Abstract:
A conductive lock washer for safely dissipating electrostatic charge which builds up on a disk surface without discharge across the head/dish gap. The lock washer, including a series of alternating inner and outer tabs, acts to reduce torque on the spindle washer damper and hoop stress on the washer itself, while still creating a conductive pathway to the top cover of the hard drive assembly.

Description:
FIELD OF THE INVENTION 
     The present invention relates to lock washers, and specifically to conductive lock washers for use in a disk drive or similar apparatus. 
     BACKGROUND OF THE INVENTION 
     Present day high capacity disk drives produce vibrations, such as created when the device is operated in upper and lower dynamic rocking modes. Such vibrations can cause the drive to malfunction. Thus, there is a need to damp the vibrations to ensure optimal drive performance. This can be done by securing the spindle motor shaft to a spindle washer damper (SWD), which is connected to a top cover, which in turn is connected to a baseplate. The SWD acts to attenuate vibrations produced within the drive thereby facilitating optimal drive performance. 
     The SWD is typically attached to the top cover of the disk drive by a non-conductive adhesive. However, the non-conductive adhesive can create an electrostatic charge problem. As electrostatic charge builds up on the spinning disk surfaces, the non-conductive adhesive creates a barrier that prevents the charge from being dissipated through the cover and baseplate. If this charge cannot be dissipated, voltage can build up across the head/disk gap until the electrostatic charge discharges across the gap, potentially causing damage to the disk surface and head, and further potentially causing data loss. 
     Additionally, as the SWD is attached to the top cover by an adhesive, torque generated by tightening the spindle motor shaft screw against the SWD and spindle can be transferred to the SWD and the adhesive. This can result in the SWD being twisted off of the top cover. In turn, a loose SWD can cause decreased performance or even failure of the disk drive, or if the failure occurs during the manufacture and assembly, it can result in costly reworks of the drive. 
     Therefore, there is a need for a conductive path from the disk surface, through the spindle motor shaft to the spindle washer damper and to the top cover. Additionally, it would be advantageous to have a torque transferring feature to help prevent the spindle motor shaft screw from applying a torque directly to the SWD and twisting it off of the adhesive and top cover during and after tightening. It would also be advantageous to have a torque locking feature, to prevent the spindle motor shaft screw from loosening during operation of the drive. 
     Others in the industry have developed ways to provide the ground path. For example, one method is to press the spindle motor shaft into the baseplate creating interference coupling which is able to dissipate any electrostatic charge to the baseplate. However, this method has the disadvantage of reduced precision in the manufacturing process, reducing spindle to baseplate alignment and potentially degrading drive performance. Others have also used a conductive adhesive to connect the shaft to the baseplate. However, this conductive adhesive is a potential source of out gassing and contamination failures. Additionally, this method does not transfer torque away from the SWD which can result in the previously mentioned manufacturing reworks caused by twisting the SWD off of its adhesive connection to the top cover. This method also allows the spindle motor shaft to work its way loose, as there is no torque locking between the spindle motor shaft and the SWD. 
     Others have also used a conductive washer placed between the spindle motor shaft screw and the SWD such that the washer also contacts the top cover to provide a conductive path. As depicted in FIG. 8, this conductive washer  20  has three outer tabs  21  that are positioned between the top cover and SWD. The nine inner tabs  22  contact the under surface of the spindle motor shaft screw head as it is tightened into the spindle shaft. However, this method does not reduce or eliminate the application of a torque on the SWD resulting in the possible disengagement of the SWD from the adhesive connecting it to the top cover, nor does it provide a locking feature to prevent loosening of the spindle motor shaft screw. 
     Accordingly, there is a need for an apparatus and method that overcomes all of the problems described above. 
     SUMMARY OF THE INVENTION 
     The present invention is designed to overcome the aforementioned problems and meet the above-described needs. 
     The present invention provides a method and apparatus for providing a conductive path between the disk and the top cover through the spindle washer damper. More specifically, in one embodiment, the lock washer is advantageously formed such that upon insertion and tightening of the spindle motor shaft screw into the spindle, the washer deforms and makes a conductive contact with the top cover, the spindle motor shaft screw and spindle washer damper. Accordingly, electrostatic discharge of voltage built up on the spinning disks can be discharged through the conductive path without a potential discharge across the head/disk gap. 
     Additionally, the lock washer is advantageously formed from a series of alternating inner tabs and outer tabs interconnected by a thin piece of resilient metal. The lock washer also has a curved cross-section, such that the only portions of the lock washer in contact with the spindle washer damper, prior to the complete tightening of the spindle motor shaft screw, are the thin pieces of resilient metal which connect the inner tabs to the outer tabs. This configuration acts to reduce hoop stresses required to deform the washer, thereby reducing the torque applied to the spindle washer damper. More specifically, as the spindle motor shaft screw is being tightened the outer tabs lift and come into contact with the under surface of the top cover. Because the outer tabs contact with the top cover at a larger radial distance from the center of the lock washer than where the thin pieces of resilient metal contact the spindle washer damper and because the outer tabs place more surface area in contact with the top cover than the thin resilient strips place in contact with the spindle washer damper, more of the torque generated by the tightening of the spindle motor shaft screw is transferred to the top cover rather than to the spindle washer damper. 
     Furthermore, in one embodiment, the curved cross-section of the lock washer is advantageously formed to allow the lock washer to be placed between the top cover and the spindle washer damper such that it does not apply any pressure between the two members prior to the insertion of the spindle motor shaft screw, and deforms upon the insertion of the screw into the spindle shaft to contact the spindle washer damper, the top cover and the screw. In addition, upon insertion of the spindle motor shaft screw into the spindle shaft, the lock washer continues to apply lifting pressure to the under side of the screw head, thereby resisting loosening of the screw during operation of the disk drive. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a free body diagram of the lock washer of the present invention; 
     FIGS. 2 through 4 are cross-sectional views of a disk drive top cover assembly showing the lock washer of the present invention at various stages of deformation as the spindle shaft screw is tightened into the spindle shaft; 
     FIG. 5 is a top view of a section of the top cover of an embodiment of the present invention; 
     FIG. 6 is a top view of a section of the top cover of an embodiment of the present invention; 
     FIG. 7 is a cross-sectional view of the disk drive top cover assembly taken along line  7 — 7  of FIG. 6, showing an outer tab of the lock washer engaging a cut out in the top cover and another outer tab engaging the under surface of the top cover; and 
     FIG. 8 is a top plan view of a prior art conductive washer. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     While this invention is susceptible of embodiments in many different forms, there is shown in the drawings and will herein be described in detail, preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspects of the invention to the embodiments illustrated. 
     With reference to FIG. 1, the lock washer  1  is shown in detail. The lock washer  1  in a preferred embodiment is made of stainless steel and has a thickness of 0.003 inches. The lock washer  1  consists of a series of inner tabs  2  and outer tabs  3 . The inner tabs  2  generally form a central aperture  4 , and the outer tabs  3  generally form an outer circumference  5 . The inner tabs  2  and outer tabs  3  are connected by a curved piece  6  which, being preferably made of stainless steel, is easily deformable. One of skill in the art will appreciate that the precise metal selected can vary, provided it is conductive and flexible. 
     With reference now to FIGS. 2-5, a disk drive assembly detail is shown. A spindle washer damper (SWD)  7  is attached to the top cover  8  of the drive by an adhesive  9 . The lock washer  1  initially rests on the SWD  7 . During assembly, a spindle motor shaft screw  10  is inserted through the top cover  8 , the central aperture  4  of the lock washer  1 , and the SWD  7  and into the spindle motor shaft  11 . 
     As shown in FIG. 5, the outer circumference  5  formed by the outer tabs  3  advantageously has a larger diameter than the hole  12  in the top cover  8 . Referring now to FIGS. 2-4, the lock washer  1  is placed in between the SWD  7  and the top-cover  8  when the two pieces are assembled. The lock washer  1  is therefore aligned with the hole in the SWD  7  as well as being aligned with the hole in the top cover  8 , thereby eliminating the need to manually align the lock washer  1  during the insertion of the spindle motor shaft screw  10 . The diameter of the central aperture  4  is larger than the diameter of the spindle motor shaft screw  10 , but smaller than the outer diameter of the head  13  of the spindle motor shaft screw  10 . 
     As shown in FIG. 3, as the spindle motor shaft screw  10  is inserted into the spindle motor shaft  11 , the head  13  of the spindle motor shaft screw  10  comes into contact with the inner tabs  2  of the lock washer  1  and begins to deform the lock washer  1 . The spindle motor shaft screw  10  forces the inner tabs  2  of the lock washer  1  down, which in turn forces the outer tabs  3  of the lock washer  1  up and into contact with the bottom surface of the top cover. 
     Referring now to FIG. 4, when the spindle motor shaft screw  10  is fully inserted into the spindle motor shaft  11 , the inner tabs  2  of the lock washer  1  are forced into contact with the SWD  7 , and the spindle motor shaft screw  11 . Additionally, the outer tabs  3  are forced into contact with the bottom surface of the top cover  8 . The lock washer  1  thus forms a conductive path between the SWD  7 , the spindle motor shaft  11 , the spindle motor shaft screw  10 , and the top cover  8 . 
     Referencing FIG. 2, the pre-assembly alignment features of an embodiment of the lock washer  1  will now be described. Prior to the insertion of the spindle motor shaft screw  10 , the lock washer  1  is aligned such that it fits within the space formed between the SWD  7  and the top cover  8 . The lock washer  1  rests upon the SWD  7  with only the curved pieces  6  having contact with the SWD  7 . In this way, there is no pressure exerted on the SWD adhesive  9  prior to the insertion of the spindle motor shaft screw  10 . 
     Referencing now FIGS. 1-5, the torque minimizing features of the lock washer  1  will now be described in detail. Due to the alignment of the inner tabs  2  and outer tabs  3 , the inner and outer circumference of the lock washer  1  do not form one continuous hoop. This way, when the lock washer  1  is deformed, the deformation occurs in the curved pieces  6  that connect the inner tabs  2  and outer tabs  3 . The force required to deform these curved pieces  6  is significantly less than the force that would be required to deform a continuous hoop. Additionally, due to the curved shape of the lock washer  1 , the outer tabs  3  contact the top cover  8 , and the inner tabs  2  contact the under surface of the spindle motor shaft screw head  13  and the SWD  7 . The larger diameter of the outer tabs  3  causes the majority of the torque generated upon insertion of the spindle motor shaft screw  10  to be transferred to the top cover  8  and away from the SWD  7  and SWD adhesive  9 . This torque transfer is due to both the larger torque associated with the larger radius of the outer tabs  3 , as well as decreased surface area in contact with the SWD  7  as compared to the top cover  8 . This combination of the reduced force required to deform the lock washer  1 , along with the transfer of the majority of the torque to the top cover  8  results in significantly less torque being applied to the SWD  7 , and the SWD adhesive  9 . 
     In another embodiment of the present invention, shown in FIGS. 6-7, the top cover  8  advantageously has slots  14  and top cover tabs  15  formed in the area where the lock washer  1  contacts it. In this embodiment, as shown in FIG. 7, as the lock washer  1  is deformed upon the insertion of the spindle motor shaft screw  10 , one or more of the outer tabs  3  will extend up into a slot  14  in the top cover  8 . Once the outer tab  3  has extended into the slot  14  in the top cover  8 , continued torque on the lock washer  1  will cause the outer tab  3  to come into contact with the edge of a top cover tab  15 . As the spindle motor shaft screw  10  continues to be inserted into the spindle motor shaft  11 , the torque generated by the tightening of the spindle motor shaft screw  10  will be transferred to the top cover  8  through the outer tab  3  that has come into contact with the top cover tab  15 . This will in turn reduce the torque that is applied to the SWD  7 , and the SWD adhesive  9 . A label or other closure may be applied over the opening in the top cover  8  to prevent particulate or other contamination from entering the drive housing. 
     Once the disk drive is assembled, the lock washer  1  will continue to exert force upon the top cover  8 , the spindle motor shaft screw  10 , and the SWD  7 . This continuing pressure will act to help prevent the spindle motor shaft screw  10  from loosening during the operation of the disk drive. Additionally, in the embodiment which utilizes the top cover tabs  15  and top cover slots  14 , any loosening of the spindle motor shaft screw  10  will result in the lock washer  1  turning and thereby cause one of the outer tabs  3  to contact the side of a top cover tab  15  which will act to prevent any further loosening of the spindle motor shaft screw  10 . 
     It will be understood that the invention may be embodied in other specific forms without departing from the spirit or central characteristics thereof The present examples and embodiments, therefore, are to be considered in all respects as illustrative and not restrictive, and the invention is not intended to be limited to the details given herein.