMARY OF THE INVENTION
To retain the smallest shaft diameter while retaining rigidity, the selection of ceramic, or tungsten carbide allows the most efficient design. However, to maintain the stiffness of the shaft of the surrounding sleeve and supported therefrom by a hydrodynamic bearing, the gap must also be adjusted and diminished. In a typical example, 3.4 mm shaft, with tighter journal gaps, can replace a 4 mm shaft saving 28% of journal power. According to this method, the shaft thickness and bearing gaps are optimized by first setting a ceramic shaft thickness, then modifying the gaps in relation to that thickness. To provide proper stiffness in the system, the gaps should be set more narrowly than expected.
Other features and advantages of the invention become apparent to a person of skill in the art who have studied the following detailed description of the preferred embodiment of the method and apparatus of the present invention given in conjunction with the accompanying drawings.
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Macleod Donald J.
Parsoneault Norbert S.
Hannon Thomas R.
Moser Patterson & Sheridan LLP
Seagate Technology LLC
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