Dynamic fluid bearing

Abstract: A dynamic fluid bearing for a spindle motor is disclosed, which provides a fluid bearing seal and support structure that simultaneously addresses the problems of preventing oil leakage, of maintaining the bearing integrity under shock, of reducing oil evaporation and of minimizing distortion of the active bearing surface. The bearing structure includes a thrust plate coupled to a shaft 480 and a bearing plate 214 and load plate 240 coupled to a sleeve 430, wherein the load plate urges the bearing plate towards the thrust plate, and wherein an adhesive 250 is interposed between the load plate and the thrust plate. The adhesive may be an UV epoxy. The sleeve is operatively coupled to a stator 494, 498, and the stator rotates the sleeve upon receiving a drive signal. The bearing further includes a diffusion barrier 270 disposed between the sleeve and the stator. A second diffusion barrier 260 is disposed between the load plate and the shaft. The bearing structure may also include an O-ring 222 disposed above the bearing plate for sealing a gap between the bearing plate and the sleeve.

Description: This invention relates in general to a dynamic fluid bearing for use in a spindle motor, for example in a disk drive. Disk drives are computer mass storage devices from which data may be read and/or to which such data may be written. In general, they comprise one or more randomly accessible rotating storage media, or disks, on which data is encoded by various means. In magnetic disk drives, data is encoded as bits of information including magnetic field reversals grouped in tracks on the magnetically-hard surface of the rotating disks. The disks are stacked in a generally parallel and spaced-apart relationship and affixed at their inner diameter ( ID ) to a common hub which is rotationally coupled to a stationary spindle shaft by a pair of bearings, typically ball bearings. With the growing trend toward even lower height form factor disk drives, the length of the spindle shaft and spacing between the upper and lower bearings becomes a significant consideration in meeting specific drive height constraints. As drive height is decreased, a proportionately shorter spindle must be accommodated within the decreased height constraints with a concomitantly shorter spacing available between the upper and lower bearings supporting the hub on the spindle. Rotary spindle motors having fluid bearings for supporting the rotary member rather than traditional ball bearings typically include a shaft having at least one axial thrust plate and a hub, which may be a rotary hub, having a sleeve portion generally enclosing the shaft and thrust plate, thus forming a journal bearing with bearing fluid disposed therein. The bearing fluid will form capillary seals at one or more ends of the shaft that are exposed to ambient air pressure. The problem with such constructions is that under certain conditions the capillary seal may break down and fluid may leak from the bearing. Disruption of the seal may be caused by shock or vibration. Under certain conditions the rotating portion of the bearing may be displaced along the axis of the shaft. In the normal course of events, lubricant flows around the end of the thrust plate from the side with decreasing clearance to the side with increasing clearance. If, however, because of sudden shock or vibration, the lubricant flow around the thrust plate is impeded, fluid will be pushed toward one end of the shaft or the other, possibly breaking down the surface tension which holds the seal in place. Leakage may also occur when there are inaccuracies in the fabrication of the patterned grooves used by the thrust plates upper and lower surfaces to maintain a desired net pressure gradient. The object of the grooves is to create a high pressure region in the middle of each thrust plate surface and to create ambient pressure zones at the inner diameter of the thrust plate, adjacent the shaft, and at the outer diameter in the gap between the readily outermost edge of the thrust plate and the sleeve. This type of pressure distribution ordinarily results in no displacement of bearing fluid, that is, the static pressures will equalize. However, fabrication inaccuracies do occur, as does tilt in the bearing, or any other physical phenomena, and these factors can alter the pressure balance in the bearing fluid resulting in flow across the bearing. The flow of bearing fluid can overcome the surface tension seal at either end of the bearing and cause the fluid to leak. The situation is particularly acute at the thrust plate end where pressure imbalances between the upper and lower surfaces of the thrust plate may create a net flow which is near the capillary seal at the upper surface of the thrust plate. Nevertheless, prior axial bearing support structures have not simultaneously address preventing oil from leaking out, maintaining the bearing integrity under shock, reducing oil evaporation and minimizing distortion of the active bearing surface. Rather, existing designs have addressed only a fraction of the requirements, e.g., only evaporation and shock induced bearing separation or distortion and evaporation. For example, U.S. Patent No. 5,490,021, issued February 6, 1996, to Johannes C. A. Muller et al., and assigned to U.S. Phillips Corporation, disclosed a dynamic groove bearing for a hard disk drive. The dynamic groove bearing includes a sleeve-like housing having a locking piece that includes a bearing disk portion which cooperates with an annular bearing disk provided on a shaft. A pressure member is adapted to compress an annular, elastically deformable sealing element to thereby seal the interface between the housing and the locking piece and to pretension the locking piece against a seat formed on the housing. However, Muller et al. does not address evaporation and shock induced bearing separation. Accordingly, the present invention provides a dynamic fluid bearing for a spindle motor having a thrust plate secured to a shaft and a bearing plate and load plate secured to a sleeve, wherein the load plate urges the bearing plate towards the thrust plate, and characterised in that an adhesive is interposed between the load plate and the bearing plate. Such a bearing with a fluid bearing seal and support structure simultaneously addresses preventing oil from leaking out, maintaining the bearing integrity under shock, reducing oil evaporation and minimizing distortion of the active bearing surface. In the preferred embodiment the adhesive is an UV epoxy, and there is a first diffusion barrier disposed between the load plate and the shaft, and a further diffusion barrier disposed between the sleeve and the shaft. An O-ring is disposed above (ie adjacent) the bearing plate for sealing a gap between the bearing plate and the sleeve. The adhesive maintains integrity between the load plate and the bearing plate when a shock condition occurs. Viewed from another aspect, the invention provides a spindle motor, comprising: a sleeve having an inner periphery and an outer periphery, the sleeve defining a disk hub on the outer periphery; a shaft rotatably supporting the sleeve; a stator for rotating the sleeve; and a dynamic fluid bearing disposed between the sleeve and the shaft, the dynamic fluid bearing further comprising a thrust plate coupled to the shaft, and a bearing plate and load plate coupled to the sleeve, wherein the load plate urges the bearing plate towards the thrust plate, and wherein an adhesive is interposed between the load plate and the bearing plate to maintain integrity between the load plate and the bearing plate when a shock condition occurs to the spindle motor. Viewed from yet another aspect the invention supplies a method for providing a dynamic fluid bearing for a spindle motor having a thrust plate secured to a shaft and a bearing plate and a load plate secured to a sleeve, wherein the load plate urges the bearing plate towards the thrust plate, the method comprising the steps of applying an adhesive between the load plate and the thrust plate and curing the adhesive to form the seal. Viewed from a yet further aspect the invention provides a disk drive, comprising: at least one storage disk; an actuator assembly including at least one actuator arm having a sensor disposed at a distal end of the actuator arm for reading and writing data on the at least one disk; a spindle shaft, coupled to the at least one storage disk, for rotating the at least one storage disk; and a spindle motor, coupled to the spindle shaft, for rotating the spindle shaft; the spindle motor further comprising: a sleeve having an inner periphery and an outer periphery, the sleeve defining a disk hub on the outer periphery, the sleeve being rotatably supported by the spindle shaft; a stator for rotating the sleeve; and a dynamic fluid bearing disposed between the sleeve and the shaft, the dynamic fluid bearing further comprising a thrust plate coupled to the shaft, and a bearing plate and load plate coupled to the sleeve, wherein the load plate urges the bearing plate towards the thrust plate, and wherein an adhesive is interposed between the load plate and the bearing plate to maintain integrity between the load plate and the bearing plate when a shock condition occurs to the spindle motor. A preferred embodiment of the invention will now be described in detail, by way of example only, with reference to the following drawings: Fig. 1 illustrates an exploded view of a magnetic storage system; Fig. 2 illustrates the configuration of a fluid bearing seal for a spindle motor in a preferred embodiment of the present invention; Fig. 3 illustrates the top cross section of the spindle; and Fig. 4 shows the bottom cross section of the spindle. The preferred embodiment provides a fluid bearing seal and support structure that may simultaneously prevent oil leakage, maintain the bearing integrity under shock, reduce oil evaporation and minimize distortion of the active bearing surface. Fig. 1 illustrates an exploded view of a magnetic storage system 100. The disk drive 100 includes a housing 112 and a housing cover 114 which, after assembly, is mounted within a frame 116. Mounted within the housing is a spindle shaft 122. Rotatably attached to the spindle shaft 122 are a number of magnetic storage disks 124. In Fig. 1, multiple disks 124 are attached to the spindle shaft 122 in spaced apart relation. The disks 124 rotate on spindle shaft 122 which is powered by a motor (not shown). Information is written on or read from the disks 124 by heads or magnetic transducers (not shown) which are supported by sliders 126. Preferably, sliders 126 are coupled to the suspensions or load springs 128. The load springs 128 are attached to separate arms 130 on an E block or comb 132. The E block or comb 132 is attached at one end of an actuator arm assembly 136. The actuator arm assembly 136 is rotatably attached within the housing 112 on an actuator shaft 138. The rotary actuator assembly 136 moves the integrated transducer/suspension assembly in an arcuate path across the surface of the storage disk 124. Fig. 2 illustrates the configuration of a fluid bearing seal 200 for a spindle motor according to the present invention. Hydrodynamic pressure is generated at the axial bearing 210 and the axial bearing plate 214 when the bearing surfaces 216, 218 are rotating with respect to each other. Therefore, bearing plate 214 must be very flat. Distortion should be minimized during and after assembly. This is accomplished by placing the O-ring 222 on top of the axial bearing plate 214. The O-ring 222 is placed at the interface between the axial bearing plate 214 and sleeve 230 to prevent oil from leaking out. Compression of the O-ring 222 is provided by load plate 240 through the fingers 242 or other means. It is important that load plate 240 should not impinge the bearing plate 214 during the assembly or thereafter. Therefore, a nominal gap, such as a tenth of a millimeter, is desired. A gap at the axial bearing 210 can suddenly form during a vertical shock. The gap at the axial bearing 210 creates a vacuum that pulls air in because the O-ring 222 is highly compliant. This mechanism is prevented by applying a few adhesive droplets 250 between the load plate 240 and the axial bearing plate 214. The adhesive 250 is applied through a hole, e.g., at least two holes 180 degrees apart to maintain balance, in the load plate 240. This allows the load plate 240 to be installed immediately after oil fill and installation of bearing plate 214 and O-ring 222. The adhesive 250 can then be applied through the a hole 252 in the load plate 240. The hole 252 allows the use of a UV cure aerobic adhesive 250. This also allows adhesive 250 application at the same station as adhesive seal application at location 242. The adhesive 250 may be an UV epoxy, wherein a UV cure of the adhesive 250 can also be performed at the same station. Diffusion barriers 260, 270 are disposed near the top of the shaft just outside of the top axial bearing 210 and at the bottom of the shaft just outside of the bottom radial bearing 272. The diffusion barriers 260, 270 typically include narrow gaps that offer significant resistance to flow and mass transfer of lubricant vapor. Figs. 3 and 4 show details at the top 300 and bottom 400 areas of the spindle respectively. In Fig. 3, the top cross section 300 of the spindle is shown. The load plate 340 which secures the opposing plate of the axial plate 314 forms a first level of barrier because of the small radial gap 360 on the inner diameter near the shaft 380. The second barrier is formed by the narrow radial gap 382 and the relatively narrow axial gap 390. Radial gap 382 is located between the load plate 340 and the sleeve 330. The axial gap 390 is located between the axial plate 314 and the load plate 340. Fig. 4 shows the bottom cross section 400 of the spindle. At the lower radial bearing end, a small axial gap 492 is provided between end of the sleeve 430 and the top end of the mount flange 496. The mount flange 496 is a press fit on the shaft 480. The stator mount 494 is a press fit on the mount flange 496 and is used for mounting the stator lamination stack 498. A long narrow annular cylindrical gap 470 is formed between the end of the sleeve 430 and the stator mount 494. Both of the above mentioned axial 492 and radial 470 gaps form effective labyrinths offering significant resistance to mass transfer of lubricant vapor. These gaps are between rotating and stationary surfaces and have been shown to be effective in this function in the range of 0.025 to 0.050 mm. Gaps smaller than 0.025 mm could be incorporated provided manufacturing and assembly level tolerances permit them without causing surface interference. Gaps larger than 0.05 mm may be used only if calculated mass transfer rates are acceptable. For example for a radial gap of 0.025 mm at a diameter of 5 mm and a length of 2 mm for the barrier, calculated evaporative loss at 70°C amounts to about 3.25 mg over 5 years. To meet mean time between failure requirements, sufficient reservoir capacity will have to be provided so that the loss of the lubricant does not affect the functional performance of the spindle bearing system.

Claims: A dynamic fluid bearing for a spindle motor having a thrust plate secured to a shaft (480) and a bearing plate (214) and load plate (240) secured to a sleeve (430), wherein the load plate urges the bearing plate towards the thrust plate, and characterised in that an adhesive (250) is interposed between the load plate and the bearing plate. The dynamic fluid bearing of claim 1 wherein the adhesive is an UV epoxy. The dynamic fluid bearing of claim 1 or 2 wherein the adhesive (250) is cured to form a seal. The dynamic fluid bearing of claim 1, 2 or 3 further comprising a diffusion barrier (260) disposed between the load plate and the shaft. The dynamic fluid bearing of claim any preceding claim further comprising a diffusion barrier disposed between the sleeve and the shaft. The dynamic fluid bearing of any preceding claim wherein the sleeve is operatively coupled to a stator (494,498), the stator rotating the sleeve upon receiving a drive signal, the bearing further comprising a diffusion barrier (270) disposed between the sleeve and the stator. The dynamic fluid bearing of claim 4, 5 or 6 wherein the diffusion barrier prevents bearing lubricant from leaking between the load plate and the shaft, the sleeve and the shaft or the sleeve and the stator as appropriate. The dynamic fluid bearing of any preceding claim wherein the adhesive maintains integrity between the load plate and the bearing plate when a shock condition occurs. The dynamic fluid bearing of claim 8 wherein the adhesive maintains integrity by absorbing tolerances of the load plate and the bearing plate. The dynamic fluid bearing of any preceding claim further comprising an O-ring (222) disposed adjacent to the bearing plate for sealing a gap between the bearing plate and the sleeve. A spindle motor, comprising: a sleeve(430) having an inner periphery and an outer periphery, the sleeve defining a disk hub on the outer periphery; a shaft (380) rotatably supporting the sleeve; a stator (494, 498) for rotating the sleeve; and a dynamic fluid bearing according to any preceding claim, disposed between the sleeve and the shaft. A disk drive, comprising: at least one storage disk (124) mounted on a disk hub; an actuator assembly (136) including at least one actuator arm (130) having a sensor disposed at a distal end of the actuator arm for reading and writing data on the at least one disk; and the spindle motor of claim 11, wherein said shaft (122) is coupled to the at least one storage disk, for rotating the at least one storage disk.

Inventors: LEE CHEN-HSIUNG, NAGARAJ HOLAVANAHALLY SESHACHA, SHIRLE NEAL BERTRAM, LEE, CHEN-HSIUNG, NAGARAJ, HOLAVANAHALLY SESHACHAR, SHIRLE, NEAL BERTRAM
Applicants: HITACHI GLOBAL STORAGE TECH, HITACHI GLOBAL STORAGE TECHNOLOGIES NETHERLANDS B.V.
Application year: 1998
Main classification: F16C33
IPC codes: F16C17, F16C33, G11B25, H02K7, H02K5, G11B19