Abstract:
A tolerance ring for applications requiring high axial static friction has contacting portions having a novel profile. The use of multiple contacting portions having a smaller size that are wedge-shaped increases rigidity and provides a directional grip that increases axial static function. Using multiple wedge-shaped contacting portions with appropriate opposite slants provides a balanced and symmetrical grip. Using inter-level wedge-shaped contacting portions with opposite slants provides a more aggressive grip while providing a tolerance ring that is more dynamically stable and has a high resonant frequency.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to bearing tolerance rings and, more specifically, pertains to tolerance rings used in cartridge bearings for actuator arms in information storage devices. 
     2. Description of Prior Art 
     A key component of any computer system is a device to store data. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are a disc that is rotated, an actuator that moves a transducer to various locations on the disc, and electrical circuitry that is used to write and read data on and from the disc. There are a variety of disc drives in use such as hard disc drives, zip drives, floppy disc drives which all utilize either rotary or linear actuators. 
     In disc drive systems, magnetic heads read and write data on the surface of co-rotating discs that are coaxially mounted on the spindle motor. The bits of information written on a disc are laid out in concentric circular tracks on the surface of the discs. The discs must rotate quickly so that the computer user does not have to wait long for a desired bit of information on the disc surface to translate to a position under the head. In modern disc drives, especially in hard disk drives, data bits and tracks must be extremely narrow and closely spaced to achieve a high information density per unit area of the disc surface. 
     The required small size and close spacing of information bits on the disc surface have the consequences on the design of the disc drive device and its mechanical components. The most important consequence is the magnetic transducer on the head must operate in extremely close proximity to the magnetic surface of the disc. Because there is relative motion between the disc surface and the head due to the disc rotation and head actuation, continuous contact between the head and disc can lead to failure of the interface. Such failure can damage the disc and head and usually causes data loss. To avoid this problem, a magnetic head is typically designed to be hydrodynamically supported by an extremely thin air bearing surface (“ABS”). When a disc rotates, air is dragged between the head and the disc surface, causing pressure, which forces the head away from the disc. At the same time, the air rushing past the head and disc produces a negative pressure area. These forces are designed to balance so that the magnetic head flies over the surface of the disc at a particularly desired fly height in very close proximity to the disc while avoiding physical contact between the head and disc. In typical systems, the spacing between the head and disc during operation is extremely small, measuring in the tens of nanometers. 
     Another consequence of the close spacing required between the bits and tracks written on the disc surface is that the spindle rotation and head actuator motion must be operated with extremely 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 disc while following the tracks. The characteristics of the actuator structure that are important to this end, 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 must be considered by the designer to minimize vibration in response to rapid angular motions and other excitations. For example, the actuator arm cannot 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 unacceptable to the user. On the other hand, the actuator arm must 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. The interface between the actuator arm and the pivot bearing must be of sufficient rigidity and strength to enable precise control of a head position during operation and to provide the boundary conditions necessary to facilitate higher natural resonant frequencies of operation of the actuator arm structure. Actuator arm stiffness must also be sufficient to limit deflection that might cause contact with the disc during mechanical shock events that may occur during operation or non-operation. The interface between the actuator arm and the pivot bearing must be of sufficient strength to prevent catastrophic structural failure such as actual slippage between the actuator arm and the actuator pivot-bearing sleeve, during large mechanical shock events. 
     In many disc drives, the actuator arm or arms are fixed to the actuator pivot bearing by a tolerance ring. Typically, tolerance rings include a cylindrical base portion and a plurality of contacting portions that are raised or recessed from the cylindrical base portion. The contacting portions are typically partially compressed during installation to create a radial preload between the mating cylindrical features of the parts joined by the tolerance ring. The radial preload compression provides frictional engagement that prevents actual slippage of the mating parts. For example, in disc drive applications, the radial compressive preload of the tolerance ring prevents separation and slippage at the interface between the actuator arm and the pivot bearing during operation and during mechanical shock events. The tolerance ring also acts as a radial spring. In this way, the tolerance ring positions the interior cylindrical part relative to the exterior cylindrical part while making up for radii clearance and manufacturing variations in the radius of the parts. 
     Additional features have been added to tolerance rings to obtain the specific advantages. For example, in U.S. Pat. No. 6,288,878 to Misso et al., circumferential brace portions have been added to the tolerance ring to increase hoop strength. U.S. Pat. No. 6,338,839 to Misso et al. discloses a tolerance ring which provides a low consistent installation force profile. 
     U.S. Pat. No. 4,790,683 to Cramer, Jr. et al. discloses the use of a conventional tolerance ring in conjunction with a cylindrical shim in applications characterized by structurally significant radial vibration or loading. The shim prevents deformation of the soft underlying material and thereby prevents undesirable partial relief of the radial compression that maintains frictional engagement of the tolerance ring. 
     State of the art tolerance rings are typically manufactured from a flat metal sheet with stamping, forming, rolling, and other steps to provide ways to recess contacting portions and a final generally cylindrical shape. The tolerance ring can be installed first into a generally cylindrical hole in an exterior part, such as an actuator arm, so that later a generally cylindrical inner part, such as an actuator pivot bearing, can be forcibly pushed into the interior of the tolerance ring to create a radial compressive preload that retains the parts by frictional engagement. In this case, the contacting portions may be recessed to a lesser radius than the base portion as well as raised to a greater radius than the base portion. Alternatively, a tolerance ring can be installed first around a generally cylindrical inner part, such an actuator pivot bearing. The inner part, together with the tolerance ring, is then forcibly pushed into the interior of the generally cylindrical hole in an exterior part, such as an actuator arm, to create a radial compressive preload that retains the parts by frictional engagement. In this case, the contacting portions of the tolerance ring are typically raised to a greater radius than the base portion. 
     There is a need in the art for a tolerance ring that can accommodate thermal mismatches that might occur between the bearing cartridge and the actuator arm as the disc drive heats from a starting temperature to an operating temperature. Moreover, there is a need for a tolerance ring that provides an increased internal diameter static friction that not only provides better performance, but prevents the tolerance ring from slipping during operation as a result of a shock event. 
     SUMMARY OF THE INVENTION 
     A tolerance ring having a substantially cylindrical base portion with a plurality of contacting portions, the base portion being at a first radius, each contacting portion having a central region with a nadir at a second radius and an apex at a third radius. Each contacting portion has at least two circumferential transition regions adjacent to the central region, the circumferential transition regions spanning from the first radius to the second and third radii over a circumferential transition length. Each contacting portion also has a first and second axial transition region, the first axial transition region spanning from the first radius to the second radius over an axial transition length, and the second axial transition region spanning from the first radius to the third radius over an axial transition length. The contacting portions are arranged on the base portion in multiple rows about the circumference of the cylindrical base. In one preferred embodiment, all the contacting portions are arranged in multiple parallel circumferential rows with the apex of the central region of each contacting portion located at an inside circumference. In a second preferred embodiment, all the contacting portions are arranged in multiple parallel circumferential rows with each axial pair of contacting portions, one in each parallel row, alternating between having the apex of their central regions at an inside circumference and at an outside circumference. In a third preferred embodiment, four parallel rows of contact portions are arranged with each axial pair in staggered rows, the contact portions of each pair alternating between having the apex of their central regions at an inside circumference and at an outside circumference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The exact nature of this invention, as well as the objects and advantages thereof, will become readily apparent from consideration of the following specification in conjunction with the accompanying drawings in which like reference numerals designate like parts throughout the figures thereof and wherein: 
         FIG. 1  is an exploded view of a disc drive actuator arm assembly, including a tolerance ring, according to an embodiment of the present invention. 
         FIG. 2  is a perspective view of a tolerance ring according to an embodiment of the present invention. 
         FIG. 3  is a detailed perspective view of a single contacting portion of the area A on the tolerance ring of  FIG. 1 . 
         FIG. 4  is an axial view of a tolerance ring according to an embodiment of the present invention. 
         FIG. 5  is a cross-sectional view of the tolerance ring of  FIG. 4 , taken along the cross-sectional line  5 - 5  in  FIG. 1 . 
         FIG. 5   a  is a magnified cross-sectional view of another tolerance ring according to the invention, taken along section line  5 - 5  in  FIG. 4 , in which the axial transition region of the contact portion has a radius of curvature. 
         FIG. 6  is a cross-sectional view along the circumference of a single contacting portion of the area A on the tolerance ring of  FIG. 1 . 
         FIG. 7  is a perspective view of a tolerance ring according to another embodiment of the present invention. 
         FIG. 8  is a perspective view of a tolerance ring according to yet another embodiment of the present invention. 
       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 purpose of explanation only. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although the invention described in this application is useful with all mechanical configurations of disc drives having rotary actuation, and is useful for all types of devices whose precise co-axial location of mating parts is desirable; it has particular application to hard disc drive systems. 
       FIG. 1  is an exploded view of an actuator arm assembly  13  which includes a bearing cartridge  23 . The bearing cartridge is cylindrical in shape and includes a shaft  24  about which the actuator arm assembly  13  rotates. The actuator arm assembly  13  has an opening or bore  27  therein. The bearing cartridge  23  fits within the bore  27  of actuator arm assembly  13 . The tolerance ring  25  fits within the space between the bore  27  and the outside diameter of the bearing cartridge  23 . 
     Actuator arm assembly  13  has a plurality of arms  15  in the comb assembly  17 . Each arm  15  typically carries at least one suspension  19 . Attached to the suspension  19  are recording heads (sliders)  21  which include magnetic transducers that magnetize the surface of the disc (not shown) to represent and store the desired data. 
     As is well known in the art of disc drives, each of the discs has a series of concentric tracks onto which the magnetic information is recorded. The sliders  21  and the magnetic transducers incorporated therein are moved over the surface of a particular disc so that a magnetic representation of data can be stored on any of the tracks on the disc. The particular actuator arm assembly  13  shown in  FIG. 1  causes transducer movement to be rotational and about the shaft  24  of bearing cartridge  23 . Rotating the actuator arm assembly  13  causes the slider  21  and the transducer in the slider to be repositioned over the surface of the disc below it. 
       FIG. 2  illustrates in perspective a tolerance ring  25  according to a preferred embodiment of the present invention. The tolerance ring  25  has a cylindrical base portion  35  and a plurality of contacting portions  33 . Elastic radial expansion and contraction of cylindrical opening  29  of the tolerance ring  25  is facilitated by an axially oriented gap  31  in the circumference of tolerance ring  25 . The contacting portions  33  have central regions or surfaces  39 , angled circumferential transition regions or surfaces  41 , and axial transition regions or surfaces  37  and  43 . 
       FIG. 3  is an expanded view of a single contacting portion  33  within the detailed region A of the tolerance ring  25  of  FIG. 2  and depicts the regions of the contacting portion with greater clarity. Contacting portion  33  has an overall axial length  47  and an overall circumferential width  45 . The circumferential transition regions  41  are steeper than the axial transition regions  43 ,  37 , thereby providing greater radial stiffness. 
     The circumferential regions  41  are generally in the shape of a four-sided figure with none of the sides being parallel. The shape results from the size and height difference between axial transition region  43  and axial transition region  37 . The figure illustrates the central region edge next to axial transition region  43  to be higher than the central region next to axial transition region  37 . 
     A first axial transition region  37  spans from a first radius of the cylindrical base portion  35  to a second radius over a first axial transition length. A second axial transition region  43  spans from the first radius of the cylindrical base portion  35  to a third radius. As illustrated in  FIG. 3 , this third radius is greater than the second radius of the first axial transition region  37 , and greater than the first radius of the cylindrical base portion  35 . It could just as well be smaller. Moreover, the second and third radii could both be smaller than the first radius, if preferred. 
     The preferred embodiment of  FIG. 2  shows a plurality of contacting portions  33  arranged in parallel circumferential rows on the outside surface of cylindrical base portion  35 . Each individual contact portion  33  is arranged so that the second axial transition length of transition region  43  is located at an inside circumference. The first axial transition length of the first axial transition region  37  of each contacting portion  33  is located at an outside circumference of the cylindrical base portion  35 . 
     Although  FIG. 2  illustrates the contacting portions  33  extending from the outside surface of cylindrical base portion  35 , the contacting portions  33  could also be located on or extending from the inside surface of cylindrical base portion  35 . Either arrangement is considered a preferred embodiment of this invention. 
       FIG. 4  is an axial view of a tolerance ring according to a preferred embodiment of the present invention. The cylindrical base portion  35  has a first radius  53 . The contact portions  33  each have two radii  51   a  and  51   b , the second radius  51   a  at a first edge is less than a third radius  51   b  at a second edge. Both radii  51   a  and  51   b  are larger than the radius  53  of cylindrical base portion  35 . However, it should be understood that both radius  51   a  and  51   b  would be smaller than the radius  53  of base portion  35 , if the contacting portions  33  were chosen to point inwardly rather than outwardly, as shown in  FIG. 4 .  FIG. 4  also illustrates the relatively narrow, steep profile of the circumferential transition regions  41  which span from a cylindrical base portion radius  53  to the contact portion radius  51   a  and  51   b  over circumferential transition lengths  41 . 
       FIG. 5  is a cross-sectional view of the tolerance ring of  FIG. 4  taken along the cross-sectional plane labelled “ 5 - 5 ” in  FIG. 4 .  FIG. 5  most clearly illustrates the wedge-shaped profile of the contact portions  33  and specifically the directional gripping created by the difference between the height  59  of the first axial transition region  37  and the height  60  of the second axial transition region  43 . 
     The first axial transition region  37  and the second axial transition region  43  are illustrated as straight-line transition regions. These regions could also have a radius of curvature, Rc 1  or Rc 2 , as shown in the magnified cross-sectional view of  FIG. 5   a . Whether the axial transition region  43  has a radius of curvature or is more of a straight line, the radius of curvature is preferably at least two and a half times the thickness T or  58 , of the material from which the tolerance ring is fabricated. 
     In a preferred embodiment, the ratio of axial transition length  57  to the overall axial length  47  ( FIGS. 3 ,  5 ) is more than the ratio of the circumferential transition length  49  ( FIGS. 4 ,  6 ) to the overall circumferential width  45  ( FIG. 3 ), but less than 250 times the ratio of the circumferential transition length  49  ( FIGS. 4 ,  6 ) to overall circumferential width  45  ( FIG. 3 ). 
     In an alternate preferred embodiment, the ratio of circumferential transition length  49  ( FIGS. 4 ,  6 ) to overall circumferential width  45  ( FIGS. 4 ,  6 ) is less than or equal to 0.4. 
     The use of many contacting portions  33  that are smaller in size improves the contact area and provides increased rigidity. The use of contact portions shaped as wedges creates a more directional gripping action, which can be used to advantage. Once the tolerance ring with these wedge-shaped contact portions is inserted into the base  27  of the actuator arm  13 , the wedge-shaped contact portions will grip the inside diameter of the bore  27  when the bearing cartridge  23  is inserted. If the contact portions  33  are located on the inside surface of the base portion  35 , the contact portions will grip the outside diameter of the bearing cartridge  23  when the bearing and tolerance ring are inserted into the bore  27  of actuator assembly  13 . 
       FIG. 7  shows a perspective view of a tolerance ring  61  according to another preferred embodiment of the present invention. The tolerance ring  61  has a cylindrical base portion  35  with a plurality of contact portions  33  located in parallel circumferential rows about the surface of base portion  35 . The contact portions  33  are arranged in pairs so that the axially aligned contact portions  33  in the parallel rows have either their first axial transition regions  37  located at an inside boundary of a circumferential row or their second axial transitional regions  43  located at an inside boundary of the circumferential row, in alternate fashion, as shown in  FIG. 6 . This alternating of the wedge-shaped contact portions creates a more balanced and symmetrical gripping force. 
       FIG. 8  is a perspective view of a tolerance ring  63  according to another preferred embodiment of the present invention. The tolerance ring  63  has a cylindrical base portion  35  and a plurality of contact portions  33  arranged in multiple parallel circumferential rows about the cylindrical base portion  35 . The preferred embodiment of  FIG. 8  illustrates four parallel rows with every two parallel rows overlapping. The contact portions are arranged in adjacent rows to create a staggered arrangement of contact portions  33 . The axially aligned pairs of contact portions  33  are arranged to have one pair with their second axial transition regions located at an inside boundary of a circumferential row, alternating with an adjacent pair of axially aligned contact portions with their first axial transition regions located at an inside boundary of a circumferential row, as illustrated in  FIG. 8 . This arrangement of contacting portions has an advantage even when the contacting portions are not wedge-shaped, but are symmetrical as shown in co-pending application U.S. Ser. No. 11/059,813 filed on Feb. 17, 2005 for Tolerance Ring With Debris-Reducing Profile. 
     Interleaving the wedge-shaped contact portions  33  in this manner creates a more dynamically stable and higher resonant frequency structure and significantly increase the gripping force. The directionally opposite wedge-shaped contact portions provide a more aggressive grip formation. Use of symmetrical contacting portions produces a more dynamically stable and higher resonant frequency structure.