Lubricant supply to hydrodynamic bearings is conventionally established by applying pressure from an external source to a lubricating liquid or gas at the bearing surface, or by centrifugal pumping of a lubricating liquid substantially uniformly into a journal between two relatively rotating members, such as a shaft and a sleeve or housing, for example. It is also conventional to establish centrifugal pumping action of the bearing by defining relief grooves or a helical groove inclined at a specified angle or angles relative to an axis or plane of rotation (as in the case of hydrodynamic thrust bearings) in one of the surfaces of the hydrodynamic bearing journal, the other surface being extremely smooth. Ideally, in the example of centrifugal pumping, unidirectional relative rotation between the shaft and the sleeve causes the lubricating liquid to be pumped into the journal and maintained therein under pressure for so long as the relative rotation is maintained.
Computer disk drives that use hydrodynamic journal bearings within disk spindle assemblies have commonly utilized one type of fluid bearing design, known in the art as a "herringbone" pattern bearing, or simply a "herringbone". This label may be attributed to a repeating, generally symmetrical pattern of Vee-shaped or chevron shaped relief grooves formed in either a shaft or in a bearing sleeve or housing. The ungrooved element has a smooth surface. Relative unidirectional rotation of the shaft and the sleeve causes the lubricating liquid to enter the legs of each Vee groove and flow toward an apex thereof, where fluid pressure from the resultant pumping action creates and maintains a hydrodynamic bearing during the relative unidirectional rotation between the shaft and its associated housing.
Almost all of the existing herringbone bearing spindle designs for self-contained bearing systems have one common characteristic, which is that they produce no lubricating liquid flow in a direction generally along an axis of relative unidirectional rotation during operation. This arrangement is provided to prevent bearing failure from starvation otherwise resulting from a net liquid pumping effect toward either side of the bearing region. Thus, these prior designs are based on the premise that the oil which originally exists in the bearing area will not be replenished during operation and any wear debris which becomes trapped within the bearing clearance between the rotating surfaces of the shaft and housing will not be carried out of it by any localized flow of the lubricating liquid. This substantially nonexistent localized flow, with its consequent entrapment of bearing wear debris, and/or old lubricant residues, etc., tends to create localized heating, wear, and breakdown of the lubricant which in turn tends to limit the useful life of the fluid bearing. Examples of this prior approach are found in Asada et al. U.S. Pat. No. 4,557,610, entitled: "Dynamic Pressure Type Fluid Bearing Device"; and, Asada et al. U.S. Pat. No. 5,112,141, entitled: "Disk Driving Apparatus".
FIG. 1 hereof illustrates in unwrap (linear) view of a conventional herringbone pumping pattern for a hydrodynamic bearing. The Vee-grooves 10 defined in a shaft 12 or a mating bearing sleeve (not shown) creates a balanced flow of lubricating liquid along legs 10A and 10B of each Vee-grove 10. An apex of each Vee-groove 10 is in line with a circular locus 15 which lies in a plane normal to an axis of relative rotation between the shaft 12 and its mating bearing sleeve. As the relative rotation proceeds in accordance with the direction of the horizontal arrow in FIG. 1, lubricating liquid enters each leg 10A and 10B of each Vee-groove 10 from respective reservoirs 16 and 18 and flows toward the apex locus 15. In this example of the prior art, each leg 10A is the same length as each leg 10B. The result is that equal flows of lubricating liquid proceed along the legs 10A and 10B from the reservoirs 16 and 18 and result in zero net flow in a direction of axial relative rotation of the bearing unit.
A variation of this prior approach is to be found in Van Roemburg U.S. Pat. No. 4,596,474, entitled: "Bearing System Comprising Two Facing Hydrodynamic Bearings". In the Van Roemburg approach, the herringbone patterns of each bearing are such that the outer legs of the Vee grooves are longer than the inner legs. This arrangement is also illustrated diagrammatically in FIG. 2 hereof. In this arrangement, the legs 10A are shorter than the legs 10B, resulting in a net axial flow toward reservoir 16 as denoted by the vertical arrows in FIG. 2 and overall pressure equilibrium. In the Van Roemburg approach, in order to replenish the lubricant along the inner legs (e.g. legs 10A of FIG. 2 hereof), another pair of oppositely threading helical feed grooves are formed on the shaft within a central reservoir region of the bearing unit. This arrangement builds up a pressure at the apex of each bearing which is greater than the pressure in the lubricating liquid built up by the inner legs and by the helical feed grooves. By providing this arrangement it is said that the lubricant is not pumped out of the bearing system. By the same token, it is not clear that this prior bearing system provides any localized circulation of bearing liquid at the bearing surfaces which would remove wear debris, spent lubricant, etc.
There have been several attempts in the prior art to provide hydrodynamic bearings which achieve a net flow of lubricating liquid in order to flush away wear debris. One example is found in U.S. Pat. No. 5,246,294 to Coda H. T. Pan, entitled: "Flow-Regulating Hydrodynamic Bearing". While this prior approach apparently achieves a net axial flow of lubricating liquid, it accomplishes this goal only with a very complicated arrangement of reservoirs, a three-way valve and multiple passages defined within the disk spindle structure, requiring complicated machining/manufacturing processes.
Thus, a hitherto unsolved need has remained for a simplified hydrodynamic bearing design which achieves unidirectional, localized lubricating liquid flows generally along an axis of relative rotation of the bearing components thereby e.g. to remove wear debris without net depletion of lubricating liquid and consequent starvation of the bearing.