The present invention relates to magnetic disc storage systems and more specifically the present invention relates to an improved fluid bearing for use in a disc drive storage system.
Magnetic disc drives are used for magnetically storing information. In a magnetic disc drive, a magnetic disc rotates at high speed and a transducing head xe2x80x9cfliesxe2x80x9d over a surface of the disc. This transducing head records information on the disc surface by impressing a magnetic field on the disc. Information is read back using the head by detecting magnetization of the disc surface. The transducing head is moved radially across the surface of the disc so that different data tracks can be read back.
Over the years, storage density has tended to increase and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage discs. For example, to achieve increased storage densities the transducing head must be placed increasingly close to the surface of the storage disc. This proximity requires that the disc rotate substantially in a single plane. A slight wobble or run-out in disc rotation can cause the surface of the disc to contact the transducing head. This is known as a xe2x80x9ccrashxe2x80x9d and can damage the transducing head and surface of the storage disc resulting in loss of data.
From the foregoing discussion, it can be seen that the bearing assembly which supports the storage disc is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair races which allow a hub of a storage disc to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor, because of low damping. Thus, there has been a search for alternative bearing assemblies for use with high density magnetic storage discs.
One alternative bearing design which has been investigated is a hydrodynamic bearing. In a hydrodynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing and a rotating member of the disc hub. In addition to air, typical lubricants include oil or ferromagnetic fluids. Hydrodynamic bearings spread the bearing interface over a large surface area in comparison with a ball bearing assembly which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Moreover, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeatable runout.
However, because the two surfaces which form the gap of the hydrodynamic bearing are not mechanically separated, the potential for impact always exists. Such impacts could occur when the motor supported by the bearing is at rest, or even more damaging, when a shock to the system occurs while the motor is either stopped or spinning. Over time, such impacts could wear down a region on one of the bearing surfaces, altering the pressure distribution and reducing bearing efficiency or induce catastrophic failure due to surface damage like galling. Moreover, particles could be generated by the scraping of one side against the other, which particles would continue to be carried about by the fluid. Such particles could build up over time, scraping the surfaces which define the hydrodynamic bearing, or being expelled into the region surrounding the motor where they could easily damage the disc recording surface.
In most instances, the surface incurring the most wear is the inner surface, which is touched or impacted when the opposed surface moves or sets down. Common materials to be used for the two facing surfaces are ceramic vs. ceramic; ceramic vs. metal; metal vs. metal; metal vs. soft metal (which includes both known materials such as bronze, and free machining materials which are hard metals having additives to make machining easier) or hard coated surface vs. ceramic or other metal. While soft metal is highly desirable for its ease of use, it is relatively easy to pit or dent. Thus a way to minimize or eliminate such problems is highly desirable.
Therefore, it is an object of the present invention to provide an improved hydrodynamic bearing which is resistant to wear and shock. More specifically, an objective of the invention is to provide a bearing in which the facing surfaces defining the gap resist the damage imposed by one surface contacting the other, either by the motor coming to rest, or by a shock imposed to the system.
These and other objectives of the invention are achieved by providing a lubricant coating on one or both of the facing surfaces defining the gap of the hydrodynamic bearing. The lubricant film would need to be very thin, in the range of about 10 xc3x85 to 1000 xc3x85, to prevent the film from materially affecting the hydrodynamic bearing gap which is typically about 1-10 xcexcm. The thin lubricant film could be a perfluoropolyether (PFPE), or a mixture of PFPE, or a phosphazine derivative. To improve the adhesion and lubricating performance, functional PFPE can be used. For a pure metallic surface like steel, phosphate esters can also be used because of its affinity with the metallic surface, e.g., ferrous. For example, the lubricant film could be PFPE, or a mixture of PFPE and a phosphazine derivative. The lubricant used should be chosen to be thermally stable, and non-reactive with the typical ambient environment. The lubricant used also has to have a very low vapor pressure so that the lubricant coating lasts for the life of the bearing, as the lubricant will be applied to the surface of the bearing only once during the manufacturing process.
As an alternative, particularly if one of the surfaces of the bearing gap is ferrous, a phosphate ester could be used. Particularly with this type of coating, the material could be quenched in the phosphate ester, providing the desired thin surface coating. A thin film of phosphate esters without quenching can also be an effective wear protective coating.
The thin lubricant film can be applied to both metallic surfaces as well as surfaces with hard coatings. The use of a hard coating allows for the usage of free machining or soft metals which are vulnerable to wear degradation, and the thin lubricant film coating protects the hard coating from wear. It should be noted that although the thin lubricant film coating is effective for both liquid and gas bearings, it is most effective for gas bearings.