High pressure barrier to oil loss by diffusion

An improved rotational motor, such as a spindle motor for a disc drive, is provided. The motor first comprises a hub having a shaft portion and a horizontal body portion. The motor also comprises a sleeve surrounding the shaft portion of the hub. A fine vertical gap is retained between the shaft and the inner diameter of the surrounding sleeve. In addition, a fine horizontal gap is provided between the upper hub portion and the top of the sleeve. The vertical gap is filled with a lubricating liquid, such as a clean oil. A capillary seal is provided in the vertical fluid gap at one end. Preferably, the capillary seal is disposed at an upper end of the shaft proximal to the horizontal gap. Novel air pumping grooves are machined along the horizontal fluid gap. When the hub is rotated, the air pumping grooves create a high pressure region in the vicinity of the capillary seal. This forms a high pressure barrier that reduces the number of oil molecules diffusing out of the capillary seal and, therefore, inhibits oil loss from the system.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fluid dynamic bearing motors. More specifically, the present invention pertains to fluid dynamic bearing motors such as are used to support and rotationally drive one or more memory discs.

2. Description of the Related Art

The computer industry employs magnetic discs for the purpose of storing information. This information may be stored and later retrieved using a disc drive system. Computer systems employ disc drive systems for transferring and storing large amounts of data between magnetic discs and the host computer. The magnetic discs are typically circular in shape (though other shapes are known), and are comprised of concentric, or sometimes spiraled, memory tracks. Each track contains magnetic data. Transitions in the magnetic data are sensed by a magnetic transducer known as a read/write head. The transducer is part of the disc drive system, and moves radially over the surface of the disc to read and/or write magnetic data.

FIG. 1presents a perspective view of magnetic media10as are commonly employed for information storage. In this view, a plurality of stacked magnetic discs10′ is shown. The discs10′ inFIG. 1are shown in vertical alignment as is common within a disc drive system. Each disc10has a central concentric opening5for receiving a spindle (shown at51inFIG. 2). A rotary motor drives the spindle51, causing the discs10of the disc pack10′ to rotate in unison.

In operation, information stored in the magnetic layer of the disc10is read by a magnetic head assembly. The magnetic head assembly is part of a disc drive system, such as the system50shown inFIG. 2.FIG. 2presents a top view of an exemplary disc drive system50, with the magnetic head assembly seen at58. The disc drive assembly50includes a servo spindle52and an actuator arm54. The servo spindle52is motorized to pivot about an axis40. More specifically, the servo spindle52is selectively positioned by a voice coil motor57which pivots the actuator arm54, causing the arm54to move through arc42. In this manner, the arm54can be positioned over any radial location “R” along the rotating disc surface.

The actuator arm54carries a flexure arm or “suspension arm”56. The suspension arm56, in turn, supports the magnetic head assembly58adjacent a surface of a disc10. The head assembly58defines a transducer that is capable of reading magnetic information from the magnetic layer of the disc10, or writing additional information on a reserved portion of the disc10. The magnetic head58is typically placed on a small ceramic block, also referred to as a slider. The slider is aerodynamically designed so that it “flies” over the disc10as the disc is rotated at a high rate of speed.

As noted, the disc10itself is supported on a drive spindle51. The drive spindle51rotates the disc10relative to the magnetic head assembly58.FIG. 3provides a perspective view of a disc drive assembly50. In this arrangement, a plurality of discs10′ are stacked vertically within the assembly50, permitting additional data to be stored, read and written. The drive spindle51receives the central openings5of the respective discs10. Separate suspension arms56and corresponding magnetic head assemblies58reside above each of the discs10. The assembly50includes a cover30and an intermediate seal32for providing an air-tight system. The seal32and cover30are shown exploded away from the disc stack10′ for clarity.

In operation, the discs10are rotated at high speeds about axis45(seen inFIG. 2). As the discs10rotate, the air bearing slider on the head58causes the magnetic head58to be suspended relative to the rotating disc10. The flying height of the magnetic head assembly58above the disc10is a function of the speed of rotation of the disc10, the aerodynamic lift properties of the slider along the magnetic head assembly58and, in some arrangements, a biasing spring tension in the suspension arm56.

Each disc10has a landing zone11where the magnetic head assembly58lands and rests when the disc drive50is turned off. When the disc drive assembly50is turned on, the magnetic head58“takes off” from the landing zone11. Each disc10also has a data zone17where the magnetic head58flies to magnetically store or read data.

As noted, the servo spindle52pivots about pivot axis40. As the servo spindle52pivots, the magnetic head assembly58mounted at the tip of its suspension arm56swings through arc42. This pivoting motion allows the magnetic head58to change track positions on the disc10. The ability of the magnetic head58to move along the surface of the disc10allows it to read data residing in tracks along the magnetic layer15of the disc. Each read/write head58generates or senses electromagnetic fields or magnetic encodings in the tracks of the magnetic disc as areas of magnetic flux. The presence or absence of flux reversals in the electromagnetic fields represents the data stored on the disc.

In order to accomplish the needed rotation of discs, an electric motor is provided. The electric motor is commonly referred to as a “spindle motor” by virtue of the drive spindle51, or “hub,” that closely receives the central opening5of a disc10.FIG. 4illustrates the basic elements of a known spindle motor design, in cross-section. The motor400first comprises a hub410. The hub410includes an outer radial shoulder412for receiving a disc (not shown inFIG. 4). The hub410also includes an inner shaft414. In this arrangement, the shaft414resides and rotates on a stable counterplate440. A sleeve420is provided along the outer diameter of the shaft414to provide lateral support to the shaft414while it is rotated.

It can be seen that a bearing surface422, or “journal surface,” is formed between the shaft410and the surrounding sleeve420. In early arrangements, one or more ball bearing systems (not shown was incorporated into the hub410to aid in rotation. Typically, one of the bearings would be located near the top of the shaft, and the other near the bottom. A raceway would be formed in either the shaft or the sleeve for holding the plurality of ball bearings. The bearings, in turn, would be lubricated by grease or oil. However, various shortcomings were realized from the mechanical bearing system, particularly as the dimensions of the spindle motor and the disc tracks became smaller. In this respect, mechanical bearings are not always scaleable to smaller dimensions. More significantly, in some conditions ball bearings generate unwanted vibrations in the motor assembly, causing the read/write head to become misaligned over the tracks. Still further, there is potential for leakage of grease or oil into the atmosphere of the disc drive, or outgassing of the components into this atmosphere.

In response to these problems, hydrodynamic bearing spindle systems have been developed. In these types of systems, lubricating fluid is placed along bearing surfaces defined around the rotating spindle/hub. The fluid may be in the form of gas, such as air. Air is popular because it avoids the potential for outgassing of contaminants into the sealed area of the head disc housing. However, air cannot provide the lubricating qualities of oil or the load capacity. Further, its low viscosity requires smaller bearing gaps and, therefore, higher tolerance standards to achieve similar dynamic performance. As an alternative, fluid in liquid form has been used. Examples include oil and ferro-magnetic fluids. A drawback to the use of liquid is that the liquid lubricant should be sealed within the bearing to avoid leakage. Any loss in fluid volume results in a reduced bearing load capacity and life for the motor. In this respect, the physical surfaces of the spindle and of the housing would come into contact with one another, leading to accelerated wear and eventual failure of the bearing system.

Returning back toFIG. 4, the motor400ofFIG. 4represents a hydrodynamic bearing system. A thrust plate430is disposed between the shaft414and the surrounding sleeve420. Fluid is injected in gaps maintained between the shaft414and surrounding parts, e.g., the counterplate440, the sleeve420, and the thrust plate430. The fluid defines a thin fluid film that cushions relative movement of hub parts.

The motor400is actuated by energizing coils in a stator in cooperation with one or more magnets. In the view ofFIG. 4, magnets450are seen disposed within the hub410, while stator coils452are provided on a base460. The magnets450and stator coils452interact to provide rotational movement of the hub410.

Additional details of fluid dynamic bearing systems are provided in U.S. patent application Ser. No. 10/099,205 filed Mar. 13, 2002, and entitled “Low Power Fluid Dynamic Bearing.” That application is commonly owned with the present application, and is incorporated herein in its entirety by reference. Of interest, that application presents various hydrodynamic motor designs wherein a thrust plate430is not employed.

As noted, it is important to retain fluid within the bearing surfaces for a hydrodynamically operated spindle motor. Various architectures have been proposed for retaining fluid within the bearing surfaces. Certain patents present a mechanical seal. For example, U.S. Pat. No. 5,347,189 entitled “Spindle Motor with Labyrinth Sealed Bearing” provides a labyrinth seal outside one of the bearings. The labyrinth seal has two parts that mate to form a tortuous flow path for fluids. This serves to inhibit the escape of grease from ball bearings. U.S. Pat. No. 5,925,955 entitled “Labyrinth Seal System” provides an alternative seal system for an electronic spindle motor.

Other patents provide for a grooved pattern that serves to retain fluid within a spindle motor. U.S. Pat. No. 6,149,159 entitled “High Pressure Boundary Seal” provides for a “herringbone pattern” of grooves along or adjacent the outer surface of the shaft. A zone of high pressure is created at or about the center of the pattern, thereby creating a high pressure boundary seal. This, in turn, prevents the flow of lubricating fluid from the interior of the motor or the bearing into the interior section of the disc drive housing. Another example is U.S. Pat. No. 5,533,812 entitled “Single Plate Hydrodynamic Bearing with Self-Balanced Fluid Level,” which offers a thrust plate having grooved surfaces.

Still another means for retaining fluid within a hydrodynamically operated bearing surface for a spindle motor is presented in U.S. Pat. No. 5,524,986. This patent is entitled “Fluid Retention Principles for Hydrodynamic Bearings.” A flexible membrane is provided at one end of the fluid gap. The spring force of the membrane allows the gap volume to adjust with fluid changes as temperature fluctuates. In this respect, the membrane is flexible, and absorbs any increase in volume of the bearing fluid. The '986 patent also introduces the principle of a capillary seal. In this respect, a capillary seal is provided at one end of the gap. The capillary seal design helps retain a volume of lubricant oil within the system necessary for continuous motor operation.

One problem presented with the capillary seal design is that an end of the bearing gap is exposed to the ambient environment of the disc drive housing. This, in turn, can lead to a slow but progressive oil loss by evaporation. The lubricant oil is selected to have a low vapor pressure to reduce evaporation. Nevertheless, over the life of the motor a noticeable amount of lubricant is lost from the capillary seal by evaporation, as well as from vapor diffusion in the gas phase.

To compensate for the oil loss, the capillary seal dimensions are designed to hold a larger amount of oil than would otherwise be necessary. However, the available reservoir volume is limited by geometrical size constraints and by requirements for seal splash robustness during shock events.

Thus, a need exists for an improved fluid dynamic bearing system for a spindle motor that retains liquid within and along the bearing surfaces. Further, there is a need for such a motor that minimizes oil loss due to evaporation. Still further, there is a need for such a motor that minimizes the amount of oil that is lost from the capillary seal over the life of the motor.

SUMMARY OF THE INVENTION

The present invention provides an improved motor arrangement. The arrangement is useful in connection with rotary electrical motors, such as spindle motors in disc drive systems. More specifically, the invention is most applicable to motors that employ fluid dynamic bearing surfaces between relatively rotating parts.

In an exemplary arrangement, the improved spindle motor first comprises a hub having a shaft portion and an upper horizontal body portion. The motor also comprises a sleeve surrounding the shaft portion of the hub. A first fine gap is retained between the shaft and the inner diameter of the surrounding sleeve. In addition, a second fine gap is provided between the upper hub portion and the top of the sleeve. The first gap typically is substantially vertical, and is filled with a lubricating liquid, such as a clean oil. The second gap is typically horizontal. However, the present invention is intended to cover any relative angle between the first and second gaps.

A capillary seal is provided in the vertical fluid gap at one end. Preferably, the capillary seal is disposed at an upper end of the shaft proximal to the upper hub portion. In addition, air pumping grooves are machined along the horizontal fluid gap. The air pumping grooves may be machined into the bottom of the upper hub portion; preferably, though, they are machined into the top of the sleeve. The air pumping grooves are used to create a high pressure region in the vicinity of the capillary seal. In this respect, the high pressure barrier reduces the number of oil molecules diffusing out of the capillary seal and, therefore, the total oil loss from the system.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an improved spindle motor arrangement. The improved motor employs novel air pumping grooves as a means for retaining liquid lubricant within a fluid bearing interface.

FIG. 5presents a cross-sectional view of an improved spindle motor arrangement500in one embodiment in which air pumping grooves526are machined. The motor500first comprises a hub510. The hub510provides a radial shoulder512for receiving and supporting a body to be rotated, such as a magnetic disc (not shown). The hub510defines a central shaft portion514and a horizontal body portion518. The shaft514is configured for constant high speed rotation. This rotation is established by a stator552which is mounted from a base560. The stator552defines an electric coil that, when energized, creates a magnetic field. The energized coil cooperates with magnets550mounted from the inner surface of the hub510to generate rotational movement of the hub510.

As noted, the shaft514is configured for high speed rotation. In this respect, the shaft514rotates over a stationary counterplate540. The interface between the bottom of the shaft514and the top of the counterplate540thus defines a thrust bearing542. Fluid such as liquid lubricant is maintained along the thrust bearing gap542to provide a fluid bearing surface. One of the top face of the counterplate540or the bottom surface of the shaft514includes a grooved pattern (seen at544in the enlarged view ofFIG. 6). The grooved pattern assists in maintaining fluid between the shaft514and the counterplate540when the shaft514rotates. When the motor500is at rest, the shaft514presses directly on the counterplate540. Fluid is then at least partially pressed into the grooves544, and also extruded around the outer diameter of the shaft514.

The motor500ofFIG. 5next comprises a sleeve520. In the arrangement ofFIG. 5, the sleeve520is stationary and is supported on the counterplate540. The sleeve520is disposed between the rotating shaft514and shoulder565of the surrounding base560. It can be seen that the interface between the rotating shaft514and the surrounding sleeve defines a fluid bearing surface522. When the motor500is energized and the shaft514and connected hub510are rotated, lubricant is drawn downward from the sleeve bearing surface522into the thrust bearing surface542. Lubricating fluid is drawn into the thrust bearing gap542under the urging of the grooved pattern544. More specifically, the lubricating fluid is drawn into the thrust bearing region542to support relative rotation between the bottom end of the shaft514and the facing surface of the counterplate540, the fluid being maintained in part in the gap542by the grooved pattern during rotation. When the shaft514comes to rest, the shaft end will rest on the plate540and, although the volume of fluid is very small, it will tend to be forced back out into the sleeve bearing gap522between the shaft514and the sleeve520. Therefore, space is preferably allowed in this gap522for this fluid.

To prevent the shaft514and connected hub510from being displaced axially too far above the counterplate540, since this is an axially upward thrust bearing542between the shaft end and the counterplate540, an opposing bias is typically introduced. This bias is utilized to prevent the thrust bearing gap542from becoming too large, which would reduce the effectiveness of the motor500. Approaches to this can be seen in the provision of a biasing magnet564facing the motor magnet550and axially spaced therefrom. By selecting a suitable size and location for this magnet564, an appropriate bias against the shaft514being axially displaced too far from the counterplate540or the base560can be optionally introduced.

In the arrangement ofFIG. 5, the fluid gap522between the shaft514and the inner diameter of the surrounding sleeve520is essentially vertical. At the same time, the fluid gap524between the upper hub portion518and the top of the sleeve520is essentially horizontal. However, the present invention is intended to cover any orientation and relative angle between the first522and second524gaps.

To inhibit the loss of liquid lubricant from the bearing gaps542,522during operation, a capillary seal516is provided at the distal end of the sleeve bearing gap522from the thrust bearing gap542. Further information concerning operation of a capillary seal within a bearing gap is disclosed in U.S. Pat. No. 5,524,986 entitled “Fluid Retention Principles for Hydrodynamic Bearings.” That patent issued to Seagate Technologies, Inc. in 1996.

To further inhibit the loss of fluid such as liquid lubricant from the bearing gaps542,522, particularly during operation of the motor500, novel pumping grooves526are provided. The pumping grooves526are positioned along an upper gap524between the horizontal body portion518of the hub510and the sleeve520. The pumping grooves526may be disposed along the surface of either the horizontal body portion518of the hub510or the sleeve520. Preferably, the grooves526are placed along the sleeve520. The pumping grooves pump fluid such as air.

FIG. 6illustrates an enlarged view of gaps522and524formed between the hub510and the surrounding sleeve520. More specifically, gap522is formed between the shaft portion514of the hub510and the sleeve520, while gap524is formed between the top of the sleeve520and the lower surface of the central body portion518of the hub510. In this enlarged view, the capillary seal516can be seen in the sleeve bearing gap522. In addition, air pumping grooves526can be seen on a top surface of the sleeve520. The air pumping grooves526serve to inhibit the evaporation of oil. Operation of this inhibitor phenomenon is as follows.

When oil evaporates from the capillary seal516in a spindle motor, an oil vapor is released. This may occur during idle periods; however, it may also occur following periods of use when the overall motor system500heats up. As the temperature of the lubricating fluid, e.g., oil, rises, the lubricating fluid volume begins to expand. Ultimately, some oil begins to transition to gas phase and diffuses outward past the capillary seal516. The resulting oil vapor typically saturates the region of the bearing gap522closest to the seal516. Given enough time to reach equilibrium, the entire volume around the seal516will become saturated with oil vapor unless the diffusion of molecules is not limited by tight gaps, or if the gap volume is too large to become fully saturated. An undesirable oil loss occurs when oil molecules migrate past the capillary seal region516and do not return.

An increase in air pressure in the volume adjacent to the capillary seal516will decrease the rate of oil molecule transfer to the outside of the capillary seal region516. Therefore, the rate of oil evaporation from the motor500can be reduced by using a “pump” to pressurize the region adjacent to the capillary seal516. The issue then becomes one of creating a pumping arrangement to increase air pressure along the gap524adjacent the capillary seal516.

According to the present invention, such a pump can be created by placing grooves in a tight gap region adjacent the capillary seal516. This is provided by placing the novel air pumping grooves526between the hub518and sleeve520or other motor component near the capillary seal516. In one arrangement, the grooves526are disposed along the bottom of the central hub portion518on a side of the capillary seal516opposite the counterplate540(seeFIG. 8). In another embodiment, and as shown inFIG. 6, the grooves526may be disposed along the top surface of the sleeve520, also on a side of the capillary seal516opposite the counterplate540. In one embodiment, the horizontal gap524is approximately 0.16 millimeters in height. In one embodiment, a portion of the motor surfaces on top of the sleeve520and under the hub518along the horizontal gap524are pre-coated with a fluid repelling coating.

The groove pattern526is configured so that air flow is guided into the capillary seal area516when the hub510is rotated. An example of such a pattern is a spiral pattern machined into the top of the sleeve520. However, any type of pattern as is used to draw air in a tight gap region is suitable to serve as the air pumping groove.

FIG. 7depicts a perspective view of an exemplary sleeve520having air pumping grooves526, in one embodiment. Preferably, the groove pattern526is spaced apart from the formed fluid meniscus of the capillary seal516and is not precoated in oil. As the sleeve520and the hub510rotate relative to one another, air is introduced by operation of the groove pattern, e.g., pattern526, in order to produce a high pressure region. Air is introduced into the upper gap region524between the horizontal hub portion518and the sleeve520. This produces a high pressure region along the capillary seal516opposite the bearing surfaces522,542. As the total pressure in the system is increased, there is a corresponding reduction of the gas phase diffusion coefficient of the oil through the reduction of the mean free path of the oil molecules. (This applies over a range of temperatures in the system, brought into the steady state condition with respect to oil diffusivity. This, in turn, reduces evaporation loss.

The diffusion of oil into the vapor phase is a function of the mean free path of the oil molecules in the gas phase versus the mean velocity of the oil molecules in the air. The general function is as follows:
D=f(λmfp,{overscore (v)})  (1)

The relationship can be mathematically defined. In the ideal gas approximation which can be applied in the range of pressures under consideration in the capillary seal system, the diffusion coefficient of the oil vapor is directly proportional to the mean free path of the oil molecules in the gas phase:Doil=13⁢λmfp⁢v_(2)
where

λmfpis the mean free path of the oil molecules in the vapor phase; and

{overscore (v)} is the mean velocity of the oil molecules in the gas phase.

The mean free path, λmfp, is proportional to system parameters, as follows:λmfp∝kTp⁢⁢σ⁢2(3)
where k is the Boltzman gas constant;

T is the temperature in the system; and

σ is the molecular cross section of the oil.

Thus, pressurizing the capillary seal region by a certain factor will decrease the oil diffusion through air by the same factor.