Abstract:
A snap ring for applications requiring cleanliness has a novel recessed interior contour that reduces debris generation during installation of the snap ring. The snap ring is suitable for applications where a reduction in debris generation is desirable, such as to retain an actuator pivot bearing in information storage devices like magnetic hard disk drives.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to snap rings designed for use in clean environments and particularly to snap rings for use in information storage devices. 
     2. Background Information 
     In hard disk drives, magnetic heads read and write data on the surfaces of co-rotating disks that are co-axially mounted on a spindle motor. The magnetically-written “bits” of written information are therefore laid out in concentric circular “tracks” on the surfaces of the disks. The disks must rotate quickly so that the computer user does not have to wait long for a desired bit of information on the disk surface to translate to a position under the head. In modern disk drives, data bits and tracks must be extremely narrow and closely spaced to achieve a high density of information per unit area of the disk surface. 
     The required small size and close spacing of information bits on the disk surface has consequences on the design of the disk drive device and its mechanical components. Among the most important consequences is that the magnetic transducer on the head must operate in extremely close proximity to the magnetic surface of the disk. However, because there is relative motion between the disk surface and the head due to the disk rotation and head actuation, continuous contact between the head and disk can lead to tribological failure of the interface. Such tribological failure, known colloquially as a “head crash,” can damage the disk and head, and usually causes data loss. Therefore, the magnetic head is typically designed to be hydrodynamically supported by an extremely thin air bearing so that its magnetic transducer can operate in close proximity to the disk while physical contacts between the head and the disk are minimized or avoided. 
     The head-disk spacing present during operation of modern hard disk drives is extremely small—measuring in the tens of nanometers. Obviously, for the head to operate so closely to the disk the head-disk interface must be kept clear of debris and contamination—even microscopic debris and contamination. In addition to tribological consequences, contamination and debris at or near the head disk interface can force the head away from the disk. The resulting temporary increases in head-disk spacing cause magnetic read/write errors. Accordingly, magnetic hard disk drives are assembled in clean-room conditions and the constituent parts are subjected to pre-assembly cleaning steps during manufacture. 
     Another consequence of the close spacing of information bits and tracks written on the disk surface is that the spindle rotation and head actuator motion must be of very high precision. The head actuator must have structural characteristics that allow it to be actively controlled to quickly seek different tracks of information and then precisely follow small disturbances in the rotational motion of the disk while following such tracks. 
     Characteristics of the actuator structure that are important include stiffness, mass, geometry, and boundary conditions. For example, one important boundary condition is the rigidity of the interface between the actuator arm and the actuator pivot bearing. 
     All structural characteristics of the actuator, including those mentioned above, must be considered by the designer to minimize vibration in response to rapid angular motions and other excitations. For example, the actuator arm can not be designed to be too massive because it must accelerate very quickly to reach information tracks containing desired information. Otherwise, the time to access desired information may be acceptable to the user. 
     On the other hand, the actuator arm must be stiff enough and the actuator pivot bearing must be of high enough quality so that the position of the head can be precisely controlled during operation. Also, the interface between the actuator arm and the pivot bearing must be of sufficient rigidity and strength to enable precise control of the head position during operation. 
     Actuator arm stiffness must also be sufficient to limit deflection that might cause contact with the disk during mechanical shock events that may occur during operation or non-operation. Likewise, the interface between the actuator arm and the pivot bearing must be of sufficient strength to prevent catastrophic structural failure such as axial slippage between the actuator arm and the actuator pivot bearing sleeve during large mechanical shock events. 
     In many disk drives, the actuator arm (or arms) is fixed to the actuator pivot bearing sleeve by a snap ring known as the actuator pivot bearing snap ring. The actuator pivot bearing snap ring typically includes one or more out-of-plane bends that function as a preloaded axial spring after assembly. The action of the actuator pivot bearing snap ring as a preloaded axial spring prevents separation and slippage at the interface between the actuator arm and the pivot bearing during operation and during mechanical shock events. 
     State of the art snap rings are typically metal parts that achieve their final shape through the use of a stamping die. The stamping die tends to slightly round the edges on one face of each snap ring. This rounding is known as stamping “die roll” and it can typically survive subsequent forming (e.g. coining) steps (if any). 
     The actuator pivot bearing snap ring may be installed with its face having edges with stamping die roll adjacent to and in contact with the actuator arm structure. In this case, the other face of the snap ring will be adjacent to and in contact with a surface of the pivot bearing sleeve. Alternatively, the actuator pivot bearing snap ring may be installed with its face having edges with stamping die roll adjacent to and in contact with the pivot bearing sleeve. In this case, the other face of the snap ring will be adjacent to and in contact with a surface of the actuator pivot bearing sleeve. 
     The actuator arm structure is typically fabricated from aluminum or an alloy of aluminum and is therefore typically softer and more easily burnished than the pivot bearing sleeve, which is typically fabricated from stainless steel. Therefore, less debris comprising aluminum are generated if a conventional snap ring is installed in an orientation such that its face having edges with stamping die roll are adjacent to and in contact with the actuator arm structure. 
     Although debris comprising aluminum may be reduced by specifying orientation of the snap ring when installed, most state-of-the-art attempts to improve post-fabrication cleanliness of disk drive components have focused on pre- and post-assembly cleaning steps and on environmental cleanliness during assembly. The industry&#39;s marked reliance on cleaning steps survives even though assembly in clean environments and post-assembly cleaning steps are not thorough in their removal of contaminants and debris. Less frequently, disk drive designers consider the generation of debris and contamination earlier in the design of sub-components. Still, such consideration is often restricted to the selection of lubricants and adhesives. 
     Consequently, there remains much scope in the art for reducing debris generation via novel changes to the basic design or assembly of various sub-components of the disk drive. Since only one of the faces of a conventional snap ring has stamping die roll, regardless of the snap ring&#39;s orientation one of its faces will be prone to generate debris (either through burnishing of the surface of the actuator arm structure or via contact with the pivot bearing sleeve). 
     Therefore, there is a need in the art for an actuator pivot bearing snap ring that can generally prevent or generally reduce the creation of debris during assembly rather than relying on debris removal by post-assembly cleaning steps. Although the need in the art was described above in the context of magnetic disk drive information storage devices, the need is also present in other applications where a snap ring is used in a clean environment that must remain as free as possible of debris and contaminants. 
     SUMMARY OF THE INVENTION 
     A snap ring comprises an interior contour that extends about an opening. The interior contour has a first segment that is defined by a first radius that is rotated about a first origin within the opening. The interior contour has at least one second segment that is defined by a second radius that is rotated about a second origin within the opening. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a snap ring according to an embodiment of the present invention. 
         FIG. 2  is a side view of a snap ring according to an embodiment of the present invention. 
         FIG. 3  is a drawing of the outer periphery of a stamping die punch used to fabricate the interior contour of a snap ring according to an embodiment of the present invention. 
         FIG. 4  depicts two instants in time during the installation of a snap ring according to an embodiment of the present invention, as used to retain an actuator pivot bearing relative to an actuator arm. 
         FIG. 4A  shows in isolation and with tilt removed either of the snap ring cross sections shown with tilt and in context in  FIG. 4 . 
         FIG. 5  depicts two instants in time during the installation of a snap ring according to another embodiment of the present invention, as used to retain an actuator pivot bearing relative to an actuator arm. 
         FIG. 5A  shows in isolation and with tilt removed either of the snap ring cross sections shown with tilt and in context in  FIG. 5 . 
     
    
    
     In these figures, similar numerals refer to similar elements in the drawing. It should be understood that the sizes of the different components in the figures may not be to scale, or in exact proportion, and are shown for visual clarity and for the purpose of explanation. 
     DETAILED DESCRIPTION 
     A snap ring for applications requiring cleanliness has a novel recessed interior contour that reduces debris generation during installation of the snap ring. 
       FIG. 1  shows a top view of an actuator pivot bearing snap ring according to an embodiment of the present invention, that illustrates several specific design features and associated nomenclature. The snap ring has an opening bounded by interior contour  10  and has an outer contour  11 . In this embodiment, the width of the snap ring in hinge region  12  is wider than in neck regions  13  and  14 . The snap ring terminates at two terminal regions  15  and  16  that may comprise tabs that include tooling holes  17  and  18 , respectively. The snap ring is typically forcefully expanded during installation which temporarily increases the circumferential gap  19  between the terminal regions  15  and  16 . The forceful expansion also causes the interior contour  10  to temporarily deform. 
     In the embodiment of  FIG. 1 , the interior contour  10  is not round in the undeformed (“free”) state but rather substantially departs from a round contour (depicted by a dashed line) near localized regions  20  and  21 . Localized regions  20 ,  21 , and  22  include locations where the snap ring would most heavily tangentially contact an internal round object (e.g. installation cone) during installation, if the interior contour of the snap ring were round in the free state like conventional snap rings typically are. Such contact localizes into regions because an initially round interior contour, when expanded during the installation process, generally departs from being round during the period of expansion. However, in the embodiment of  FIG. 1  contact is at least partially spread within or away from localized regions  20  and  21  because material is removed to recess the interior contour in these regions. 
     In the embodiment of  FIG. 1 , the interior contour  10  includes a segment that comprises at least half of the interior contour  10  and is defined by sweeping a radius centered at origin  1 . The magnitude of the angle between a line passing through origin  1  and extending towards contact region  20  (or  21 ) and a downward pointing vertical line (also passing through origin  1 ) is represented in  FIG. 1  by the Greek letter α. 
       FIG. 2  shows a side view of an actuator pivot bearing snap ring according to an embodiment of the present invention. The snap ring has thickness  38  and has one or more out-of-plane bends  32 ,  33  that cause regions  35  to have vertical stature relative to regions  30  and  31 —enabling the snap ring to perform as an axial spring. The snap ring is axially compressed in its installed state, so that it has an axial preload after installation. This preload is maintained after installation because the preloaded snap ring contacts constraining surfaces of the parts to be relatively retained (e.g. actuator arm structure and actuator pivot bearing sleeve). In disk drive applications, depending on the orientation of the snap ring after installation, either surfaces  34  will contact the actuator arm structure while surfaces  36  and  37  contact one or more surfaces of the actuator pivot bearing sleeve, or surfaces  34  will contact one or more surfaces of the actuator pivot bearing sleeve while surfaces  36  and  37  contact the actuator arm structure. 
       FIG. 3  is a drawing of the outer contour  70  of a stamping die punch used to fabricate the interior contour of a snap ring according to an embodiment of the present invention. In this embodiment, the majority of the interior contour of the snap ring is stamped (i.e. “punched”) to an ordinary radius  71 . However, near regions  20  and  21  (described earlier) the snap ring is stamped to a recessing radius  73 . In the embodiment of  FIG. 3 , the origin of the recessing radius  73  is shifted horizontally from the origin of the ordinary radius  71  by a horizontal shift  74  and is shifted vertically by a vertical shift  75 . If the origins were chosen to be coincident, and if recessing radius  73  were chosen to be greater in length than ordinary radius  71 , then the radial reach of recessing radius  73  would exceed the radial reach of ordinary radius  71  everywhere on the interior contour. In that case, the boundary between a segment of the interior contour defined by ordinary radius  71  and any segment of the interior contour defined by recessing radius  73  might be characterized by an undesirable sharp radial transition. 
     A sharp radial transition can be avoided by including a transition segment of varying radius or by choosing the relationship between the recessing radius  73 , the ordinary radius  71 , the horizontal shift  74 , and the vertical shift  75 , as follows:
 
Radius 73 =Radius 71 +√{square root over (Shift 74   2 +Shift 75   2 )}
 
     In order for the recessing radius  73  to cause an area of contact between the snap ring and the installation cone and/or actuator pivot bearing sleeve flange to shift and spread, and thereby decrease the associated contact pressure at the interfaces to reduce the propensity for scratching or galling of the contacting surfaces, the recessing radius  73  must reach further into the radial width of the snap ring in a region of expected contact than the ordinary radius  71  does. In the embodiment of  FIG. 3 , because the origins of ordinary radius  71  and recessing radius  73  are not coincident, the radial reach of recessing radius  73  is not assured to be greater than the radial reach of ordinary radius  71  merely by virtue of being greater in length. Rather, in the embodiment of  FIG. 3 , such reach is only assured if the recessing radius  73  is chosen to satisfy the following inequality:
 
Radius 73 &gt;√{square root over ((Radius 71  cos α+Shift 75 ) 2 +(Radius 71  sin α+Shft 74 ) 2 )}{square root over ((Radius 71  cos α+Shift 75 ) 2 +(Radius 71  sin α+Shft 74 ) 2 )}
 
where α represents the magnitude of the angle from a downward pointing vertical line passing through the origin of the ordinary radius to another line drawn from the origin of the ordinary radius to either of regions  20 ,  21  (where contact would occur if the interior contour were stamped at a radius equal to the ordinary radius everywhere along the contour except in the region of gap  19 ).
 
     In certain embodiments, the choice of recessing radius  73 , horizontal shift  74 , and vertical shift  75  may be further constrained by a design requirement that at least half of the interior circumference of the snap ring be defined by ordinary radius  71 . 
     In certain embodiments, the recessing radius  73  is further constrained to not exceed a reach where a resulting narrowness of the snap ring (in the radial direction) significantly weakens the snap ring such that its strain during installation or removal is concentrated in a localized region of weakness. In a particular embodiment, this constraint on the recessing radius  73  can be expressed in terms of a design requirement that the recessing radius  73  may not exceed a reach where the resulting ratio of the width, cubed, of the snap ring (in the radial direction) in a region of contact, divided by the distance from that region of contact to a tooling hole (e.g. one of tooling holes  17  or  18 ), becomes less than half of the minimum ratio of the cubed width of the snap ring (measured anywhere) divided by the distance from where that width is measured to said tooling hole. That is, in this particular embodiment, recessing radius  73  can not be chosen so large that: 
     
       
         
           
             
                
               
                 
                   w 
                   c 
                   3 
                 
                 
                   d 
                   c 
                 
               
                
             
             &lt; 
             
               0.5 
               ⁢ 
               
                 
                    
                   
                     
                       w 
                       3 
                     
                     d 
                   
                    
                 
                 min 
               
             
           
         
       
     
     where w c  is the width of the snap ring (in the radial direction) in a region of contact, d c  is the lever arm distance from the aforementioned region of contact to a tooling hole (e.g. one of tooling holes  17  or  18 ), w is the width of the snap ring at any arbitrary point on the snap ring, and d is the distance from the arbitrary point on the snap ring where w is measured to said tooling hole. 
       FIG. 4  depicts two instants during the installation of a snap ring fabricated according to an embodiment of the present invention, to retain an actuator pivot bearing  45  relative to an actuator arm structure  47 . To provide greater detail in  FIG. 4 , only the top portion of an actuator arm structure  47  is shown. The rest of the actuator arm structure  47  appears cut away in  FIG. 4 . Also to provide greater detail in  FIG. 4  only the portion of the actuator arm structure  47  that falls to the left of axis of rotation  44  is shown. A portion of the actuator pivot bearing sleeve  45  is also visible in  FIG. 4 . Only the portion of the actuator pivot bearing sleeve that protrudes above the top surface  48  of actuator arm structure  47  can be seen, and only the portion of the actuator pivot bearing sleeve  45  that falls to the left of axis of rotation  44  is shown. 
     The actuator pivot bearing sleeve  45  is meant to be retained relative to the actuator arm structure  47  by a snap ring to be installed in grove  61 . The axial preload of the snap ring will exert an upward force on the underside of top flange  46  of the actuator pivot bearing sleeve  45 , and a downward force on the top surface  48  of the actuator arm structure  47 . 
     Temporarily mounted on the top flange  46  of the actuator pivot bearing sleeve  45  is a snap ring installation cone  40 . The snap ring installation cone  40  is only mounted during installation of the snap ring. Only the portion of the snap ring installation cone  40  that falls to the left of axis of rotation  44  is shown in  FIG. 4 . The snap ring installation cone  40  has an upper conical surface  41 , a lower cylindrical surface  42 , and a bottom edge  43 . The snap ring installation cone  40  and the actuator pivot bearing sleeve  45  are typically fabricated from stainless steel, and the actuator arm structure  47  is typically fabricated from aluminum or an alloy of aluminum. 
     A cross-section  80  of a snap ring fabricated according to an embodiment of the present invention is shown in  FIG. 4 . Cross section  80  is taken at a location on the snap ring where the snap ring contacts the snap ring installation cone  40 . The circumferential locations and extent of the regions of contact in this embodiment depend on the choices made for the design parameters defined with respect to  FIG. 3  (i.e. recessing radius  73 , horizontal shift  74 , and vertical shift  75 ). 
     Note that bottom edge  52  of the snap ring has stamping die roll and is shown to be in sliding contact with the upper conical surface  41  of snap ring installation cone  40 . Note also that snap ring cross section  80  is tilted by an angle  84  so that the portion of the cross section corresponding to top edge  81  ends up being the most proximate portion of the snap ring cross section  80  to the axis of rotation  44 . 
     Cross section  80  tilts during circumferential expansion of the snap ring because of accompanying torsional deflection. The torsional deflection occurs during circumferential expansion of the snap ring because the snap ring is not flat but rather is fabricated, as previously described, with out-of-plane bends that cause certain regions to have vertical stature relative to other regions. The more the snap ring is circumferentially expanded, the greater will be the angle  84  of tilt. So when the snap ring is pushed further down the snap ring installation cone  40  to a new position where the cone has a larger diameter, not only is the snap ring further circumferentially expanded (causing temporary growth in gap  19 ), but the angle  84  of tilt of cross section  80  will increase also. 
     Cross section  83  in  FIG. 4  is the same as cross section  80  except cross section  83  is depicted at a slightly later instant in time during the installation process, where both the temporary radial expansion and tilt of the snap ring are greater. Accordingly, top edge  81  of snap ring cross section  83  is depicted to be in sliding contact with the lower cylindrical surface  42  of snap ring installation cone  40 , at the location on the snap ring where cross section  83  (and  80 ) is taken. During the final phase of the snap ring installation process, the contacting surface of top edge  81  of snap ring cross section  83  slides over the bottom edge  43  of snap ring installation cone  40 , and over the bottom edge of actuator pivot bearing flange  46 , at location  89 , to “snap in” to actuator pivot bearing groove  61 . 
     Tribological problems in magnetic disk drives sometimes have non-obvious causes that, once known, understood, and accounted for, give one disk drive manufacturer a competitive edge over another. The present inventors recognized that the final “snapping in” phase of the snap ring installation process can shear metal fragments from the edges of the snap ring installation cone  40  and the actuator pivot bearing sleeve flange  46 , and such fragments can later contaminate the head-disk interface and ultimately lead to a head crash and possibly to data loss. Their solution to this problem is novel. 
       FIG. 4A  shows in isolation and with tilt removed either of the snap ring cross sections that are shown with tilt and in the context of adjacent parts during installation in  FIG. 4 . In the embodiment of  FIG. 4  and  FIG. 4A , the edge lacking die roll (i.e. top edge  81  of the snap ring) is deliberately rounded (e.g. by a separate coining step) in at least a region of contact to provide a curved edge profile that can be approximately characterized by radius of curvature  82 . In a preferred embodiment, the cross-sectional profile in a region of contact is adequately and practically blunted if the radius of curvature  82  is chosen to be in the design range of 40% to 85% of the thickness of the snap ring. 
       FIG. 5  depicts two instants during the installation of a snap ring fabricated according to an embodiment of the present invention, to retain an actuator pivot bearing  45  relative to an actuator arm structure  47 .  FIG. 5  is meant to be generally similar to  FIG. 4 , except for a change to the geometry of the snap ring. 
     To provide greater detail in  FIG. 5 , only the top portion of an actuator arm structure  47  is shown. The rest of the actuator arm structure  47  appears cut away in  FIG. 5 . Also to provide greater detail in  FIG. 5  only the portion of the actuator arm structure  47  that falls to the left of axis of rotation  44  is shown. A portion of the actuator pivot bearing sleeve  45  is also visible in  FIG. 5 . Only the portion of the actuator pivot bearing sleeve that protrudes above the top surface  48  of actuator arm structure  47  can be seen, and only the portion of the actuator pivot bearing sleeve  45  that falls to the left of axis of rotation  44  is shown. 
     Temporarily mounted on the top flange  46  of the actuator pivot bearing sleeve  45  is a snap ring installation cone  40 . The snap ring installation cone  40  is only mounted during installation of the snap ring. Only the portion of the snap ring installation cone  40  that falls to the left of axis of rotation  44  is shown in  FIG. 5 . The snap ring installation cone  40  has an upper conical surface  41 , a lower cylindrical surface  42 , and a bottom edge  43 . 
     A cross-section  85  of a snap ring fabricated according to an embodiment of the present invention is shown in  FIG. 5 . Cross section  85  is taken at a location on the snap ring where the snap ring contacts the snap ring installation cone  40 . The circumferential locations and extent of the regions of contact in this embodiment depend on the choices made for the design parameters defined with respect to  FIG. 3  (i.e. recessing radius  73 , horizontal shift  74 , and vertical shift  75 ). 
     Note that bottom edge  52  of the snap ring has stamping die roll and is shown to be in sliding contact with the upper conical surface  41  of snap ring installation cone  40 . Note also that snap ring cross section  85  is tilted by an angle  84  so that the portion of the cross section corresponding to top edge  86  ends up being the most proximate portion of the snap ring cross section  85  to the axis of rotation  44 . 
     Cross section  88  in  FIG. 5  is the same as cross section  85  except cross section  88  is depicted at a slightly later instant in time during the installation process, where both the temporary radial expansion and tilt of the snap ring are greater. Accordingly, top edge  86  of snap ring cross section  88  is depicted to be in sliding contact with the lower cylindrical surface  42  of snap ring installation cone  40 , at the location on the snap ring where cross section  88  (and  85 ) is taken. During the final phase of the snap ring installation process, the contacting surface of top edge  86  of snap ring cross section  88  slides over the bottom edge  43  of snap ring installation cone  40 , and over the bottom edge of actuator pivot bearing flange  46 , at location  89 , to “snap in” to actuator pivot bearing groove  61 . 
       FIG. 5A  shows in isolation and with tilt removed either of the snap ring cross sections that are shown with tilt and in the context of adjacent parts during installation in  FIG. 5 . In the embodiment of  FIG. 5  and  FIG. 5A , the edge lacking die roll (i.e. top edge  86  of the snap ring) is deliberately beveled (e.g. by a separate coining step) in at least a region of contact to provide a flattened edge profile that can be approximately characterized by bevel angle  87  and bevel depth  90 . In a preferred embodiment, the cross-sectional profile in a region of contact is adequately and practically blunted if the bevel angle  87  is chosen to be in the design range of 10° to 40° and the bevel depth is chosen to be in the design range of 60% to 85% of the thickness of the snap ring. The bevel angle and depth can be deliberately formed (e.g. by a separate coining step) within these design ranges during manufacture, after the stamping step that creates the interior radius of the snap ring. 
     Stamping (as well as coining) is generally accomplished using a press that pushes on a die that includes a die block and a die punch. A feeder advances the material to be stamped (e.g. a metal sheet) into or through the die, and a “stripper” clamps the material during stamping. The die punch is pressed against the die block or through a hole in the die block (in which case the punch must be smaller than the hole by a clearance). The clearance must be carefully selected to avoid the formation of burrs in the material that is stamped, yet also to ensure adequate life of the die components. 
     Any beveling that might occur incidentally due to stamping (i.e. so-called “die break”) depends upon the stamping clearance and also depends upon the choice of the material to be stamped. Therefore, “die break” can not be controlled without affecting (potentially adversely) material properties, the avoidance of burrs, and the life of the die components. A bevel that is intentionally formed (e.g. by a separate coining step) can be controlled without these adverse consequences and also does not present a jagged edge that is characteristic of “die break”. 
     While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and detail may be made therein without departing from the scope of the invention.