Patent Document

CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to Singapore patent application number 200007673-7, filed on Dec. 23, 2000, entitled ELECTRIC SPINDLE MOTOR WITH MAGNETIC BEARING AND HYDRODYNAMIC BEARING, which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an electric spindle motor, in particular it relates to an electric spindle motor with an integrated magnetic bearing and hydrodynamic bearing for use in data storage devices such as hard disk drives, optical drives or other precision instruments. 
     BACKGROUND OF THE INVENTION 
     Electric spindle motors are widely used in precision instruments such as hard disk drives, optical drives, magnetic-optical drives, printers or similar devices to meet high speed, high precision, low acoustic noise and low power consumption requirements of these applications. Conventional ball bearing spindle motors have been attempted for use in the above applications. Due to the drawbacks such as higher non-repeatable runout as well as higher acoustic noise caused by imperfect geometry on the inner race, outer race and the rolling elements, conventional ball bearing spindle motors are unlikely suitable for the next generation of precision instruments such as hard disk drives. Fluid film bearings have no direct surface contact during operation hence may be an alternative solution in replacement of the ball bearings for use in hard disk drives. One of the major difficulties for the application of fluid film bearing in hard disk drives is the liquid lubricant leakage which may degrades the performance of the fluid bearing. Lubricant leakage will also contaminate the surfaces of the disks leading to failures of the hard disk drive. 
     U.S. Pat. No. 5,283,491 to Jabbar et al and U.S. Pat. No. 5,760,509 to Chung et al disclose spindles with aerodynamic bearings which may have no risk of lubricant leakage. However, aerodynamic bearings have relatively lower load capacity and lower stiffness compared with the hydrodynamic bearings at similar range of geometric configuration. Furthermore, the rotor part and the stator part are electrically insulated hence there is no path for electrical discharge during operation of the spindle. This may result in possible damage of relevant part of the spindle such as the magneto-resistive (MR) head and cause failure of the hard disk drives. Aerodynamic bearings also have relative higher wear ratio, especially during the starting and stopping period of the spindle motor. 
     To overcome the above mention drawbacks in prior inventions, a hybrid bearing system, that is, an integration of hydrodynamic journal bearings and magnetic thrust bearings is provided by the present application. The present invention minimizes the friction between the bearing matting surfaces in a hydrodynamic thrust bearing therefore is power saving; fast starting and stopping with reduced contamination and cost-effective. 
     SUMMARY OF THE INVENTION 
     It is a first aspect of the present invention that an electric spindle motor disclosed has an improved radial load capacity and a reduced friction resistance. 
     It is a second aspect of the present invention that an integrated structure is disclosed for an electric spindle motor using a hydrodynamic journal bearing together with a magnetic bearing. 
     In accordance with the first and second aspects above, the electric spindle motor of the present invention comprises a stationary sub-assembly having a base and a first supporting means mounted on the base; a rotary sub-assembly having a rotating axis and a second supporting means for movably receiving the first supporting means; a hydrodynamic journal bearing interposed between the first and the second supporting means; and a magnetic bearing set positioned between the stationary sub-assembly and the rotary sub-assembly. The magnetic bearing set has at least one pair of electromagnets and at least one pair of thrust plates interactively associated with the at least one pair of electromagnets for controllably suspending the rotary sub-assembly from the stationary sub-assembly. 
     Preferably, the hydrodynamic journal bearing further comprises a first section, a second section and a gap therebetween for containing a lubricant. 
     Preferably, the electric spindle motor further comprises a sealing means for insulating the hydrodynamic journal bearing and discharging static electric charges from the rotary sub-assembly. 
     Preferably, the electric spindle motor further comprises at least one pair of permanent magnets respectively mounted on the at least on pair of thrust plates whereby providing a pre-load for keeping the rotary sub-assembly resting on the stationary sub-assembly. 
     Alternatively, the pair of electromagnets are offset from the pair of thrust plates along an axial direction of the rotating axis of the rotary sub-assembly. 
     Alternatively, the pair of electromagnets being offset from the pair of thrust plates along an redial direction of the rotating axis of the rotary sub-assembly. 
     Alternatively, the first supporting means comprises a shaft and the second supporting means comprises a sleeve. 
     Alternatively, the first supporting means comprises a sleeve and the second supporting means comprises a shaft. 
     The electric spindle motor having integrated hydrodynamic journal bearing and magnetic bearing of the present invention is capable of high speed and high accuracy running, providing increased radial load and stiffness, and reduces the friction resistance therefore improves the performance and power saving. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional view of an electric spindle motor according to a first embodiment of the present invention; 
     FIG. 2 is a cross sectional view of an electric spindle motor according to a second embodiment of the present invention; 
     FIG. 3 is a cross sectional view of an electric spindle motor according to a third embodiment of the present invention; 
     FIG. 4 is a cross sectional view of an electric spindle motor according to a forth embodiment of the present invention; 
     FIG. 5 is a cross sectional view of an electric spindle motor according to a fifth embodiment of the present invention; 
     FIGS. 6A and 6B are enlarged views showing the magnetic seals of the electric spindle motor according to the present invention; and 
     FIG. 7 is a functional block diagram of a data storage device comprising an electrical spindle motor according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIG. 1, the electric spindle motor according to a first embodiment of the present invention for use in a hard disk drive comprises a stationary sub-assembly and a rotary sub-assembly. The term “stationary sub-assembly” here refers to the parts of the electric spindle motor mounted to the housing of the hard disk drive, and the term “rotary sub-assembly” refers to the parts of the electrical spindle motor for carrying a plurality of disks and is movable relative to the stationary sub-assembly. The stationary sub-assembly includes a base  14  and a shaft  12  mounded onto the base  14  through an opening  16 . A stator lamination  32  and a coil winding  34  are formed on the stationary sub-assembly. The rotary sub-assembly is rotatable about a rotating axis  100  and includes a hub  30  and a sleeve housing  22  mounted onto the hub  30  for supporting a shaft sleeve  20 . The rotary sub-assembly and the stationary sub-assembly are movably connected through the shaft sleeve  20  and the shaft  12 . A hydrodynamic bearing including a first section  24  and a second section  26  separated by a gap  28  and is formed between the inner surface of the shaft sleeve  20  and the outer surface of the shaft  12 . A lubricant (not shown) can be filled in the gap  28 . The hydrodynamic bearing set provide radial load capacity and radial stiffness and guide the rotating portion of the spindle together with its load rotating around the shaft  12 . 
     The rotary sub-assembly further includes a yoke  38  and a magnet  36  which correspond to the stator lamination  32  and the coil winding  34  to form a driving sub-assembly for driving the rotary sub-assembly rotating relative to the stationary sub-assembly at a range of predetermined angular velocity. 
     A pair of thrust plate  41  and  42  are securely attached to the hub  30 . A pair of thrust bearing stators  43  and  44  with their respective pair of coils  45  and  46  are fixed to the base  14  through an extension  48 . The pair of thrust bearing stators  43  and  44  are placed in between and are interactively associated with the pair of thrust plates  41  and  42  to form a magnetic bearing set  40  which includes a first thrust bearing  49  and a second thrust bearing  50 . The thrust bearing stators  43  and  44  are back-to-back oriented with each other and axially facing their associated thrust plate  41  and  42 , respectively. A first magnetic force of the thrust bearing  49  is generated between the thrust plate  41  and the thrust bearing stator  43 , and is substantially parallel to the axial direction of the shaft  12  when an electric current is applied to the coil  45 . Similarly, a second magnetic force of the thrust bearing  50  is also generated between the thrust plate  42  and the thrust bearing stator  44  along the axial direction of the shaft  12  when a current is applied to the coil  46 . The first and second magnetic forces can axially suspend and maintain the rotary sub-assembly in a non-contact position from the stationary sub-assembly. The first and second forces enable the spindle motor to have a fast starting and stopping without bearing surface rubbing, and a low friction resistance during rotation. Risks of bearing wear and particulate generation will be much reduced. In addition, a radial magnetic force can be generated by the magnetic bearing set which may keep the axis of the thrust bearing stators  43  and  44  aligned with the thrust plates  41  and  42 . This radial force cooperates with the hydrodynamic journal bearing to increase the radial load capacity and stiffness, therefore enhances the performance of the hydrodynamic bearing set. 
     It is appreciated that under the inventive concept of the present invention, the structure of the electric spindle motor may also be altered by mounting the shaft onto the rotary sub-assembly and mount the sleeve onto the stationary sub-assembly. 
     A first seals  52  and a second  54  are applied at two ends of the hydrodynamic bearing. The first seal  52  includes a magnet ring  56 , a sealing ring  58 , a first cylindrical surface of the shaft  12  and a ferrofluid  60 . The second seal  54  includes a magnet ring  62 , a sealing ring  64 , a second cylindrical surface of the shaft  12  and the ferrofluid  60 . The magnetic force captures the ferrofluid  60  within the gaps between the shaft  12  and the magnetic sealing rings  56  and  64 . The magnetic seals effectively prevent the lubricant from leaking out of the hydrodynamic bearings. A first absorber  66  and a second absorber  68  are provided at the ends of the hydrodynamic journal bearing for absorbing any liquid from the hydrodynamic journal bearing and preventing the liquid from contaminating the disk surfaces due to the evaporation at certain condition such as low environment pressure. 
     A second embodiment of the present invention shown in FIG. 2 discloses an electric spindle motor including magnetic thrust bearings  149  and  150 , which comprises a pair of thrust plate  141 ,  142 , a pair of thrust bearing stators  143 , 144 , and a pair of coils  145 ,  146 . This embodiment is configured same as the first embodiment except that a magnetic thrust bearing set  140  is formed by separating the two magnetic thrust bearings  149  and  150 . 
     A third embodiment of the present invention shown in FIG. 3 discloses an electric spindle motor including magnetic thrust bearings  249  and  250 , which comprises a pair of thrust plate  241 ,  242 , a pair of thrust bearing stators  243 ,  244 , and a pair of coils  245 ,  246 . In this embodiment, the pair of thrust bearing stators  243  and  244  are placed side-by-side and facing outwardly along a direction substantially perpendicular to the rotating axis  201 . The pair of thrust plates  241  and  242  are in a form of ring shape and are placed surrounding the respective pair of thrust bearing stators  243  and  244 . Two pairs of magnetic poles  241   a ,  241   b  and  242   a ,  242   b  are formed at protrusions on the pair of plates  241  and  242  respectively. The pair of thrust bearing stators  243 ,  244  are offset with the pair of thrust plate  241 ,  242  along an axial direction of the rotating axis  201 , and are inwardly offset in between the pair of thrust plate  241 ,  242 , whereby two alignment forces can be generated. The two forces are thrust forces to keep the rotating assembly at a desired axial position. Simultaneously, attraction forces are generated between the pair of thrust bearing stators  243 ,  244  are offset with the pair of thrust plate  241 ,  242  along a radial direction, which is cooperative with the hydrodynamic journal bearing for withstanding the radial load the electric spindle motor. 
     A fourth embodiment of the present invention shown in FIG. 4 discloses an electric spindle motor with configurations same as the third embodiment, except that the pair of thrust plate  341 ,  342  are outwardly offset from the pair of thrust bearing stators  343 ,  344  for generating alignment forces alone the axial direction of the rotating axis  301  to keep the rotating assembly at a desired axial position. 
     FIG. 5 shows a fifth embodiment of the present invention. A pair of annular permanent magnets  441   b  and  442   b  are mounted onto the respective pair of thrust plates  441  and  442 . A thrust force an be generated the by the pair of permanent magnets  441   b  and  442   b  when the pair of thrust bearing stators  443  and  444  are not activated. The thrust force serves to provide pre-load to keep the rotary sub-assembly rest on the stationary sub-assembly. This will reduce the risk of damaging spindle motors during shipment. The magnetic thrust bearings can be activated to suspend the rotary sub-assembly from the stationary sub-assembly when the electric spindle motor is desired to work. 
     FIGS. 6A and 6B shows two alternatives of the sealing means for use with the electric spindle motor of the present invention. In FIG. 6A, the ferrofluid  60  are retained in annular groove while in FIG. 6B the ferrofluid  60  are retained in a wedge. 
     As shown in FIG. 7, a data storage device  700  includes an electric spindle motor  710  as described above; at least one data storage disk  712 ; at least one read/write assembly  714 ; read/write assembly controller  716 ; a spindle motor controller  718 ; a data processing electronics  720  and an interface  722  connected to a computer  730 . At the moment of starting, the spindle motor controller  718  sends a first signal to activate the electromagnets of the electric spindle motor  710  to suspend the rotary sub-assembly from the stationary sub-assembly; and a second signal to cause the rotary sub-assembly to rotate. Likely, at the moment of stopping, the spindle motor controller  718  sends a third signal to the electric spindle motor  710  to stop the rotary sub-assembly from rotating; and with a fourth signal to the electromagnets to have the rotary sub-assembly rest again on the rotary sub-assembly.

Technology Category: f