Hydrodynamic Bearing

This invention relates to precision hydrodynamic bearings.

One important limitation to increasing track density of computer disk drives is spindle bearing performance. A disk drive whose spindle bearing has low runout can accommodate higher track densities which results in more data storage capacity per disk.

The kinematics of the spin axis of a spindle bearing determine the precision of the bearing. As the journal spins relative to the sleeve, the spin axis may trace out a path or orbit. The motion of this axis typically has components that are synchronous with the spin and repetitive in nature. These motions are termed repetitive runout. Other components of spin axis motion may be asynchronous and nonrepetitive with respect to spin. These components are termed nonrepetitive runout. As a general rule, spindle bearing precision is increased as repetitive and nonrepetitive runouts are decreased.

Ball bearing spindle systems make up the majority of prior art disk drives. The kinematics of the rolling elements in ball bearings result in relatively large nonrepetitive runout. This results from the fact that the lubricant film thicknesses in ball bearings are very thin providing little attenuation of geometric defects in the bearing. In addition, ball bearings produce forces on the disk drive structure to which it is attached which are of relatively high frequency and large amplitude.

Hydrodynamic spindle bearing designs are also known. The Hewlett-Packard Model No. 9154A, 3.5 inch micro-Winchester disk drive incorporates a hybrid hydrodynamic-ball bearing spindle. The performance of this bearing is degraded by the incorporation of the ball bearings. The Phillips video 2000 videocassette recorder utilizes a hydrodynamic bearing which employs grease as the lubricant limiting operation to low speeds. Other known hydrodynamic spindle bearings for disk drives employ a ferromagnetic fluid as the lubricant for the bearing. This fluid is retained or sealed in the bearing by magnetic fields set up in pole pieces at each end of the bearing. Unless the magnetic fields and clearances are very precisely matched at each end of the bearing, one seal will be stronger than the other and when the bearing heats up, the lubricant can be spilled. See U.S. Pat. No. 4,526,484.

In general, the hydrodynamic bearing according to the instant invention includes a rotatable shaft/thrust plate combination disposed within a sleeve forming a first clearance space between the shaft and the sleeve and a second clearance space between the thrust plate and the sleeve. The external faces of the thrust plate are exposed to air and the clearance spaces are filled with a liquid lubricant. The sleeve includes pressure equalization ports connecting the first clearance space and the second clearance space. In a preferred embodiment, the bearing includes surface tension dynamic seals between axially extending surfaces of the thrust plate and sleeve. These axially extending surfaces of the thrust plate and sleeve diverge toward the ends of the bearing to form the dynamic seal. The divergence may be a straight taper having an angle of approximately 2.degree.. The pressure equalization ports include axially extending passageways in communication with radially extending passageways to connect the first and second clearance spaces. The radially extending passageways may be located near the center of the bearing. The bearing may also include relief patterns in opposed sleeve/thrust plate faces to generate inwardly directed radial forces.

In one embodiment of the invention, the bearing includes a cylindrical sleeve including a portion having a smaller inside diameter. A shaft including a portion having a diameter adapted to form a first clearance space with respect to the smaller diameter portion of the sleeve fits within the sleeve. A pair of thrust plates are disposed on the shaft to form second clearance spaces with respect to radially extending faces of the smaller diameter portion of the sleeve, the external faces of the thrust plate being exposed to the air. The clearance spaces are filled with a liquid lubricant. The smaller diameter portion of the sleeve includes plural axially extending passageways in liquid communication with radially extending passageways interconnecting the first and second clearance spaces. Surface tension seals are provided between the thrust plates and sleeve.

Another aspect of the invention is a method for introducing lubricant into the hydrodynamic bearing to avoid incorporating air. The bearing is placed in a vacuum chamber above a liquid lubricant and the chamber is evacuated to a pressure below atmospheric pressure. The bearing is submerged into the lubricant and the pressure in the chamber is raised to atmospheric pressure which forces the lubricant into the clearance spaces in the bearing. After the bearing is filled, it can be exposed to ultrasonic energy to expel any residual air. The vacuum chamber can also be repeatedly cycled between a high and a low pressure to expel residual air.

In another, particularly preferred embodiment of the invention, the bearing incorporates both external and internal surface tension seals at each end of the bearing. In this embodiment, there is an air space between the two ends of the bearing. This embodiment results in a reduced evaporation rate from the seals, improved moment stiffness, and faster thermal transient response.

In yet another aspect of the invention, the shaft and sleeve include mating tapered portions at each end of the bearing defining lubricant filled clearance spaces for supporting radial and axial loads. Each clearance space is sealed by an internal and an external surface tension dynamic seal and pressure equalization ports are provided to connect the internal and external seals. In this embodiment, the shaft is a continuous unit without a separate thrust plate portion. No O-ring seals are required.

The hydrodynamic bearing of the instant invention achieves lower levels of runout than ball bearings as a result of a thick film of lubricant which separates the sliding metal surfaces. This film provides a high degree of viscous damping which significantly attenuates nonrepetitive runout to levels which are less than state of the art rolling element bearings. In addition, the bearing generates forces on the structure attached to it which are low frequency and low amplitude relative to ball bearings. This reduction in the forcing function bandwidth and amplitude minimizes other vibrations in the disk drive and further improves tracking performance. The pressure equalization ports reduce pressure differentials which are caused by pumping actions inside the bearing. Because of the pressure balancing, the bearing does not tend to pump lubricant in a preferential manner through one seal or the other. Thus, only the external pressure differential across the bearing influences the position of the dynamic seal interfaces. The surface tension seals of the present invention do not leak nor do they generate solid debris.

FIG. 1 is a cross-sectional view of the bearing of the invention;

FIG. 2 is an elevational view of the sleeve portion of the bearing;

FIG. 3 is an expanded view of a portion of FIG. 1;

FIG. 4 is an expanded view of a portion of FIG. 1;

FIG. 5 is an expanded view of a portion of FIG. 4;

FIG. 6 is a schematic illustration of the method of filling the bearing with lubricant;

FIG. 7 is a cross-sectional view of a particularly preferred embodiment of the present invention; and

FIG. 8 is a cross-sectional view of an embodiment of the invention utilizing a tapered shaft.

A hydrodynamic bearing 10 shown in FIG. 1 includes a sleeve 12 including a portion of smaller inside diameter 14. A journal or shaft 16 fits within the sleeve 12 forming a first clearance space 18. The journal 16 may include a recess 20. Thrust plates 22 and 24 rest on the journal 16 and are sealed by means of O-ring seals 26. The thrust plates 22 and 24 form second clearance spaces 28 with respect to radially extending surfaces of the smaller inside diameter portion 14 of the sleeve 12. The portion 14 of the sleeve 12 also includes axially extending passageways 30 and radially extending passageways 32. As shown in FIG. 2 the passageways 30 and 32 are arranged around the circumference of the sleeve 12. Four sets of passageways 30 and 32 are shown in FIG. 2 but more or fewer may be employed. FIG. 2 also shows spiral relief patterns 34. These relief patterns cooperate with patterns on the journal to generate radially directed inward hydrodynamic pressure.

Relative rotation between the journal 16 and the sleeve 12 is provided for by the clearance spaces 18 and 28. Suitable dimensions for the clearance spaces 18 and 28 are 0.0002 to 0.001 inches and 0.0005 to 0.002 inches, respectively. These clearance spaces are filled with a lubricant such as oil which reduces wear between the journal and sleeve and provides a medium through which a hydrodynamic pressure field may be generated. Relative rotation or radial motion between the journal 16 and sleeve 12 is required to set up the hydrodynamic pressure field. The hydrodynamic bearing 10 supports loads by metal-to-metal contact when there is no relative motion. During normal operation, the spinning of the journal 16 sets up a steady pressure field around the clearance spaces which pushes the journal and sleeve apart and thus prevents metal-to-metal contact. The hydrodynamically pressurized film provides the stiffness needed to support the radial load of the disk, motor and associated hardware. Note that the hydrodynamic film stiffness is a measure of the resistance of the clearance space to change size under the influence of a load.

Axial loads along the journal 16 spin axis are supported by the hydrodynamic pressure field in the clearance spaces 28 between the thrust plate faces and the sleeve portion 14. The amount of separation between the thrust plate faces and sleeve is controlled by the hydrodynamic film stiffness and the applied axial load (usually the weight of the entire rotating assembly). Pressure building geometries such as the relief pattern 34 shown in FIG. 2 are employed to generate film stiffness of sufficient magnitude.

The sealing of the lubricant within the hydrodynamic bearing 10 will now be described in conjunction with FIGS. 1, 4 and 5. There are two types of seals in the bearing 10, namely, static and dynamic seals. Static seals 26 which are preferably O-ring seals prevent lubricant leakage between the thrust plates 22 and 24 and the journal 16. They are called static seals in that there is no relative rotation or sliding between the thrust plates 22 and 24 and the journal 16. Dynamic sealing is required in the clearance space 36 between the thrust plates and the sleeve. These seals must not leak or generate solid debris. Sealing is provided by surface tension-capillary seals in which a lubricant-air interface 38 provides the surface forces.

As shown in FIG. 5, two components, the liquid-gas (lubricant-air) interface 38 and the solid surfaces of the thrust plates and sleeve make up each seal. Surface tension forces directed axially away from each end of the bearing indicated by the arrows 40 balance the forces due to pressure differentials which may be applied across each interface as indicated by the arrows 42 and a force due to gravity. The magnitude of the axial surface tension forces depends on the wetted perimeter of the liquid-gas interface 38, the surface tension (a property of the liquid lubricant), the taper angle and the contact angle. The forces due to pressure differentials are dependent on the pressure differentials and the lubricant-air interface area. Since the solid boundaries of the seal are tapered, the wetted perimeter and area of the interface vary with the axial position of the interface. As a result, the axial position of the interface varies with pressure differences applied to the bearing until the surface tension forces and pressure forces balance. Stability of the interface is sensitive to the angle of taper. A taper angle of approximately 2.degree. has been experimentally determined to be optimum for insuring interface stability.

During bearing 10 operation, it is necessary that the pressures be nearly the same at the lubricant side of each lubricant-air interface 38. This pressure balance is provided by the pressure equalization ports 30 and 32 which connect the clearance spaces 18 and 28. Without the equalization ports, pumping actions inside the bearing may set up pressure differentials. For example, the thrust plates 22 and 24 produce an inwardly directed radial pumping action. The equalization ports tend to equalize the pressures. Furthermore, the passages should maintain a constant radial position in the neighborhood of the thrust plates. This requirement prevents large pressure gradients from developing in the passages due to the centrifugal pumping effects caused by the thrust plates. The bearing 10 is thus pressure balanced and does not tend to pump the lubricant in a preferential manner through one seal or the other. Only the external pressure differential across the bearing, therefore, influences the position of the interfaces. The equalization ports coupled with the surface tension dynamic seals result in a hydrodynamic bearing of higher precision with respect to runout relative to conventional bearing designs.

Lubricant must be introduced into the bearing in such way that a minimal amount of air is trapped in the bearing. This is necessary because trapped air in the bearing expands as the bearing heats up and tends to push the lubricant out of the bearing. A method for filling the bearing with lubricant so as to minimize the amount of trapped air will be described in conjunction with FIG. 6. First of all, the bearing 10 is placed within a vacuum chamber 50 above the level of a liquid lubricant 52. The vacuum chamber 50 is then evacuated to a suitable pressure below atmospheric such as 5.mu. of mercury. The bearing 10 is then submerged within the lubricant 52, after which the pressure in the chamber 50 is allowed to rise to atmospheric pressure. As the pressure rises, lubricant is forced into the bearing through the clearance spaces between the thrust plates and sleeve. Residual air bubbles in the bearing may be removed by applying ultrasonic energy to the chamber 50 within an ultrasonic tank 54. If necessary, additional residual air may be removed by repeated cycling of the pressure in the chamber 50 between a high and a low pressure.

FIG. 7 is a particularly preferred embodiment of the invention having several advantages as compared to the embodiment of FIG. 1. A bearing 70 includes a shaft 72 with thrust plates 74 and 76. The shaft 72 with attached thrust plates 74 and 76 rotates within a sleeve 78. The sleeve 78 includes a portion having increased inside diameter to create an air space 80. The bearing 70 includes external surface tension seals 82 and internal surface tension seals 84. The external surface tension seals 82 and internal surface tension seals 84 are connected by pressure equalization ports 86. The surface tension seals 82 and 84 and the pressure equalization ports 86 are filled with a lubricant. As with the embodiment of FIG. 1, the surface tension seals are created by diverging, axially extending surfaces.

The embodiment of FIG. 7 results in reduced evaporation rate of the lubricant from the seals. When the orientation of a bearing changes, the position of the surface tension seals along the spin axis also changes. In the case in which the oil-air interface moves into the bearing, a film of oil is left on the region of the metal which was previously covered by the lubricant of the seal. This film of oil is then exposed to air and has a large amount of surface area compared to the seal oil-air interface area. As a result of this increased surface area, the evaporation of the oil is reduced.

When a bearing is not operating, the position of the seals is determined by the pressure difference between the two sealed regions of the bearing which are connected together by the pressure equalization port or balance tube. The internal fluid pressure difference is controlled by the elevation difference between the two regions of the bearing and the specific weight of the lubricant fluid. The external pressure differences due to variations in air pressure around the bearing are usually negligible. Thus seal position and the change in seal position are controlled primarily by the elevation changes in the bearing. Splitting the lubricated regions of the bearing of FIG. 7 into two separate and shorter zones reduces the range of possible elevation differences and also the resulting range of seal position changes. This design thus reduces the wetted area of the bearing and the evaporation rate.

The bearing of FIG. 7 also provides higher moment stiffness. The higher stiffness results from the fact that the length of the bearing can be made longer relative to the bearing of FIG. 1. Moment stiffness is proportional to the length of the bearing squared when all of the other bearing characteristics are held constant. The bearing of FIG. 7 can be longer than the bearing of FIG. 1 because the seal areas are split into separate zones so that the central region of the bearing can be lengthened without affecting the behavior of the seals.

Another advantage of the embodiment of FIG. 7 is faster thermal transient response of the lubricant. It is desirable to have the lubricant come up to temperature as fast as possible during start up. When the lubricant oil is warm, it has a lower viscosity than when it is cold and thus the torque requirements are less when the oil is warm. Accordingly, when the oil can be made to heat up quickly, a shorter period of high load on the driving motor results which is very desirable for some applications. The faster thermal response of the bearing of FIG. 7 results from the reduction of oil volume in this bearing design and the resulting increase in bearing power to oil volume ratio.

FIG. 8 is yet another embodiment of the present invention. A bearing 100 includes a spindle shaft 102 which has tapered portions 104 and 106. These tapered portions mate with tapered bearing shells 108 and 110 which reside within a spindle housing or sleeve 112. The spaces between the tapered shaft and tapered bearing shells are filled with a liquid lubricant. The lubricant is sealed by external surface tension or capillary seals 114 and 116, and internal capillary seals 118 and 120. An equalization port 122 connects the seals 114 and 118, and an equalization port 124 connects the seals 116 and 120.

Because of the tapered surfaces, both radial and axial loads are supported by the bearing. The spindle housing and shaft surfaces are a single contiguous unit without any parting line. No O-ring seals are required since no secondary leakage is possible with the tapered arrangement. The tapered portions of the bearing shaft or the tapered bearing shell surfaces include herringbone patterns which generate a net liquid flow due to machining tolerances. This net liquid flow in the bearing is compensated for by a flow in the opposite direction through the equalization ports 122 and 124.

The bearing shells 108 and 110 have grooves on their outer surfaces. These bearing shells are shrink fitted into the spindle housing 112 and the grooves cooperate with the housing 112 to create the equalization ports.